Density dependence in total egg production per spawner for marine fish

A paradigm of fisheries science holds that spawning stock biomass (SSB) is directly proportional to total egg production (TEP) of fish stocks. This “SSB–TEP proportionality” paradigm has been a basic premise underlying the spawner–recruitment models for fisheries management and numerous studies on recruitment mechanisms of fish. Studies on maternal effects on reproductive potential of a stock have progressed during the last few decades, leading to doubt concerning the paradigm. Nonetheless, a direct test of the paradigm at multidecadal scales has been difficult because of data limitations in the stock assessment systems worldwide. Here, we tested the paradigm for marine fish based on a novel combination of two independent 38‐year time series: fishery‐dependent stock assessment data and fishery‐independent egg survey data. Through this approach, we show that the SSB–TEP proportionality is distorted by density dependence in total egg production per spawner individual (TEPPS) or spawner unit weight (TEPPSW) at a multidecadal scale. The TEPPS/TEPPSW exponentially declined with biomass and thus was density‐dependent for Japanese sardine, a small pelagic species exhibiting a high level of population fluctuation, in the western North Pacific. By contrast, the TEPPS/TEPPSW was sardine‐density‐dependent for Japanese anchovy, another small pelagic species exhibiting a moderate level of population fluctuation well‐known for being out of phase with sardine. Our analysis revealed intraspecific (sardine) and interspecific (anchovy) density dependence in TEPPS/TEPPSW, which was previously unaccounted for in spawner–recruitment relationships. Such density‐dependent effects at the time of spawning should be considered in fisheries management and studies on recruitment mechanisms.     

Compensatory density dependence in fish populations: importance, controversy, understanding and prognosis

Density‐dependent processes such as growth, survival, reproduction and movement are compensatory if their rates change in response to variation in population density (or numbers) such that they result in a slowed population growth rate at high densities and promote a numerical increase of the population at low densities. Compensatory density dependence is important to fisheries management because it operates to offset the losses of individuals. While the concept of compensation is straightforward, it remains one of the most controversial issues in population dynamics. The difficulties arise when going from general concepts to specific populations. Compensation is usually quantified using some combination of spawner–recruit analysis, long‐term field monitoring or manipulative studies, and computer modelling. Problems arise because there are limitations to each of these approaches, and these limitations generally originate from the high uncertainty associated with field measurements. We offer a hierarchical approach to predicting and understanding compensation that ranges from the very general, using basic life‐history theory, to the highly site‐specific, using detailed population models. We analyse a spawner–recruit database to test the predictions about compensation and compensatory reserve that derive from a three‐endpoint life‐history framework designed for fish. We then summarise field examples of density dependence in specific processes. Selected long‐term field monitoring studies, manipulative studies and computer modelling examples are then highlighted that illustrate how density‐dependent processes led to compensatory responses at the population level. Some theoretical and empirical advances that offer hope for progress in the future on the compensation issue are discussed. We advocate an approach to compensation that involves process‐level understanding of the underlying mechanisms, life‐history theory, careful analysis of field data, and matrix and individual‐based modelling. There will always be debate if the quantification of compensation does not include some degree of understanding of the underlying mechanisms.     

Evidence of density-dependent effects on population variation of Japanese sardine (Sardinops melanosticta) off Korea

Interannual variation of some biological parameters for the Japanese sardine, Sardinops melanosticta, population in Korean waters was investigated using scientific surveys and fisheries information since the late 1970s. The abundance and geographical coverage of sardine eggs were high (peak in 1986) and broad when spawning biomasses were high in the mid 1980s, and vice versa in the late 1970s and the early 1990s. Also, feeding and spawning areas based on fisheries information exhibited the same pattern of expansion/contraction as seen in ichthyoplankton surveys. Annual Gonadal Somatic Index (GSI) in spawning season (February to April) and the size at age 1 of sardine were reduced during the high abundance period. It is suggested that density‐dependent effects on the reproduction and growth of the sardine population in Korean waters existed.     

Interactive effects of life history and season on size‐dependent growth in juvenile Atlantic salmon

Size‐dependent growth (SDG) is an important process in structuring populations as well as determining life history outcomes. Despite its importance, there have been few investigations from observational studies focusing on the interaction between life history decisions and SDG. In this study, we used data on individually tagged Atlantic salmon from both the laboratory and the field to investigate differences in SDG among two life history groups, parr and smolts. In the laboratory, we found little evidence of SDG in parr but seasonally dependent SDG in the smolt group. Smolts showed at strong compensatory response over the winter months just prior to the smolt transformation window. In the field, we found little evidence of SDG early in ontogeny (i.e., age 0+ fall and winter). There was some evidence of depensatory growth (positive SDG) during the age 1+ spring among both life history groups that may reflect random habitat variation or the monopolisation of resources. After the age 1+ spring, we found that smolts were more likely to show a compensatory effect (negative SDG) than parr. This effect was strongest, as they approached the smolt window in the spring of their age 2+ year. These results suggest (i) SDG is common in Atlantic salmon; however, the form and extent of life history depends on (ii) season and (iii) life history. For individuals that adopt a smolt life history, trade‐offs between freshwater survival and sea survival may lead to a convergent growth pattern, as they approach the smolt migration window.     

Density‐ and size‐dependent mortality in fish early life stages

The importance of survival and growth variations early in life for population dynamics depends on the degrees of compensatory density dependence and size dependence in survival at later life stages. Quantifying density‐ and size‐dependent mortality at different juvenile stages is therefore important to understand and potentially predict the recruitment to the population. We applied a statistical state‐space modelling approach to analyse time series of abundance and mean body size of larval and juvenile fish. The focus was to identify the importance of abundance and body size for growth and survival through successive larval and juvenile age intervals, and to quantify how the dynamics propagate through the early life to influence recruitment. We thus identified both relevant ages and mechanisms (i.e. density dependence and size dependence in survival and growth) linking recruitment variability to early life dynamics. The analysis was conducted on six economically and ecologically important fish populations from cold temperate and sub‐arctic marine ecosystems. Our results underscore the importance of size for survival early in life. The comparative analysis suggests that size‐dependent mortality and density‐dependent growth frequently occur at a transition from pelagic to demersal habitats, which may be linked to competition for suitable habitat. The generality of this hypothesis warrants testing in future research.     

Disentangling density‐dependent effects on egg production and survival from egg to recruitment in fish

Understanding of density‐dependent effects is key to achieving sustainable management of self‐regulating biological resources such as fish stocks. Traditionally, density‐dependent effects on population abundance in fish have been considered to occur from hatching to recruitment, based on the paradigm of proportionality between spawning stock biomass and total egg production. Here, we demonstrate how the existence of intraspecific and interspecific density dependence in egg production changes the current understanding of density‐dependent processes in the life history of fish, by disentangling density‐dependent effects on egg production and survival from egg to recruitment, using sardine (Sardinops melanostictus, Clupeidae) and anchovy (Engraulis japonicus, Engraulidae) as model species. For sardine, strong intraspecific density‐dependent effects occurred in egg production, but no density‐dependent effects occurred or if any they were weak enough to be masked by environmental factors from hatching to recruitment. In contrast, for anchovy, interspecific density‐dependent effects occurred in egg production. In the survival after hatching, anchovy experienced stronger intraspecific density‐dependent effects than currently recognized. This analysis could overturn the current understanding of density‐dependent effects in the life history, highlighting contrasts between the effects on individual quality and population abundance and between the model species. We propose to reconsider the basis of fisheries management and recruitment studies based on the revised understanding of density‐dependent effects in the life history of the respective species.     

Economic,ecological and genetic impacts of marine stock enhancement and sea ranching: A systematic review

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.     

The density dilemma: limitations on juvenile production in threatened salmon populations

Density‐dependent processes have repeatedly been shown to have a central role in salmonid population dynamics, but are often assumed to be negligible for populations at low abundances relative to historical records. Density dependence has been observed in overall spring/summer Snake River Chinook salmon Oncorhynchus tshawytscha production, but it is not clear how patterns observed at the aggregate level relate to individual populations within the basin. We used a Bayesian hierarchical modelling approach to explore the degree of density dependence in juvenile production for nine Idaho populations. Our results indicate that density dependence is ubiquitous, although its strength varies between populations. We also investigated the processes driving the population‐level pattern and found density‐dependent growth and mortality present for both common life‐history strategies, but no evidence of density‐dependent movement. Overwinter mortality, spatial clustering of redds and limited resource availability were identified as potentially important limiting factors contributing to density dependence. The ubiquity of density dependence for these threatened populations is alarming as stability at present low abundance levels suggests recovery may be difficult without major changes. We conclude that density dependence at the population level is common and must be considered in demographic analysis and management.     

Addition of structural complexity – contrasting effect on juvenile brown trout in a natural stream

In a field experiment, we examined the effects of structural complexity in the form of added artificial plastic plants and shredded plastic bags on growth and abundance of juvenile brown trout (Salmo trutta). Just after emergence, the added complexity had a positive effect on the density, biomass and condition factor of young‐of‐the‐year (0+) brown trout. This difference in density was not present six weeks later. In contrast, both young‐of‐the‐year and older brown trout generally tended to be larger in the simple habitat. Hence, our data suggest that increased complexity initially is beneficial for young‐of‐the‐year individuals probably due to lower risk of predation and increased densities of prey. However, as density increases in the complex environment, it may induce negative density‐dependent effects, here reflected in smaller sized fish in the complex environment. This might force fish to redistribute to habitats with lower densities of conspecifics as they grow larger. We propose that habitat complexity can increase survival of yearlings in early phases and thereby also affect the overall population structure of brown trout in natural streams.     

Density‐dependent territory size and individual growth rate in juvenile Atlantic salmon (Salmo salar)

Whether territoriality regulates population size depends on the flexibility of territory size, but few studies have quantified territory size over a broad range of densities. While juvenile salmonids in streams exhibit density‐dependent mortality and emigration, consistent with space limitation, there has been relatively little study of how territory size and individual growth rate change over a broad range of densities, particularly in field experiments. Consequently, we manipulated the density (range = 0.25–8 m^?2) of young‐of‐the‐year (YOY) Atlantic salmon (Salmo salar) in mesh enclosures erected in a natural stream to test whether (i) territory size is fixed, decreases continuously or decreases towards an asymptotic minimum size as density increases; and (ii) individual growth rate decreases as a negative power curve with density as in observational field studies. Territory size decreased with increasing density, consistent with an asymptotic minimum size of about 0.13 m² for a 5‐cm fish. Individual growth rate also decreased with density, although the magnitude of decrease was steeper than in observational studies. Our results suggest a limit to how small territories can be compressed, which will set the upper limit to the local density in a habitat. The density‐dependent changes in territory size and individual growth rate will both play a role in the regulation of stream salmonid populations.     

Reserve site selection for data‐poor invertebrate fisheries using patch scale and dispersal dynamics: a case study of sea cucumber (Cucumaria frondosa)

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The food limitation hypothesis for juvenile marine fish

Coastal zones are productive areas that serve as nursery grounds for a large number of marine species. However, the processes involved in survival success during the juvenile phase are not well‐known. Some authors suggest that the availability of prey is important to support the production of pre‐recruit fish whose fitness is enhanced through optimal feeding conditions. Accordingly, recruitment is limited by the carrying capacity of the nursery habitat. In contrast, other authors state that the carrying capacity of the nursery grounds is not fully exploited, suggesting that there is no effect of food limitation. This study combines an overview of the literature, focused on flatfish that are especially dependent on coastal and estuarine nursery grounds, an extension to other marine fishes and a modelling approach on growth and survival of juvenile fish to explore the controversy of food limitation in their nursery grounds. We demonstrate that the relative lack of growth limitation observed for young marine fishes at the individual scale is related to an observational bias: fish have been affected by size‐selective mortality linked to food limitation, but only surviving individuals are observed. As the population is skewed towards the faster‐growing juveniles, the growth of survivors remains close to optimal, even when food resources are limited. Food limitation is of major influence in determining the carrying capacity of the nursery habitat. To sustain marine fish populations and related fisheries, management action is needed to protect coastal and estuarine areas and maintain or restore nursery productivity.     

When in life does density dependence occur in fish populations?

Fisheries advice is based on demographic calculations, which assume that density‐dependent processes regulating recruitment occur only in early life. This assumption is challenged by laboratory and lake studies and some recent indications from marine systems that demonstrate density‐dependent regulation late in life. By accounting for spatial dynamics of a population, something that has previously been ignored in models of fish, we show that density‐dependent regulation is determined by the size of the habitat: in small habitats, for example small lakes, regulation occurs late in life, while it can occur early in large habitats. When regulation happens late in life, fisheries yield is maximized by exploitation of mainly juvenile fish, while exploiting mature fish maximizes yield if regulation happens early. We review and interpret observations of density dependence in the light of the theory. Our results challenge the current assumption that density dependence always occurs early in life and highlights the need for an increased understanding of density‐dependent processes. This can only come about by a change of focus from determining stock‐recruitment relationships towards understanding when and how density‐dependent regulation occurs in nature.     

Opportunities to improve fisheries management through innovative technology and advanced data systems

Fishery‐dependent data are integral to sustainable fisheries management. A paucity of fishery data leads to uncertainty about stock status, which may compromise and threaten the economic and food security of the users dependent upon that stock and increase the chances of overfishing. Recent developments in the technology available to collect, manage and analyse fishery‐relevant data provide a suite of possible solutions to update and modernize fisheries data systems and greatly expand data collection and analysis. Yet, despite the proliferation of relevant consumer technology, integration of technologically advanced data systems into fisheries management remains the exception rather than the rule. In this study, we describe the current status, challenges and future directions of high‐tech data systems in fisheries management in order to understand what has limited their adoption. By reviewing the application of fishery‐dependent data technology in multiple fisheries sectors globally, we show that innovation is stagnating as a result of lack of trust and cooperation between fishers and managers. We propose a solution based on a transdisciplinary approach to fishery management that emphasizes the need for collaborative problem‐solving among stakeholders. In our proposed system, data feedbacks are a key component to effective fishery data systems, ensuring that fishers and managers collect, have access to and benefit from fisheries data as they work towards a mutually agreed‐upon goal. A new approach to fisheries data systems will promote innovation to increase data coverage, accuracy and resolution, while reducing costs and allowing adaptive, responsive, near real‐time management decision‐making to improve fisheries outcomes.     

Improvements in larviculture of Crangon crangon as a step towards its commercial aquaculture

The European brown shrimp, Crangon crangon, is a highly valued commercial species in Europe. These shrimp fisheries are characterized by strong seasonal variations in average landings. Attempts to ‘catch and hold’ wild adults in land‐based rearing systems have been proven to be very difficult, due to inadequate feed, slow growth and high mortality. In this study, we have optimized design and operation procedures of a small‐scale static larval rearing system for the culture of C. crangon larvae. Focus was on optimizing larval survival via water temperature, feed selection, feeding regime and density. This is the first report that shows that C. crangon larvae can be reared at high densities of 300 larvae L^‐1 with high survival of 73.5 (± 5.4)% under laboratory conditions. In these systems, larvae can be fed exclusively Artemia nauplii according to a feeding regime which is adjusted based on major moulting events. Addition of microalgae may further increase survival by 10%. The information gathered during this research can be applied to further optimize larval development in either flow‐through or recirculation systems.     

Responses of scallop biodeposits to bioturbation by a deposit‐feeder Apostichopus japonicus (Echinodermata: Holothuroidea): does the holothurian density matter?

The deposit‐feeding holothurians can reduce the negative impact of bivalve farming by feeding and reworking bivalve wastes (biodeposits) in the context of co‐culture. To test effects of the bioturbation by holothurians of different densities on bivalve wastes, a stocking density regime of the sea cucumber Apostichopus japonicus (35.4 ± 1.2 g, mean ± SE) was set at 0.0, 6.6, 13.2 ind m^?2, and responses of the biotic parameters including chlorophyll a concentration (Chl. a), bacterial biomass and the abiotic ones as oxidation‐reduction potential (ORP), organic matter (OM), organic carbon (OC), total nitrogen (TN) and organic phosphorus (OP) in biodeposits discharged by Japanese scallop Patinopecten yessoensis were investigated. Results showed that A. japonicus grew in a density‐dependent manner, and the density‐dependent effect on both biotic and abiotic parameters in biodeposits were also observed. Apostichopus japonicus stimulated a transfer process from reduction to weak reduction state of the biodeposits with a trend of higher density holothurians stimulating the process more. Furthermore, A. japonicus significantly controlled the bacterial abundance and Chl. a, as well as reducing the contents of OM, OC, TN and OP in the biodeposits. Yet, the response of abiotic parameters delayed rather than that of biotic parameters, underling the biotic parameters could be more sensitive to bioturbation than the abiotic ones. Our study suggests that the bioturbation of A. japonicus plays an important role in retarding organic waste accumulation and cleansing nutrients in bivalve farming wastes under co‐culture condition and the bioremediation capacity may be closely dependent on its stocking density in practice.     

Foraging behaviour of juvenile pink salmon (Oncorhynchus gorbuscha) and size‐dependent predation risk

Two hypotheses related to effects of juvenile pink salmon (Oncorhynchus gorbuscha) foraging behaviour and size on their predation risk were evaluated using field data collected in Prince William Sound, Alaska 1 995–97. My results supported the hypothesis that low macrozooplankton density leads to dispersion of juvenile salmon from shallow nearshore habitats and greater predation risk, but zooplankton type was an important factor. When the biomass of large copepods (primarily Neocalanus spp.) declined, salmon dispersed from shallow nearshore habitats, and mean daily individual predator consumption of salmon increased by a factor of 5. A concomitant five‐fold increase in the probability of occurrence of salmon in predator stomachs supported the notion that increased predation on salmon was caused by a greater overlap between predator and prey when salmon dispersed offshore, not an increase in the number of salmon consumed per feeding bout. The results also generally supported the hypothesis that the timing of predation events modifies the nature of size‐dependent predation losses of salmon to different predator groups (small and large planktivores and piscivores). Size‐dependent vulnerabilities of salmon to predators were a function of both predator and prey sizes. When simulated predation was shifted from May to June, the vulnerability of salmon became more dependent on their growth than initial size. But, the size‐ and growth‐dependent vulnerabilities of salmon differed more among predator groups than between May and June, suggesting that changes in the composition of predator fields could more strongly affect the nature of size‐dependent predation than changes in the timing of predation losses.     

Fish life history,angler behaviour and optimal management of recreational fisheries

To predict recreational‐fishing impacts on freshwater fish species, it is important to understand the interplay between fish populations, anglers and management actions. We use an integrated bioeconomic model to study the importance of fish life‐history type (LHT) for determining (i) vulnerability to over‐exploitation by diverse angler types (generic, consumptive and trophy anglers), who respond dynamically to fishing‐quality changes; (ii) regulations [i.e., minimum‐size limits (MSLs) and licence densities] that maximize the social welfare of angler populations; and (iii) biological and social conditions resulting under such socially optimal regulations. We examine five prototypical freshwater species: European perch (Perca fluviatilis), brown trout (Salmo trutta), pikeperch (Sander lucioperca), pike (Esox lucius) and bull trout (Salvelinus confluentus). We find that LHT is important for determining the vulnerability of fish populations to overfishing, with pike, pikeperch, and bull trout being more vulnerable than perch and brown trout. Angler type influences the magnitude of fishing impacts, because of differences in fishing practices and angler‐type‐specific effects of LHT on angling effort. Our results indicate that angler types are systematically attracted to particular LHTs. Socially optimal minimum‐size limits generally increase with LHT vulnerability, whereas optimal licence densities are similar across LHTs. Yet, both regulations vary among angler types. Despite this variation, we find that biological sustainability occurs under socially optimal regulations, with one exception. Our results highlight the importance of jointly considering fish diversity, angler diversity and regulations when predicting sustainable management strategies for recreational fisheries. Failure to do so could result in socially suboptimal management and/or fishery collapse.     

Coupling dynamic energy budget and population dynamic models to inform stock enhancement in fisheries management

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.