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Fish stocking and harvest regulations are frequently used to maintain or enhance freshwater recreational fisheries and contribute to fish conservation. However, their relative effectiveness has rarely been systematically evaluated using quantitative models that account for key size- and density-dependent ecological processes and adaptive responses of anglers. We present an integrated model of freshwater recreational fisheries where the population dynamics of two model species affect the effort dynamics of recreational anglers. With this model, we examined how stocking various fish densities and sizes (fry, fingerlings, and adults) performed relative to minimum-length limits using a variety of biological, social, and economic performance measures, while evaluating trade-offs. Four key findings are highlighted. First, stocking often augmented the exploited fish population, but size- and density-dependent bottlenecks limited the number of fry and fingerlings surviving to a catchable size in self-sustaining populations. The greatest enhancement of the catchable fish population occurred when large fish that escaped early bottlenecks were stocked, but this came at the cost of wild-stock replacement, thereby demonstrating a fundamental trade-off between fisheries benefits and conservation. Second, the relative performance of stocking naturally reproducing populations was largely independent of habitat quality and was generally low. Third, stocking was only economically advisable when natural reproduction was impaired or absent, stocking rates were low, and enough anglers benefitted from stocking to offset the associated costs. Fourth, in self-sustaining fish populations, minimum-length limits generally outperformed stocking when judged against a range of biological, social and economic objectives. By contrast, stocking in culture-based fisheries often generated substantial benefits. Collectively, our study demonstrates that size- and density-dependent processes, and broadly the degree of natural recruitment, drive the biological, social, and economic outcomes of popular management actions in recreational fisheries. To evaluate these outcomes and the resulting trade-offs, integrated fisheries-management models that explicitly consider the feedbacks among ecological and social processes are needed.

Forage fish support the largest fisheries in the world but also play key roles in marine food webs by transferring energy from plankton to upper trophic-level predators, such as large fish, seabirds, and marine mammals. Fishing can, thereby, have far reaching consequences on marine food webs unless safeguards are in place to avoid depleting forage fish to dangerously low levels, where dependent predators are most vulnerable. However, disentangling the contributions of fishing vs. natural processes on population dynamics has been difficult because of the sensitivity of these stocks to environmental conditions. Here, we overcome this difficulty by collating population time series for forage fish populations that account for nearly two-thirds of global catch of forage fish to identify the fingerprint of fisheries on their population dynamics. Forage fish population collapses shared a set of common and unique characteristics: high fishing pressure for several years before collapse, a sharp drop in natural population productivity, and a lagged response to reduce fishing pressure. Lagged response to natural productivity declines can sharply amplify the magnitude of naturally occurring population fluctuations. Finally, we show that the magnitude and frequency of collapses are greater than expected from natural productivity characteristics and therefore, likely attributed to fishing. The durations of collapses, however, were not different from those expected based on natural productivity shifts. A risk-based management scheme that reduces fishing when populations become scarce would protect forage fish and their predators from collapse with little effect on long-term average catches. Significance Forage fish provide substantial benefits to both humans and ocean food webs, but these benefits may be in conflict unless there are effective policies governing human activities, such as fishing. Collapses of forage fish induce widespread ecological effects on dependent predators, but attributing collapses to fishing has been difficult because of natural fluctuations of these stocks. We implicate fishing in forage fish stock collapses by showing that high fishing rates are maintained when stock productivity is in rapid decline. As a consequence, the magnitude and frequency but not duration of stock collapses are far greater than expected from natural fluctuations. Risk-based management policies would provide substantial ecological benefits with little effect on fishery catches.

The white snook (Centropomus viridis ) is an emerging aquaculture species with high market acceptance, exhibiting catadromous and protandric hermaphroditic characteristics in adulthood. This study aimed to preliminarily characterize certain hematological and biochemical parameters, as well as blood cell morphology, for identifying possible variations between sexes maintained under aquaculture recirculating system (RAS) conditions. The white snook broodstock was anesthetized with clove oil, and biometric values, as well as sex classification, were measured. Then, blood samples were collected from 14 females (7132 ± 1610 g) and 20 males (2200 ± 0.963 g) via caudal vessel puncture to analyze selected hematological parameters, blood biochemistry, and cellular morphology. Fulton’s condition factor (K) showed no differences between sexes, indicating a healthy fish status. Females showed significantly higher serum cholesterol, glucose, and triglyceride levels than males. Also, hematocrit (HCT) and mean corpuscular volume (MCV) were elevated in females. No sex-related differences were observed in red or white cell counts or in blood cell dimensions. Morphological characterization identified erythrocytes, thrombocytes, and three types of leukocytes: lymphocytes (small and large lymphocytes), neutrophils, and monocytes, with no eosinophils or basophils detected in either sex. These findings provide fundamental reference values for the hematological and biochemical profiles of C. viridis broodstock in captivity and highlight sex-specific differences relevant for reproductive and health monitoring. However, it should be considered that the sample size used to establish reference ranges for the species is small, so it is recommended to implement a monitoring plan for this and other broodstocks of this emerging species.

In the current experiment the effects of brown seaweeds (Sargassum aquifolium, SA) on blood biochemical, antioxidant, innate immune, and mRNA gene expression responses of Tilapia (Oreochromis niloticus) under high stocking rate were examined. The fish were allocated into equal six experimental groups: low stocking density (LSD) (the fish reared at 10 g L−1 and fed an un-supplemented diet), High stocking density (HSD) (the fish reared at 20 g L−1 and fed an un-supplemented diet), HSD + SA50, HSD + SA100, HSD + SA150, and HSD + SA200 (the fish reared at 20 g L−1 and fed Sargassum-supplemented diet at level 50, 100, 150, 200 g kg−1, respectively). After 56 days of the feeding trial, the fish at the HSD treatment revealed remarkedly reduced growth and feed efficiency compared to those at the LSD treatment, but the HSD + SA50 and HSD + SA100 fish groups showed a marked enhancement in performance and efficiency of consumed diets comparable to the LSD treatment. Under overcrowded stress, the fish fed diets enriched with Sargassum significantly diminished ammonia releasing levels within the entire environment. Serum total protein, albumin, liver enzymes, and lipase levels had significantly improved in HSD + SA150 and HSD + SA200 fish groups compared to those in the HSD treatment. Superoxide dismutase, catalase, and glutathione peroxidase exhibited high values at HSD + SA200- and LSD-treated groups than the HSD-treated group. However, HSD suppressed all plasma antioxidant activity. The high stocking rate remarkedly recorded higher plasma malondialdehyde levels, and Sargassum treatments significantly alleviated such increases. The HSD and HSD + SA50 fish groups significantly reduced plasma lysozyme, complement C3, IgA, and IgM levels and revealed higher intestinal aerobic and anaerobic bacterial counts than those at the LSD treatment; however, enriched tilapia diets with a high level of Sargassum diminished these negative effects. The HSD fish group significantly upregulated the expression of tumor necrosis factor-alpha (TNF-a), and downregulated interleukin 10 (IL10), catalase (CAT), and glutathione (GST) genes. Sargassum dietary administration, specifically at 50 g kg−1 level notably alleviated the adverse effects of high stocking rate on the cytokines. In conclusion, enriched tilapia diets with Sargassum treatment at levels ranging from 50 up to 100 g kg−1 alleviated overcrowded stress, oxidative stress, and immunosuppression caused by high stocking rate in Tilapia (O. niloticus).

Wild stocks of Pacific salmonids have experienced sharp declines in abundance over the past century. Consequently, billions of fish are released each year for enhancing abundance and sustaining fisheries. However, the beneficial role of this widely used management practice is highly debated since fitness decrease of hatchery-origin fish in the wild has been documented. Artificial selection in hatcheries has often been invoked as the most likely explanation for reduced fitness, and most studies to date have focused on finding signatures of hatchery-induced selection at the DNA level. We tested an alternative hypothesis, that captive rearing induces epigenetic reprogramming, by comparing genome-wide patterns of methylation and variation at the DNA level in hatchery-reared coho salmon (Oncorhynchus kisutch) with those of their wild counterparts in two geographically distant rivers. We found a highly significant proportion of epigenetic variation explained by the rearing environment that was as high as the one explained by the river of origin. The differentially methylated regions show enrichment for biological functions that may affect the capacity of hatcheryborn smolts to migrate successfully in the ocean. Shared epigenetic variation between hatchery-reared salmon provides evidence for parallel epigenetic modifications induced by hatchery rearing in the absence of genetic differentiation between hatchery and natural-origin fish for each river. This study highlights epigenetic modifications induced by captive rearing as a potential explanatory mechanism for reduced fitness in hatchery-reared salmon.

The effects of dietary Nano selenium (Se) and garlic extract on the performance of grass carp under different stocking densities were examined. Fish (2.10 ± 0.09 g) were stocked into 18 tanks: fish were fed a diet supplemented with 1 mg Nano Se and 1 g garlic kg−1 diet (Diet1) or a diet supplemented with 2 mg Nano Se and 2 g garlic kg−1 diet (Diet2) and reared at low (LD, 24 fish per tank), medium (MD, 48 fish per tank), and high (HD, 96 fish per tank) stocking densities. After 60 days, growth was highest in Diet2‐LD and Diet2‐MD groups and lowest in Diet1‐HD group. Results showed an extreme low or high density induced stress responses. At all densities, serum cortisol and glucose levels were significantly lower and amylase and lipase activities were higher in fish fed diet 2. The protease, catalase, and GPx activities were highest in the Diet2‐MD group. The highest SOD activity was observed in medium‐density groups. The serum malondialdehyde level was lowest in Diet2‐LD and Diet2‐MD groups. Thus, higher dietary levels of Nano Se and garlic are recommended to suppress stress and improve growth, digestive activity, and antioxidant capacity in grass carp. In addition, farming fish at medium density is the most optimal aquaculture practice.

Significance The conventional parametric approach to modeling relies on hypothesized equations to approximate mechanistic processes. Although there are known limitations in using an assumed set of equations, parametric models remain widely used to test for interactions, make predictions, and guide management decisions. Here, we show that these objectives are better addressed using an alternative equation-free approach, empirical dynamic modeling (EDM). Applied to Fraser River sockeye salmon, EDM models ( i ) recover the mechanistic relationship between the environment and population biology that fisheries models dismiss as insignificant, ( ii ) produce significantly better forecasts compared with contemporary fisheries models, and ( iii ) explicitly link control parameters (spawning abundance) and ecosystem objectives (future recruitment), producing models that are suitable for current management frameworks. It is well known that current equilibrium-based models fall short as predictive descriptions of natural ecosystems, and particularly of fisheries systems that exhibit nonlinear dynamics. For example, model parameters assumed to be fixed constants may actually vary in time, models may fit well to existing data but lack out-of-sample predictive skill, and key driving variables may be misidentified due to transient (mirage) correlations that are common in nonlinear systems. With these frailties, it is somewhat surprising that static equilibrium models continue to be widely used. Here, we examine empirical dynamic modeling (EDM) as an alternative to imposed model equations and that accommodates both nonequilibrium dynamics and nonlinearity. Using time series from nine stocks of sockeye salmon ( Oncorhynchus nerka ) from the Fraser River system in British Columbia, Canada, we perform, for the the first time to our knowledge, real-data comparison of contemporary fisheries models with equivalent EDM formulations that explicitly use spawning stock and environmental variables to forecast recruitment. We find that EDM models produce more accurate and precise forecasts, and unlike extensions of the classic Ricker spawner–recruit equation, they show significant improvements when environmental factors are included. Our analysis demonstrates the strategic utility of EDM for incorporating environmental influences into fisheries forecasts and, more generally, for providing insight into how environmental factors can operate in forecast models, thus paving the way for equation-free mechanistic forecasting to be applied in management contexts.

Interbreeding between hatchery-reared and wild fish, through deliberate stocking or escapes from fish farms, can result in rapid phenotypic and gene expression changes in hybrids, but the underlying mechanisms are unknown. We assessed if one generation of captive breeding was sufficient to generate inter- and/or transgenerational epigenetic modifications in Atlantic salmon. We found that the sperm of wild and captive-reared males differed in methylated regions consistent with early epigenetic signatures of domestication. Some of the epigenetic marks that differed between hatchery and wild males affected genes related to transcription, neural development, olfaction, and aggression, and were maintained in the offspring beyond developmental reprogramming. Our findings suggest that rearing in captivity may trigger epigenetic modifications in the sperm of hatchery fish that could explain the rapid phenotypic and genetic changes observed among hybrid fish. Epigenetic introgression via fish sperm represents a previously unappreciated mechanism that could compromise locally adapted fish populations.

Increasing aquaculture production requires high-density farming, which induces stress, necessitating supplements to mitigate its effects and ensure fish health. The aim of this study was to examine how CaNa.sub.2 EDTA (EDTA) affects the growth, immune response and antioxidant activity in Nile tilapia (Oreochromis niloticus). The fish were raised at three different stocking densities: low (LD = 2.00 kg/m.sup.3 ), medium (MD = 3.50 kg/m.sup.3 ), and high (HD = 5.00 kg/m.sup.3). Each density group was fed with one of four levels of EDTA supplementation (E0 = 0 g/kg, E1 = 5 g/kg, E2 = 10 g/kg, and E3 = 15 g/kg) for 60 days. Each diet was tested in triplicate (n = 66 fish per replicate in LD, 116 per replicate in MD, and 166 per replicate in HD). After 60 days, the results of this study declared that LD group showed better growth than the MD and HD groups, and among all groups, those fed the E1 diet grew better than those on other diets. The study found significant changes in the chemical composition of the fish and the activity of digestive enzymes across all treatments. Antioxidant enzyme levels and cortisol were higher in the HD group compared to the LD and MD groups. However, fish in the HD group fed the E1 diet had the lowest levels of antioxidant enzymes and cortisol. Malondialdehyde levels were higher in the HD group compared to the LD and MD groups, with the lowest levels seen in fish on the E1 diet in the HD group. The expression of Somatostatin-1 did not increase in the MD group compared to the LD and HD groups. The gene expression levels of pro-opiomelanocortin-[alpha] and Interleukin 1-[beta] were not significantly affected by either stocking density or EDTA supplementation. In conclusion, EDTA supplementation improved growth and antioxidant response in tilapia, with the best results seen at a dose of 5 g/kg in the high-density group, suggesting that this approach could be beneficial in intensive tilapia farming.

This study evaluated the relationships between plankton and floc abundance and the growth performance of female climbing perch ( Anabas testudineus ) reared in a biofloc system. A completely randomized design was used with three stocking densities (100, 125, and 150 fish per 250 L), each replicated four times. Specific growth rate, absolute weight and length gain, survival, plankton abundance, and floc density were assessed using simple linear regression. Plankton abundance differed across treatments. Although plankton abundance varied among treatments, all correlations with growth and survival parameters were negligible (r < 0.04), and floc density showed similarly minimal association with biological responses. The intermediate stocking density of 125 fish per 250 L resulted in the most favourable performance, suggesting that behavioural interaction and density-related factors exerted greater influence on growth outcomes than plankton or floc availability. These findings confirm the suitability of biofloc technology for climbing perch culture and underscore the importance of stocking-density optimization in future applications.