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Article

The Effect of Temporal and Environmental Conditions on Catch Rates of the Narrow-Barred Spanish Mackerel Setnet Fishery in Khanh Hoa Province, Vietnam

by
Nghiep Ke Vu
1 and
Khanh Quoc Nguyen
1,2,*
1
Institute of Marine Science and Fishing Technology, Nha Trang University, 2 Nguyen Dinh Chieu, Nha Trang 57105, Vietnam
2
Fisheries and Oceans Canada, St. John’s, NL A1C 5X1, Canada
*
Author to whom correspondence should be addressed.
Fishes 2025, 10(6), 257; https://doi.org/10.3390/fishes10060257
Submission received: 26 April 2025 / Revised: 13 May 2025 / Accepted: 22 May 2025 / Published: 1 June 2025
(This article belongs to the Special Issue Effects of Climate Change on Marine Fisheries)
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Abstract

Small-scale inshore fisheries significantly contribute to the total landing volumes and have an important role in Vietnamese socioeconomic development, food security, livelihoods, and social well-being. The setnet fishery has been used throughout coastal communities of Vietnam for many decades. Being a passive fishing gear, the catch efficiency of setnet depends on various conditions such as fish density, season, oceanography, environment, and others. However, very little information exists about the relationship between catch rates and national conditions. Recognizing this research gap, this study examined the effect of temporal and environmental conditions on the catch rates of the narrow-barred Spanish mackerel (Scomberomorus commerson) setnet fishery using long-term data from 2005 to 2016. Overall, the catch of narrow-barred Spanish mackerel decreased over the course of the study. The generalized additive model (GAM) showed that catch rates were significantly affected by sea surface temperature (SST), which peaked at 27 °C. After this temperature point, the catch rates significantly decreased. Temporal variables also contributed to the catch variation. The setnet caught the highest yield in April and May, and more fish were caught during periods of low nightlight intensity than during high illuminated periods. Our study contributes to the understanding of critical factors affecting the catch rates of valuable species, which helps to determine the optimal fishing process of the setnet fishery within the shifting of marine heatwaves.
Keywords:
narrow-barred Spanish mackerel; marine heatwaves; sea surface temperature; CPUE; setnet fishery
Key Contribution: The study shows that sea surface temperature significantly affects the catch rate of the narrow-barred Spanish mackerel setnet fishery. Year, season, and moon phase variables also contribute to the explanation of the catch variation. Insights into the temporal and environmental variables affecting the catch rates of narrow-barred Spanish mackerel in this study can help better management and operation of the setnet fishery to contribute to sustainable fishery development using small-scale fishing methods in the context of climate change and shifting of marine heatwaves.

1. Introduction

Setnets are traditionally fished as stationary, uncovered pound fishing gear [1]. There are different setnet structures depending on target species, geography, and traditional practices [1]. However, the fishing mechanism of setnets is similar, which relies on fish behavior in response to the natural diel movements or seasonal migration or prey-predator behavior (e.g., foraging and avoidance behavior). Fish schools are guided by the net wall, called the leader, to gradually swim into the playground through the special entrance, and are eventually trapped in the codend. Fishermen can either remove the codend or use an additional net (small purse seine net) to harvest the trapped fish [2]. This passive fishing method has been classified as environmentally friendly, sustainable, and energy-saving fishing gear, which has been widely used throughout coastal areas around the world for decades [1,3,4,5,6]. Setnet captures live fish, which are typically of higher market quality and price. However, the catch efficiency of setnet is typically low compared to other fishing methods such as gillnet, purse seine, and trawl [7,8,9]. This gear can also result in high bycatch and undersized catch rates [3]. Catch rates of setnet are therefore influenced by a number of factors such as location, oceanographical condition, season, sea surface temperature (SST), and natural light condition [6,10,11].
Marine fisheries have a major contribution to socioeconomic development, provide nutrients, animal protein, and micronutrients, and are an important source of income throughout the coastal provinces of Vietnam [12,13]. Although offshore fishing has been invested in and developed during the past 15 years, small-scale inshore fisheries are important [9,12,14]. In Vietnam, setnets have traditionally been used throughout the coastal communities to capture a variety of pelagic species such as narrow-barred Spanish mackerel (Scomberomorus commerson), moontail bullseye (Priacanthus hamrur), skipjack tuna (Katsuwonus pelamis), spotted mackerel (Scomberomorus guttatus), Indian mackerel (Rastrelliger kanagurta), redtail scad (Decapterus kurroides), and others [15,16]. In the past, this artisanal and subsistence fishing method significantly contributed to hunger and poverty alleviation and provided animal protein nutrition in the local diet. Several hundred setnets operated throughout the fjords, estuaries, bays, and lagoons in the 1990s [15,17]. However, the setnet fishery has shrunk during the past 15 years because of low catch performance, alternation of other fishing methods, and offshore fishing development [16].
Narrow-barred Spanish mackerel, which is a primary catch composition of the setnet fishery, is a highly migratory species belonging to the family Scombridae [18,19]. This species is widely distributed from the Indo-West Pacific to South Africa, the Red Sea, Australia, and the Mediterranean Sea [20,21,22,23]. In 2020, the global total catch of narrow-barred Spanish mackerel was approximately 295 thousand tonnes [18]. This species can grow up to 70 kg and reach a size of 240 cm fork length. They can live up to 22 years [20]. The juvenile narrow-barred Spanish mackerel live inshore and surrounding the islands. Once mature and reaching migratory sizes, they migrate to offshore and deeper waters [18]. As a thermally hypersensitive species, the tolerant water temperature is from 13 °C to 33 °C at a depth varying between 5 m and 169 m [22]. Narrow-barred Spanish mackerel hunts a diversity of food compositions, including anchovies (Anchoviella), slipmouths (Leiognathus), clupeids (Sardinella), squid (Todarodes pacificus), shrimps (Penaeus), and more [22]. In Vietnam, narrow-barred Spanish mackerel is captured by various fishing methods such as gillnet, purse seine, and setnet [7,8,16]. Total landings in 2023 were ~36 thousand tonnes, accounting for $12.5 million [13]. However, the narrow-barred Spanish mackerel population has declined over time, particularly in the coastal areas because of overfishing, anthropogenic stressors, and environment shifting, requiring an effective management and conservation of this stock.
Global climate change has substantially influenced marine biodiversity, which results in the migration of many tropical and sub-tropical species toward the south and north poles [24]. Seawater heating directly affects population structure, distribution, and migratory routes, greatly influencing the catch rates of many kinds of fishing gear, including setnet [2,25]. The SST in the South China Sea has significantly increased during the past century [26]. Lu and Lee (2014) [6] showed that the average SST increases by 0.014 °C annually, which is higher than the global average increase of 0.011 ± 0.003 °C/year. Some studies conducted in this region showed the impact of heatwaves on marine species and fisheries. For instance, the SST rise has significantly affected the abundance of anchovies, Indian mackerel, Japanese scad, and Japanese jack mackerel [27,28,29]. However, very little is known about the effect of environmental conditions on the catch rates of fishing gear, particularly passive fishing gear like setnets.
In addition to the environmental factors, the catch rates of most fishing gear depend on the season. For example, the catch efficiency of narrow-barred Spanish mackerel gillnet is highest in August and September [7]. However, pole-and-line gear captures the highest amount of yellowfin (Thunnus albacares) and bigeye (Thunnus obesus) tuna in April and May, and the lowest catch in October [30]. Despite being the traditional fishing method in Vietnam, how the catch efficiency varies according to seasons and local SST has not been examined and documented. This poses a challenge for fishing management as well as predicting the optimal fishing condition of this fishery.
To assess the effectiveness of setnet on fishery sustainability—so that local people can rely on fishing for a relatively stable profit and livelihoods—it is important to understand how temporal and environmental conditions affect the catch efficiency. Our study aims to analyze patterns of setnet catch rates over a 12-year period. We examine the relationships between the catches and time series (e.g., year), season (month), moon phase, and SST.

2. Materials and Methods

2.1. Study Site

The study was undertaken in Khanh Hoa province, which is located along the central coast of Vietnam and is about 400 km from Ho Chi Minh City. The province has a 400 km coastline covering the Nha Trang Bay, which includes more than 200 islands and two marine protected areas (Ran Trao and Nha Trang Bay). Coral reefs can be found in many areas, cover an area of greater than 2500 km2, and are divided into two categories of coastal and oceanic reefs, which include approximately 997 species belonging to 647 genera and 174 families of 6 major groups of organisms [31]. This environment provides the ideal living conditions for more than 300 fish species, 170 Mollusca, 68 Crustacea, and 37 Echinodermata [31]. This natural condition is highly suitable for the setnet operation. The tourist industry is the biggest contributor to the local economy with an amount of ~$600 million annually. However, marine fisheries and aquaculture provide significant income and employment for local people. Total landings in 2024 were 104,180 tonnes with an export value of $729 million [13]. Khanh Hoa’s sea is located in the upwelling region, having rich and abundant marine resources, which is one of the main fishing grounds of Vietnam [31]. In addition to the most valuable oceanic tunas captured by pole-and-line fishing methods from offshore, multiple economic species are harvested along the nearshore areas using a variety of fishing methods such as trawl, gillnet, and purse seine. Moreover, the river systems (e.g., Cai river) and canals from the inland along with fjords and islands at the bays and lagoons create deep channels and basins separated by sills, creating good conditions for the marine harbor construction and supporting a favorable environment for fishery development.

2.2. Data Collection

The daily catch data from 2005 to 2016, including species and weight, were collected from the Bich Hai Fishing Cooperative, Nha Trang City, Khanh Hoa province, Vietnam, through a mandatory logbook. This fishing cooperative owned the setnet for many years. The setnet was fixed at the location 12°9′51″ N and 109°19′49″ E in the south of Hon Tre island (Figure 1). The net structures, including wings (leaders), entrance, and codend (pound), were placed in the water columns by anchors to the sea bed, supported by globular polyvinyl chloride (PVC) floats, and kept vertical in shape by weights (rocks and concrete). The setnet was located in the fishing ground in January and used during the fishing season until October, before it was removed from the location for maintenance. The wings were constructed with nylon multifilament of 210D/90, had a stretched mesh size of 250 mm, and extended from the shoreline to fix across the movement direction of fish schools. The net height was equal to the depth of the fishing ground, stretching from 5 to 30 m. The codend was directly connected to the wings. The special structure of the entrance ensured that fish were trapped once they entered the codend. The codend was constructed with a 90 mm stretched mesh size. Finally, a lift net made of nylon multifilament of 210D/9-210D/21 with a mesh size of 30 mm was used to harvest fish kept in the codend. Figure 2 illustrates the Bich Hai setnet at Bich Dam, Nha Trang City, where we collected the catch data.
The SST was collected from the Vietnam National Center for Hydrometeorological Forecasting (NCHMF), which measured the SST every two hours at the station in Nha Trang Bay.

2.3. Statistical Analysis

The plot of SST was used to provide the overall temperature in Nha Trang Bay, where we collected the setnet fishery data. The yearly SST anomaly was calculated as the difference between the temperature at a given year and the average temperature over a longer reference period (i.e., all available data), following the equation A n o i = S S T i − m e a n ( S S T t o t a l ) , where Anoi is the anomaly of year i, SSTi is the STT of year i, and mean(SSTtotal) is the average temperature over a long period.
The generalized additive model (GAM) was used to estimate the effect of temporal and environmental conditions on the catch per unit effort (CPUE) of narrow-barred Spanish mackerel over time, where CPUE was defined as the total catch (kg) of narrow-barred Spanish mackerel per standardized fishing day (24 h). The GAM was used because the shape of the non-linear relationships between CPUE and predictors was unknown. GAM, including a class of equations with a loess function, allows the generalization of data into smooth curves by local fitting to subsections of the data [32]. The predictors in this model included temporal factors (e.g., year, DoY (day of the year), and moon phase) and environmental conditions (e.g., SST). In addition, the soak time of setnet varied from 20 to 26.5 h; thus, we included fishing effort in the model as an offset. The GAM formula for CPUE was as follows:
log ⁡ C P U E ~   s Y e a r + s D o Y + s M o o n p h a s e + s S S T + o f f s e t ( log ⁡ e f f o r t )
where CPUE (kg per fishing day) represents the catch per unit effort as defined above. s is a thin-plate smoothing-spline function. The DoY was converted from the date (continuous variable) to fit in the model. The moon phase was included in the model because the foraging behavior of narrow-barred Spanish mackerel was known to be influenced by the prevailing natural night light [7,8]. Moonphase was a ratio scale that ranged from 0 for the new moon (darkest time) to 1 for the full moon (the most illuminated time). Because of the potential non-linear relationships of CPUE with Year, DoY, and SST, cubic regression splines defined by a modest-sized set of knots spread evenly through the covariate values were applied. They are penalized by the conventional integrated square second derivative cubic spline penalty. The cyclic cubic regression spline was employed for the moon phase variable, which is a penalized cubic regression spline whose ends match, up to the second derivative. To avoid overfitting while obtaining spatially relevant responses, the maximum number of knots for the predictor was set at four (k = 4) for the moon phase and SST variables and 11 for year and DoY, allowing the smoother to divide the response from each explanatory variable into suitable parts. Outliers were examined based on Cook’s distances [33] using the CookD function in the package predictmeans [34]. The fit model was analyzed without outliers. We tested and found that the best model fit was produced using a gamma error structure with a log link, which had the smallest AICc value. Residual linearities were checked using the lag.plot function in R, and homogeneities of the variance were visually inspected from the plot of predicted values vs. residual values, then fitted to Q-Q plots to check normality (Figure A1).
Finally, the Kolmogorov–Smirnov test was used to compare the narrow-barred Spanish mackerel weight frequency distributions between years with a significance level of 0.05. Data preparation, plotting figures, and data analysis were performed by the open statistical software R V.4.3.2 [35].

3. Results

The daily SST in Nha Trang Bay fluctuated between 21.4 and 31.9 °C between winter and summer months in recent years (Figure 3A). As expected, near-surface water temperatures are highest during the summer and coldest in the winter. The SST during fishing days varied from 24.3 to 31.6 °C. The SST anomaly time series also showed high levels of variation over the 2005–2016 period (Figure 3B). The SST anomaly showed that the coldest year was 2008, with a value 0.52 °C lower than the average. In contrast, the SST was warmest in 2016 with an anomaly of 0.53 °C higher than the average. The period of 2012–2016 was considered a warm period, having a positive anomaly.
A total of 653 fishing days were conducted from 2005 to 2016. The fishing season was from February to September. The number of fishing days ranged from 21 days to 114 days per year, with an average of 55 days per year (Table 1). In total, 384.05 tonnes and 19 species were captured over the course of the study (Table 2). As expected, most species were the pelagic species, some rockfish and mollusk species were also caught. Narrow-barred Spanish mackerel dominated at 231.84 tonnes (Table 1), accounting for 60.4%, and thus only this species was included in the analysis. Other substantial contributions to the total catch included skipjack tuna (10.39%), swordfish (4.85%), and spotted mackerel (4.48%).
The average CPUE of narrow-barred Spanish mackerel varied between 1.8 kg per day and 3650.5 kg per day (mean: 355.05 kg per day). The average individual weight of narrow-barred Spanish mackerel significantly varied between years (p-value < 0.001) and ranged from 1.76 kg to 13.4 kg (mean: 4.67 kg) (Figure 4). The highest average weight of narrow-barred Spanish mackerel was in 2009 (5.98 kg per individual), followed by 2007 (5.72 kg per individual) and 2006 (5.38 kg per individual), but in 2016 and 2014, the lowest weights of individuals were recorded, with 3.72 and 3.64 kg, respectively.
The GAM analysis showed that the variations in the CPUE for narrow-barred Spanish mackerel could be explained by temporal (e.g., year, day of the year, and moon phase) and environmental (e.g., sea surface temperature) conditions (Table 3). The partial effects of the predictors in the model are shown in Figure 5. Overall, the CPUE of narrow-barred Spanish mackerel decreased over the course of the study. The CPUE of narrow-barred Spanish mackerel was highest in 2007 (mean: 842.38 kg per day) and lowest in 2013 (mean: 79.42 kg per day) (Figure 5A). The model revealed that CPUE varied between months, with the highest CPUE in April and May, then dropping to 144.17 kg per day in June, which was the lowest catch in the year (Figure 5B). Moon phase had a significant effect on the CPUE of narrow-barred Spanish mackerel. Specifically, CPUE significantly decreased with the higher lunar illumination levels (Figure 5C). Importantly, the model showed that the CPUE was influenced by the SST, with peaks of CPUE at 27 °C (Figure 5D). Although narrow-barred Spanish mackerel was captured in a wide range of SST (24.3–31.6 °C), 93% of the catch was distributed between 25.6 °C and 29.2 °C. Of these four explanatory variables, the STT was the most important predictor of CPUE, followed by day of the year, moon phase, and finally year, based on the deviances explained and adjusted R2 values derived from the single explanatory variable model. The total deviance explained by the models for narrow-barred Spanish mackerel was 53.2%, and the adjusted R2 was 0.62, indicating that the majority of variation in CPUE could be explained by the GAMs model (Table 3).

4. Discussion

The study is the first in-depth examination of the commercial setnet fishery, including the temporal and environmental conditions in Vietnam in general and Khanh Hoa province in particular. Results showed that the catch rates of inshore setnet fishery are associated with the temperature. This relates to fish migration and foraging behavior [23,25,36]. While catch rates of the setnet fishery were highest in April and May, which is consistent with the temporal distribution of narrow-barred Spanish mackerel in the South China Sea [23]. Overall, the mean CPUE of narrow-barred Spanish mackerel gradually decreased from 442.6 kg in 2005 to 249.9 kg in 2016. This reduction can be attributed to either population decline due to overfishing [37] and climate change [38] or the development of other offshore fisheries, which catch migratory fish from distances before they can come to the coastal areas. Lastly, the lunar regime is significantly correlated to the CPUE. Insights into how temporal and environmental variables affect the catch rates of narrow-barred Spanish mackerel in this study can help the better operation of the setnet fishery to contribute to efficient fisheries and sustainable development using small-scale fishing methods.
The migration, foraging behavior, habitat selection, and spatial distribution of narrow-barred Spanish mackerel are affected by water temperature [23], resulting in the variation of the catch rates of the passive gear [23,25]. Our study revealed that water temperature was the important factor influencing the catch rates of narrow-barred Spanish mackerel and that the catch rates of setnet are highest when the SST is around 27 °C, which is consistent with previous documents proposed by [3,11,23,25]. Those studies show that the optimal water temperature for the fishing grounds of narrow-barred Spanish mackerel varies from 25 to 29 °C. The catch rates of gillnet, trammel net, and purse seine are associated with the SST within the thermal range, but then significantly decrease with higher temperatures, e.g., >29 °C [7,25], which is consistent with our results.
The scientific information on narrow-barred Spanish mackerel stock in Vietnam has not been fully documented. Thus, the effective management and conservation of this fish stock is difficult and challenging. To develop sustainable fisheries, the government of Vietnam deployed the national development program on efficient and sustainable fishing for the period from 2022 to 2025, toward 2030, in which it emphasizes a Total Allowable Catch (TAC) and quota allocation management system [39]. The key component to perform this ecosystem-based management is to rely on the biological and demographic characteristics. Besides the survey data, which is costly and poorly collected in Vietnamese marine resources [40], the commercial catch can provide valuable data for the stock assessment model using a catch length frequency method [41]. The estimates of abundance and biomass of narrow-barred Spanish mackerel for stock assessment and management purposes are based on commercial catch rates using passive fishing gear such as gillnet and setnet, with the assumption of constant catchability. Existing stock assessment models do not incorporate temperature effects on catch rate [41]. Potential bias as a result of temperature and related behavioral changes in the catch rate of setnet might affect spatial and temporal comparisons used to understand population dynamics, particularly as environmental conditions continue to change. For example, the stock assessment model can produce overestimates or underestimates if the sample is collected during highly productive fishing time or poorly productive fishing time, respectively, without considering in situ water temperature. Our study determines the effect of temporal and environmental conditions, which is vital information to provide state-of-the-art advice to stock assessment scientists and managers to better manage this fish stock toward sustainable fishery development using small-scale fishing methods. Specifically, the scientists and managers can forecast the migration, distribution, and fishing grounds of narrow-barred Spanish mackerel based on the optimal catch temperature in this study. In addition, the commercial-based sampling for stock assessment can be performed more efficiently and economically if the data collection is conducted in the high catch rate period (e.g., April–June). Finally, fisheries managers can restrict the fishing season within the curtain period with a minimal impact on the total catch of setnet fishermen to conserve and protect the natural resources based on the temporal effect on catch rates.
In addition to the SST and as the passive fishing gear, the catch rates of the setnet fishery are largely dependent on the tide and coastal currents [1]. The tides in Nha Trang Bay are irregular. For instance, from October to March, the tide is low in the morning, but it has an opposite pattern between April and September. The strongest tides in the year are from April and June [42], which corresponds with the highest CPUE period as shown in this study. Moreover, there are seasonal migratory patterns of narrow-barred Spanish mackerel in the South China Sea, where they migrate from offshore to the coastline during the southwest monsoon season, resulting in an upwelling period foraging the anchovy and spawning [6,17,43,44]. This could explain the seasonal effect of catch rates of the coastal setnet fishery in this study. As a poikilotherm and high migratory species, any change in the living environment can highly affect growth rates, mortality, and recruitment of narrow-barred Spanish mackerel [20]. The global SST has increased by 0.85 °C from 1880 to 2012 [45], where the South China Sea is one of the most affected places by the ENSO regime [46]. The higher SST influences seasonal migration, resulting in a decrease of CPUE in the short-term, but also has impacts on the population abundance in the long-term. For example, changing environmental conditions and habitats, as a result of climate change or marine heatwaves, could affect the distribution of narrow-barred Spanish mackerel populations throughout the West Pacific Ocean, resulting in a northward expansion of narrow-barred Spanish mackerel reducing their presence in traditional fishing grounds as a result of changing water temperatures [23]. Potential differences in the migration and seasonal distribution of narrow-barred Spanish mackerel due to different water temperatures in this study are an important consideration for both fisheries stock assessment and understanding climate change implications, particularly in southern and rapidly warming areas of the species’ distributions.
Like other pelagic fish species, narrow-barred Spanish mackerel is more responsive to the nocturnal light condition and less foraging movement, and stays further shore at the deeper water columns during the illuminated nights [36], resulting in lower CPUE than the low moonlight intensity period (e.g., 25th–30th and 1st–5th of lunar month). In contrast, Narrow-barred Spanish mackerel reportedly experiences shallower nighttime distributions during new moons [23], thus being closer to setnet, resulting in higher catch rates, presumably in association with the diel distribution of prey [41,42]. This finding is consistent with other pelagic species and fisheries, such as the gillnet fishery catching narrow-barred Spanish mackerel [7], tuna handline fishery [30,45], purse seine fishery [8,46], and stick-held falling net [47]. The catch rates of those fishing methods usually decreased with the high lunar illumination levels.
One of the negative impacts of the setnet fishery is catching juvenile and immature species [3]. The common approach to maintaining fishing sustainability and excluding small-sized individuals is to improve gear selectivity by employing larger codend mesh sizes, which retain the trapped fish [11,48,49]. Refs. [49,50,51] showed that the multi-species selectivity of setnet depends on the way individual species respond to the net wall, but the selectivity curve has a similar pattern across the species. Our results show that the mean weight of narrow-barred Spanish mackerel caught by the setnet is 4.67 kg, indicating that a large proportion of caught individuals is smaller than the first maturity size of 4.55 kg [19] and that juvenile individuals are not able to pass through the existing mesh sizes. Large proportions of small sizes of narrow-barred Spanish mackerel were captured recently, indicating a reduction among the large-sized population. This is challenging for sustainable fishery development and the maintenance of the healthy stock of narrow-barred Spanish mackerel. The minimum stretched mesh size regulation is not applicable for the setnet fishery under the current management regime, e.g., the Vietnamese Law on Fisheries [52]. Thus, updating the regulation on minimum mesh sizes of setnet and minimum landing sizes of narrow-barred Spanish mackerel to better manage this fishery is crucial to mitigate the incidental catch of juveniles and improve the selectivity of setnet to conserve natural resources.
In the past, the setnet was one of the most important inshore fisheries in Nha Trang City, Vietnam, providing significant income and seafood to the local communities [16,17]. However, this fishing technology has been less favorable in the recent two decades due to low catch rates and profits, as well as the development of other efficient fishing methods. Fortunately, fisheries managers, agencies, and fishermen have realized that well-managed small-scale artisanal fisheries can contribute to sustainable fishery development [16,53]. Importantly, recreational fisheries using setnet have attracted a lot of tourists and participants, which helps to improve the income of setnet fishermen and supports the local tourist development [54]. Therefore, some effort, resources, and several projects on the maintenance of the existing setnet fishery have been deployed in the last few years [16,50]. Encouraging the environmentally friendly setnet fishery is likely a useful way to conserve the coastal fishery resources toward sustainable fishery development and maintaining a healthy ecosystem for future generations. Finally, collaborating amongst fishermen and stakeholders (e.g., tourist companies) could help to improve the income and fishing profits for setnet fishermen by promoting the recreational fishing service as a new tourist product for visiting tours. As such existing co-benefit, the maintenance of setnet fishery in Khanh Hoa province is possible and requires very little effort.

5. Conclusions

In Vietnam, small-scale and coastal fisheries have significantly contributed to the total landing volume and value. It also has an important role in food security and subsistence, employment, and livelihood. However, the passive fishing method such as setnet, where the catch efficiency relies on fish behavior and movement, is highly affected by environmental conditions. Moreover, the challenge of sustainable fishing development is to keep a balance between economic fishing benefits and maintaining healthy, stable fish populations. The South China Sea is one of the areas that is highly impacted by climate change and rising seawater. This threatens the fish habitat and population, resulting in the reduction of catch rates. Our results show that the sea surface temperature is a primary factor affecting the catch rate of the narrow-barred Spanish mackerel setnet fishery. Our findings highlight the importance of monitoring and addressing the catch variation in response to the temporal and environmental conditions to ensure the long-term sustainability of coastal fisheries.

Author Contributions

Conceptualization, N.K.V. and K.Q.N.; methodology, N.K.V. and K.Q.N.; validation, N.K.V. and K.Q.N.; formal analysis, N.K.V. and K.Q.N.; data curation, N.K.V. and K.Q.N.; writing—original draft preparation, N.K.V. and K.Q.N.; writing—review and editing, N.K.V. and K.Q.N.; visualization, N.K.V. and K.Q.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

This study did not target or involve endangered or protected species. Experiments were conducted as part of a commercial fishing practice, which did not require an experimental permit. The animals were naturally killed, the same as under normal fishing conditions.

Data Availability Statement

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

Acknowledgments

The authors are grateful to the Vietnam National Center for Hydrometeorological Forecasting and the Bich Hai Fishing Cooperative for their valuable cooperation during the data collection. The Institute of Marine Science and Fishing Technology supported the effort and provided resources to conduct this study. Vang Nguyen helped with data collection. Phu Tran consulted on every process of the experimental design, data collection, and statistical analysis, and commented on the early draft. This study could not have been done without their substantial assistance. Finally, the authors thank the anonymous reviewers and editor for valuable comments and edicts during the peer-reviewed process, as well as the Fishes Editorial Office for the helpful services during publication.

Conflicts of Interest

The author declares no conflicts of interest.

Appendix A

Fishes 10 00257 g0a1
Figure A1. The homogeneity of the variance (A), independence of the model residuals (B), and normality of the model residuals (C) for the GAM analysis. There are no convincing evidence of heterogeneity. No evidence of upward or downward trend in residuals, thus residuals are independent. There are few outliers, but not far from the qqlines. Thus, assumption of normality is met.
Figure A1. The homogeneity of the variance (A), independence of the model residuals (B), and normality of the model residuals (C) for the GAM analysis. There are no convincing evidence of heterogeneity. No evidence of upward or downward trend in residuals, thus residuals are independent. There are few outliers, but not far from the qqlines. Thus, assumption of normality is met.
Fishes 10 00257 g0a1

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Figure 1. Map of Vietnam shows the study area (red square) in the Khanh Hoa province. *s in the inlet panel show the location of Ha Noi (north) and Ho Chi Minh City (south).
Figure 1. Map of Vietnam shows the study area (red square) in the Khanh Hoa province. *s in the inlet panel show the location of Ha Noi (north) and Ho Chi Minh City (south).
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Figure 2. The setnet used in the Bich Hai Fishing Cooperative (A) and its schematic net plan (B).
Figure 2. The setnet used in the Bich Hai Fishing Cooperative (A) and its schematic net plan (B).
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Figure 3. The daily sea surface temperature (SST) (°C) from 1 January 2005 to 31 December 2016 (A) and yearly anomaly (B). Each open dot in panel (A) denotes the mean SST for each day. The blue curve represents the monthly mean SST. Red squares in the top panel show the yearly mean SST. The color gradient in panel (B) represents the SST, with negative anomalies in blue and positive anomalies in red.
Figure 3. The daily sea surface temperature (SST) (°C) from 1 January 2005 to 31 December 2016 (A) and yearly anomaly (B). Each open dot in panel (A) denotes the mean SST for each day. The blue curve represents the monthly mean SST. Red squares in the top panel show the yearly mean SST. The color gradient in panel (B) represents the SST, with negative anomalies in blue and positive anomalies in red.
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Figure 4. The weight distribution of narrow-barred Spanish mackerel caught by the setnet from 2005 to 2016. The color gradient represents the average weight distribution of narrow-barred Spanish mackerel in each year with darker color indicating larger individual.
Figure 4. The weight distribution of narrow-barred Spanish mackerel caught by the setnet from 2005 to 2016. The color gradient represents the average weight distribution of narrow-barred Spanish mackerel in each year with darker color indicating larger individual.
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Figure 5. The prediction of the effect of year (A), DoY (B), moon phase (C), and SST (D) on the CPUE of narrow-barred Spanish mackerel caught by the setnet derived from the GAMs. Solid curves indicate mean values obtained from the models. Dashed curves represent 95% confidence intervals.
Figure 5. The prediction of the effect of year (A), DoY (B), moon phase (C), and SST (D) on the CPUE of narrow-barred Spanish mackerel caught by the setnet derived from the GAMs. Solid curves indicate mean values obtained from the models. Dashed curves represent 95% confidence intervals.
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Table 1. Summary of fishing effort and catch of narrow-barred Spanish mackerel caught by the setnet fishery from 2005 to 2016.
Table 1. Summary of fishing effort and catch of narrow-barred Spanish mackerel caught by the setnet fishery from 2005 to 2016.
Year# of Fishing DaysTotal Catch (kg)Month# of Fishing DaysTotal Catch (kg)
20055323,456.9261304.5
20064826,726.038720,181.5
20072117,689.9417793,758.8
20087126,054.7514562,719.2
20097010,832.3610214,704.7
20105016,032.076412,235.3
201111444,671.986023,170.5
20126925,205.99123769.9
2013362859.2Sum653231,844
20146628,619.3
2015272698.2
2016286998.3
Sum653231,844.5
Table 2. Catch composition in total and percent of the setnet from 2005 to 2016. Species are ordered by scientific name.
Table 2. Catch composition in total and percent of the setnet from 2005 to 2016. Species are ordered by scientific name.
Common NameSpecies NameTotal Catch (ton)Percent (%)
Target species (retained)
Narrow-barred Spanish mackerelScomberomorus commerson231.8460.4
Incidental catch (retained)
Flat needlefishAblennes hians0.890.2
Unicorn leatherjacket filefishAluterus monoceros2.430.6
Frigate tunaAuxis thazard thazard5.611.5
Limpid-wing flyingfishCheilopogon unicolor3.450.9
Redtail scadDecapterus kurroides8.982.3
Shortfin scadDecapterus macrosoma7.21.9
Japanese scadDecapterus maruadsi4.781.2
Black-spotted grouperEpinephelus epistictus2.130.6
KawakawaEuthynnus affinis1.460.4
Skipjack tunaKatsuwonus pelamis39.910.4
SquidLoligo chinensis3.460.9
Red snapperLutjanus sanguineus4.211.1
Torpedo scadMegalaspis cordyla6.271.6
Moontail bullseyePriacanthus hamrur7.822.0
Bigeye scadSelar crumenophthalmus6.731.8
Bigfin reef squidSepioteuthis lessoniana1.840.5
Mottled spinefootSiganus fuscescens5.321.4
Spotted mackerelSomber autralasicus17.24.5
Bigeye barracudaSphyraena forsteri3.71.0
SwordfishXiphias gladius18.624.8
Other species 0.210.1
Sum 384.05100
Table 3. The GAM analysis of the effect of year, DoY, moon phase, and SST on CPUE.
Table 3. The GAM analysis of the effect of year, DoY, moon phase, and SST on CPUE.
GCVAdjusted R2Deviance ExplainedAICPredictorsedfRef.dfFp
1.8160.6253.2%8777.755s(Year)9.851106.805<0.001
s(DoY)7.43897.364<0.001
s(Moon phase)1.30921.5990.012
s(Temperature)2.88334.9850.002
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Vu, N.K.; Nguyen, K.Q. The Effect of Temporal and Environmental Conditions on Catch Rates of the Narrow-Barred Spanish Mackerel Setnet Fishery in Khanh Hoa Province, Vietnam. Fishes 2025, 10, 257. https://doi.org/10.3390/fishes10060257

AMA Style

Vu NK, Nguyen KQ. The Effect of Temporal and Environmental Conditions on Catch Rates of the Narrow-Barred Spanish Mackerel Setnet Fishery in Khanh Hoa Province, Vietnam. Fishes. 2025; 10(6):257. https://doi.org/10.3390/fishes10060257

Chicago/Turabian Style

Vu, Nghiep Ke, and Khanh Quoc Nguyen. 2025. "The Effect of Temporal and Environmental Conditions on Catch Rates of the Narrow-Barred Spanish Mackerel Setnet Fishery in Khanh Hoa Province, Vietnam" Fishes 10, no. 6: 257. https://doi.org/10.3390/fishes10060257

APA Style

Vu, N. K., & Nguyen, K. Q. (2025). The Effect of Temporal and Environmental Conditions on Catch Rates of the Narrow-Barred Spanish Mackerel Setnet Fishery in Khanh Hoa Province, Vietnam. Fishes, 10(6), 257. https://doi.org/10.3390/fishes10060257

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