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Coupling dynamic energy budget and population dynamic models to inform stock enhancement in fisheries management
Abstract
Extensive applications of fishery stock enhancement worldwide bring up broad concerns about its negative effects, creating a pivotal need for science‐based assessment and planning of enhancement strategies. However, the lack of mechanistic understanding of enhanced population dynamics, particularly the density‐dependent processes, leads to compromise in model development and limits the capacity in predicting enhancement effects. Here, we developed an individual‐based model based on dynamic energy budget theory and full life‐history processes, to understand the mechanism of density dependence in population dynamics that emerge from individual‐level processes. We demonstrated the utility of the model framework by applying it to an extensively enhanced species, Chinese prawn ( Fenneropenaeus chinensis , Penaeidae). The model could yield projections reflecting the observed trajectory of population biomass and yields. The model also delineated the key effects of density dependence on the vital rates of growth, fecundity and starvation mortality. Regarding the manifold effects of stock enhancement, we demonstrated a dampened shape in population biomass and yields with increasing magnitude of enhancement, and trade‐offs between the ecological and economic objectives, that is, pursuing high benefit might compromise the wild population without proper management. Furthermore, we illustrated the possibility of combining stock enhancement and harvest regulation in promoting population recovery while maintaining fisheries yields. We highlight the potential of the proposed model for understanding density dependence in enhancement programme, and for designing integrated management strategies. The approach developed herein may serve as a general approach to assess the population dynamics in stock enhancement and inform enhancement management.
... In this context, population models, such as agent-based models (ABMs), can help to unveil how individual variability scales to the population level and discover emergent properties DeAngelis and Grimm, 2014;Guscelli et al., 2019;Hahn et al., 2012). ABMs are increasingly combined with mechanistic bioenergetic models at the individual level (e.g., Arnould-Pétré et al., 2021;Bueno-Pardo et al., 2020;Liao et al., 2021;Liu et al., 2023;Martin et al., 2012;Pethybridge et al., 2013;Rakel et al., 2020). The rationale behind is that the proximate mechanisms that scale from the individuals to the population may involve between-individual differences in energy-driven processes, as metabolic functioning, life history strategies and behavioral performance. ...
... In the case of the 69 fish monitored, the remarkable variability in growth trajectories was well explained by the estimated differences between individuals in two main energy fluxes: energy assimilation (related with the compound parameter ∀) and energy mobilization (related with energy conductance v). These findings indicate that individual variability in DEB parameters is significant, thus highlighting the potential of combining agent based population models with DEB (Arnould-Pétré et al., 2021;Bueno-Pardo et al., 2020;Liao et al., 2021;Liu et al., 2023;Martin et al., 2012;Pethybridge et al., 2013;Rakel et al., 2020). ...
Population dynamics is influenced by between-individual variability. Dynamic Energy Budget (DEB) theory is an appealing framework for assessing such a variability, yet DEB parameters have rarely been estimated at the individual level. Bayesian hierarchical models show promise for inferring individual variability in DEB parameters , thought computational challenges have limited their use due to the need to solve differential equations. Timely, Stan has emerged as a general-purpose statistical tool for fitting dynamic models. This paper introduces an analytical strategy using Bayesian parametric inference and hierarchical modelling to estimate individual-specific DEB parameters. Two biologically relevant DEB parameters were successfully estimated for 69 Gilt-head breams (Sparus aurata) with up to 11 measures of length and wet weight each. The estimated between-individual variability in these two DEB parameters explained well the observed patterns in length and weight at between-and within-individual levels. Moreover, data-simulation experiments highlighted the potential and limitations of our approach, suggesting that improved data collection could enable to increase precision and the number of DEB parameters that can be estimated at the individual level. This strategy can better represent between-individual variability in DEB parameters, which ultimately may improve forecasting of population dynamics after integrating DEB into population models.
Density-dependent growth, which might influence the effects of fisheries on a population, is often ignored when management strategies are evaluated, mainly due to a lack of appropriate models readily available to be implemented. To improve on this, we investigated if somatic growth in Norwegian spring-spawning herring (Clupea harengus) depends on cohort density using a formulation of the von Bertalanffy growth function on cohorts from 1921 to 2014 and found a significant negative correlation between estimated asymptotic length and density. This clearly indicates density-dependent effects on growth, and we propose a model that can be used to predict the size-at-age of Norwegian spring-spawning herring as a function of herring density (the abundance of two successive cohorts) in short-term predictions of catch advice, and in Management strategy evaluations, including estimation of their reference points such as FMSY.
Farmed anguillid eels are frequently stocked into natural fresh waters to enhance eel resources, but little is known about what happens to these eels or their interactions with wild eels after stocking. A recent study observed a depressed survival and growth rate of farmed Japanese eels when they were reared with wild eels, which indicated that wild eels might interfere with the survival and growth of farmed‐and‐stocked eels through intraspecific competition. To contribute to improving eel stocking efficiency, the growth of farmed‐and‐stocked Japanese eels was compared among four rivers with different wild eel densities using mark‐and‐recapture studies. Based on the 2‐year recapture survey after stocking, it was found that the density of the farmed‐and‐stocked eels was not significantly different among rivers. The daily growth rates of farmed‐and‐stocked eels in the rivers with lower wild eel density were significantly higher than those of the eels stocked into the rivers with higher wild eel density. The farmed‐and‐stocked eels moved significantly greater distances downstream than wild eels that showed sedentary behaviour. This and previous studies indicate that significant questions remain about the effectiveness of stocking farmed eels into water bodies where naturally recruited wild eels are present.
The correct prediction of the shape and strength of density dependence in productivity is key to predicting future stock development and providing the best possible long‐term fisheries management advice. Here, we identify unbiased estimators of the relationship between somatic growth, recruitment and density, and apply these to 80 stocks in the Northeast Atlantic. The analyses revealed density‐dependent recruitment in 68% of the stocks. Excluding pelagic stocks exhibiting significant trends in spawning stock biomass, the probability of significant density dependence was even higher at 78%. The relationships demonstrated that at the commonly used biomass limit of 0.2 times maximum spawning stock size, only 32% of the stocks attained three quarters of their maximum recruitment. This leaves 68% of the stocks with less than three quarters of their maximum recruitment at this biomass limit. Significantly lower recruitment at high stock size than at intermediate stock size was seen in 38% of the stocks. Density dependence in late growth occurred in 54% of the stocks, whereas early growth was generally density‐independent. Pelagic stocks were less likely to exhibit density dependence in recruitment than demersal and benthic stocks. We recommend that both the degree to which productivity is related to density and the degree to which the relationship changes over time should be investigated. Both of these aspects should be considered in evaluations of whether sustainability and yield can be improved by including density dependence in forecasts of the effects of different management actions.
Biological reference points (BRPs) derived from per-recruit analyses are commonly used in inferring stock status and serve as the target or threshold in fisheries management. However, the estimation of BRPs may be impacted by the variability in life history processes, and particularly, individual growth rates often display substantial seasonal oscillations but are seldomly considered in per-recruit analyses. Using four commercial fish species Lophius litulon , Saurida elongata , Hexagrammos otakii , and Larimichthys polyactis in coastal China Seas as examples, this study examined the effects of seasonal growth variability on per-recruit analyses and on the estimation of BRPs. We developed an individual-based modeling framework to simulate growth patterns with and without variations at the seasonal and the individual levels and adopted two common assessment methods, age-based analysis and length-frequency analysis, to estimate growth parameters regarding data availability in data-rich or data-poor fisheries, respectively. We found that ignoring seasonality could lead to substantial errors in the estimation of BRPs for the small-size species H. otakii and L. polyactis in our evaluation; when seasonal growth was considered, the estimation could be largely improved. Length-frequency analysis might yield considerably less reliable estimations than age-based method. The time of year when fast growth occurs determines positive or negative bias in estimation, and the amplitude of seasonal growth determines the degree of biases. In general, ignoring the seasonality of growth when there is can lead to underestimated growth parameter K and trigger biases that propagate in stock assessment and management, whereas incorporating seasonality falsely in assessment when there is no seasonal variation will have little influences on the estimation of BRPs. This study contributes to demonstrate the risk of ignoring seasonality in stock assessment and the approaches accounting for seasonal variability in fishery management.
Many salmonid populations are of conservation concern, and the release of hatchery-produced juveniles is a frequently used measure to alleviate declines and increase harvest opportunities. While such releases may be of conservation value for some populations, stocking may also decrease the effective population size and subsequently impose additional strain on already threatened populations. In this study, we assessed how the cohort-wise effective number of breeders in five populations of Atlantic salmon (Salmo salar) were affected by supplementation. Altogether, 19 cohorts were studied (2–7 cohorts per population) by estimating the proportion hatchery-released individuals and the effective number of wild and captive breeders in each cohort of the respective populations. We show that the effect of releasing captive-bred individuals varies both between populations and between years within the same population. A Ryman–Laikre effect—where the effective number of breeders has decreased as a consequence of supplementation—was observed for 11 cohorts. We discuss how supplementation can be adapted to optimize the effective population size, demonstrate that evaluation of supplementation can be reliably achieved, and show that supplementation programmes that lead to high proportions of hatchery-origin fish on spawning grounds are more likely to induce a Ryman–Laikre effect.
Marine ecosystems, particularly in high‐latitude regions such as the Arctic, have been significantly affected by human activities and contributions to climate change. Evaluating how fish populations responded to past changes in their environment is helpful for evaluating their future patterns, but is often hindered by the lack of long‐term biological data available. Using otolith increments of Northeast Arctic (NEA) cod (Gadus morhua ) as a proxy for individual growth, we developed a century‐scale biochronology (1924‐2014) based on the measurements of 3894 fish, which revealed significant variations in cod growth over the last 91 years. We combined mixed‐effect modeling and path analysis to relate these growth variations to selected climate, population and fishing‐related factors. Cod growth was negatively related to cod population size and positively related to capelin population size, one of the most important prey items. This suggests that density‐dependent effects are the main source of growth variability due to competition for resources and cannibalism. Growth was also positively correlated with warming sea temperatures but negatively correlated with the Atlantic Multidecadal Oscillation, suggesting contrasting effects of climate warming at different spatial scales. Fishing pressure had a significant but weak negative direct impact on growth. Additionally, path analysis revealed that the selected growth factors were interrelated. Capelin biomass was positively related to sea temperature and negatively influenced by herring biomass, while cod biomass was mainly driven by fishing mortality. Together, these results give a better understanding of how multiple interacting factors have shaped cod growth throughout a century, both directly and indirectly.
Dynamic energy budget (DEB) theory provides a framework for quantifying metabolic processes and biological rates. DEB models have been widely applied to aquaculture species, but this type of model has great potential for application to fisheries for stock assessment and enhancement. The shrimp Fenneropenaeus chinensis, widely distributed along the coast of China and Korea, is the most important fisheries and aquaculture species in China. With the AmP method, DEB parameters were estimated for the population along the coast of China. The parameter estimation achieved an overall goodness of fit with MRE of 0.131 and SMSE of 0.178. In comparison with similar species, the values of a few main parameters are relatively high including reserve capacity (Em), somatic maintenance (ṗM) and allocation fraction to growth and somatic maintenance (κ). This may reflect an adaptation to variation of environmental conditions. The model can predict the physiological behaviours including respiration, ammonia excretion and feeding rates reasonably well. It shows overall capability to predict the growth and reproduction with acceptable confidence in three main geographic regions. There are clear differences between the female and male with much faster growth rate of the former. Validations of the model have shown that it can adequately predict growth of the shrimp in both its natural distribution waters and land-based culture systems. This study provides important information for further development of modeling tools which can contribute to estimating the carrying capacity for stock enhancement and optimizing production from integrated multi-trophic aquaculture.
Global fishery resources have generally declined, although the rapidity has decreased, and trends have reversed in specific instances following judiciously well-informed intervention. Appropriate management is necessary for the optimum utilisation of fish stocks. Small yellow croaker Larimichthys polyactis is an important fisheries species in the northwest Pacific Ocean, and a major decline in its abundance has been attributed to overfishing and environmental changes. For understanding energetics of small yellow croaker in responses to varying environmental conditions, a dynamic energy budget (DEB) model was applied to this species in the Yellow and East China Seas. The model was parameterised with the observed data including age-at-birth, age-since-birth-at-puberty, life span, fecundity, age-length and length-weight relationship. The model parameterisation has achieved an acceptable goodness of fit, with both low mean relative error and symmetric mean squared error of 0.083. Applications of the model have shown that it can reasonably reproduce the energetics of small yellow croaker in its main biogeographic distribution. Growth simulations in both length and weight were within the range of observations. The simulated fecundity matched the observation reasonably well. We discuss the need for further improvement of the model and additional collection of environmental data.
China is the world’s biggest fishing nation and a major player in the global seafood trade. Its fisheries development can decisively influence the global seafood trade, food security and marine conservation. In recent years, significant changes have taken place in China’s fisheries management priorities, policies and regulations. In this paper, we review the evolving fisheries management practices in China to delineate changes in the management policies, methods and their performances from 1949 to 2019. We determined that the following issues impede the development, implementation and enforcement of fisheries policies and regulations, namely the large size of the fishing fleet, large and poorly organized fisheries population, the “hidden” fishing capacity, uniform management approaches that sometimes fail to account for local conditions, lack of clearly defined and allocated fishing rights, limited data quality and availability, insufficient fisheries monitoring programmes, absence of a robust scientific input framework and insufficient stakeholder involvement. Combining those problems with China’s current management initiatives, we propose recommendations for China’s future fisheries reforms. We hope this paper can inform China’s marine fisheries policies and provide valuable references for further researches related to China’s sustainable fisheries management.
Hatchery release is one of the most popular management tools in fisheries, forestry and wild life management, while its negative impacts on wild populations are a global concern. Research and monitoring of its impacts are generally lacking, and the usefulness of hatchery release for fisheries and conservation objectives is unclear. Here, I evaluated positive and negative impacts of worldwide marine stock enhancement and sea ranching programmes in a systematic review associated with meta-analyses with the goal of reducing bias of the review. Vast numbers of individuals of more than 180 species are released into the wild each year, but most studies are at experimental stages to assess its potential, and empirical studies are sparse for evaluating the impact on fishery production. Most cases are economically unprofitable except for a few successful cases or unevaluated. The effects of releasing juveniles can be dwarfed by the magnitude of natural recruitment when the spawning stock produces much larger recruitment than released juveniles. Density-dependent growth caused by competition of food can be substantial, and growth rates of hatchery and wild fish and other competitive species can simultaneously be reduced when stocking exceeded the carrying capacity. Relative reproductive success can vary depending on the species, seed quality and environmental factors. Empirical studies show evidence of substantial gene flow from hatcheries, but fitness reduction in stocked populations has not been reported. The results represent the current state of worldwide marine stock enhancement and sea ranching activity and provide key information for growing fields of artificial propagation and conservation.
In this paper, maximum sustainable yield (MSY) parameters for the Baltic Sea sprat (Sprattus sprattus) are estimated in relation to pressure from cod (Gadus morhua) predation and the influence of density dependence on sprat growth. This study is based on long-term deterministic and stochastic simulations in which sprat density-dependent growth and predation mortality are considered. The resultant model is a relatively simple tool that allows for streamlined analyses of problems typically approached using complex multispecies models. The analysis indicates that estimates of the MSY parameters (i.e., MSY and FMSY) and equilibrium biomass differ significantly between approaches that hold growth and natural mortality constant and those that allow for density-dependent growth and natural mortality. Based on the cod biomass observed in the 1980s, the MSY parameters estimated by a model that accounts for density-dependent growth and by a model assuming constant growth may differ by a factor of 2. As such, the MSY parameters decline (approximately linearly) with the size of the cod stock.
Hufnagl, M., Huebert, K. B., and Temming, A. 2013. How does seasonal variability in growth, recruitment, and mortality affect the performance of length-based mortality and asymptotic length estimates in aquatic resources? – ICES Journal of Marine Science, 70: 329–341.
We tested the sensitivity of eight methods for estimating total mortality from size frequencies (modified Wetherall; Powell; Beverton and Holt; Jones and van Zalinge; Hoenig; Ssentongo and Larkin; seasonal and non-seasonal Length Converted Catch Curve) to violations of basic assumptions, such as seasonal growth, mortality, recruitment and variable asymptotic length L∞ or growth parameter K. For each method, bias was estimated by simulating length frequency distributions with different combinations of known L∞, Z and K values, calculating θ (Z/K) and L∞ estimates, and comparing the true input with the estimated output values. Input mortality was generally underestimated by all methods and in 27% of all simulations no method provided estimates within θ ± 1. Spring recruitment especially negatively influenced the mortality estimate. A decision tree was developed that provides general guidance in selecting appropriate methods despite violated assumptions, but species-specific case studies are recommended. An example of a species-specific study is provided for the brown shrimp, Crangon crangon. Despite inherent limitations for all methods, the results illustrate that estimates of θ and Z for brown shrimp can be improved substantially by selecting suitable methods and correcting for observed bias.
The sardine Sardinops sagax population in the southern Benguela has undergone substantial fluctuations in size over the past 50 years, collapsing from an apparently large population in the 1950s to low levels in the mid-1960s, remaining low for the next two decades, and recovering from the late 1980s to a population size that is now similar to or larger than that which occurred during the 1950s. Marked changes in condition and reproductive parameters of sardine have also occurred during this period; condition and standardised gonad mass are higher and length-at-maturity is lower at low population size compared with high population size. The correspondence between the temporal patterns in condition, reproductive parameters and population size are strongly suggestive of density-dependence, and indicate a compensatory response arising from reduced intra-specific competition. This is likely to have resulted from greater per capita food intake, improved body condition and hence faster growth, thus enabling fish to achieve maturation at a presumably younger age and smaller size. Biological parameters did not vary in or out of phase with time-series of sea surface temperature in the southern Benguela, weakening the hypothesis of environmentally mediated changes in these parameters and hence providing support for the hypothesis of a direct density-dependent response by sardine.
Rigorous assessment of species and ecosystem biology underpins responsible marine stock enhancement. Estimation of limits to stocking density, based on ecosystem productivity and energetic requirements of stocked species, can be used to gauge the appropriate magnitude of release densities, minimizing waste of resources, and the possibility for adverse stocking effects. A generalized mass-balance model (generalized predatory impact model) for stocking density estimation has been developed. The approach is based around the principles of ECOPATH and accounts for dynamic estimation of stocking-related ecosystem relationships at fine temporal (days) and spatial scales. The main parameter inputs include probability distributions for key biological and life-history traits of stocked species and estimates of primary productivity for the target ecosystem. The energetic requirements of stocked fish are evaluated in terms of growth and mortality as well as ontogenetic transitions in diet, habitat use, morphology, and migration. The theoretical carrying capacity for a stocked species within a given arena is assessed from primary productivity, levels of predation by stocked fish on different trophic groups, and a specified level of acceptable trophic impact. A Monte Carlo analysis of uncertainty is used to provide a probability distribution of stocking densities for a given trophic impact. The model is applied for stocking juveniles of snook (Centropomus undecimalis) in Sarasota, FL, USA, and mulloway (Argyrosomus japonicus) in Georges River, NSW, Australia. The model is useful for estimating an appropriate stocking density when planning pilot-scale fish releases. Such releases should be carefully monitored to validate model assumptions and determine density-dependent and other environmental effects.
Five hatcheries in Prince William Sound, Alaska, release more than 500 million juvenile pink salmon Oncorhynchus gorbuscha each year, constituting one of the largest salmon hatchery programs in the world. Before the program was initiated in 1974, pink salmon catches were very low, averaging 3 million fish per year between 1951 and 1979. Since 1980 the catch has averaged more than 20 million fish per year. However, catches in three other areas in Alaska with substantial fisheries for pink salmon (southeast Alaska, Kodiak Island, and the southern Alaska Peninsula) also increased equivalently during the same period, and the hatchery production did not become the dominant factor in Prince William Sound until the mid-1980s, long after the wild population had expanded. A hatchery program in the Kodiak area provides useful contrast to the Prince William Sound program because it is smaller and more isolated from the major wild-stock-producing areas of Kodiak Island. The evidence suggests that the hatchery program in Prince William Sound replaced rather than augmented wild production. Two likely causes of the replacement were a decline in wild escapement associated with harvesting hatchery stocks and biological impacts of the hatchery fish on wild fish. Published papers disagree on the impact of the 1989 Exxon Valdez oil spill, but none of the estimates would account for more than a 2% reduction in wild-stock abundance, and the decline in wild stocks began well before the oil spill. No evidence in the Kodiak area program suggests any impact on wild stocks. This analysis suggests that agencies considering the use of hatcheries for augmenting salmonids or other marine species should be aware of the high probability that wild stocks may be adversely affected unless the harvesting of the hatchery fish is isolated from the wild stocks and the hatchery and wild fish do not share habitat during their early ocean life.
Individual-based models (IBMs) predict how dynamics at higher levels of biological organization emerge from individual-level processes. This makes them a particularly useful tool for ecotoxicology, where the effects of toxicants are measured at the individual level but protection goals are often aimed at the population level or higher. However, one drawback of IBMs is that they require significant effort and data to design for each species. A solution would be to develop IBMs for chemical risk assessment that are based on generic individual-level models and theory. Here we show how one generic theory, Dynamic Energy Budget (DEB) theory, can be used to extrapolate the effect of toxicants measured at the individual level to effects on population dynamics. DEB is based on first principles in bioenergetics and uses a common model structure to model all species. Parameterization for a certain species is done at the individual level and allows to predict population-level effects of toxicants for a wide range of environmental conditions and toxicant concentrations. We present the general approach, which in principle can be used for all animal species, and give an example using Daphnia magna exposed to 3,4-dichloroaniline. We conclude that our generic approach holds great potential for standardized ecological risk assessment based on ecological models. Currently, available data from standard tests can directly be used for parameterization under certain circumstances, but with limited extra effort standard tests at the individual would deliver data that could considerably improve the applicability and precision of extrapolation to the population level. Specifically, the measurement of a toxicant's effect on growth in addition to reproduction, and presenting data over time as opposed to reporting a single EC50 or dose response curve at one time point.
Ecological Modelling j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e c o l m o d e l a b s t r a c t The 'ODD' (Overview, Design concepts, and Details) protocol was published in 2006 to standardize the published descriptions of individual-based and agent-based models (ABMs). The primary objectives of ODD are to make model descriptions more understandable and complete, thereby making ABMs less subject to criticism for being irreproducible. We have systematically evaluated existing uses of the ODD protocol and identified, as expected, parts of ODD needing improvement and clarification. Accordingly, we revise the definition of ODD to clarify aspects of the original version and thereby facilitate future standardization of ABM descriptions. We discuss frequently raised critiques in ODD but also two emerg-ing, and unanticipated, benefits: ODD improves the rigorous formulation of models and helps make the theoretical foundations of large models more visible. Although the protocol was designed for ABMs, it can help with documenting any large, complex model, alleviating some general objections against such models.
The relative roles of competition and predation in demographic density de-pendence are poorly known. A tractable experimental design to determine such effects and their interactions for demersal (seafloor oriented) fishes and similar sedentary species is cross-factoring multiple densities of new recruits with the presence and absence of predators. This design allows one to distinguish between density-dependent mortality due to com-petition alone, predation alone, or an interaction between the two, especially when sup-plemental field observations are available. To date, 14 species of marine fish have been examined with some variant of this design, and for 12 species predation was demonstrated to be the sole or major cause of density dependence. However, as competition may be slow acting relative to predation, the importance of competition can be underestimated in short-term experiments. On the Great Barrier Reef, we conducted a long-term field experiment in which multiple densities of new recruits of a planktivorous damselfish were cross-factored with the presence or absence of resident piscivorous fish on patch reefs. During the first 10 months, no density-dependent mortality was detected, regardless of whether resident predators were present or absent. By the end of the experiment at 17 months, per capita mortality was strongly density dependent and highly compensatory in both predator treat-ments; all reefs ultimately supported nearly the same adult density regardless of experi-mental treatment. Examination of treatment effect sizes suggested that competition was the main source of density-dependent mortality, with predation being merely a proximate agent of death. We hypothesize that predators were ineffective in this system compared with similar studies elsewhere because prey density was low relative to ample prey refuges provided by highly complex corals. Combined with previous studies, these findings indicate that density-dependent mortality in demersal marine fishes is often caused by interplay of predation and competition, whose roles may be altered by variation in habitat complexity and larval supply. These conclusions are relevant to marine fisheries models, which typically assume that density dependence is due solely to intraspecific competition.
Marine stock enhancement is a set of management approaches involving the release of cultured organisms to enhance or restore fisheries. Such practices, including sea ranching, stock enhancement, and restocking, are widespread, of variable success, and often controversial. A set of principles aimed at promoting responsible development of restocking, stock enhancement, and sea ranching has been proposed by Blankenship and Leber [American Fisheries Society Symposia 15: 167–175 (1995)], and has gained widespread acceptance as the 'Responsible Approach'. Fisheries science and management, in general, and many aspects of fisheries enhancement have developed rapidly since the responsible approach was first formulated. Here we provide an update to the Responsible Approach in light of these developments. The updated approach emphasizes the need for taking a broad and integrated view of the role of enhancements within fisheries management systems; using a stakeholder participatory and scientifically informed, accountable planning process; and assessing the potential contribution of enhancement and alternative or additional measures to fisheries management goals early on in the development or reform process. Progress in fisheries assessment methods applicable to enhancements and in fisheries governance provides the means for practical implementation of the updated approach.
State and federal agencies in the United States annually release millions of hatchery salmon and steelhead into public waters.
Many of the hatchery programs are located in areas where the wild populations are now listed under the U.S. Endangered Species
Act (ESA) (16 U.S.C. §§1531–1544). These hatchery programs pose genetic and ecological risks to wild fish populations. Genetic
risks occur when hatchery and wild fish interbreed and usually occur within a taxonomic species. Ecological risks occur when
the presence of hatchery fish affects how wild fish interact with their environment or with other species and may affect whole
species assemblages. This paper reviews some of the factors that contribute to ecological risks. Important contributing factors
include the relative abundance of hatchery and wild fish in natural production areas, hatchery programs that increase density-dependant
mortality, residual hatchery fish, some physical advantages that hatchery fish can have over wild fish, and life history characteristics
that may make some species especially vulnerable to the effects of ecological risks. Many of these risk factors can be mitigated
by management activities that reduce the level of interactions between hatchery and wild fish. This paper concludes by recommending
twelve mitigation strategies that may be useful when agencies need to bring hatchery programs into compliance with the take
provisions of the ESA.
The Chinese shrimp (Fenneropenaeus chinensis), one of the important fishery resources in North China, has sharply declined. Artificial propagation and release of F. chinensis have been developed in recent decades in order to supplement wild stocks and improve economic profit. For investigation of the effects of stock enhancement in Jinzhou Bay, we traced this program by sampling 574 female shrimp after spawning and recaptured 293 individuals post-release. Among recaptured individuals, 21 were identified as being hatchery-raised via mtDNA control region and six microsatellite markers identification, accounting for 7.2%, indicating that the hatchery-released juvenile shrimps could survive in the natural environment and contribute to F. chinensis population replenishment in the wild. To address genetic impacts from stock enhancement, we compared the genetic variations between spawners used for enhancement release and local recaptured individuals. High genetic diversities were maintained in F. chinensis stocks, but local recaptured individual stock seemed to exhibit lower genetic diversity. Deviation from Hardy-Weinberg equilibrium and positive values in the inbreeding coefficient in all microsatellite loci and stocks indicated that homogenization and inbreeding may have occurred. More appropriate strategies to restore F. chinensis populations should be discussed for future fishery management.
Facing an alarming continuing decline of wild sea cucumber resources, management strategies were developed over the past three decades to sustainably promote development, maintenance, or regeneration of wild sea cucumber fisheries. In New Caledonia (South Pacific), dedicated management efforts via restocking and sea ranching programs were implemented to cope with the overharvesting of the sandfish Holothuria scabra and the recent loss of known populations. In order to investigate genetic implications of a major H. scabra restocking program, we assessed the genetic diversity and structure of wild stocks and hatchery-produced sandfish and compared the genetic outcomes of consecutive spawning and juvenile production events. For this, 1,358 sandfish collected at four sites along the northwestern coasts of New Caledonia, as well as during five different restocking events in the Tiabet Bay, were genotyped using nine polymorphic microsatellite markers. We found that wild H. scabra populations from the northwestern coast of New Caledonia likely belonged to one panmictic population with high level of gene flow observed along the study scale. Further, this panmictic population displayed an effective size of breeders large enough to ensure the feasibility of appropriate breeding programs for restocking. In contrast, hatchery-produced samples did suffer from an important reduction in the effective population size: the effective population sizes were so small that genetic drift was detectable over one generation, with the presence of inbred individuals, as well as more related dyads than in wild populations. All these results suggest that dedicated efforts in hatcheries are further needed to maintain genetic diversity of hatchery-produced individuals in order to unbalance any negative impact during this artificial selection.
A new approach for estimating the fishing mortality benchmark F msy (fishing pressure that corresponds to maximum sustainable yield) is proposed. The approach includes density-dependent factors. The analysis considers 53 data-rich fish stocks in the Northeast Atlantic. The new F msy values are estimated from an ensemble of data sources: (i) applying traditional surplus production models on time-series of historic stock sizes, fishing mortalities, and catches from the current annual assessments; (ii) dynamic pool model (e.g. age-structured models) estimation for stocks where data on density-dependent growth, maturity, and mortality are available; (iii) extracts from multispecies and ecosystem literature for stocks where well-tested estimates are available; (iv) the "Great Experiment" where fishing pressure on the demersal stocks in the Northeast Atlantic slowly increased for half a century; and (v) linking F msy to life history parameters. The new F msy values are substantially higher (average equal to 0.38 year À1) than the current F msy values (average equal to 0.26 year À1) estimated in stock assessments and used by management, similar to the fishing pressure in the 1960s, and about 30% lower than the fishing pressure in 1970-2000.
In the environmental risk assessment of chemicals, ecological modelling can facilitate the quantification of specific protection goals, for instance by the extrapolation of untested situations from toxicity test results. In particular, individual-based models (IBMs) are gaining resonance in the regulatory community as a mean to predict how populations perform under environmental perturbation. In this regard, the hope is that standardised or general designs will increase the applicability and acceptance of model based risk assessments. Strong moves in the direction of generalised IBM designs have been made based on metabolic theories such as the dynamic energy budget (DEB) theory. Here, we parameterise the standard DEB model for the earthworm Eisenia fetida and test the individual-based DEB model by comparing predictions with published data on life-histories measured for individual worms and populations. Overall, the comparison showed good agreement between the simulations and the data, suggesting that the standard DEB model, more precisely DEBs ‘kappa-rule’, characterizing the energy partitioning between somatic and reproductive processes, is applicable to earthworms. The parametrized model is able to predict growth and reproduction for different food quality and availability.
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 a new 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. This article is protected by copyright. All rights reserved.
The artificial propagation and release of Fenneropenaeus chinensis have been conducted over the past three decades in China. To examine the current population genetic diversity and variability of F. chinensis, six broodstock populations and two wild populations from Chinese coastal waters were collected, and nine mi-crosatellite loci were analyzed. High genetic diversity was found for the eight F. chinensis populations. Inbreeding coefficient (F is) estimates were all positive (0.215-0.317), suggesting that inbreeding occurs in this species. A total of 36 (50%) tests deviated from Hardy-Weinberg equilibrium, and all these loci showed evidence of a null allele. Low genetic differentiation among some F. chinensis populations was detected, which is consistent with the microsatellite DNA results of previous studies. A changing geographic pattern was detected for the F. chinensis population, and stock enhancement activity should be focused on F. chinensis because positive inbreeding coefficients were found.
Similar or very contrasted puzzling population dynamics between anchovy and sardine occur worldwide. Underlying factors are not well understood, but insights towards different biological traits are suggested, in particular trophic specialisation, leading to different responses to environmental conditions. Based on most striking differences in biological and life history traits, i.e. size, spawning and feeding, we calibrated a bioenergetics model, based on the Dynamic Energy Budget theory, for Engraulis encrasicolus and Sardina pilchardus in the Bay of Biscay. Starting from the anchovy model, differences in traits were successively integrated to build the sardine model through a novel exploratory approach by scenarios. We used a robust method for parameter estimation, the Evolution Strategies, with a large dataset of length and mass at age, as well as energy density, which is the first time in such a model calibration. Energy density data proved to be particularly well suited to assess the quality of DEB model predictions and parameter set estimates. Insights in respective physiology were drawn from analysis of parameter values and predictions of the model. We showed that anchovy and sardine have distinct strategies with respect to energy acquisition and especially to allocation to spawning. Anchovy are characterised by higher metabolic rates and requirements. This species is more likely to benefit from periods of high food availability to carry out both growth, spawning and reserve storage. Sardine have less demanding food requirements and metabolic costs. Sardine take advantage of larger reserves storage capacity to decouple spawning and prey blooms and to lengthen spawning period, and thus display a more capital breeding spawning behaviour. Overall, our model outputs distinguish between anchovy that tend towards an almost “all or nothing” energetic strategy, and sardine that tend to carry out lower metabolic activities but on a more regular basis. This first modelling demonstration of a bioenergetics difference between these two species, and the explanation it brings in the understanding of their respective reproduction strategies, opens new perspectives in the interpretation of their differential responses at the population scale to environment variability.
The determination of the optimum stocking density of hatchery-reared juveniles in relation to the carrying capacity of the nursery ground is important to the success of any stock enhancement programs. In order to estimate the surplus productivity of nursery grounds available to support hatchery-reared fish, a population growth model for juvenile Japanese flounder Paralichthys olivaceus was developed by extending an ecophysiology-based model on growth of juvenile fishes. Parameters of the model were adjusted using field-collected data from Ohno Bay in northeastern Japan. This model predicted the optimum stocking density, namely the maximum number of hatchery-reared juvenile Japanese flounder released into the bay without decreasing the growth of conspecific wild juveniles owing to food limitation. The optimum stocking density for juveniles of 89 mm total length was estimated to be 2000 individuals per 250,000 m² in Ohno Bay in 1989. Under the model, the most important factors affecting the optimum stocking density of Japanese flounder were the abundance of mysids and wild juveniles at the time of release. The effect of abundance of a competing flatfish species on growth of wild Japanese flounder was less important.
Eight microsatellite markers were used to analyze genetic diversity, level of inbreeding, and effective population size of spawner and recaptured populations of Chinese shrimp (Fenneropenaeus chinensis) during stock enhancement in the Bohai Bay in 2013. A total of 254 and 238 alleles were identified in the spawner and recaptured populations, respectively, and the numbers of alleles (Na) were 8–63 and 6–60, respectively. The numbers of effective alleles (Ne) were 2.52–21.60 and 2.67–20.72, respectively. The polymorphism information content ranged from 0.529 to 0.952. The observed heterozygosity (Ho) values (0.638–0.910 and 0.712–0.927) were lower than the expected heterozygosity (He) values (0.603–0.954 and 0.625–0.952), which indicated that the two populations possessed a rich genetic diversity. In 16 tests (2 populations×8 loci), 13 tests deviated from the Hardy-Weinberg equilibrium. Fis values were positive at seven loci and the inbreeding coefficients (F) of the two populations estimated by trioML were 13.234% and 11.603%, suggesting that there was a relatively high degree of inbreeding. A certain level of inbreeding depression had occurred in the Chinese shrimp population. Fst values ranged from 0 to 0.059, with a mean of 0.028, displaying a low level of genetic differentiation in the two populations. Effective population sizes (3 060.2 and 3 842.8) were higher than the minimum number suggested for retaining the evolutionary potential to adapt to new environmental conditions. For enhancement activity in 2014, the ideal number of captured shrimp spawners should have ranged from 7 686 to 19 214 to maintain genetic diversity and effective population size. Further strategies to adjust the balance of economic cost, fishing effort and ideal number of shrimp spawners to maintain a satisfactory effective population size for ensuring the sustainability of Chinese shrimp are proposed.
We used an integrated bio-economic model to explore the nature of tradeoffs between conservation of fisheries resources and their use for socioeconomic benefit, as realized through the stock enhancement of recreational fisheries. The model explicitly accounted for the dynamics of wild, stocked, and naturally recruited hatchery-type fish population components, angler responses to stocking, and alternative functional relationships that defined conservation and socioeconomic objectives. The model was set up to represent Florida’s red drum (Sciaenops ocellatus) fishery as a case study. Stock enhancement produced strong trade-offs characterized by frontiers indicating that maximizing socioeconomic objectives could only be achieved at great losses to conservation objectives when the latter were based exclusively on abundance of wild-type fish. When naturally recruited hatchery-type fish were considered equivalent to wild fish in conservation value, this tradeoff was alleviated. Frontier shapes were sensitive to alternative assumptions regarding how conservation objectives were formulated, differential harvesting of stocked and wild-type fish, and potential inherent stakeholder satisfaction from the act of stocking. These findings make more explicit the likely opportunity costs associated with recreational stock enhancement and highlight the utility of trade-off frontiers for evaluating management actions.
The Dynamic Energy Budget theory unifies the commonalities between organisms, as prescribed by the implications of energetics, and links different levels of biological organisation (cells, organisms and populations). The theory presents simple mechanistic rules that describe the uptake and use of energy and nutrients and the consequences for physiological organisation throughout an organism's life cycle, including the energetics of ageing and contact with toxic compounds. This new edition includes a new chapter on evolutionary aspects, and discusses methods to quantify entropy for living individuals, isotope dynamics, a mechanism behind reserve dynamics, and toxicity of complex mixtures of compounds. An updated ageing module now also applies to demand systems, new methods for parameter estimation, adaptation of substrate uptake, the use of otiliths for reconstruction of food level trajectories, the differentiated growth of body parts (such as tumours and organs) linked to their function, and many more topics. © Cambridge University Press 1993, 2000 and
An integrated socioecological model was developed to evaluate the potential for stock enhancement
with hatchery fishes to achieve socioeconomic and conservation objectives in recreational fisheries.
As a case study, this model was applied to the red drum Sciaenops ocellatus recreational fishery in
the Tampa Bay estuary, Florida, U.S.A. The results suggest that stocking of juvenile fish larger than
the size at which the strongest density dependence in mortality occurs can help increase angler satisfaction
and total fishing effort (socioeconomic objectives) but are likely to result in decreases to the
abundance of wild fishes (a conservation objective). Stocking of small juveniles that are susceptible to
density-dependent mortality after release does not achieve socioeconomic objectives (or only at excessive
cost) but still leads to a reduction of wild fish abundance. The intensity and type of socioeconomic
gains depended on assumptions of dynamic angler-effort responses and importance of catch-related
satisfaction, with greatest gains possible if aggregate effort is responsive to increases in abundance and
satisfaction that are greatly related to catch rates. These results emphasize the view of stock enhancement,
not as a panacea but rather as a management tool with inherent costs that is best applied to
recreational fisheries under certain conditions.
Fisheries enhancements are a set of management approaches involving the use of aquaculture
technologies to enhance or restore fisheries in natural ecosystems. Enhancements are widely used
in inland and coastal fisheries, but have received limited attention from fisheries scientists. This
paper sets out 10 reasons why fisheries scientists should care about understanding and managing
enhancements. (1) Enhancements happen, driven mostly by resource users and managers rather than
scientists. (2) Enhancements create complex fisheries systems that encompass and integrate everything
fisheries stakeholders can practically manage. (3) Enhancements emerge in fisheries where the scope
for technical and governance control is high, and they synergistically reinforce both. (4) Successful
enhancements expand management options and achievable outcomes. (5) Many enhancements fail or
do ecological harm but persist regardless. (6) Effective science engagement is crucial to developing
beneficial enhancements and preventing harmful ones. (7) Good scientific guidance is available to
aid development or reform of enhancements but is not widely applied. (8) Enhancement research
advances, integrates and unifies the fisheries sciences. (9) Enhancements provide unique opportunities
for learning about natural fish populations and fisheries. (10) Needs, opportunities and incentives for
enhancements are bound to increase.
Ecosystems can "flip" and, as a result of reinforcing feedback mechanisms, "lock" into alternative stable states. We studied this process in a kelp-forest ecosystem in Maine, USA, for nearly four decades and found two stable states: one dominated by green sea urchins and crustose coralline
algae and the other by erect fleshy macroalgae. Herbivory by urchins drives algal deforestation, but declined after fishing for sea urchins began in 1987. As the fishery expanded northeastward, so did phase shifts to macroalgae. By 2000, macroalgae dominated nearly all of coastal Maine. Monitoring
newly settled sea urchins between 1996 and 2002 revealed sites with thousands of settlers per square meter per year, but virtually none survived to become adults. Algal succession to densely branched morphologies may create nursery habitat for settling crabs that prey on settling sea urchins.
Experiments intended to restore herbivory to prefishing levels, through translocation of 51,000 adult sea urchins over two consecutive years at multiple release sites (with controls), resulted in complete urchin mortality both years as a result of predation by large migratory Cancer borealis
Stimpson, 1859 crabs. Population densities of this crab increased fivefold coastwide soon after the macroalgal phase shift. Persistent absence of urchins (even in no-take reserves) probably resulted from predation on newly settled and/or adult urchins. Fisheries-induced declines in herbivory
may therefore have improved recruitment potential for predatory crabs that then became the region's new apex predator. Cascading sequential processes of herbivory, recruitment, and predation create reinforcing feedback, effectively locking this ecosystem into alternative stable states.
Raid, T., Kornilovs, G., Lankov, A., Nisumaa, A-M., Shpilev, H., and Järvik, A. 2010. Recruitment dynamics of the Gulf of Riga herring stock: density-dependent and environmental effects. – ICES Journal of Marine Science, 67: 1914–1920.
The Gulf of Riga and open-sea stocks of the Baltic herring have displayed remarkably consistent inverse recruitment and stock development patterns since the 1970s: the open-sea stocks steadily declined, whereas the Gulf stock increased rapidly in the early 1990s, reaching a peak abundance in the early 2000s and exceeding the level of the 1970s by a factor of 2–3. The increase was accompanied by a decline in the mean weight-at-age and the condition factor. The estimated decline (by 30–40%) in the average annual consumption rate per individual and changes observed in the zooplankton community suggest that density-dependent effects may have increased since the 1970s. The current period of high stock sizes is also characterized by greater recruitment variability. Historical fecundity investigations have established that the average egg production per individual has decreased in all age groups by 20–50%, along with a decrease in mean weight and condition. Yet, the effect on recruitment has been low so far, because lower fecundity has been compensated by the greater abundance and population fecundity has been maintained at the original level. Recruitment appears to be more influenced by environmental conditions than by spawning-stock biomass.
The goal of this study is to identify the mechanisms and measure the strengths of interactions within and among size classes in experimental populations of rainbow trout, Onchorynchus mykiss. The metric that we used to assess the density-dependent effects was based on consumptive allometry and predator-prey theory. We demonstrate that the interactions among size classes were asymmetrical, favoring larger-bodied individuals. Descriptions of diet and spatial resource use, measures of prey availability, and risk to intra-specific interactions allowed assessment of the relative contributions of exploitative and interference competitive interactions among size classes. Growth of the larger classes was strongly density-dependent and driven primarily by exploitative competition. Growth of the smallest size class was controlled by a combination of exploitative competition within and among size classes and interference competition with larger-bodied conspecifics. This combination of interactions among size classes within populations resulted in a body-size-based asymmetry favoring the larger size classes. Survival of all size classes was positively related to both body size and growth rate. We speculate that the net result of these processes within size-structured populations is compensatory, leading to stable population dynamics.
The demand for fish is expected to rise substantially by 2020. Although aquaculture must provide much of the additional fish, it remains to be seen whether restored or enhanced capture fisheries can also help fill the projected gap in supply. The key challenges for capture fisheries involve reducing fishing effort, removing excess fishing capacity and building the institutional arrangements needed to restore spawning biomass to more productive levels, and to reverse degradation of the supporting habitats. Two interventions, based largely on hatchery technology, have the potential to reduce the time needed to rebuild some severely over-exploited fisheries, or improve the productivity of other ‘healthy’ fisheries. These interventions are ‘restocking’, which involves releasing cultured juveniles to restore spawning biomass to levels where the fishery can once again support regular harvests, and ‘stock enhancement’, which involves release of cultured juveniles to overcome recruitment limitation. However, despite the potential of these interventions, few restocking and stock enhancement programmes have met expectations. The main problems have been a pre-occupation with bio-technical research at the expense of objective analysis of the need for the intervention, and failure to integrate the technology within an appropriate management scheme that has the participation and understanding of the users. The papers presented at the Special Symposium on this subject at the Seventh Asian Fisheries Forum provide a series of valuable lessons to guide objective assessment of the potential for restocking and stock enhancement. They also show how to implement these interventions responsibly and effectively where they are deemed to add value to other forms of management. Above all, these studies demonstrate that restocking and stock enhancement programmes are applied in complex human–environment systems, involving dynamic interactions between the resource, the technical intervention and the people who use it.
1. Dynamic Energy Budget (DEB) theory was designed to understand the dynamics of biological systems from cells to populations and ecosystems via a mass balance approach of individuals. However, most work so far has focused on the level of the individual. To encourage further use of DEB theory in a population context, we developed DEB-IBM, a generic individual-based model (IBM) that is based on DEB theory.
2. The generic IBM is implemented as a computer program using NetLogo, a free software platform that is accessible to biologists with little programming background. The IBM uses DEB to represent assimilation, maintenance, growth and reproduction of individuals. The model description follows the overview, design and details (ODD) protocol, a generic format for describing IBMs, and thereby provides a novel and accessible introduction to DEB theory and how it works in a population context.
3. Dynamic Energy Budget-individual-based model can be used to explore properties of both individual life-history traits and population dynamics, which emerge from the set of DEB parameters of a species, and their interaction with environmental variables such as food density. Furthermore, DEB-IBM can be adapted to address specific research questions, for example by including spatial effects. A user manual explains how this can be done.
4. Dynamic Energy Budget-individual-based model is designed to both facilitate use and testing DEB theory in a population context and to advance individual-based modelling by basing the representation of individuals on well-tested physiological principles.
Some salmon hatchery programs intentionally integrate the wild and hatchery population by taking naturally spawned fish as some fraction of the broodstock and allowing hatchery progeny to constitute some fraction of the adults spawning in the wild. This circumvents some ecological concerns about the effects of hatchery fish on the "wild" population while still reaping some of the benefits of increased potential for harvest, but it increases some genetic concerns. Here, we model phenotypic evolution in the integrated population to investigate the effects on natural spawning fitness at the joint selection and demographic equilibrium. We find a potential, but not a certainty, depending on quantitative aspects of the management interacting with biological characteristics of the stock, for substantial erosion of natural spawning fitness, compared with the original wild population, including the possibility of runaway selection driving natural spawning fitness effectively to zero. The vulnerability to such evolutionary deterioration increases with the magnitude of the contribution of hatchery breeding to the total production and increases with harvest. The response of the selection equilibrium to increasing contribution of hatchery progeny to the broodstock can exhibit a catastrophic discontinuity.
A size-structured, bioenergetics model was implemented to examine the effects of short-term envi-ronmental changes on European anchovy, Engraulis encrasicolus, in the North-western Mediterranean Sea. The model approach was based on Dynamic Energy Budget (DEB) theory and details the acquisi-tion and allocation of energy (J d −1) during an organisms' full life-cycle. Model calibration was achieved using biometric data collected from the Gulf of Lions between 2002 and 2011. Bioenergetics simu-lations successfully captured ontogenetic and seasonal growth patterns, including active growth in spring/summer, loss of mass in autumn/winter and the timing and amplitude of multi-batch spawn-ing events. Scenario analysis determined that vital rates (growth and fecundity) were highly sensitive to short-term environmental changes. The DEB model provided a robust foundation for the implementation of an individual-based population model (IBM) in which we used to test the responses of intrinsic and density-independent population growth rates (r) to observed and projected environmental variability. IBM projections estimate that r could be reduced by as much as 15% (relative to that estimated under mean conditions) due to either a 5% (0.8 • C) drop in temperature (due to a reduced spawning duration), a 18% (25 mg zooplankton m −3) depletion in food supply, a 30% increase in egg mortality rates, or with the phytoplankton bloom peaking 5 weeks earlier (in late-February/Winter). The sensitivity of r to short-term (1 year) and long-term (4–10 year) environmental changes were similar, highlighting the importance of first-year spawners. In its current form, the models presented here could be incorporated into spatially-explicit, higher-trophic (predator–prey and end-to-end ecosystem), larval-dispersal and toxicokinetic models or adapted to other short-lived foraging fish (clupeid) species.
Over the past 20 years, major advances have clarified how ecological patterns inform theory, and how in turn theory informs applied ecology. Also, there has been an increased recognition that the problem of scale at which ecological processes should be considered is critical if we are to produce general predictions. Ecological dynamics is always stochastic at small scales, but variability is conditional on the scale of description. The radical changes in the scope and aims of ecology over the past decades reflect in part the need to address pressing societal issues of environmental change. Technological advances in molecular biology, global positioning, sensing instrumentation and computational power should not be overlooked as an explanation for these radical changes. However, I argue that conceptual unification across ecology, genetics, evolution and physiology has fostered even more fertile questions. We are moving away from the view that evolution is played in a fixed ecological theatre: the theatre is being rapidly and relentlessly redesigned by the players themselves. The maintenance of ecosystem functions depends on shifts in species assemblages and on cellular metabolism, not only on flows of energy and matter. These findings have far reaching implications for our understanding of how ecosystem function and biodiversity will withstand (or not) environmental changes in the 21st century.
I reviewed nine marine stocking programs for which biological or economic measures of success were available. Only one, the Japanese chum salmon program, appears to be a clear economic success. Programs for pink salmon in Alaska, chinook and coho salmon in the U.S. and Canada, lobster in the U.K. and France, cod in Norway, and Kemp's ridley sea turtle are clear economic failures. No economic data were available for striped mullet in Hawaii or red drum in Texas. Incomplete and conflicting economic data for flounder in Japan provide no clear evidence. Marking was successfully used in a number of projects to establish that the stocked individuals survived, but it was far more difficult to establish that stocking effected a net increase in population size. Marking should be standard procedure for establishment of survival; control areas should be the method for determination of net increase in abundance. I suggest that stocking programs be made subject to peer review by scientists without a vested interest in the success of marine enhancement. The economics of stocking should be compared with that of alternatives such as habitat protection, fishery regulation, and stricter enforcement. Density-dependent processes in the ocean pose difficult obstacles for marine stocking programs, and none of the projects reviewed showed clear evidence of increasing total abundance. It appears that a coalition of vested interests including politicians, users, and technology advocates has little desire for critical evaluation and that many stocking programs will continue to receive substantial public funds even if shown to be uneconomical.
Most fisheries models are based on the assumption that population regulation occurs exclusively in the prerecruit phase of the life cycle, but increasing evidence indicates that density-dependent body growth in the recruited phase and its interaction with size-dependent reproductive development can play an important role in regulation. I use comparative analyses and population modeling to explore the respective roles in regulation, and the interactions between density-dependent processes in pre- and postrecruit phases of the life cycle. Of 16 study populations, 14 show significant density dependence and therefore regulation in either (9) or both (5) phases. when standardized by habitat area, the density-dependent parameters of both phases are correlated, but the density-dependent growth parameter is a better predictor of average biomass density than the equivalent parameter of the spawner-recruit relationship. Population modeling shows that, in the absence of exploitation (i.e., near carrying capacity), 11 of the 16 populations respond most strongly to relaxation of prerecruit density dependence, whereas 5 respond most strongly to relaxation of density-dependence in postrecruit growth. Growth regulation is less important when population density is reduced below carrying capacity. Fishing erodes compensatory reserve in the recruited phase by truncating the age and size distribution. The spawner-recruit relationship therefore dominates compensation in heavily exploited populations. Growth-mediated regulation in the recruited phase is likely to be important when populations are closer to carrying capacity and therefore particularly relevant to the assessment of harvest reserves, stock rebuilding measures, and fisheries enhancements.
Hatchery releases of the prawn (shrimp), Penaeus chinensis, have been made in China for 20 years, both within and outside the natural distribution of the species. The number of released prawns reached a peak of 5.213 billion in 1991 and currently remains at ∼600 million p.a., providing annual landings of around 720 t. Catches of released prawns have contributed >90% to total landings in one region, the Haiyangdao Fishing Ground in the north Yellow Sea. Although recapture rates of hatchery released juveniles varied greatly between release sites, they reached an average of 9.2% at the Haiyangdao Fishing Ground from 1985 to 1992, and 9% in Xiangshan Bay from 1984 to 1995 and 5.4% in Dongwuyang Bay from 1986 to 1995, two bays in the East China Sea. The ratio of production costs to revenue from the catches of released prawns reached 1:8.5 for the stock enhancement programmes in the Yellow Sea, and averaged 1:5.6 in the translocation of prawns outside their natural range in the East China Sea. Although releases of hatchery-reared prawns in China have succeeded in augmenting production of commercial prawn fisheries, they have not increased the size of the wild prawn populations in the Yellow-Bohai Sea region due to over-fishing of pre-spawning animals. Similarly, they have not succeeded in establishing self-replenishing populations in areas when P. chinensis was translocated outside its natural distribution. Instead, the various prawn stock enhancement initiatives have led to successful sea ranching operations, i.e., releasing juveniles and harvesting them when they reach market size but before they reach sexual maturity. However, rebuilding the wild prawn stock through management regulations promises to be a more cost-effective approach to increasing prawn production than the current sea ranching operations. ‘Put and take’ releases are an inappropriate substitute for more urgently needed fisheries management actions, such as habitat restoration and reduction in fishing effort, and management to allow releases of hatchery-reared prawns to contribute to spawning biomass.
a b s t r a c t Harvest restrictions and stock enhancement are commonly proposed management responses for sus-taining degraded fisheries, but comparisons of their relative effectiveness have seldom been considered prior to making policy choices. We built a population model that incorporated both size-dependent har-vest restrictions and stock enhancement contributions to explore trade-offs between minimum length limits and stock enhancement for improving population sustainability and fishery metrics (e.g., catch). We used a Murray cod Maccullochella peelii peelii population as a test case, and the model incorporated density-dependent recruitment processes for both hatchery and wild fish. We estimated the spawning potential ratio (SPR) and fishery metrics (e.g., angler catch) across a range of minimum length limits and stocking rates. Model estimates showed that increased minimum length limits were much more effective than stock enhancement for increasing SPR and angler catches in exploited populations, but length limits resulted in reduced harvest. Stocking was predicted to significantly increase total recruitment, population sustainability, and fishery metrics only in systems where natural reproduction had been greatly reduced via habitat loss, fishing mortality was high, or both. If angler fishing effort increased with increased fish abundance from stocking efforts, fishing mortality was predicted to increase and reduce the benefits real-ized from stocking. The model also indicated that benefits from stock enhancement would be reduced if reproductive efficiency of hatchery-origin fish was compromised. The simulations indicated that stock enhancement was a less effective method to improve fishery sustainability than measures designed to reduce fishing mortality (e.g., length limits).
Whereas the effects of density‐dependent growth and survival on population dynamics are well‐known, mechanisms that give rise to density dependence in animal populations are not well understood. We tested the hypothesis that the trade‐off between growth and mortality rates is mediated by foraging activity and habitat use. Thus, if depletion of food by prey is density‐dependent, and leads to greater foraging activity and risky habitat use, then visibility and encounter rates with predators must also increase.
We tested this hypothesis by experimentally manipulating the density of young rainbow trout ( Oncorhynchus mykiss ) at risk of cannibalism, in a replicated single‐factor experiment using eight small lakes, during an entire growing season.
We found no evidence for density‐dependent depletion of daphnid food in the nearshore refuge where most age‐0 trout resided. Nonetheless, the proportion of time spent moving by individual age‐0 trout, the proportion of individuals continuously active, and use of deeper habitats was greater in high density populations than in low density populations. Differences in food abundance among lakes had no effect on measures of activity or habitat use.
Mortality of age‐0 trout over the growing season was higher in high density populations, and in lakes with lower daphnid food abundance. Therefore, population‐level mortality of age‐0 trout is linked to greater activity and use of risky habitats by individuals at high densities. We suspect that food resources were depleted at small spatial and temporal scales not detected by our plankton sampling in the high density treatment, because food‐dependent activity and habitat use by age‐0 trout occurs in our lakes when food abundance is experimentally manipulated ( Biro, Post & Parkinson, in press ).
Some models of marine no‐take reserves predict that reserves can enhance fishery yield. However, empirical evidence of this remains inconclusive. One reason for this may be the disregard for density‐dependent body growth in most models. Density‐dependent body growth links the number and size of individuals, and thus could influence the biomass of fishery yield.
We developed an age‐ and size‐structured model of an exploited population and analysed the effect of implementing a no‐take reserve of varying size.
Protecting part of a population from exploitation in a no‐take reserve results in a rapid build‐up of biomass inside the reserve because of increased survival. However, when body growth is density‐dependent it also results in reduced length at a given age within the no‐take reserve because of crowding effects. This prediction is backed up by empirical observations.
If there is export of individuals (here larvae) from the no‐take reserve, length at a given age will also decrease in the fished part of the population outside the reserve. An increase in the number of exploitable individuals thus results in decreased individual body mass. The positive effect of larval drift on fished population size and catch numbers will therefore rarely translate into an increase in equilibrium yield biomass.
Synthesis and applications . When body growth is density‐dependent, implementation of no‐take reserves affects the body size of both protected and exploitable individuals. Although reserves can have several benefits besides increasing yields, our study shows that, if density‐dependent somatic effects are important, a general increase in yield biomass cannot be expected. In populations with density‐dependent body growth, reserves are more likely to decrease yield biomass unless the population is severely overexploited. Analyses of the efficiency of marine reserves as a means of enhancing the yield of fisheries need to account for ecological processes, and density‐dependent body growth is likely to be key.
Conservation biologists often ignore density dependence because at-risk populations are typically small relative to historical levels. However, if populations are reduced as a result of impacts that lower carrying capacity, then density-dependent mortality may exist at low population abundances. Here, we explore this issue in threatened populations of juvenile chinook salmon (Oncorhynchus tshawytscha). We followed the fate of more than 50 000 juvenile chinook in the Snake River Basin, USA to test the hypothesis that their survival was inversely associated with juvenile density. We also tested the hypotheses that non-indigenous brook trout and habitat quality affect the presence or strength of density dependence. Our results indicate that juvenile chinook suffer density-dependent mortality and the strength of density dependence was greater in streams in which brook trout were absent. We were unable to detect an effect of habitat quality on the strength of density dependence. Historical impacts of humans have greatly reduced population sizes of salmon, and the density dependence we report may stem from a shortage of nutrients normally derived from decomposing salmon carcasses. Cohorts of juvenile salmon may experience density-dependent mortality at population sizes far below historical levels and recovery of imperiled populations may be much slower than currently expected.