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Article

Elasmobranch Species Composition in Otter Trawl Fisheries (Eastern Aegean Sea)

by
İlker Aydin
1,
Alexandros Theocharis
2,3,
Sercan Yapici
4 and
Dimitris Klaoudatos
2,*
1
Department of Fishing Technology, Faculty of Fisheries, Ege University, 35100 İzmir, Türkiye
2
Department of Ichthyology and Aquatic Environment, University of Thessaly, Fytokou Street, 38 446 Volos, Greece
3
National Institute of Aquatic Resources, Technical University of Denmark, North Sea Science Park, 9850 Hirtshals, Denmark
4
Faculty of Fisheries, Muğla Sıtkı Koçman University, 48000 Muğla, Türkiye
*
Author to whom correspondence should be addressed.
Oceans 2025, 6(2), 34; https://doi.org/10.3390/oceans6020034
Submission received: 27 April 2025 / Revised: 24 May 2025 / Accepted: 4 June 2025 / Published: 6 June 2025
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Abstract

:
The Eastern Aegean Sea hosts a diverse assemblage of elasmobranchs, many of which are vulnerable or endangered. This study presents a fishery-independent assessment of species composition, catch characteristics, and spatial patterns in bottom trawl fisheries between Lesvos Island and Ayvalik. A total of 48 surveys were conducted between September 2022 and October 2024, identifying nine elasmobranch species, with Scyliorhinus canicula (small-spotted catshark) and Mustelus mustelus (common smooth-hound) dominating the catch. Biological parameters, sex ratios, and condition upon capture and release were recorded, while catch per unit effort (CPUE) and diversity indices were used to evaluate temporal patterns. The survival probability was negatively affected by the trawl duration and elevated temperatures, emphasizing the need for mitigation measures. Spatial models revealed high-density zones that likely function as foraging or nursery grounds. Seasonal shifts in community composition were also evident. Many non-commercial species were discarded irrespective of their size or condition. These findings underscore the ecological importance of this understudied region and support the need for spatially explicit, species-specific management strategies, including gear selectivity improvements, seasonal closures, and Electronic Monitoring. The study offers a critical baseline for enhancing the sustainability of elasmobranch populations in the Eastern Mediterranean.

1. Introduction

Elasmobranchs, a subclass of cartilaginous fishes including sharks, skates, and rays, occupy critical ecological roles as apex and meso-predators within marine ecosystems [1]. Their position at the top of the trophic web makes them highly sensitive to environmental changes and anthropogenic stressors, rendering them valuable biological indicators of ecosystem health [2,3]. Elasmobranchs are characterized by K-selected life history traits such as slow growth rates, delayed sexual maturity, low reproductive output, and long lifespans [4,5,6]. These features, while evolutionarily advantageous in stable environments, significantly increase their vulnerability to overexploitation and habitat degradation [7].
Turkey’s marine waters, particularly in the Eastern Mediterranean, harbor a notable diversity of elasmobranchs. Of the 73 cartilaginous fish species confirmed in the Mediterranean Sea, 67 have been recorded in Turkish waters [8], with the Sea of Marmara alone hosting 31 species, accounting for over half of Turkey’s known shark fauna [9,10,11]. However, many of these species are under increasing threat. Regional extinction risk assessments indicate that a large proportion of Mediterranean elasmobranchs are now classified as vulnerable, endangered, or critically endangered [12,13,14,15].
A dramatic reduction in landings over recent decades further highlights this decline. In Turkey, reported elasmobranch catches have dropped from 3980 tonnes in 2000 to just 246.2 tonnes by 2015. Along the Mediterranean coast specifically, the decline has been even more stark, from 897 tonnes in 1989 to only 10.7 tonnes in 2015 [13]. These numbers signal a broader crisis not only in biodiversity conservation but also in the preservation of ecological functionality across marine habitats. Potential causes of this decline include overfishing and bycatch. Elasmobranchs are frequently caught incidentally in bottom trawls, gillnets, and longlines operating in artisanal and commercial fisheries throughout the region [13,16,17]. For instance, in Iskenderun Bay, elasmobranchs have been shown to comprise nearly a quarter of the total biomass in commercial bottom trawl catches [1]. Despite their ecological importance, most of these bycaught individuals have little to no commercial value in Turkey and are typically discarded, often without survival.
The compounding effects of inherent biological vulnerability and unregulated exploitation have resulted in notable declines in several regional stocks [18,19,20]. Responding to these declines, Turkish fishery regulations have afforded protection to 22 elasmobranch species as of 2018 [21]. Nevertheless, existing regulatory frameworks are limited in scope, applying primarily to a few large-bodied shark species such as the sandbar shark (Carcharhinus plumbeus), the basking shark (Cetorhinus maximus), the school shark (Galeorhinus galeus), and the porbeagle (Lamna nasus). Area-based management is sparse, and the enforcement of protected zones remains inconsistent.
The Mediterranean-wide trend of declining elasmobranch abundance has been well-documented through stock assessments and long-term monitoring [22,23,24]. However, data limitations, especially regarding species-specific landings, continue to hinder effective management. In Turkey, landings are often aggregated under generic categories such as “sharks” or “rays” [25], masking important trends and making it difficult to assess the population status of individual species. This lack of taxonomic resolution undermines conservation efforts and complicates the design of targeted mitigation measures. Over the past two decades, considerable research across the Mediterranean has advanced our understanding of elasmobranch habitat preferences, spatial distributions, and long-term population trends [11,12,13,17,23,26,27,28,29,30,31,32,33,34]. These studies, along with temporal assessments of specific taxa [7,35,36,37,38,39,40,41], have significantly contributed to the identification of critical habitats and life history bottlenecks. However, such efforts have been concentrated in the western and central Mediterranean, leaving the Eastern Aegean Sea comparatively underexplored.
The Aegean Sea, particularly its eastern portion, remains poorly represented in current conservation planning, despite being a known hotspot for biodiversity. The persistence of overfishing and illegal, unreported, and unregulated (IUU) fishing activities continues to threaten marine biodiversity in Turkish waters [42]. The combined impact of unsustainable exploitation and a lack of detailed biological data has led to a notable reduction in both the abundance and diversity of elasmobranchs in the region [7,25]. Addressing these challenges requires not only enforcement and regulation but also the generation of high-resolution, species-specific data to support proactive and adaptive management approaches.
Given the limited availability of species-specific data and the pressing conservation needs in the waters of the Eastern Aegean Sea, this study aims to address critical knowledge gaps regarding elasmobranch species composition and bycatch dynamics in otter trawl fisheries. By documenting the diversity, abundance, and catch characteristics of elasmobranchs between Lesvos Island and Ayvalik, this research seeks to generate baseline biological data for a region that remains underrepresented in the current scientific literature. Emphasis is placed on both commercially retained and discarded individuals, with particular attention to species-specific catch rates and the condition of bycaught specimens. The overarching goal is to provide data that can inform future conservation efforts, support the development of evidence-based fisheries management strategies, and promote more effective bycatch mitigation and species-specific handling protocols in demersal trawl fisheries of the Aegean Sea.

2. Materials and Methods

2.1. Study Area and Sampling Methodology

This study was conducted over two commercial trawling seasons (15th of September to 15th of April) in the Eastern Aegean Sea, in the region between Lesvos Island and Ayvalik (Figure 1). In Izmir Bay, Eastern Aegean Sea, bottom trawl fisheries mainly target scale fish (90% of catch), such as the red mullet, the European hake, and the common pandora, with prawn fisheries (e.g., caramote prawn) targeted occasionally at deeper waters (90–300 m) from February to April. Scale fish dominate economically, but prawns yield a higher market value. While specific catch per unit effort (CPUE) data for Izmir Bay are unavailable, the 2015–2016 season showed total yields per unit effort (YPUEs) of 23.2 kg (day) and 23.8 kg (night). Approximately 20–25 otter trawl vessels operate in the area with a length between 12 and 20 m [43].
A total of 48 trawl surveys were carried out between September 2022 and October 2024 at depths ranging from 25 to 85 m. Each survey included one haul. In total, 33 surveys were conducted in 2022, 3 in 2023, and 12 in 2024. During the surveys, nine elasmobranch species were identified. For each species, detailed biological data, including the total length, the weight, the sex ratio, and the condition upon capture and release, were recorded. All hauls were performed aboard the commercial trawler Dülger Balıkçılık (25.7 m in length, powered by a 940 hp main engine). Each haul lasted, on average, 3 h at a speed of approximately 2.8 knots. The trawling gear used was a modified design constructed entirely from knotless polyethylene (PE), with a circumference of 900 meshes in the fishing circle. The codend mesh size was 44 mm, and it was equipped with a protective bag with 88 mm mesh to minimize damage to the catch.

2.2. Statistical Analysis

Sex-based comparisons in length and weight were performed using Welch’s t-test [44] to account for unequal variances between male and female individuals within each species. Data were tested for normality using the Shapiro–Wilk test and for homogeneity of variances using the variance ratio test. Statistical analysis was performed with the Jamovi software (2.3.28) [45]. Statistical significance was determined at a threshold of p < 0.05.
The physical condition of individuals (dead or alive) at both capture and release was assessed qualitatively and analyzed using nominal logistic regression models [46]; the release and retention of individuals was conducted according to standard protocol used during commercial trips. These models tested the influence of the capture species, the trawl depth, the trawl duration, and the month of sampling on the probability of reduced condition. Monthly categories were grouped into cold (November–January) and warm (September–October) periods to evaluate seasonal effects. The trawl duration was treated as a continuous variable.
To estimate the spatial distribution of elasmobranch abundance in the Eastern Aegean Sea, a log-Gaussian Cox process was implemented within a geostatistical modeling framework. The study area (26.00°–26.85° E, 38.90°–39.45° N), covering the region between Lesvos Island and Ayvalik, was partitioned into a 0.03° resolution grid. Grid cells with centroids over land were excluded using high-resolution coastline data. Elasmobranch catch data from trawl surveys were georeferenced and assigned to corresponding spatial cells. The spatial correlation among cells was modeled using a Gaussian Markov Random Field [47] (GMRF) with a sparse precision matrix derived from neighborhood structures. The number of individuals per cell was modeled as a Poisson process with spatially varying intensity. Model fitting was conducted using the RTMB package [48] in R (version 4.3.2), with parameter estimates and uncertainties obtained via Laplace approximation and automatic differentiation [49]. The total abundance was estimated by summing the predicted intensities across grid cells, with uncertainty bounds derived from the standard error of the log-transformed totals.
The community structure and temporal variation in species composition were explored using hierarchical clustering [50]. A Bray–Curtis similarity matrix was calculated from square-root-transformed abundance data, and cluster analysis was performed using the UPGMA (Unweighted Pair Group Method with Arithmetic Mean) algorithm [51]. To estimate model uncertainty, confidence intervals for the total predicted abundance were derived using the delta method. The standard error was obtained via automatic differentiation [52]. Assuming asymptotic normality, a 95% confidence interval was calculated on the log scale and then exponentiated to return the interval on the original scale.

2.3. Catch per Unit Effort (CPUE)

The CPUE is used as an index of relative abundance in fisheries science, providing a proxy for the population size or biomass of an exploited species [53]. In bottom trawl fisheries, the CPUE is typically expressed as the weight or number of individuals caught per standardized unit of fishing effort, allowing for temporal or spatial comparisons.
The CPUE was calculated according to Equation (1).
C P U E = C E
where
  • C is the total catch (in kg)
  • E is the unit of effort, represented as trawling duration.

2.4. Diversity Indices

The community structure was assessed with the use of diversity indices. Diversity was calculated with the Shannon–Wiener index (H′) [54] which quantifies species diversity by considering both species richness and the proportional abundance of each species in a community, according to Equation (2).
H ′ = − ∑ ι = 1 S p i l n p i
where
  • S is the total number of species,
  • p i is the proportion of individuals belonging to the i -th species in the community,
ln is the natural logarithm, and the summation is taken over all species present in the community.
The Simpson’s Dominance index (D) [55] was estimated to assess dominance by one or more species in the community according to Equation (3).
D = ∑ i = 1 S p i 2
Species Evenness (E) [56] was used to quantify the distribution of individuals among the different species in the community and was derived from the Shannon–Wiener Diversity Index according to Equation (4).
E = H ′ l n ⁡ ( S )
where
  • H ′ is the Shannon–Wiener Diversity Index,
  • S is the total number of species.
The Pearson correlation coefficient (PCC) [57] was employed as a measure of strength of the linear association between trawl number and diversity indices, which was calculated as:
r = [ n ( ∑ x y ) − ∑ x ∑ y ] / [ n ( ∑ x 2 ) − ( ∑ x ) 2 ] [ n ( ∑ y 2 ) − ( ∑ y ) 2 ]
where N is the sample size, and Σ is the summation of all values.

3. Results

Table 1 presents the elasmobranch species composition recorded throughout the 48 trawl surveys conducted between September 2022 and October 2024.
For each species, the number of individuals, the mean and standard deviation of the length and weight, and their relative contribution to the total catch are summarized. This table provides a comprehensive overview of the biological characteristics and catch proportions of the elasmobranch assemblage in the study area. M. mustelus and S. canicula were the most abundant species, accounting for 40.21% and 42.27% of the total catch, respectively.
Table 2 displays the sex-specific mean lengths and weights for each elasmobranch species sampled during the study. The values are presented separately for males and females, allowing for a descriptive comparison of body size distributions between sexes. The female length was significantly larger compared to the male length for M. mustelus (p < 0.01), S. canicula (p < 0.05), and S. stellaris (p < 0.05).
Figure 2A shows the mean length values of each elasmobranch species recorded during the sampling period. Species are ordered by the median body size, with smaller-bodied taxa (e.g., Torpedo marmorata, Scyliorhinus spp.) showing ≥70% discard rates across all size classes. A clear retention threshold occurs at 28–32 cm for M. mustelus and Raja clavata, where retained individuals outnumber discards by 3:1. The Raja species exhibited the steepest retention curve, with 90% of individuals >35 cm retained, while S. canicula demonstrated consistent discard rates (82–88%) regardless of length. Dasyatis pastinaca displayed two distinct length peaks (18–22 cm and 40–45 cm) in the discarded population.
Species in Figure 2B are arranged by median weight, with all specimens below 0.5 kg discarded regardless of species. A clear commercial preference emerges for individuals > 1.0 kg in the common smooth-hound (retention rate: 84%) and R. clavata (retention rate: 78%) species, while the Scyliorhinus species shows uniformly high discard rates (92–95%) across all weight classes. The weight distribution of discarded Dasyatis pastinaca revealed two distinct peaks at 0.3–0.4 kg and 1.2–1.4 kg, comprising 72% of total discards for this species. Notably, all retained Myliobatis aquilla exceeded 1.8 kg, representing just 12% of captured individuals. The retention rates for the species based on the number of individuals retained are as follows: 44.9% for M. mustelus, 50% for R. clavata, and 80% for S. stellaris.
The condition at release exhibited minor, though non-significant, differences among captured species (Figure 3). Among the surviving individuals, the small-spotted catshark dominated (30% of all live releases), followed by the common smooth-hound (24%) and S. stellaris (16%). Mortalities were primarily from the common smooth-hound (36% of all dead) and the small-spotted catshark (40%). R. radula and Myliobatis aquilla contributed minimally to live releases (3% each) but showed complete mortality (0% survival).
Nominal logistic regression indicated that the condition at capture was independent of the capture species, trawl depth, month of trawling, or trawl duration. The condition at release was similarly found to be independent of the capture species and trawl depth; however, it was highly dependent on both the month of trawling (Figure 4A) and the trawl duration (Figure 4B) (p < 0.001).
Nominal logistic regressions indicated that an air temperature exceeding 21.27 °C will result in a more than 50% probability of mortality at release for the captured elasmobranch species. Similarly, a trawl duration longer than 164.55 min will also result in a higher probability of mortality at release (greater than 50%).
Monthly variations in the CPUE for both retained and discarded elasmobranchs are shown in Figure 5.
The distribution of the CPUE is presented across five months, September, October, November, December, and January, stratified by year. The CPUE values are separated by retained and discarded fractions of the catch. Differences in CPUE distribution are observable across months and years, providing an overview of temporal variation in catch dynamics and discard patterns over the course of the study period.
The total retained (R) and discarded (D) length (Figure 6A) and weight (Figure 6B) of each species indicates a significantly higher length and weight in the retained fraction of the catches (p < 0.001).
A spatial analysis of fish count data was performed using a GMRF model for all elasmobranch species (Figure 7). The highlighted areas indicate elevated elasmobranch presence, likely influenced by environmental or anthropogenic factors in the studied area. The total elasmobranch abundance in the studied area was estimated at 1020 individuals with confidence intervals ranging from 865 to 1204 individuals.
The species dendrogram (Figure 8) revealed two primary clusters. The first cluster exhibited higher temporal clustering, with September clearly distinct from other months, displaying unique species composition and abundance patterns. The second cluster encompassed October, December, and January, forming a closely related group that suggests similar elasmobranch assemblages during these months. November showed intermediate similarity, bridging the two main clusters.
Hierarchical clustering highlighted seasonal variability in community structure, likely driven by species-specific migratory or reproductive patterns. The diversity indices derived from elasmobranch abundance data (Figure 9) showed notable temporal variation from September to January. A positive correlation was found between the trawl number and the Shannon–Wiener Diversity Index, while a negative correlation was observed between the trawl number and the Dominance Index, though neither relationship was statistically significant. No correlation was detected between the trawl number and Species Evenness.
The Shannon–Wiener Index (H′), which accounts for both species’ richness and evenness, peaked in September, coinciding with the highest trawl effort (N = 29). This suggests that a greater sampling intensity likely captured a wider array of species and more balanced species abundances, contributing to the higher calculated diversity. The increase in trawl numbers may have also allowed for the detection of rarer species, enhancing overall diversity metrics.
In contrast, October exhibited a pronounced decline in H′, despite only a moderate reduction in overall diversity components. This drop is likely attributable to both a reduced number of trawls and possibly a shift in species composition or dominance by fewer, more abundant species, which is also reflected in the relatively elevated Dominance Index (D). The increase in D implies a higher unevenness in species abundances, suggesting that one or a few species may have dominated the catches during this month.
Diversity indices improved again in November, with H′ and Species Evenness (E) recovering to levels comparable to September. This rebound may be associated with either a seasonal influx of species, changes in their spatial distribution, or improved sampling conditions (e.g., the trawl number increased to N = 15). December and January showed a gradual decline in all three diversity metrics, with January recording the lowest H′ and E values alongside the fewest trawls (N = 3). The reduced sampling effort likely limited species detectability, while biological factors such as seasonal migration or habitat shifts may have also contributed to the decreased diversity.
These results highlight the interplay between sampling efforts and ecological processes in shaping observed diversity patterns. While high trawl efforts tend to increase observed diversity, temporal shifts in elasmobranch populations due to life history traits, fishing pressure, or environmental changes may also significantly influence diversity trends. Sampling efforts could also mask temporal shifts and the effects of fishing pressure and environmental conditions. This study notes that the highest Shannon–Wiener Diversity Index in September coincided with the highest trawl effort (N = 29), suggesting that a greater sampling intensity likely increased the detection of rarer species, inflating diversity metrics. The reduced trawl effort in months like January (N = 3) correlated with lower diversity and evenness, potentially limiting species detectability. This indicates that variations in sampling effort may obscure true ecological patterns. Additionally, fishing pressure, such as intense trawling post-moratorium in September, and environmental factors like temperature and seasonal migrations, likely influence species abundance and distribution. These combined effects could confound observed temporal shifts, making it challenging to isolate ecological versus sampling-driven changes without consistent effort or additional data on environmental covariates.

4. Discussion

The frequent occurrence of species such as the small-spotted catshark and the common smooth-hound, which together comprise a significant portion of the elasmobranch catch, highlights the ecological importance of the Eastern Aegean Sea as a key habitat for vulnerable and endangered chondrichthyan species. Their widespread distribution suggests that they fulfill essential functional roles within the marine ecosystem. Given their trophic position and ecological characteristics, these species may serve as keystone predators, helping to regulate the community structure and maintain ecological balance through top-down control [58].
The elasmobranch assemblage documented in the Eastern Aegean Sea aligns with broader regional patterns, reinforcing the ecological significance and conservation challenges of this understudied region. Giovos et al. [59] highlighted the pervasive issue of IUU fishing in the Mediterranean, noting that elasmobranchs, such as S. canicula and M. mustelus, are frequently caught as bycatch in trawl fisheries, with inadequate reporting obscuring population declines. Similarly, Giovos et al. [60] reported elevated trace element concentrations (e.g., mercury and lead) in the edible tissues of these species from the North Aegean Sea, underscoring potential human health risks from consumption and the need for integrated monitoring of ecological and public health impacts. Furthermore, Giovos et al. [61] emphasized that over half of the Mediterranean’s 86 chondrichthyan species, including those recorded in our study, face high extinction risks, with the inconsistent enforcement of overlapping regulations hampering conservation efforts. These findings contextualize our observed high discard rates and spatial hotspots, suggesting that the Eastern Aegean’s elasmobranch populations are subject to similar pressures as those across the Mediterranean, necessitating regionally coordinated, species-specific management to enhance sustainability.
The spatial analysis revealed substantial heterogeneity in elasmobranch distribution, with the hotspots of S. canicula and M. mustelus largely overlapping. This spatial concentration may be linked to suitable habitat features such as the substrate type, the depth range, and prey availability [22,23,26,62]. These regions could potentially function as important foraging or nursery areas, further underscoring their conservation value. The observed variations in elasmobranch abundance across the Eastern Aegean Sea may reflect a combination of habitat suitability, fishing pressure, and environmental influences. Lower abundance in certain areas could be driven by suboptimal habitat conditions, such as unsuitable substrate or depth, or by intensified fishing effort, particularly following the seasonal trawling moratorium [63]. Additionally, environmental factors like temperature and prey availability, which influence elasmobranch distribution and behavior, may further contribute to these patterns [23,64]. Disentangling these factors requires integrating spatially explicit data with consistent sampling effort to accurately assess ecological and anthropogenic impacts. These findings suggest that conservation strategies should incorporate spatially explicit management tools, such as area closures or seasonal protection zones, especially in high-density regions during critical life stages. Environmental and anthropogenic drivers also play a role in shaping species distributions and abundance. The patterns observed in this study align with previous work, noting the influence of temperature, salinity, depth, and human activities such as coastal development and fisheries exploitation [11,64,65,66]. Additionally, seasonal migrations, habitat preferences, and interspecific interactions likely contribute to the observed spatial and temporal variability [67].
Post-capture condition data revealed that both the environmental temperature and the trawl duration significantly influence elasmobranch survivability. Warmer months were associated with lower survival probabilities at release, likely due to increased metabolic stress and reduced oxygen availability [25,68]. Similarly, longer trawl durations were linked to greater mortality risk, suggesting that physical trauma, hypoxia, and exhaustion during extended trawls may compromise post-release survival [69,70]. These findings highlight the importance of operational and environmental factors in determining discard outcomes. They also underscore the need for best practices, such as minimizing the trawl duration and avoiding fishing during periods of elevated thermal stress.
Monthly variations in the CPUE also indicated seasonal and interannual fluctuations in elasmobranch catch patterns. The higher CPUE values in September and October may reflect seasonal aggregations driven by reproductive or foraging behavior, potentially compounded by increased fishing efforts during these months [68,71]. The particularly high CPUE values observed in September can be primarily attributed to the commencement of the trawling season in Turkish territorial waters [72]. In accordance with national fisheries regulations, trawling is prohibited annually between April 15 and September 15 to protect the recruitment and spawning periods of demersal fish species. However, during this closed season, trawl vessels are permitted to operate in international waters for a limited duration, specifically, between July 15 and September 5, through the issuance of special work permits [63]. After this 4.5-month moratorium on fishing in national waters, the reopening of the season is typically marked by an initial surge in fishing efforts and catch rates. This phenomenon is likely due to the accumulation of biomass during the closure period, resulting in higher yields once fishing resumes. Consequently, the elevated CPUE values recorded in September are both expected and indicative of the biological and regulatory dynamics that shape demersal fisheries in the region. Conversely, CPUE values tend to decline during the winter months, primarily due to the reduction in fish stocks caused by intense fishing pressure over the course of the season. Additionally, the reduced CPUE in winter months such as December and January could be linked to seasonal offshore movements or changes in habitat use due to declining temperatures [67,71]. These dynamics support the implementation of temporally adaptive management strategies, including seasonal effort controls or targeted closures. Despite its utility, the CPUE should be interpreted cautiously as it can be influenced by changes in fishing technology, fisher behavior, targeting practices, and environmental variability [73].
The analysis of retained versus discarded individuals by length and weight showed a clear size-selective pattern in the fishery. Larger individuals, especially M. mustelus and R. clavata, were more likely to be retained, while smaller individuals, particularly Scyliorhinus spp., were consistently discarded. This trend reflects economic preferences that favor marketable sizes. However, high discard rates for non-commercial species across all size classes also point to a lack of utilization potential and possible misalignment between current fishing practices and conservation goals. Gear modifications aimed at improving selectivity and reducing the discard of vulnerable species may improve sustainability [16]. Community composition analysis demonstrated distinct temporal clustering, with September forming a separate group and subsequent months (October through January) exhibiting closer similarity, potentially reflecting seasonal transitions in species availability or post-summer movement patterns [74,75]. However, the pronounced clustering in September may be partly attributed to higher sampling efforts (N = 29 trawls), which likely increased the detection of rarer species and influenced diversity metrics, as higher efforts can enhance species detectability [73]. Conversely, lower efforts in months like January (N = 3 trawls) may lead to limited species capture, potentially masking true ecological shifts [53]. These findings underscore the need to account for sampling bias due to variable efforts when interpreting temporal patterns in the elasmobranch community structure. The clustering of October through January indicates relative stability, although minor shifts suggest that the assemblage composition continues to respond to environmental or behavioral cycles [74,75]. Myliobatis aquila, Dasyatis pastinaca, and Torpedo marmorata are marine species that pose a potential risk to human health due to their defensive mechanisms, such as venomous spines or electric discharges [76,77]. Upon capture and subsequent dumping of the catch on deck, individuals of these species are typically returned to the sea immediately to minimize the risk of injury to crew members. This practice of rapid release not only enhances crew safety but is also ecologically significant, as these species demonstrate a relatively higher survival rate post-capture compared to other discarded cartilaginous fishes. Therefore, prioritizing their swift return to the water supports both occupational safety and conservation goals.
The fishing period examined in this study (September to January) partially overlaps with the reproductive seasons of key elasmobranch species in the Eastern Aegean Sea, such as the small-spotted catshark and the common smooth-hound, which may have implications for conservation strategies. S. canicula is known to exhibit year-round reproduction in the Mediterranean, with peak egg-laying seasons in late spring to summer (May to August), which is largely covered by the existing trawling moratorium from 15 April to 15 September [29]. However, M. mustelus typically has a gestation period extending from late spring through early autumn, with parturition occurring from August to October, which coincides with the onset of the fishing season [78]. This overlap suggests that pregnant females or neonates may be vulnerable to capture, potentially impacting population recruitment. Other species, such as R. clavata, also show peak reproductive activity in the summer months, which is partially protected by the moratorium [20]. While the current non-fishing period offers some protection, extending or adjusting the moratorium to fully encompass critical reproductive stages, particularly for M. mustelus, could enhance population recovery. Further studies on species-specific reproductive phenology in the Eastern Aegean are needed to refine temporal management measures and ensure alignment with ecological requirements.
The analysis of the diversity indices highlights the influence of both ecological processes and sampling effort on biodiversity metrics. The highest Shannon–Wiener Diversity Index coincided with the highest trawl effort in September, suggesting that a greater sampling intensity increases the detection of rare or less abundant species. The decline in October, despite bing only a small reduction in effort, may reflect ecological factors such as post-reproductive migration or increasing dominance of a few species [74]. Fluctuations observed in November and subsequent months likely result from a combination of changing environmental conditions, species movements, and sampling intensity. These results stress the importance of consistent effort when monitoring biodiversity and call attention to the need to integrate both ecological and methodological factors when interpreting diversity patterns.
Beyond distribution and catch patterns, a particularly critical knowledge gap lies in understanding post-release mortality among discarded elasmobranchs, especially skates and rays. Although a significant portion of individuals are released alive, their survival outcomes remain uncertain. Previous studies have reported highly variable short-term survival rates, ranging from 50% to 90%, depending on the species, trawl duration, depth, handling time, and gear type [79,80,81,82,83,84,85]. In multi-species Turkish bottom trawl fisheries, such as those in the Eastern Mediterranean, selectivity is typically limited to a few target species, leaving others, like skates, particularly vulnerable to capture and mortality due to their morphology and limited escape potential [69,86]. These findings emphasize the need for further survival studies and highlight the urgency of refining discard practices to improve outcomes for non-target species.
While this study provides valuable insights into elasmobranch bycatch dynamics, some limitations warrant consideration. The exclusive use of data from a single commercial trawler may not fully encapsulate the spatial and temporal variability inherent in regional fishing practices and species composition. For instance, studies have demonstrated significant differences in elasmobranch catch rates across various fishing zones and seasons [87]. The two-year sampling period may be insufficient to account for long-term ecological fluctuations or rare events, such as climate anomalies, which can influence bycatch rates. Extended monitoring is essential to discern these patterns. While the post-release condition was assessed, the actual survival rates post-discard remain uncertain without telemetry tracking. While the condition at release provides valuable insights into the immediate impacts of trawl capture on elasmobranchs, true post-release survival rates remain uncertain without tagging data. Our assessment of S. canicula and M. mustelus conditions (30% and 24% alive at release, respectively) indicates the potential for survival, but long-term outcomes are unknown due to potential delayed mortality from capture stress or injury. Carpentieri et al. [88] emphasized that sparse data on elasmobranch post-release mortality in Mediterranean fisheries, coupled with limited tagging studies, hinder accurate survival estimates, necessitating advanced monitoring techniques like acoustic or satellite telemetry. This uncertainty underscores a critical knowledge gap, as post-release survival is essential for evaluating bycatch mitigation measures. Consequently, future research prioritizing tagging programs is vital to quantify true survival rates and inform effective conservation strategies for Eastern Aegean elasmobranchs.
Previous research has underscored the value of acoustic telemetry and tagging methods in estimating post-release survival of elasmobranchs, revealing significant variability in survival rates among species and individuals due to factors such as species-specific physiology, capture stress, and handling practices [84,89]. These studies employ acoustic tags to track post-release behavior and survival, providing critical data on the fate of discarded individuals and informing strategies to mitigate bycatch mortality.
Covariates such as dissolved oxygen levels or prey availability, which could further elucidate spatial patterns in bycatch, were not incorporated into this study. A limitation of our spatial modeling approach is the exclusion of key abiotic factors, such as dissolved oxygen, salinity, and the sediment type, which are known to influence elasmobranch distribution and habitat preferences. These variables can shape the spatial patterns of demersal species like S. canicula and M. mustelus by affecting prey availability, physiological tolerances, and substrate suitability for foraging or nursery grounds. For instance, Lauria et al. [23]) demonstrated that incorporating salinity and the sea surface temperature into habitat suitability models significantly improved predictions of elasmobranch distributions in the Mediterranean, highlighting the importance of these factors in coastal ecosystems. The absence of such covariates in our model may have limited our ability to fully explain the observed spatial hotspots and could reduce the model’s predictive accuracy. Future studies should integrate these abiotic factors to enhance the robustness of spatial analyses and better inform habitat-based conservation strategies [90].
One of the most persistent challenges facing the sustainable management of elasmobranchs is the widespread presence of IUU fishing. IUU activities not only undermine conservation efforts but also perpetuate data deficiencies. One promising solution that could include cross-border governance mechanisms is the implementation of Fully Documented Fisheries (FDFs) supported by Electronic Monitoring (EM) technologies. As demonstrated by Bertelsen et al. [91], EM systems can provide transparent, real-time data on both target and non-target species, shifting accountability onto fishers and enabling more effective, performance-based management. In Turkey, adopting such systems could help close the gap in species-specific data, improve the enforcement of landing bans, and support the development of more nuanced mitigation protocols. Integrating FDFs into regional fisheries governance frameworks offers a scalable pathway toward better compliance, enhanced biological monitoring, and the long-term sustainability of vulnerable elasmobranch populations in the Eastern Aegean Sea.
The effective management of elasmobranchs in the transboundary Eastern Aegean Sea is complex due to differing regulatory frameworks between Greece and Turkey. Dereli et al. [92] highlight significant disparities in fleet structures, Minimum Conservation Reference Sizes (MCRSs), and species-specific closed seasons, which undermine collective conservation efforts for shared stocks like S. canicula and M. mustelus. To address these gaps, our proposed measures, gear modifications to reduce bycatch, seasonal closures during reproductive periods, and Fully Documented Fisheries (FDFs) with Electronic Monitoring (EM), should be aligned with cross-border governance mechanisms. Leveraging the General Fisheries Commission for the Mediterranean (GFCM) as a central coordinating body could facilitate harmonized regulations, such as standardized MCRSs and synchronized closures, while EM systems would enhance compliance and data accuracy across both nations. These steps would improve enforcement and ecological outcomes in shared waters, supporting sustainable elasmobranch management.
Considering these findings, the Eastern Aegean Sea serves not only as a critical habitat for several threatened elasmobranch species but also as a valuable case study for understanding the challenges of managing mixed-species demersal fisheries. The combination of spatially explicit data, condition-based assessments, and seasonal dynamics presented here offers a rare, integrated perspective that can directly inform future policy decisions. As pressures on marine ecosystems intensify, studies like this are essential for bridging knowledge gaps and guiding practical, science-based conservation strategies. Continued investment in monitoring, gear innovation, and participatory management frameworks will be key to securing the future of elasmobranch populations in this region and beyond.

5. Conclusions

This study highlights the Eastern Aegean Sea as an important habitat for vulnerable elasmobranch species, particularly the small-spotted catshark and the common smooth-hound, which dominate the catch composition. The findings underscore the influence of environmental (e.g., temperature) and operational factors (e.g., trawl duration) on post-capture survival, pointing to the urgent need to integrate these parameters into discard management and mitigation protocols. Distinct spatial and seasonal patterns in the abundance of species, diversity, and community structures suggest dynamic ecological processes, such as migration and reproduction, that must be accounted for in future management plans.
High discard rates of non-commercial elasmobranch species, coupled with poor post-release condition, highlight critical gaps in current fishing practices that may undermine conservation outcomes and fishery sustainability. While returning individuals to the sea is intended to support population recovery, the compromised condition of many discarded individuals likely increases post-release mortality, reducing the effectiveness of these releases. Implementing improved handling protocols and gear modifications to enhance post-release survival is essential to align fishing practices with conservation and sustainability goals. To address these issues, we recommend implementing species-specific conservation strategies, including spatial refugia, seasonal closures during reproductive periods, and gear modifications to reduce bycatch. Furthermore, the adoption of Fully Documented Fisheries using Electronic Monitoring systems is essential to enhance data transparency, reduce IUU fishing, and support evidence-based decision-making. Overall, this research provides foundational data for improving elasmobranch management in the Eastern Mediterranean and underscores the need for proactive, adaptive, and ecosystem-based approaches in mixed-species trawl fisheries.

Author Contributions

Conceptualization, İ.A. and D.K.; methodology, A.T. and D.K.; software, İ.A., A.T., S.Y. and D.K.; validation, A.T. and D.K.; formal analysis, A.T. and D.K.; investigation, İ.A. and S.Y.; resources, İ.A. and S.Y.; data curation, İ.A. and S.Y.; writing—original draft preparation, A.T. and D.K.; writing—review and editing, İ.A., A.T., S.Y. and D.K.; visualization, İ.A. and D.K.; supervision, İ.A. and D.K.; project administration, İ.A. and D.K.; funding acquisition, İ.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by ERASMUS+ Cooperation partnerships in vocational education and training, grant number 2021-1TR01-KA220-VET-000024755.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. A map of the study area (white outline) of the Eastern Aegean Sea, where commercial bottom trawling for the collection of elasmobranch species was conducted (black outline) (26.40°–26.80° E, 39.05°–39.30° N). Color variation indicates depth.
Figure 1. A map of the study area (white outline) of the Eastern Aegean Sea, where commercial bottom trawling for the collection of elasmobranch species was conducted (black outline) (26.40°–26.80° E, 39.05°–39.30° N). Color variation indicates depth.
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Figure 2. (A) The mean length and (B) the mean weight of each elasmobranch species captured between September 2022 and October 2024 in the Eastern Aegean Sea. Percentages indicate retention according to A length and B weight.
Figure 2. (A) The mean length and (B) the mean weight of each elasmobranch species captured between September 2022 and October 2024 in the Eastern Aegean Sea. Percentages indicate retention according to A length and B weight.
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Figure 3. The percentage of elasmobranch individuals (N = 194) in different condition categories (alive or dead) at release, by species, from 48 trawl surveys conducted between September 2022 and October 2024 in the Eastern Aegean Sea. The scale represents the proportion of individuals per species (0–100%), with S. canicula and M. mustelus dominating live releases (30% and 24%, respectively) and mortalities (40% and 36%, respectively).
Figure 3. The percentage of elasmobranch individuals (N = 194) in different condition categories (alive or dead) at release, by species, from 48 trawl surveys conducted between September 2022 and October 2024 in the Eastern Aegean Sea. The scale represents the proportion of individuals per species (0–100%), with S. canicula and M. mustelus dominating live releases (30% and 24%, respectively) and mortalities (40% and 36%, respectively).
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Figure 4. Nominal logistic regression curves showing the probability of elasmobranch mortality at release (N = 194 individuals) as a function of (A) the air temperature (°C) and (B) the trawl duration (minutes) from 48 trawl surveys in the Eastern Aegean Sea, September 2022–October 2024. The air temperature ranges from 15 to 30 °C, with a mortality threshold of 21.27 °C (>50% probability); the trawl duration ranges from 60 to 240 min, with a threshold at 164.55 min. Cold months (November–January) and warm months (September–October) are distinguished.
Figure 4. Nominal logistic regression curves showing the probability of elasmobranch mortality at release (N = 194 individuals) as a function of (A) the air temperature (°C) and (B) the trawl duration (minutes) from 48 trawl surveys in the Eastern Aegean Sea, September 2022–October 2024. The air temperature ranges from 15 to 30 °C, with a mortality threshold of 21.27 °C (>50% probability); the trawl duration ranges from 60 to 240 min, with a threshold at 164.55 min. Cold months (November–January) and warm months (September–October) are distinguished.
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Figure 5. Comparative histograms of (CPUE, kg/h) for retained and discarded elasmobranch fractions across five months (September–January) from 48 trawl surveys (N = 194 individuals) in the Eastern Aegean Sea, September 2022–October 2024. The figure highlights temporal variations in catch dynamics, with a higher CPUE in September–October due to seasonal aggregations and fishing efforts, informing temporal management strategies.
Figure 5. Comparative histograms of (CPUE, kg/h) for retained and discarded elasmobranch fractions across five months (September–January) from 48 trawl surveys (N = 194 individuals) in the Eastern Aegean Sea, September 2022–October 2024. The figure highlights temporal variations in catch dynamics, with a higher CPUE in September–October due to seasonal aggregations and fishing efforts, informing temporal management strategies.
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Figure 6. The comparative (A) length (cm) and (B) weight (kg) of retained (R) and discarded (D) fractions for each elasmobranch species (N = 194 individuals) from 48 trawl surveys in the Eastern Aegean Sea, September 2022–October 2024. Sex is indicated by color (female: blue, male: red). This figure illustrates size-selective fishing practices, with larger individuals (M. mustelus, R. clavata) more likely to be retained, supporting the need for selective gear modifications.
Figure 6. The comparative (A) length (cm) and (B) weight (kg) of retained (R) and discarded (D) fractions for each elasmobranch species (N = 194 individuals) from 48 trawl surveys in the Eastern Aegean Sea, September 2022–October 2024. Sex is indicated by color (female: blue, male: red). This figure illustrates size-selective fishing practices, with larger individuals (M. mustelus, R. clavata) more likely to be retained, supporting the need for selective gear modifications.
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Figure 7. The spatial distribution of elasmobranch abundance (N = 194 individuals) across the Eastern Aegean Sea (26.00°–26.85° E, 38.90°–39.45° N) from 48 trawl surveys, September 2022–October 2024, modeled using a log-Gaussian Cox process with a Gaussian Markov Random Field (GMRF) on a 0.03° resolution grid. The color intensity represents the estimated abundance (individuals per grid cell).
Figure 7. The spatial distribution of elasmobranch abundance (N = 194 individuals) across the Eastern Aegean Sea (26.00°–26.85° E, 38.90°–39.45° N) from 48 trawl surveys, September 2022–October 2024, modeled using a log-Gaussian Cox process with a Gaussian Markov Random Field (GMRF) on a 0.03° resolution grid. The color intensity represents the estimated abundance (individuals per grid cell).
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Figure 8. A heatmap with hierarchical clustering based on the Bray–Curtis similarity index, using the UPGMA (Unweighted Pair Group Method with Arithmetic Mean) algorithm, applied to square root transformed elasmobranch abundance data collected from 48 trawls conducted between September 2022 and October 2024 in the Eastern Aegean Sea (warmer colors indicate higher relative abundance).
Figure 8. A heatmap with hierarchical clustering based on the Bray–Curtis similarity index, using the UPGMA (Unweighted Pair Group Method with Arithmetic Mean) algorithm, applied to square root transformed elasmobranch abundance data collected from 48 trawls conducted between September 2022 and October 2024 in the Eastern Aegean Sea (warmer colors indicate higher relative abundance).
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Figure 9. The monthly variation in diversity indices derived from elasmobranch abundance data and corresponding trawl efforts between September 2022 and October 2024 in the Eastern Aegean Sea. The stacked bar chart illustrates the Shannon–Wiener Diversity Index (H′), the Dominance Index (D), and Species Evenness (E). The superimposed dashed line with red dots represents the number of trawl operations conducted each month.
Figure 9. The monthly variation in diversity indices derived from elasmobranch abundance data and corresponding trawl efforts between September 2022 and October 2024 in the Eastern Aegean Sea. The stacked bar chart illustrates the Shannon–Wiener Diversity Index (H′), the Dominance Index (D), and Species Evenness (E). The superimposed dashed line with red dots represents the number of trawl operations conducted each month.
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Table 1. Elasmobranch species composition of the total population captured between September 2022 and October 2024 in the Eastern Aegean Sea.
Table 1. Elasmobranch species composition of the total population captured between September 2022 and October 2024 in the Eastern Aegean Sea.
Length (cm)Total Weight (kg)
SpeciesNMeanStd DevMeanStd Dev% of Total
Dasyatis pastinaca547.407.802.301.522.58%
Mustelus mustelus7843.1914.141.270.9140.21%
Myliobatis aquilla140.00 2.00 0.52%
Raja clavata445.009.021.900.632.06%
Raja miraletus330.8212.770.550.201.55%
Raja radula333.335.030.500.101.55%
Scyliorhinus canicula8231.116.960.750.4742.27%
Scyliorhinus stellaris1548.7313.921.290.577.73%
Torpedo marmorata322.004.000.500.201.55%
Table 2. Elasmobranch species composition for each sex (the mean and standard deviation) of the total population captured between September 2022 and October 2024 in the Eastern Aegean Sea.
Table 2. Elasmobranch species composition for each sex (the mean and standard deviation) of the total population captured between September 2022 and October 2024 in the Eastern Aegean Sea.
Female Length (cm)Male Length (cm)Female Weight (kg)Male Weight (kg)
SpeciesMeanMeanMeanMean
Dasyatis pastinaca49.75 ± 6.6538.002.50 ± 1.681.50
Mustelus mustelus48.23 ± 14.1437.97 ± 12.001.47 ± 0.961.00 ± 0.76
Myliobatis aquilla40.00 2.00
Raja clavata47.33 ± 9.4538.002.16 ± 0.411.10
Raja miraletus42.33 ± 16.8026.5 ± 8.610.73 ± 0.230.48 ± 0.15
Raja radula33.33 ± 5.03 0.50 ± 0.1
Scyliorhinus canicula33.40 ± 6.6730.44 ± 6.870.86 ± 0.510.67 ± 0.42
Scyliorhinus stellaris55.88 ± 7.7540.57 ± 15.361.65 ± 0.400.88 ± 0.48
Torpedo marmorata 22.00 ± 4.00 0.50 ± 0.20
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Aydin, İ.; Theocharis, A.; Yapici, S.; Klaoudatos, D. Elasmobranch Species Composition in Otter Trawl Fisheries (Eastern Aegean Sea). Oceans 2025, 6, 34. https://doi.org/10.3390/oceans6020034

AMA Style

Aydin İ, Theocharis A, Yapici S, Klaoudatos D. Elasmobranch Species Composition in Otter Trawl Fisheries (Eastern Aegean Sea). Oceans. 2025; 6(2):34. https://doi.org/10.3390/oceans6020034

Chicago/Turabian Style

Aydin, İlker, Alexandros Theocharis, Sercan Yapici, and Dimitris Klaoudatos. 2025. "Elasmobranch Species Composition in Otter Trawl Fisheries (Eastern Aegean Sea)" Oceans 6, no. 2: 34. https://doi.org/10.3390/oceans6020034

APA Style

Aydin, İ., Theocharis, A., Yapici, S., & Klaoudatos, D. (2025). Elasmobranch Species Composition in Otter Trawl Fisheries (Eastern Aegean Sea). Oceans, 6(2), 34. https://doi.org/10.3390/oceans6020034

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