Reproductive resilience: a paradigm shift in understanding spawner‐recruit systems in exploited marine fish

A close relationship between adult abundance and stock productivity may not exist for many marine fish stocks, resulting in concern that the management goal of maximum sustainable yield is either inefficient or risky. Although reproductive success is tightly coupled with adult abundance and fecundity in many terrestrial animals, in exploited marine fish where and when fish spawn and consequent dispersal dynamics may have a greater impact. Here, we propose an eco‐evolutionary perspective, reproductive resilience, to understand connectivity and productivity in marine fish. Reproductive resilience is the capacity of a population to maintain the reproductive success needed to result in long‐term population stability despite disturbances. A stock's reproductive resilience is driven by the underlying traits in its spawner‐recruit system, selected for over evolutionary timescales, and the ecological context within which it is operating. Spawner‐recruit systems are species specific, have both density‐dependent and fitness feedback loops and are made up of fixed, behavioural and ecologically variable traits. They operate over multiple temporal, spatial and biological scales, with trait diversity affecting reproductive resilience at both the population and individual (i.e. portfolio) scales. Models of spawner‐recruit systems fall within three categories: (i) two‐dimensional models (i.e. spawner and recruit); (ii) process‐based biophysical dispersal models which integrate physical and environmental processes into understanding recruitment; and (iii) complex spatially explicit integrated life cycle models. We review these models and their underlying assumptions about reproductive success vs. our emerging mechanistic understanding. We conclude with practical guidelines for integrating reproductive resilience into assessments of population connectivity and stock productivity.     

How to provide scientific advice for ecosystem‐based management now

In this paper, I argue that we have at hand what is needed to provide scientific advice for ecosystem‐based management of small pelagics and other species groups now. The ingredients for this advice are (i) large marine ecosystems as spatial management units; (ii) maintaining ecosystem productivity and exploiting at multispecies maximum yield as overarching management objectives; (iii) assessment of ecosystems by evaluating changes in primary productivity; (iv) an operational management procedure in which single‐species catch proposals are adjusted to ecosystem productivity using a set of control rules. Inspection of historic landings for small pelagics and other small species in the Northeast Atlantic (ICES area) reveals that most likely fisheries exploitation does not, and never did, exceed system productivity in most LMEs and is therefore overall sustainable, although not necessarily for individual stocks.     

Towards ecosystem-based fisheries management

Performance measures and reference points for the management of target species are now widely used in the fisheries of the developed world. To move us closer to an ecosystem‐based fisheries management framework, we look at the prospects for expanding current single‐species approaches for target species, by considering nontarget species. We also examine the development of performance measures and reference points for emergent ecosystem‐level properties. We conclude that the expansion of single‐species reference points to take account of the nontarget species of a fishery is tractable and desirable. In contrast, the use of performance measures for a single or a small selection of ecosystem metrics is not possible at present, owing to the absence of a clear understanding of their dynamics and a lack of underlying theory to explain their behaviour. However, recent methods that aggregate a wide range of metrics to provide an overall picture of the ecosystem status show promise and have a particular attraction because they have the potential to provide a framework for establishing a dialogue on management issues between all interested parties.     

Ecosystems say good management pays off

Understanding the strengths and weaknesses of alternative assessment methods, harvest strategies and management approaches are an important part of operationalizing single‐species and ecosystem‐based fisheries management. Simulations run using two variants of a whole‐of‐ecosystem model for the Southern and Eastern Scalefish and Shark Fishery (SESSF) area shows that (a) data‐rich assessments outperform data‐poor assessments for target species and that this performance is reflected in the values of many system‐level ecosystem indicators; (b) ecosystem and multispecies management outperforms single‐species management applied over the same domain; (c) investment in robust science‐based fisheries management pays dividends even when there are multiple jurisdictions, some of which are not implementing effective management; and (d) that multispecies yield‐oriented strategies can deliver higher total catches without a notable decline in overall system performance, although the resulting system structure is different to that obtained with other forms of ecosystem‐based management.     

Efficiency of fisheries is increasing at the ecosystem level

Managing fisheries presents trade‐offs between objectives, for example yields, profits, minimizing ecosystem impact, that have to be weighed against one another. These trade‐offs are compounded by interacting species and fisheries at the ecosystem level. Weighing objectives becomes increasingly challenging when managers have to consider opposing objectives from different stakeholders. An alternative to weighing incomparable and conflicting objectives is to focus on win–wins until Pareto efficiency is achieved: a state from which it is impossible to improve with respect to any objective without regressing at least one other. We investigate the ecosystem‐level efficiency of fisheries in five large marine ecosystems (LMEs) with respect to yield and an aggregate measure of ecosystem impact using a novel calibration of size‐based ecosystem models. We estimate that fishing patterns in three LMEs (North Sea, Barents Sea and Benguela Current) are nearly efficient with respect to long‐term yield and ecosystem impact and that efficiency has improved over the last 30 years. In two LMEs (Baltic Sea and North East US Continental Shelf), fishing is inefficient and win–wins remain available. We additionally examine the efficiency of North Sea and Baltic Sea fisheries with respect to economic rent and ecosystem impact, finding both to be inefficient but steadily improving. Our results suggest the following: (i) a broad and encouraging trend towards ecosystem‐level efficiency of fisheries; (ii) that ecosystem‐scale win–wins, especially with respect to conservation and profits, may still be common; and (iii) single‐species assessment approaches may overestimate the availability of win–wins by failing to account for trade‐offs across interacting species.     

Evaluation of a spatially sex‐specific assessment method incorporating a habitat preference model for blue marlin (Makaira nigricans) in the Pacific Ocean

Blue marlin, widely distributed throughout the Pacific Ocean, are sexually dimorphic, have certain preferred habitats, and migrate seasonally. These characteristics have been ignored in previous stock assessment models. A population dynamics model that includes spatial structure, and sex and age structure was therefore constructed and fitted to fisheries data for blue marlin, along with information on the relative density of the population over space derived from a habitat preference model that uses the oceanographic and biological variables sea‐surface temperature, mixed layer depth, sea‐surface height anomaly, and chlorophyll‐a concentration. Monte Carlo simulation was then used to examine the estimation performance of the stock assessment method. Estimates of management‐related quantities including current spawning stock biomass are substantially biased when the assessment method ignores seasonal movement and sexual dimorphism. We also found that (i) uncertainty about the relationship between catch rate and abundance influences estimation performance to a larger extent than uncertainty in catches, (ii) the outcomes of the assessment are sensitive to the values assumed for natural mortality and stock‐recruitment steepness, and (iii) the ratio of current spawning stock biomass to that at pre‐exploitation equilibrium appears to be the most robust among the quantities considered. We conclude that assessment methods for blue marlin in the Pacific Ocean need to take account of seasonal migration and sex structure to improve stock assessments.     

Salmon shark connections: North Pacific climate change,indirect fisheries effects,or just variability?

A variety of changes are occurring in the ecosystems of the North Pacific Ocean and Bering Sea, but information about the mechanisms of change has been relatively limited, due in part to the region’s remoteness and subarctic conditions. Any number of ecosystem components or indicators could be used to exemplify this dilemma, but here we point to the salmon shark (Lamna ditropis, Lamnidae) as an example of a species that can potentially mediate considerable ecosystem change due to its high trophic level, but for which some basic information is lacking despite attracting some interesting research and widespread rumours and anecdotal evidence of increased abundance. Increases in the abundance of sharks such as salmon sharks in this region during the 1990s, if true, may help explain other observed changes such as declines in ocean survival rates of Pacific salmon (Oncorhynchus spp., Salmonidae) in the region and declines in some wild salmon stocks. Mechanisms that could cause salmon shark increases in Alaskan coastal waters include: (i) increases in sea temperature since the 1970s allowing distributional shifts of this species and changes in the abundance or distribution of their prey; (ii) the 1992 banning of high seas drift gillnets; and (iii) indirect fisheries effects such as competitive release of salmon sharks in the North Pacific transition region and towards the more southern geographic extent of their annual migration as the result of fishery‐related reductions in blue sharks (Prionace glauca, Carcharhinidae) and other pelagic predators. The relative plausibility of these alternative explanations can be evaluated using combinations of existing ecosystem models and empirical research and monitoring programmes including local and indigenous observations.     

A minimal model of the variability of marine ecosystems

In marine ecology, population abundance time series often reveal patterns, such as decadal pseudo-cycles, on scales that make them difficult to mathematically characterize by means of conventional models based on functional responses. We propose here a simple non-deterministic model based on three strong ecological constraints, without any use of functional responses or external factors: (i) mass balance constraints between flows and biomass, (ii) a satiation constraint relating inflows and biomass and (iii) an inertia principle restricting the variation of populations of a given species according to its lifespan. This model reproduces in a robust manner observed patterns of variability and can be used to question the relevance of other modelling approaches of ecosystem dynamics with regard to determinism, constraints and stochasticity. Referring to a non-deterministic model without any functional relationships and environmental or anthropogenic forcing can help in avoiding misleading advice based on the belief that we can explain the causes of observed patterns, which may simply result from basic structural constraints within which the ecosystem functions.     

Inclusion of discards in stock assessment models

A large portion of the catch in many stocks may comprise discards which need to be accounted for in assessments in order to avoid bias in estimates of fishing mortality, stock biomass and reference points. In age‐structured assessment models, discards are sometimes treated as a separate fleet or are added to the landings before fitting so that information about discard behaviour and sampling error is lost. In this paper, an assessment model is developed to describe the discard process with size as a covariate while retaining age‐structured population dynamics. Discard size selection, high grading and bulk dumping of fish at sea are modelled so that the temporal dynamics of the process can be quantified within the assessment. The model is used to show that discarding practices have changed over time in a range of Northeast Atlantic demersal fish. In some stocks, there is a substantial increase in high grading and evidence for bulk discarding which can be related to regulatory measures. The model offers a means of identifying transient effects in the discard process that should be removed from both short‐term forecasts and equilibrium reference point calculations.     

Harvest models and stock co‐occurrence: probabilistic methods for estimating bycatch

A primary goal of ecosystem‐based fishery management is to reduce non‐target stock impacts, such as incidental harvest, during targeted fisheries. Quantifying incidental harvest has generally incorporated fishery‐dependent catch data, yet such data may be biased by gear non‐retention, observation difficulties, and non‐random harvest patterns that collectively lead to an impartial understanding of non‐target stock capture. To account for such issues and explicitly recognize the combined influence of ecological and harvest factors contributing to incidental capture within targeted fisheries, we demonstrate a probabilistic modelling framework that incorporates: (i) background rates of target and non‐target stock co‐occurrence as the primary ecological basis for incidental harvest; (ii) the probability of harvesting at localities exhibiting co‐occurrences; (iii) the probability of selecting for non‐target species with fishery gear; and, (iv) as a function of harvest effort, the overall probability of incidental capture for any non‐target stock contained in the species pool available for harvest. To illustrate application of the framework, simulation models were based on fishery‐independent data from a freshwater fishery in Ontario, Canada. Harvest simulations of empirical stock data indicated that greatest species‐specific capture values were over 4000 times more likely than for species with lowest values, indicating highly variable capture probabilities because of the combined influence of stock heterogeneity and harvest dynamics. Estimated bycatch–effort relationships will allow forecasting incidental harvest on the basis of effort to evaluate future shifts in fishing activity against specific ecosystem‐based fishery management objectives, such as reducing the overall probability of bycatch while maintaining target landings.     

Necessary elements of precautionary management: implications for the Antarctic toothfish

We review the precautionary approach to fisheries management, propose a framework that will allow a systematic assessment of insufficient precaution and provide an illustration using an Antarctic fishery. For a single‐species fishery, our framework includes five attributes: (1) limit reference points that recognize gaps in our understanding of the dynamics of the species; (2) accurate measures of population size; (3) ability to detect population changes quickly enough to arrest unwanted declines; (4) adequate understanding of ecosystem dynamics to avoid adverse indirect effects; and (5) assessment of the first four elements by a sufficiently impartial group of scientists. We argue that one or more of these elements frequently fail to be present in the management of many fisheries. Structural uncertainties, which characterize almost all fisheries models, call for higher limit points than those commonly used. A detailed look into the five elements and associated uncertainties is presented for the fishery on the Antarctic toothfish in the Ross Sea (FAO/CCAMLR Area 88.1, 88.2), for which management was recently described as ‘highly precautionary’. In spite of having features that make the Ross Sea fishery ideal for the application of the precautionary approach, gaps in our knowledge and failure to acknowledge these gaps mean that current regulation falls short of being sufficiently precautionary. We propose some possible remedies.     

Methods of assessing extinction risk in marine fishes

The decline and disappearance of species from large parts of their former geographical range has become an important issue in fisheries ecology. There is a need to identify which species are at risk of extinction. The available approaches have been subject to considerable debate – particularly when applied to commercially exploited species. Here we have compiled methods that have been used or may be used for assessing threat status of marine organisms. We organize the methods according to the availability of data on the natural history, ecology and population biology of species. There are three general approaches to inferring or assessing extinction risk: (i) correlative approaches based on knowledge of life histories and ecology; (ii) time‐series approaches that examine changes in abundance; and (iii) demographic approaches based on age‐ or stage‐based schedules of vital rates and fisheries reference points. Many methods are well suited to species that are highly catchable and/or have relatively low productivity, but theory is less well developed for assessing extinction risk in species exhibiting narrow geographical distributions or ecological specialization. There is considerable variation in both definitions of extinction risk and the precision and defensibility of the available risk assessment methods, so we suggest a two‐tiered approach for defining and assessing extinction risk. First, simple methods requiring a few easily estimated parameters are used to triage or rapidly assess large numbers of populations and species to identify potentially vulnerable populations or species. Second, the populations and species identified as vulnerable by this process can then be subject to more detailed and rigorous population analysis explicitly considering sources of error and uncertainty.     

Exploring uncertainty in population viability analysis and its implications for the conservation of a freshwater fish

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Skating on thin ice: Identifying the need for species‐specific data and defined migration ecology of Rajidae spp.

Skates (class Chondrichthyes; subclass Elasmobranchii; order Rajiformes; family Rajidae) comprise one quarter of extant chondrichthyans, yet have received little attention in the scientific literature likely due to their relatively low economic value and difficulties in species identification. The absence of species‐specific information on catch, life history and migration of skates has often precluded the development of single‐species stock assessments and led to the use of cursory multispecies assessments, which lack the ability to track species‐specific catch and abundance trends. This has resulted in undetected local extirpations, as has been previously reported for common (Dipturus batis, Rajidae), white (Rostroraja alba, Rajidae) and long‐nose (Dipturus oxyrhinchus, Rajidae) skates in the Irish Sea. Here, we (a) use case studies to illustrate how the perception of skate population structure and stock status has historically been masked through multispecies assessment and management practices, (b) review current information on the movement of skates and identify gaps in knowledge, and (c) identify biases associated with the use of various tagging technologies, which have confounded our understanding of movement and migration ecology of skates. Considering that over 40% of extant Rajidae species are listed as “Data Deficient” by the International Union for Conservation of Nature, we illustrate a critical need to broaden the current understanding of skate life history, movement and migration ecology by providing recommendations on the further application of electronic tags and biogeochemical natural tags in movement studies and highlight the benefits that studies using these approaches have for novel management frameworks.     

Coupling ecosystem and individual‐based models to simulate the influence of environmental variability on potential growth and survival of larval sprat (Sprattus sprattus L.) in the North Sea

To investigate the impact of changing environmental conditions in the North Sea on the distribution and survival of early life stages of a marine fish species, we employed a suite of coupled model components: (i) an Eulerian coupled hydrodynamic/ecosystem (Nutrients, Phyto‐, Zooplankton, Detritus) model to provide both 3‐D fields of hydrographical properties, and spatially and temporally variable prey fields; (ii) a Lagrangian transport model to simulate temporal changes in cohort distribution; and (iii) an individual‐based model (IBM) to depict foraging, growth and survival of fish early life stages. In this application, the IBM was parameterized for sprat (Sprattus sprattus L.) and included non‐feeding (egg and yolk‐sac larval) stages as well as foraging and growth subroutines for feeding (post‐yolk sac) larvae. Sensitivity analyses indicated that the angle of visual acuity, assimilation efficiency and the maximum food consumption rate were the most critical intrinsic model parameters. As an example, we applied this model system for 1990 in the North Sea. Results included not only information concerning the interplay of temperature and prey availability on larval fish survival and growth but also information on mechanisms underlying larval fish aggregation within frontal zones. The good agreement between modelled and in situ estimates of sprat distribution and growth rates in the German Bight suggested that interconnecting these different models provided an expedient tool to scrutinize basic processes in fish population dynamics.     

A novel spatiotemporal stock assessment framework to better address fine‐scale species distributions: Development and simulation testing

Characterizing population distribution and abundance over space and time is central to population ecology and conservation of natural populations. However, species distribution models and population dynamic models have rarely been integrated into a single modelling framework. Consequently, fine‐scale spatial heterogeneity is often ignored in resource assessments. We develop and test a novel spatiotemporal assessment framework to better address fine‐scale spatial heterogeneities based on theories of fish population dynamic and spatiotemporal statistics. The spatiotemporal model links species distribution and population dynamic models within a single statistical framework that is flexible enough to permit inference for each state variable through space and time. We illustrate the model with a simulation–estimation experiment tailored to two exploited marine species: snow crab (Chionoecetes opilio, Oregoniidae) in the Eastern Bering Sea and northern shrimp (Pandalus borealis, Pandalidae) in the Gulf of Maine. These two species have different types of life history. We compare the spatiotemporal model with a spatially aggregated model and systematically evaluate the spatiotemporal model based on simulation experiments. We show that the spatiotemporal model can recover spatial patterns in population and exploitation pressure as well as provide unbiased estimates of spatially aggregated population quantities. The spatiotemporal model also implicitly accounts for individual movement rates and can outperform spatially aggregated models by accounting for time‐and‐size varying selectivity caused by spatial heterogeneity. We conclude that spatiotemporal modelling framework is a feasible and promising approach to address the spatial structure of natural resource populations, which is a major challenge in understanding population dynamics and conducting resource assessments and management.     

A new role for MSY in single-species and ecosystem approaches to fisheries stock assessment and management

In 1977, Peter Larkin published his now‐famous paper, ‘An epitaph for the concept of maximum sustained yield’. Larkin criticized the concept of single‐species maximum sustained yield (MSY) for many reasons, including the possibility that it may not guard against recruitment failure, and the impossibility of maximising sustainable yields for all species simultaneously. However, in recent years, there has been a fundamental change in the perception of the fishing mortality associated with MSY (F_MSY) as a limit to be avoided rather than a target that can routinely be exceeded. The concept of F_MSY as a limit is embodied in several United Nations Food and Agriculture Organization (FAO) agreements and guidelines, and has now been incorporated into the US Magnuson–Stevens Fishery Conservation and Management Act. As a result, the United States now requires the development of overfishing definitions based on biological reference points that treat the F_MSY as a limit reference point and must also define a lower limit on biomass below which rebuilding plans with strict time horizons must be developed. This represents a major paradigm shift from the previously mandated (but often unachieved) objective to simply maintain fishing mortalities at levels below those associated with recruitment overfishing. In many cases, it requires substantial reductions in current fishing mortality levels. Therefore, the necessity of the new paradigm is continually questioned. This paper draws on examples from several fisheries, but specifically focuses on the recent US experience illustrating the practical difficulties of reducing fishing mortality to levels below those corresponding to MSY. However, several studies suggest that even more substantial reductions in fishing mortality may be necessary if ecosystem considerations, such as multispecies interactions, maintenance of biodiversity and genetic diversity, and reduction of bycatch and waste, are taken into account. The pros and cons of moving beyond single‐species assessment and management are discussed. A US plan for improving stock assessments indicates that even a ‘basic’ objective such as ‘adequate baseline monitoring of all managed species’ may be extremely costly. Thus, the suggestion of Larkin (1983, 1997) that the costs of research and management should not exceed 10–20% of the landed value of the catch may preclude comprehensive ecosystem management. More importantly, neither single‐species nor ecosystem‐based fisheries management is likely to improve appreciably unless levels of fishing capacity are aligned with resource productivity, as is currently being promoted by FAO and several individual nations.     

Fishing impact and environmental status in European seas: a diagnosis from stock assessments and ecosystem indicators

Stock‐based and ecosystem‐based indicators are used to provide a new diagnosis of the fishing impact and environmental status of European seas. In the seven European marine ecosystems covering the Baltic and the North‐east Atlantic, (i) trends in landings since 1950 were examined; (ii) syntheses of the status and trends in fish stocks were consolidated at the ecosystem level; and (iii) trends in ecosystem indicators based on landings and surveys were analysed. We show that yields began to decrease everywhere (except in the Baltic) from the mid‐1970s, as a result of the over‐exploitation of some major stocks. Fishermen adapted by increasing fishing effort and exploiting a wider part of the ecosystems. This was insufficient to compensate for the decrease in abundance of many stocks, and total landings have halved over the last 30 years. The highest fishing impact took place in the late 1990s, with a clear decrease in stock‐based and ecosystem indicators. In particular, trophic‐based indicators exhibited a continuous decreasing trend in almost all ecosystems. Over the past decade, a decrease in fishing pressure has been observed, the mean fishing mortality rate of assessed stocks being almost halved in all the considered ecosystems, but no clear recovery in the biomass and ecosystem indicators is yet apparent. In addition, the mean recruitment index was shown to decrease by around 50% in all ecosystems (except the Baltic). We conclude that building this kind of diagnosis is a key step on the path to implementing an ecosystem approach to fisheries management.     

Comparing species and ecosystem-based estimates of fisheries yields

Three methods are described to estimate potential yields of commercial fish species: (i) single-species calculation of maximum sustainable yields, and two ecosystem-based methods derived from published results for (ii) energy flow and for (iii) community structure. The requirements imposed by food-web fluxes, and by patterns of relative abundance, provide constraints on individual species. These constraints are used to set limits to ecosystem-based yields (EBY); these limits, in turn, provide a comparison with the usual estimates of maximum sustainable yields (MSY). We use data on cod and haddock production from Georges Bank for the decade 1993–2002 to demonstrate these methods. We show that comparisons among the three approaches can be used to demonstrate that ecosystem based estimates of yields complement, rather than supersede, the single-species estimates. The former specify the significant changes required in the rest of the ecosystem to achieve a return to maximum sustainable levels for severely depleted commercial fish stocks. The overall conclusion is that MSY defines changes required in particular stocks, whereas EBY determines the changes required in the rest of the ecosystem to realize these yields. Species specific MSY only has meaning in the context of the prey, predators and competitors that surround it.     

Recruitment,distribution boundary and habitat temperature of an arcto‐boreal gadoid in a climatically changing environment: a case study on Northeast Arctic haddock (Melanogrammus aeglefinus)

Climate change is expected to have major effects on the distribution and abundance of fish. In spite of extensive research on the topic in high‐latitude marine ecosystems, the mechanistic understanding of how temperature impacts recruitment and distribution of arcto‐boreal fish stocks remains elusive. Exemplified by an arcto‐boreal gadoid in the Barents Sea, the Northeast Arctic (NEA) haddock (Melanogrammus aeglefinus), we investigate the effect of ecosystem temperature (here temperature from a fixed reference section) on abundance and distribution boundaries between 1981 and 2008. During this time interval there has been a trend of increasing temperature in the ecosystem. We compare the ecosystem temperature with the species habitat temperature of NEA haddock (i.e., ambient temperature of the population) – two temperature approaches representing the indirect and direct environmental impacts on fish, respectively. In addition to the temperature effects, density‐dependent effects on distribution boundaries are considered. The study is based on swept area density estimates and spatial temperature data collected annually in winter surveys. We found a positive relationship between ecosystem temperature and abundance, a connection related to both direct and indirect mechanisms with short‐term and long‐term pathways. Distribution boundaries are, on a year‐to‐year basis, more related to abundance than ecosystem temperature. The long‐term trends, however, indicate a north‐eastward shift in distribution boundaries, probably indirectly related to the coinciding ecosystem temperature increase. In spite of the gradual increase in ecosystem temperature, the abundance of 4‐ to 7‐ year old NEA haddock expanded into colder waters. Thus, our results show how different the two temperature approaches may be.