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Article

Ichthyoplankton Composition and Environmental Drivers in the Sanquianga Tapaje Estuarine System, Eastern Tropical Pacific

by
Juan José Gallego-Zerrato
1,3,
Andrés Cuellar
2 and
Alan Giraldo
3,*
1
Programa de Doctorado en Ciencias Biología, Departamento de Biología, Facultad de Ciencias Naturales y Exactas, Universidad del Valle, Cali 760032, Colombia
2
Investigación y Monitoreo, Territorial Pacífico, Parques Nacionales Naturales, Cali 760032, Colombia
3
Grupo de Investigación en Ciencias Oceanográficas, Departamento de Biología, Facultad de Ciencias Naturales y Exactas, Universidad del Valle, Cali 760032, Colombia
*
Author to whom correspondence should be addressed.
Diversity 2025, 17(9), 649; https://doi.org/10.3390/d17090649
Submission received: 21 August 2025 / Revised: 13 September 2025 / Accepted: 14 September 2025 / Published: 16 September 2025
(This article belongs to the Special Issue Marine Zooplankton and Ichthyoplankton Ecology: Patterns, Processes, and Perspective)
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Abstract

Estuaries are vital coastal ecosystems that support fish during key life stages such as spawning, feeding, and early development. This study investigates ichthyoplankton composition and abundance in the Sanquianga Tapaje estuarine system, located in the southern Eastern Tropical Pacific (ETP) of Colombia. Zooplankton samples were collected using bongo nets at 11 stations across four river mouths (Tapaje, Amárales, Sanquianga, and Guascama), alongside measurements of oceanographic parameters at 1 and 10 m depths. A total of 357 fish larvae were identified, representing 23 species and 11 families, with Engraulidae, Gobiidae, and Carangidae dominating the assemblage. Water column conditions reflected typical tropical estuarine dynamics, influenced by tidal action and freshwater input. Spearman’s rank correlation revealed strong positive associations between larval abundance and surface salinity (rs = 0.81, p = 0.003), as well as dissolved oxygen saturation and concentration (rs > 0.68, p < 0.021). Diversity indices (Shannon, Pielou, Whittaker) indicated high species turnover in transitional zones, and larval hotspots were associated with outer estuarine zones. Salinity and dissolved oxygen emerged as key drivers of ichthyoplankton distribution. These findings underscore the ecological complexity and nursery function of tropical estuaries, offering baseline data to inform biodiversity conservation, ecosystem monitoring, and adaptive management in Colombia’s Pacific region and across the ETP.

1. Introduction

Estuarine systems in the Eastern Tropical Pacific (ETP) function as biologically productive transition zones between freshwater and marine environments, supporting the early developmental stages of numerous fish species. These habitats are shaped by dynamic hydrological and geomorphological processes that generate spatial gradients in salinity, temperature, turbidity, and nutrient availability, often dominated by extensive mangrove forests. The estuary–mangrove complexes in this region, characterized by tidal channels and nutrient-rich waters, provide essential ecological functions for fish during early life stages [1,2,3]. These habitats offer shelter and abundant food resources, thereby increasing the survival probability of eggs and larvae [4,5], and their environmental gradients—particularly salinity, temperature, oxygen concentration, turbidity, and plankton biomass—strongly regulate the abundance, composition, and spatial distribution of ichthyoplankton [6,7,8,9,10,11,12].
Within these systems, fish larvae play a pivotal role in energy transfer, acting as primary consumers of phytoplankton and zooplankton and contributing to the trophic structure of estuarine food webs, and their spatial patterns of distribution often emerge from interactions between hydrodynamic processes, habitat heterogeneity, and species-specific tolerances [13,14]. Furthermore, their survival and recruitment success directly influence future fish stock dynamics and biomass levels [15,16,17], with significant implications for artisanal and subsistence fisheries that depend on these resources [18]. Although methodological constraints have limited the number of studies on fish larvae, accounting for only 1% of estuarine ichthyofauna research [17], larval assemblages remain particularly valuable for ecological assessment due to their sensitivity to local environmental conditions and limited dispersal capacity. Moreover, larval assemblages also offer valuable insights into spawning activity, habitat quality, and ecosystem integrity—parameters that are essential for guiding conservation and resource management efforts, and the occurrence and spatial distribution may serve as useful ecological indicators of habitat suitability, environmental gradients, and nursery function, particularly in systems characterized by strong freshwater–marine connectivity [19,20,21,22,23].
Despite the ecological and fisheries relevance of estuarine ichthyoplankton, scientific understanding of their composition, structure, and spatiotemporal dynamics in tropical estuarine environments remains limited. In the Eastern Tropical Pacific (ETP), most ichthyoplankton studies have focused on coastal lagoons, bays, and gulfs, such as those in the Mexican Pacific [24,25,26], the Nicoya and Dulce Gulfs in Costa Rica [27,28], and select sites in Colombia and Ecuador, including Ensenada de Utría, Málaga Bay, and the Guayaquil Gulf [29,30,31]. These studies collectively highlight the ecological importance of estuarine systems as nursery habitats and reveal consistent patterns of larval assemblage structure shaped by environmental gradients. For instance, Gordo et al. [24] provided the first systematic inventory of ichthyoplankton in Jalisco and Colima, identifying over 100 taxa and emphasizing the dominance of Cynoglossidae and Carangidae in coastal upwelling zones. Navarro-Rodríguez et al. [25] demonstrated that seasonal shifts in temperature and salinity modulate larval abundance and diversity in Laguna El Quelele, with Engraulidae and Eucinostomus sp. as key taxa. In Bahía de Banderas, Navarro-Rodríguez et al. [26] found that larval density peaks in spring and is strongly associated with zooplankton biomass and coastal–oceanic gradients. Ramírez et al. [27] reported high larval abundance in the Gulf of Nicoya but found no evidence of mangroves serving as spawning grounds, suggesting tidal transport as a key driver. Molina-Ureña [28] identified distinct larval assemblages in Nicoya and Golfo Dulce, with Engraulids and Sciaenids dominating neritic zones and Myctophids prevalent offshore. In Bahía Málaga, Medina-Contreras et al. [29] documented high larval densities and temporal variability, with Carangidae and Sciaenidae as dominant families, while Valencia et al. [30] in Ensenada de Utría revealed larval abundance peaked during rainy seasons—indicating complex trophic responses to hydrographic shifts in the northern part of the Colombian pacific coast. Finally, Calderón-Peralta et al. [31] showed that larval richness and abundance in the Guayaquil Gulf are structured by salinity and water transparency, with Engraulidae and Gobiidae dominating the inner estuary and Sciaenidae more prevalent externally. These findings underscore the dynamic nature of tropical estuarine systems and the need for spatially and temporally resolved studies to understand larval ecology and inform conservation strategies.
In the southern sector of the ETP in Colombia, the Sanquianga–Tapaje estuarine system exemplifies a low-gradient, flood-prone coastal landscape shaped by high precipitation and alluvial river discharge [32,33,34,35]. This system forms part of the Parque Nacional Natural Sanquianga (PNN Sanquianga), a designated National Natural Park along Colombia’s Pacific coastal region. Given its geomorphological complexity and environmental variability, the Sanquianga–Tapaje system offers a strategic opportunity to examine how environmental gradients influence ichthyoplankton composition and distribution, contributing to the sustainable management of hydrobiological resources [19,20,21,22]. In this study, we examine the composition, abundance, and environmental drivers of fish larvae in the Sanquianga–Tapaje estuarine system, a region of high biodiversity and strategic ecological value in the ETP. Our objectives were guided by the following research questions: (1) What are the abundance and species composition of fish larvae within the Sanquianga and Tapaje estuarine system? (2) How do salinity, temperature, dissolved oxygen, turbidity, and food availability affect the distribution and abundance of fish larvae in this estuarine environment? and (3) Is there a spatial trend in the variation of abundance, composition, or diversity within fish larval assemblages across the Sanquianga–Tapaje system? We hypothesize that spatial variations in oceanographic conditions within the estuarine system significantly influence the abundance and species composition of fish larvae, with salinity and dissolved oxygen playing a central role in regulating their spatial variability. To test these hypotheses, we conducted a spatially explicit survey of ichthyoplankton across multiple stations within the Sanquianga–Tapaje estuarine system. Environmental variables were measured concurrently to assess their influence on larval abundance and composition. This integrative approach enables a detailed characterization of larval fish assemblages and their ecological responses to estuarine gradients, contributing to a broader understanding of nursery habitat dynamics in tropical estuarine systems.

2. Materials and Methods

2.1. Study Area and Sampling Procedures

This study was conducted within the Sanquianga–Tapaje estuarine system, which forms part of the Parque Nacional Natural Sanquianga, a protected area located on the southern Colombian Pacific coast (2°22′–2°04′ N, 78°76′–75°37′ W), within the easternmost sector of the Eastern Tropical Pacific (Figure 1) [36].
The Sanquianga Tapaje estuarine system spans approximately 80,000 hectares of the Pacific alluvial plain and is characterized by flat, flood-prone terrain dissected by numerous channels and estuaries formed by the deltas of the Sanquianga, Patía, La Tola, Aguacatal, and Tapaje rivers. The interaction between substantial river discharge and tidal influence (tidal range: 4 m) has shaped a complex geomorphological system comprising estuaries, coastal channels, and tide-influenced mangrove forests [36]. The region experiences an average air temperature of 26 °C and annual precipitation of approximately 3250 mm, distributed across four rainy periods; the most intense occurs from April to November, followed by a secondary, less pronounced phase from December to March [37,38]. These conditions define the ecological framework under which ichthyoplankton sampling was conducted and support the most extensive and structurally developed mangrove forest in the Colombian Pacific [39,40].
Sampling was conducted from 14–16 August 2019, during the peak rainy season in the Sanquianga–Tapaje estuarine system. This period was selected due to logistical constraints, including restricted access imposed by entry permits required by the Sanquianga National Natural Park and temporary public order disruptions in the region. Despite these limitations, August represents one of the most hydrologically active phases of the year, characterized by elevated freshwater discharge, increased nutrient input, and enhanced estuarine productivity—conditions known to support spawning activity and larval abundance in tropical fish species. We acknowledge that single-date sampling may not capture the full temporal variability in species composition and larval abundance, particularly given the asynchronous reproductive cycles of estuarine fishes. However, this sampling window provides a representative snapshot of larval assemblages during a biologically dynamic period and serves as a baseline for future multi-seasonal or interannual comparisons.
Zooplankton sampling was conducted at 11 stations located at the mouths of the Tapaje (T), Amárales (A), Sanquianga (S), and Guascama (G) rivers (Figure 1). At each station, a single surface tow (1 m depth) was performed during daylight hours using a bongo net (mouth diameter: 30 cm; length: 180 cm) fitted with 300 and 500 μm mesh sizes. This mesh size is widely used in tropical estuarine studies to capture fish larvae across preflexion and postflexion stages, while minimizing clogging and ensuring efficient filtration in turbid environments. Replicate tows were not feasible due to logistical constraints, including restricted access to the Sanquianga National Natural Park and temporary public order conditions in the region. To minimize methodological variability, standardized procedures were applied across all stations, including consistent tow duration (10 min), towing speed (2 knots), and net specifications. Non-continuous water samples (discrete samples) were collected at depths of 1 m and 10 m using a Niskin bottle and analyzed for temperature, salinity, dissolved oxygen, and chlorophyll-a concentration using a previously calibrated multiparametric YSI Exxo probe. Zooplankton samples were preserved in a 4% formalin–seawater solution and transported to the Oceanographic Sciences Laboratory at Universidad del Valle (UV), Cali, Colombia. In the laboratory, fish larvae were separated from the zooplankton samples, counted, and identified to the lowest possible taxonomic level (based on morphological characters) using the guides of Beltrán-León & Ríos-Herrera [41] and Richards [42].
Filtered water volume was quantified using a Hydrobios® (Altenholz, Germany) flowmeter mounted on each net mouth, following manufacturer specifications. Larval abundance was standardized to individuals per 1000 m−3. Zooplankton biomass was estimated from half of the sample collected with the 300 μm mesh net. Biological material was concentrated onto pre-dried and pre-weighed cellulose filters using a vacuum filtration system, dried at 60 °C for 24 h, and weighed on an analytical balance with 0.0001 g precision. Dry biomass was calculated by weight difference and standardized per unit volume.

2.2. Data Analysis

Fish larvae abundance data were log-transformed to improve visualization and reduce skewness caused by extreme values [43]. Oceanographic variables—temperature, salinity, dissolved oxygen, and chlorophyll-a—were spatially interpolated using the inverse distance weighting (IDW) method in Surfer v11® software, with a power parameter of 2 and a search radius encompassing all sampling stations. The resulting maps are intended for visualization purposes only and should be interpreted as qualitative representations of spatial gradients rather than statistically validated surfaces [44]. To assess relationships between fish larvae abundance, zooplankton biomass, and environmental parameters, Spearman’s rank correlation was employed. This non-parametric test evaluates monotonic associations without assuming normality, making it suitable for ecological datasets with heterogeneous distributions [43].
Species richness and sampling completeness were estimated using the first-order jackknife estimator, which adjusts for unseen taxa by accounting for the frequency of singletons (taxa occurring in only one sample). This method provides a robust lower-bound estimate of true richness, especially in under-sampled or diverse communities [45,46]. Alpha diversity was quantified using the Shannon diversity index (H′), which incorporates both species richness and evenness to reflect community complexity [47]. This index is sensitive to rare species and widely used in marine biodiversity assessments. Species evenness was evaluated using Pielou’s index (J′), which measures the uniformity of species abundance distribution and was calculated using the natural logarithm base. A jackknife-based correction was applied to reduce bias in similarity estimates [46]. Pairwise differences in H′ and J′ among stations were tested using permutation procedures (999 iterations), which evaluated whether observed values deviated significantly from those expected under the null hypothesis. These resampling procedures are distribution-free and suitable for small sample sizes [43]. Beta diversity was calculated using Whittaker’s index (β = γ/α − 1) to quantify compositional turnover across sites. This metric reflects spatial heterogeneity in ichthyoplankton assemblage structure across sampling stations and is particularly useful for identifying ecological gradients or habitat discontinuities [47,48]. All statistical analyses and diversity computations were performed using Statistica® v8.0 and PAST® v4.03 software, which offer robust platforms for ecological data exploration and hypothesis testing [44]. Shannon diversity (H′) and Pielou’s evenness (J′) were calculated using the natural logarithm (ln) base, as implemented in PAST® v4.03 software.
Spatial variability was described based on observed ranges and station-wise comparisons of ecological and environmental metrics; no formal spatial statistics were applied. Correlations between abiotic factors and larval abundance were assessed using Spearman’s rank correlation, a non-parametric method suitable for small sample sizes and non-normal data. However, spatial autocorrelation was not formally tested, and results may be influenced by spatial structure.

3. Results

3.1. Ichthyoplankton Composition and Zooplankton Biomass

A total of 357 fish larvae were collected, representing 11 families and 23 species. The most abundant families were Engraulidae (39.5%), Gobiidae (26.61%), and Carangidae (15.13%). Dominant species included Cetengraulis mysticetus, Gobiidae sp.1, Gobiidae sp.4, and Bairdiella sp. The Gobiidae family exhibited the highest taxonomic richness, with six species identified, followed by Carangidae (four species) and Sciaenidae (three species) (Table 1).
Zooplankton dry biomass exhibited substantial spatial variation across sampling stations, ranging from 0.34 to 180.43 g 1000 m−3, with the highest values observed inside of the Tapaje River. Fish larvae abundance showed a heterogeneous spatial distribution, with elevated concentrations generally associated with outer estuarine zones (Figure 2).
Sampling completeness, as estimated by the first-order jackknife richness estimator, yielded a value of 23.7 ± 3.9, indicating a satisfactory level (65% to 83%) of representativeness in the larval fish assemblage across the surveyed estuarine zones. Shannon diversity indices revealed notable differences among sampling sites. Tapaje exhibited the highest diversity (H′ = 2.02), followed by Guascama (H′ = 1.99), Sanquianga (H′ = 1.77), and Amarales (H′ = 1.63) (Table 2). Permutation-based pairwise comparisons indicated statistically significant differences between Amarales and Guascama (p = 0.0001), Amarales and Tapaje (p = 0.0001), and Sanquianga and Tapaje (p = 0.049). No significant differences were observed between Amarales and Sanquianga (p = 0.432) or between Guascama and Tapaje (p = 0.801).
Evenness, assessed using Pielou’s index, followed a similar pattern. Guascama (J = 0.87) and Sanquianga (0.85) exhibited higher equitability compared with Amarales (0.60). All comparisons involving Amarales were statistically significant (p = 0.0001). A significant difference was also found between Guascama and Tapaje (p = 0.0023), whereas comparisons between Guascama and Sanquianga (p = 0.6766) and Sanquianga and Tapaje (p = 0.241) were not statistically significant. Furthermore, Beta diversity, calculated using Whittaker’s index, revealed the greatest dissimilarity between Amarales and Tapaje (βw = 0.50), followed closely by Amarales and Sanquianga (βw = 0.48). Lower dissimilarity values were observed between Guascama and Sanquianga (βw = 0.33) and between Sanquianga and Tapaje (βw = 0.33), suggesting greater compositional similarity among these latter sites (Table 2).

3.2. Environmental Conditions

Marked spatial variability was observed in sea surface temperature (SST) and sea surface salinity (SSS) across the Sanquianga Tapaje estuarine system (Figure 3). SST ranged from 25.53 to 29.81 °C, while SSS varied between 21.39 and 30.74. In contrast, temperature and salinity at 10 m depth exhibited lower variability, with T10m ranging from 28.34 to 29.85 °C and S10m from 26.70 to 30.74. Water transparency, assessed via Secchi disk depth, ranged from 1.0 to 2.5 m, indicating moderate turbidity throughout the study area (Figure 3).
Chlorophyll-a concentrations showed minimal spatial variation, with surface values ranging from 0.21 to 0.29 mg/m3 and subsurface values (10 m) from 0.20 to 0.30 mg/m3 (Figure 3). Dissolved oxygen concentrations on the surface ranged from 5.1 to 6.6 mg/L, with the lowest values recorded near the Tapaje river mouth. At a 10 m depth, oxygen levels ranged from 5.6 to 6.9 mg/L, with the lowest concentrations observed in the inner zones of the Amárales and Tapaje river mouths (Figure 3).

3.3. Statistical Associations

Spearman’s rank correlation (rs) analysis showed significant positive associations between fish larvae abundance and surface salinity (rs = 0.81, p = 0.003), surface dissolved oxygen saturation (rs = 0.68, p = 0.021), and surface dissolved oxygen concentration (rs = 0.69, p = 0.019) (Table 3). These findings suggest that larval abundance was highest in regions with elevated salinity and oxygen availability, particularly in outer estuarine zones. In contrast, no significant relationships were detected between fish larvae abundance and zooplankton dry biomass, chlorophyll-a concentration, water temperature, or turbidity (p > 0.05) (Table 3), indicating that larval distribution may be more strongly influenced by physicochemical gradients than by primary productivity or prey biomass.

4. Discussion

In coastal ecosystems, the connectivity between lacustrine and marine habitats generates pronounced variability in physicochemical conditions. A key hydrodynamic feature in estuarine systems is the formation of a stratified water column, commonly referred to as a “salt wedge,” where denser seawater intrudes beneath freshwater discharged from rivers [5,49,50]. This stratification facilitates the retention of nutrients originating from river basins and adjacent riparian forests, supporting a diverse planktonic community that underpins local food webs [51,52]. The Sanquianga Tapaje estuarine system exhibits a typical tropical seasonal regime, with alternating rainy and dry seasons that modulate freshwater input and tidal dynamics [38,53]. Combined with the region’s geomorphological complexity and a tidal range of approximately 4 m [1,4], these factors generate environmental gradients along estuarine channels, similar to patterns observed in other tropical systems [54,55,56]. These gradients play a pivotal role in structuring fish larvae assemblages.
In this study, fish larvae abundance was positively correlated with surface salinity and dissolved oxygen concentrations, with the highest densities recorded in the outer zones of the river mouths—areas more strongly influenced by marine waters. These findings align with previous research indicating that salinity is a key determinant of larval distribution in estuarine environments, while low dissolved oxygen levels are associated with increased mortality and reduced growth rates [57,58,59,60,61,62]. Similar spatial patterns have been documented in Amazonian estuaries, where fish larvae tend to concentrate in outer estuarine zones during the rainy season, coinciding with elevated salinity levels [56,63,64,65]. This trend also mirrors seasonal migrations of adult fish, which move toward river mouths and deltas during periods of high freshwater discharge, resulting in reduced fish abundance in the estuarine interior [54,66,67].
While salinity and dissolved oxygen emerged as primary drivers of larval abundance in the Sanquianga Tapaje system, other environmental variables—including temperature, turbidity, and food availability—also influence early life stage viability. Elevated temperatures can accelerate metabolic rates in fish larvae, potentially reducing growth and survival [68]. Turbidity, often driven by suspended particles, may affect habitat selection and visual foraging efficiency, thereby altering predator–prey dynamics [69,70]. However, when turbidity is associated with increased particulate food availability, larval survival and growth may be enhanced [70,71]. Despite the ecological relevance of zooplankton as a trophic resource for early-stage fish larvae, our results revealed no significant correlation between zooplankton biomass and larval abundance. This lack of association may stem from several interacting factors. First, there may be a temporal mismatch between peak zooplankton availability and larval presence, particularly given the limited temporal resolution of our sampling [72]. Second, differences in vertical distribution and microhabitat preferences between zooplankton and ichthyoplankton could reduce spatial overlap, thereby weakening observed correlations [73]. Third, the use of a 300 µm mesh net may have under-sampled smaller zooplankton taxa such as nauplii and rotifers, which are critical food sources for preflexion larvae [74]. This methodological constraint likely introduced a sampling bias, limiting our ability to detect fine-scale trophic relationships. Future studies should consider finer mesh sizes and temporally resolved sampling to better capture zooplankton dynamics and their potential influence on larval fish assemblages
The ichthyoplankton assemblage observed in this study was characterized by low larval densities, small body sizes (preflexion stages), and limited taxonomic diversity—patterns typical of shallow tropical estuarine systems [75,76,77]. Assemblages were numerically dominated by a few taxa, particularly Engraulidae, Gobiidae, and Sciaenidae, consistent with findings from other tropical estuaries where these families, along with Clupeidae, are commonly dominant [63,75,78]. These taxa are considered estuarine residents or coastal migrants that utilize estuaries as nursery habitats [49,79,80]. The occasional presence of non-resident taxa, such as Pomacentridae, may be attributed to advective transport, salt wedge inversion, tidal mixing, or episodic shifts in food resource availability [81,82,83]. These dynamics underscore the importance of hydrological and trophic connectivity in shaping larval assemblage structure and highlight the need for continued monitoring to understand the ecological implications of environmental variability in tropical estuarine systems.
The estimated species richness observed in our study aligns with findings from tropical estuarine systems where targeted sampling captured dominant larval taxa but often underrepresented rare species due to patchy distributions and transient inputs [72]. The use of Shannon and Pielou’s indices to assess diversity and evenness has proven effective in revealing site-specific ecological patterns, as demonstrated in recent studies across Indo-Pacific and Neotropical estuaries [73,84]. Broader temporal analysis of diversity would be required to assess assemblage stability; however, taken together, these diversity metrics underscore the ecological complexity of the Sanquianga Tapaje system. Beta diversity metrics, such as the Whittaker index, have been increasingly applied to characterize spatial turnover in larval assemblages, particularly in systems with strong hydrodynamic gradients [85]. These metrics, when combined with abundance data, offer a multidimensional view of larval fish ecology and reflect the dynamic interplay between habitat connectivity, salinity intrusion, and larval richness, evenness, and turnover [86,87].
The spatial structure of ichthyoplankton assemblages in the Sanquianga Tapaje estuarine system reflects the influence of physicochemical gradients shaped by tidal dynamics and freshwater discharge variability. As previously discussed, salinity and dissolved oxygen emerged as primary environmental drivers of larval abundance, with higher densities observed in outer estuarine zones where marine influence prevails. These findings are consistent with patterns reported in other tropical estuaries, where larval distributions are modulated by salinity intrusion and oxygen availability [57,60,62]. However, the limited temporal resolution of our sampling constrains the ability to fully capture the influence of tidal dynamics and freshwater inflow. Accordingly, such factors should be considered tentative hypotheses that warrant further investigation through temporally resolved sampling and hydrodynamic modeling.

5. Conclusions

A comprehensive understanding of the early life stages of fish in tropical estuarine systems is fundamental to advancing biodiversity conservation and sustainable fisheries management. In the Sanquianga Tapaje estuarine system, the strong correlation between fish larvae abundance and key physicochemical variables—particularly salinity and dissolved oxygen—highlights the need for continuous monitoring of these parameters to anticipate population dynamics and inform protective measures. The presence of both resident and non-resident larval families reinforces the role of estuaries as essential nursery and refuge habitats, facilitating coastal–estuarine connectivity and supporting broader population resilience. These findings contribute to a growing body of evidence that tropical estuaries function as dynamic ecological corridors, warranting targeted conservation strategies that integrate hydrological, biological, and climatic variability.
Although most larvae were identified only to the genus or family level due to their early developmental stages and limited morphological differentiation, this constraint reflects broader challenges in tropical ichthyoplankton research—particularly the scarcity of regional identification guides and diagnostic resources. To advance species-level resolution and improve ecological assessments, future efforts should prioritize the development of comprehensive reference collections and the integration of molecular tools such as DNA barcoding. These improvements will enhance taxonomic precision and support more effective monitoring of estuarine nursery habitats [88,89,90].
The estimated species richness indicates that the sampling effort effectively captured a substantial portion of the local ichthyoplankton assemblage, although rare taxa may have remained undetected. The Shannon diversity index reflected localized environmental conditions and habitat complexity, which likely influence larval recruitment, retention, and overall diversity. Evenness, assessed using Pielou’s index (J′), revealed site-specific patterns: Amarales was dominated by a few taxa, suggesting reduced equitability, whereas Guascama and Sanquianga exhibited more balanced assemblages, indicative of more heterogeneous larval inputs. These trends were consistent with beta diversity patterns (Whittaker index), which showed greater dissimilarity between Amarales and both Tapaje and Sanquianga, while Guascama and Sanquianga shared more similar ichthyoplankton compositions.
To investigate ecological associations, non-parametric Spearman rank correlations were employed to examine relationships between larval abundance and both biotic and abiotic variables. This method was chosen for its robustness in handling small sample sizes and non-normal data distributions, while also offering a broader ecological context that complements the observed patterns in larval abundance. However, spatial autocorrelation was not formally assessed, and we acknowledge that this may have influenced correlation outcomes due to potential pseudoreplication. Likewise, multicollinearity among abiotic variables was not evaluated (e.g., via VIF or PCA), owing to the limited sample size. These analytical constraints are recognized as limitations; nonetheless, the observed associations highlight the ecological sensitivity of larval assemblages to underlying physicochemical gradients.
From a management perspective, recognizing these spatial patterns is essential for designing effective conservation strategies and monitoring programs. Protecting estuarine nursery habitats requires not only preserving physical conditions but also maintaining the ecological processes that sustain larval diversity and recruitment. Future studies with expanded spatial and temporal replication should incorporate spatial statistics and multivariate approaches to refine ecological inference and support more targeted conservation efforts.

Author Contributions

Conceptualization, A.G. and A.C.; methodology, A.G.; software, J.J.G.-Z. and A.G.; validation, J.J.G.-Z. and A.C.; formal analysis, J.J.G.-Z. and A.G.; investigation, J.J.G.-Z. and A.C.; resources, A.G. and A.C.; data curation, A.C. and A.G.; writing—original draft preparation, J.J.G.-Z. and A.G.; writing—review and editing, A.G. and A.C.; visualization, J.J.G.-Z. and A.G.; supervision, A.G.; project administration, A.G. and A.C.; funding acquisition, A.C. and A.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Parque Nacional Natural Sanquianga y la Universidad del Valle. An APC waiver was granted by the journal’s editorial board.

Institutional Review Board Statement

The study was conducted in accordance with the Declaration of Helsinki, and the sampling protocol was approved by Parques Nacionales Naturales de Colombia (PNN Sanquianga).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

We thank Felipe Muriel (PNN Sanquianga) and Diego Córdoba (Universidad del Valle) for their invaluable support during fieldwork and laboratory activities. Zooplankton sampling and oceanographic data collection were conducted by officials from Colombia’s National Natural Parks, in collaboration with the Oceanographic Sciences Research Group at Universidad del Valle.

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) Geographic location of the Sanquianga Tapaje estuarine system (red dot) within the Eastern Tropical Pacific (ETP). The dotted line delineates the extent of the ETP. (B) Detailed view of the Sanquianga Tapaje estuarine sustem showing the distribution of sampling stations: Guascama (G), Sanquianga (S), Amárales (A), and Tapaje (T). Image source: Google Earth®, Landsat/Copernicus, SIO, NOAA, U.S. Navy, NGA, GEBCO. 31 December 2020, altitude 50 km, image center: 2°31′42.80″ N–78°13′12.45″ W.
Figure 1. (A) Geographic location of the Sanquianga Tapaje estuarine system (red dot) within the Eastern Tropical Pacific (ETP). The dotted line delineates the extent of the ETP. (B) Detailed view of the Sanquianga Tapaje estuarine sustem showing the distribution of sampling stations: Guascama (G), Sanquianga (S), Amárales (A), and Tapaje (T). Image source: Google Earth®, Landsat/Copernicus, SIO, NOAA, U.S. Navy, NGA, GEBCO. 31 December 2020, altitude 50 km, image center: 2°31′42.80″ N–78°13′12.45″ W.
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Figure 2. Spatial variation of zooplankton biomass (A) and fish larvae abundance (B) at the river mouths of the estuarine system of Sanquianga National Natural Park (PNNS): Guascama (G), Sanquianga (S), Amárales (A), and Tapaje (T).
Figure 2. Spatial variation of zooplankton biomass (A) and fish larvae abundance (B) at the river mouths of the estuarine system of Sanquianga National Natural Park (PNNS): Guascama (G), Sanquianga (S), Amárales (A), and Tapaje (T).
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Figure 3. Spatial variation in temperature (T), salinity (S), chlorophyll-a (Clo-a), and dissolved oxygen (DO) in the Sanquianga Tapaje estuarine system at depths of 1 m and 10 m in August 2019. Sampling locations: Guascama (G), Sanquianga (S), Amárales (A), and Tapaje (T).
Figure 3. Spatial variation in temperature (T), salinity (S), chlorophyll-a (Clo-a), and dissolved oxygen (DO) in the Sanquianga Tapaje estuarine system at depths of 1 m and 10 m in August 2019. Sampling locations: Guascama (G), Sanquianga (S), Amárales (A), and Tapaje (T).
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Table 1. Taxonomic list of ichthyoplankton associated with the Sanquianga Tapaje estuarine system, indicating relative abundance (%). Bolded values represent the relative abundance of each Family. Non-bolded values correspond to relative abundance of each taxon.
Table 1. Taxonomic list of ichthyoplankton associated with the Sanquianga Tapaje estuarine system, indicating relative abundance (%). Bolded values represent the relative abundance of each Family. Non-bolded values correspond to relative abundance of each taxon.
Family/Specie %
Clupeidae 0.84
Opisthonema sp.0.84
Engraulidae 39.50
Anchoa sp.1.96
Cetengraulis mysticetus37.54
Hemiramphidae 0.28
Hemiramphus sp.0.28
Carangidae 15.12
Caranx sp.4.76
Chloroscombrus orqueta5.60
Oligoplites saurus4.48
Seriola sp.0.28
Haemulidae 1.40
Anisotremus sp.1.40
Sciaenidae 14.00
Bairdiella sp.8.96
Menticirrhus sp. 11.40
Menticirrhus sp. 23.64
Gobiosocidae 0.28
Gobiesox sp.0.28
Gobiidae 26.61
Gobulus crescentalis1.40
Microgobius sp.0.57
Gobiidae sp. 112.04
Gobiidae sp. 20.28
Gobiidae sp. 30.28
Gobiidae sp. 412.04
Ephippidae 0.28
Chaetodipterus zonatus0.28
Pomacentridae 1.13
Stegastes sp.0.29
Chromis sp.0.84
Achiridae 0.56
Achirus sp.0.56
Table 2. Summary of diversity metrics for ichthyoplankton assemblages across sampling stations in the Sanquianga Tapaje estuarine system. Metrics include species richness (S), abundance (N, ind/1000 m3), Shannon diversity index (H′), Pielou Evenness index (J′), and Whittaker beta diversity index (βw). Values are based on standardized sampling effort. G: Guascama, S: Sanquianga, A: Amarales, T: Tapaje.
Table 2. Summary of diversity metrics for ichthyoplankton assemblages across sampling stations in the Sanquianga Tapaje estuarine system. Metrics include species richness (S), abundance (N, ind/1000 m3), Shannon diversity index (H′), Pielou Evenness index (J′), and Whittaker beta diversity index (βw). Values are based on standardized sampling effort. G: Guascama, S: Sanquianga, A: Amarales, T: Tapaje.
Diversity MetricsGSAT
Taxa (S)1081513
Abundance (N)19985696811
Shannon (H’)1.991.771.632.02
Evenness (J’)0.870.850.600.79
ComparisonsG-SG-AG-TS-AS-TA-T
Whittaker (βw)0.330.360.480.480.330.50
Permutation H’ (p value)0.05750.0007 *0.80730.43220.0494 *0.0001 *
Permutation J’ (p value)0.67660.0001 *0.0023 *0.0001 *0.24120.0001 *
* Statistically significant difference (p < 0.05).
Table 3. Spearman rank correlation coefficients (rs) between fish larval abundance and selected abiotic and biotic variables across sampling stations in the Sanquianga Tapaje estuarine system. Abiotic variables include transparency (m), temperature (°C), salinity (psu), oxygen saturation (%), dissolved oxygen (mg·L−1), and turbidity (NTU); biotic variables include chlorophyll-a concentration (µg·L−1) and zooplankton biomass (mg·m−3).
Table 3. Spearman rank correlation coefficients (rs) between fish larval abundance and selected abiotic and biotic variables across sampling stations in the Sanquianga Tapaje estuarine system. Abiotic variables include transparency (m), temperature (°C), salinity (psu), oxygen saturation (%), dissolved oxygen (mg·L−1), and turbidity (NTU); biotic variables include chlorophyll-a concentration (µg·L−1) and zooplankton biomass (mg·m−3).
Variablersp Level
Transparency0.450.161
Sea Surface Temperature−0.030.937
Sea Surface Salinity0.810.003 *
Dissolved Oxygen Saturation0.680.021 *
Dissolved Oxygen Concentration0.690.019 *
Phytoplankton Biomass (Chlorophyll-a)0.290.393
Turbidity−0.200.555
Zooplankton Biomass0.350.285
* Statistically significant correlations (p < 0.05).
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Gallego-Zerrato, J.J.; Cuellar, A.; Giraldo, A. Ichthyoplankton Composition and Environmental Drivers in the Sanquianga Tapaje Estuarine System, Eastern Tropical Pacific. Diversity 2025, 17, 649. https://doi.org/10.3390/d17090649

AMA Style

Gallego-Zerrato JJ, Cuellar A, Giraldo A. Ichthyoplankton Composition and Environmental Drivers in the Sanquianga Tapaje Estuarine System, Eastern Tropical Pacific. Diversity. 2025; 17(9):649. https://doi.org/10.3390/d17090649

Chicago/Turabian Style

Gallego-Zerrato, Juan José, Andrés Cuellar, and Alan Giraldo. 2025. "Ichthyoplankton Composition and Environmental Drivers in the Sanquianga Tapaje Estuarine System, Eastern Tropical Pacific" Diversity 17, no. 9: 649. https://doi.org/10.3390/d17090649

APA Style

Gallego-Zerrato, J. J., Cuellar, A., & Giraldo, A. (2025). Ichthyoplankton Composition and Environmental Drivers in the Sanquianga Tapaje Estuarine System, Eastern Tropical Pacific. Diversity, 17(9), 649. https://doi.org/10.3390/d17090649

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