Stock-recruitment Relationship Research Articles

Accounting for post-release mortality in data-limited, length-based assessments for four demersal fish species with varying life histories

Released (or discarded) fish from recreational and commercial line fisheries can experience significant levels of post-release mortality (PRM), affecting sustainability. Although PRM effects are often considered in data-rich (integrated model) assessments, this is uncommon in data-limited assessments. This study investigates impacts of PRM when applying data-limited, length-based assessment methods for four reef-dwelling demersal fish species in temperate Western Australia (Glaucosoma hebraicum, Chrysophrys auratus, Choerodon rubescens and Epinephelides armatus). These species differ considerably in their biology and susceptibility to PRM. For each species simulations were undertaken to (1) evaluate the reliability of a length-based catch curve (LBCC) method for estimating fishing mortality (F) and selectivity parameters, and (2) predict the impacts of different PRM levels on values of relative biomass, i.e. ratios of fished to unfished equilibrium spawning biomass (Brel), estimated using a length-based equilibrium analysis (LBEA). LBEA extends length-based per-recruit analysis to incorporate a stock-recruitment relationship as well as allow for PRM. On average, estimates of F and selectivity parameters from LBCC for each species were unbiased when applied to simulated data. Precision decreased, however, when LBCC was fitted to data simulated with recruitment variation. For a given F, increased PRM levels reduced Brel for all species, with the extent of impacts varying depending on species biology. LBCC was then applied to two actual datasets, firstly to lengths for all fish that had either been retained or released during historical recreational fishing trips (to estimate gear selectivity), and secondly to length data for retained fish from recent charter fishing (to estimate F and selectivity of landings). The resultant estimates were then used in LBEA to estimate Brel with and without accounting for PRM. Relative impacts of PRM varied among species depending on their biology and size limits. While PRM was predicted to have limited impact on Brel for C. rubescens (as fish are not fully selected by the gear until they attain a relatively large size), accounting for PRM resulted in estimates of Brel (at their current estimated F levels) being reduced by 31 % for G. hebraicum, 36 % for C. auratus and 23 % for E. armatus. It is recommended that, where evidence of PRM exists for a species, this be accounted for in data-limited assessments.

Occurrence and Strength of Instantaneous and Intracohort Density-Dependence in Northeast Atlantic Fish Stocks.

Biological reference points (BRPs) used in fisheries management do not include density-dependent (DD) growth, with DD processes only considered in the stock recruitment relationship. Not accounting for DD on somatic growth has led to criticism that such BRPs underestimate the compensatory effects of DD at low stock size, and therefore risk foregone catch opportunities. Here, we analyse 81 stocks from the Northeast Atlantic for evidence of DD growth, defined as the process in which stock size affects somatic weight. We evaluate the following questions: (1) How many stocks have experienced instantaneous DD growth and do stocks of the same species display similar trends? (2) Is there a common instantaneous DD growth relationship shared by all stocks? (3) For stocks exhibiting significant instantaneous DD growth, can we quantify the strength of the relationship? (4) Is DD growth operating as an intra-cohort process as opposed to an instantaneous effect? Results reveal that only the weight of recruits exhibits a common instantaneous DD growth while the other responses analysed show a positive, noncompensatory effect, suggesting that other processes are at work. All responses examined showed significant temporal autocorrelation, which, when not accounted for, suggest apparent instantaneous DD growth in several stocks. Comparison of instantaneous against intracohort DD growth showed an increase in the number of stocks with significant DD growth, although, as for instantaneous DD growth, this declined greatly when temporal autocorrelation was accounted for. Our results counteract the a priori assumption that DD growth compensation is related only to stock biomass or density, suggesting that DD growth should be dealt case-by-case. Consequently, management practices that aim to fish down stock biomass with the anticipation of triggering DD growth will be associated with greater asymmetric risks than keeping biomass at levels where replacement yield does not rely on it.

The ecological and socioeconomic implications of future total allowable catch decisions and illegal fishing scenarios for South Africa’s West Coast rock lobster fishery

The West Coast rock lobster fishery, vital to South Africa, is in a critical state with only 1.1 % of its original exploitable biomass remaining. This study evaluates effects of various total allowable catch (TAC) decisions on biomass and economic performance, taking illegal fishing, currently a serious problem, into account. TAC options included two sustainable options and an option (1500 mt), which could only be feasible if illegal fishing is stopped. Biomass projections, from 2022 to 2050, are modelled from a length-based model incorporating a Beverton and Holt stock-recruit relationship. The impacts of different TAC decisions on biomass, net seasonal incomes (NSI) and net present value (NPV) are assessed. Sensitivity to different discount rates is also considered. Biomass projections suggest an imminent collapse if legal fishing limits are set too high or if IUU fishing is not substantially reduced. The NPVs for the entire sectors are found to be highest under a 1500 mt TAC but unless accompanied by a halt to IUU fishing, would result in unsustainable catch rates and rapidly declining NSI in areas where many quota holders are located. Under the 550 mt TAC scenario, there is less risk of biomass declines but the quotas for many stakeholders would be economically unviable. Lower TAC options are forecast to lead to the functional exclusion of many rights holders and reduction in employment. The most secure and economically successful strategy is considered to be adjusting annual TACs in response to the effectiveness of controlling IUU fishing and changes in biomass.

Robust fisheries management strategies under deep uncertainty

Fisheries worldwide face uncertain futures as climate change manifests in environmental effects of hitherto unseen strengths. Developing climate-ready management strategies traditionally requires a good mechanistic understanding of stock response to climate change in order to build projection models for testing different exploitation levels. Unfortunately, model-based projections of fish stocks are severely limited by large uncertainties in the recruitment process, as the required stock-recruitment relationship is usually not well represented by data. An alternative is to shift focus to improving the decision-making process, as postulated by the decision-making under deep uncertainty (DMDU) framework. Robust Decision Making (RDM), a key DMDU concept, aims at identifying management decisions that are robust to a vast range of uncertain scenarios. Here we employ RDM to investigate the capability of North Sea cod to support a sustainable and economically viable fishery under future climate change. We projected the stock under 40,000 combinations of exploitation levels, emission scenarios and stock-recruitment parameterizations and found that model uncertainties and exploitation have similar importance for model outcomes. Our study revealed that no management strategy exists that is fully robust to the uncertainty in relation to model parameterization and future climate change. We instead propose a risk assessment that accounts for the trade-offs between stock conservation and profitability under deep uncertainty.

What stock-per recruit target can be applied to Japanese fisheries resources under large uncertainties in the stock-recruitment relationship?

What stock-per recruit target can be applied to Japanese fisheries resources under large uncertainties in the stock-recruitment relationship?

On the probable distribution of stock-recruitment resilience of Pacific saury (Cololabis saira) in the Northwest Pacific Ocean

Abstract Determining how resilient a stock-recruitment relationship is to environmental variation is crucial for fisheries management. Steepness is a key factor characterizing the resilience of a fish stock and, hence, for establishing management reference points. This study estimates the distribution of steepness for Pacific saury using a simulation approach based on evolutionary ecology and reproductive biology, and how it changes in response to environmental change. The median estimated steepness is 0.82 (80% probable range 0.59, 0.93) based on the best available biological information, which suggests that Pacific saury can produce a relatively high proportion of unfished recruitment when depleted to 20% of unfished spawning biomass. Elasticity analysis indicates that steepness for Pacific saury is most sensitive to the survival rate of early life stages, mean body weight, growth, and length-at-maturity. Environmental change could substantially impact steepness, with unfavorable conditions related to survival rates, length-at-maturity, mean body weight, and growth potentially leading to a reduction in resilience. Understanding these impacts is crucial for the assessment and management of Pacific saury. Our numerical simulation approach provides an analytical tool applicable for calculating the steepness distribution in other small pelagic fish influenced by increases in sea surface temperature due to global warming.

The response of marine fish population productivity to juvenile habitat modifications depends upon life histories

AbstractCoastal and estuarine habitats are essential for both growth and survival at juvenile stages for a large proportion of marine fish but are exposed to a variety of threats. However, the quantitative impacts of anthropic‐mediated nursery degradation on exploited population dynamics and productivity are rarely addressed. We developed a comprehensive steepness‐based parameterization of the stock–recruitment relationship that directly depends upon juvenile habitat quality and surface area and used a generic age‐ and stage‐structured model to simulate exploited population dynamics under nursery degradation/restoration scenarios. Population responses to juvenile habitat modification were estimated for three species with contrasting life histories. Modifying the surface area of nursery habitat has a predictable direct scaling effect on population size and affects biomass and fishing yield, with identical responses observed for various life histories. Modification of habitat quality affects both the population size and the shape of the productivity curve, leading to more complex responses of biomass and fishing yields that depend upon combinations of life history traits. Populations with low steepness and hence lower resiliency to fishing are the most sensitive to the degradation of habitat quality. Our study emphasizes the critical impact of juvenile habitat modification on population dynamics and productivity and the need to integrate habitat consideration into the management of exploited marine species. Our modelling framework is generalizable and can be extended to handle many diverse life histories and case studies to quantify the consequences of coastal habitat degradation/restoration for the dynamics of exploited marine fish populations.

Stock-recruitment relationships and maximum sustainable yield (MSY): examples of TAC species

Stock-recruitment relationships and maximum sustainable yield (MSY): examples of TAC species

Bridging proxy reference points to sustainable yields to support data-limited fisheries

Many of the world fisheries, especially small-scale fisheries, are in data-limited situations, for which proxy Biological Reference Points (BRPs) derived from per-recruit analyses are widely used to inform fisheries management. However, due to limited information on stock recruitment, per-recruit analyses usually assume constant recruitment and the influences of this key assumption have not been fully understood. In this study, we combined per-recruit analyses and stock-recruit relationship to build equilibrium models for eight species with different life history traits, and used the management strategy evaluation (MSE) framework to assess the uncertainty of proxy BRPs in achieving fisheries sustainability. Our results showed substantial variations in the performances of proxy BRPs on maintaining yield and biomass, and even conservative BRPs, e.g. F0.1 caused overexploitation in half of the cases (4 out of 8). The divergence of the performance between proxy BRPs and FMSY could be substantial, depending on steepness of stock-recruit relationships (correlation ranged from –0.55 to 0.9 for yield, and 0.53 to 0.96 for biomass). Accordingly, we presented an empirical approach for remediation, using correction coefficient to adjust the traditional proxy BRPs. The empirical approach showed satisfactory performances in the validation processes with an independent dataset. This paper demonstrates that proxy BRPs may introduce risks in achieving fisheries sustainability, whereas the errors can be readily adjusted with empirical estimates of steepness for most data-limited fisheries.

Microgeographic variation in demography and thermal regimes stabilize regional abundance of a widespread freshwater fish.

Predicting the persistence of species under climate change is an increasingly important objective in ecological research and management. However, biotic and abiotic heterogeneity can drive asynchrony in population responses at small spatial scales, complicating species-level assessments. For widely distributed species consisting of many fragmented populations, such as brook trout (Salvelinus fontinalis), understanding the drivers of asynchrony in population dynamics can improve the predictions of range-wide climate impacts. We analyzed the demographic time series from mark-recapture surveys of 11 natural brook trout populations in eastern Canada over 13 years to examine the extent, drivers, and consequences of fine-scale population variation. The focal populations were genetically differentiated, occupied a small area (~25 km2 ) with few human impacts, and experienced similar climate conditions. Recruitment was highly asynchronous, weakly related to climate variables and showed population-specific relationships with other demographic processes, generating diverse population dynamics. In contrast, individual growth was mostly synchronized among populations and driven by a shared positive relationship with stream temperature. Outputs from population-specific models were unrelated to four of the five hypothesized drivers (recruitment, growth, reproductive success, phylogenetic distance), but variation in groundwater inputs strongly influenced stream temperature regimes and stock-recruitment relationships. Finally, population asynchrony generated a portfolio effect that stabilized regional species abundance. Our results demonstrated that population demographics and habitat diversity at microgeographic scales can play a significant role in moderating species responses to climate change. Moreover, we suggest that the absence of human activities within study streams preserved natural habitat variation and contributed to asynchrony in brook trout abundance, while the small study area eased monitoring and increased the likelihood of detecting asynchrony. Therefore, anthropogenic habitat degradation, landscape context, and spatial scale must be considered when developing management strategies to monitor and maintain populations that are diverse, stable, and resilient to climate change.

Testing novel methods for short-term forecasting of European glass eel recruitment

Recruitment forecasting constitutes a major issue in population dynamics, especially in stock assessments. In many cases, high recruitment stochasticity thwarts the determination of a stock-recruitment (SR) relationship. When no SR relationship is available, recruitments can be projected from past trends using methods such as the geometric means of the past recruitments. This approach implicitly assumes stability in upcoming years and might not be appropriate in the case of regime shifts. Recruitment forecasting is particularly critical when predicting fishing opportunities for recruitment fisheries. The fishery of glass European eels (Anguilla anguilla) is an example of a recruitment fishery. The French glass eel fishery, which is currently guided by two models to forecast upcoming recruitments and derive fishing opportunities, has failed to achieve management targets. This study develops new models that forecast glass eel recruitments without assumptions about the SR relationship, and with flexible assumptions about future recruitment evolutions. Tests based on multiple criteria found the best performance in a random slope model, which provides flexibility to track recruitment dynamics while balancing the interests of fisheries, management and conservation. This model has wide potential for trend projections in natural resource management, economics and other fields.

Pragmatic approaches to modeling recruitment in fisheries stock assessment: A perspective

Our tasks in performing stock assessment are to estimate what is in the population now, what is likely to be there in the near-future, and how we should best extract yield without imperiling future reproduction or yield. There are many considerations for modeling recruitment in a stock assessment, and I walk through the typical sequence of decisions an analyst must make when setting up their stock assessment. I begin with the deceptively simple decision of the first modeled age, progress through the treatment of fecundity, and into the important decision of parameterizing the stock recruit relationship (SRR). Subsequent assessment steps such as projections and estimation of reference points build on these earlier decisions. The consequences of many of these decisions are amplified or obviated, depending on whether or not a null SRR model (mean or median with deviations) is implemented. Examples of expanding or attenuating consequences are provided within each decision. The potential for misspecification related to these decisions is noted, and advice to reduce that misspecification as well as suggested diagnostics are offered. As a high level summary, I recommend fitting the null SRR, using weight as a surrogate for fecundity and conducting sensitivity analysis if there are data suggesting significant departure from isometry. If estimating a non-null SRR, move away from the steepness parameterization (return to the original α,β parameterization), and do not begin at age 0. Lastly, exercise caution when incorporating covariates –they may be hitchhikers posing as drivers.

Recruitment of Engraulis japonicus and its environmental drivers: An EDM analysis

Anchovies are the key trophic links between secondary production and higher trophic levels, and their recruitment changes have the potential to cascade and affect higher trophic levels. Therefore, understanding the stock-recruitment relationships and the external drivers of recruitment has important ecological and fishery management implications. In this study, we employed empirical dynamic modelling (EDM) to analyze the recruitment dynamics of Japanese anchovy (Engraulis japonicus) in the Yellow Sea large marine ecosystem (YSLME), based on data from surveys of the YSLME. The results indicated that biological effects between the YSLME Japanese anchovy stock and its recruitment were deterministic and bidirectional (interplay of causality in both directions). Japanese anchovy recruitment was also driven by other external factors, chief among these was fishing pressure which plays a decisive role in overall recruitment of the YSLME. In contrast, climate related factors act more locally on recruitment. We conclude that the decline in Japanese anchovy biomass is mainly caused by overfishing. This has important implications for rebuilding and subsequent management of Japanese anchovy stock: under current climate pressure, the EDM showed that reducing fishing pressure would promote recovery of stock biomass and recruitment.

The robustness of our assumptions about recruitment: A re-examination of marine recruitment dynamics with additional data and novel methods

Recruitment is the number of young fish entering a population and is an essential process in age-structured stock assessment models. Many of the models on which stock assessments are based assume some level of influence of spawning biomass on recruitment, and estimation of this functional relationship has been a focus of substantial research. However, recruitment is also influenced by environmental fluctuations that induce autocorrelated patterns, trends, and shifts in deviations from the stock-recruitment relationship. Consequently, previous meta-analyses of stock-recruitment relationships have had trouble calibrating the relationship. We revisited the findings of Szuwalski et al. (2015) to determine if their results are robust to the addition of data to the RAM Legacy Stock Assessment database and choice of analysis method (the PELT algorithm, MARSS models, and Bayesian change point detection). Since the publication of Szuwalski et al. (2015), the number of stocks in the database has doubled. We determined the primary influence of spawning biomass on recruitment and examined the recruitment time series for regime shifts for 432 stocks. Our results indicated that 57% of stocks did not have a significant correlation between spawning biomass and recruitment over the observed biomasses. Environmental conditions played a larger role in recruitment variation than spawning biomass. The presence, location, and number of regime shifts in recruitment time series was highly dependent on the detection method, with Bayesian change point detection identifying the fewest regime shifts. Despite the sensitivity to the detection method, 46% of stocks without a significant correlation between spawning biomass and recruitment are estimated to have experienced at least one regime shift as determined by the PELT algorithm Our analyses suggest that effective methods for modeling and forecasting large variations in recruitment over time are needed, particularly given that climate change is predicted to impact the frequency and magnitude of regime shifts.

The interactions between fishing mortality, age, condition and recruitment in exploited fish populations in the North Sea

A quantity of studies have documented how greater condition and older age of individual fish enhances reproductive success via a variety of processes from egg production to larval survival. If these relations translate into relationships between stock demography and recruitment, maintaining a “healthy” age composition may be a key step towards ensuring stock productivity. We examined the correlation between indicators of the age and size composition and the residual-variation from the stock-recruitment relationship (i.e. the variation not explained by the spawning stock biomass) in eleven commercially exploited fish stocks in the North Sea. We also analyzed to what extent spawner age and size composition responded to variations in fishing mortality. Before detrending to accommodate any temporal patterns, recruitment residuals correlated negatively with the relative age composition in all eleven stocks (plaice, whiting, and saithe being significant). However, after detrending the recruitment residuals, the correlations weakened and only nine out of eleven stocks correlated negatively. Lastly, we also conducted a delay difference analysis, which yielded a more or less even distribution between negative and positive correlations, and in addition two stocks (cod and Norway pout) came out showing significant positive correlations between recruitment residuals and age composition. In contrast, eight of the eleven stocks showed positive correlations between spawner weight and recruitment residuals and correlations were significant for plaice, cod, and saithe. After detrending the data, the significant correlations disappeared, but the overall tendency towards positive correlations between spawner weight and recruitment residuals prevailed. Overall, these results suggest that fish condition is more likely to show positive relationships with recruitment success than age composition. Lastly, it was found that the majority of stocks showed a strong link between fishing mortality and the two indicators of age composition, and when looking across stocks, the age composition appears to have returned to the same level as in the 1960 s. Conversely, spawner weight at age have not recovered, and considering the indications of a positive link between spawner weight and recruitment, recruitment success of the North Sea stocks may remain low in spite of the apparently successful rebuilding of the age composition.

Combining output control and fishing pressure limitations improves the management of the Japanese sardine Sardinops melanostictus

The stock biomass of the Japanese sardine Sardinops melanostictus in the Pacific Ocean shows large fluctuations in response to environmental variations and fishing activities; this is reflected in annual catch amounts. Harvest policy in Japan determines the current allowable biological catch (ABC) by using catch data and marine resource indices. The total allowable catch (TAC) for sardine is based on the ABC, but there is a time delay from the collection and analysis of data to its implementation. We have developed a new harvest control rule (HCR) into which fishing pressure limitations are incorporated. The rule calls for fishing to stop when fish landings reach the TAC or the fishing effort and the fishery catchability reach the limit. We have evaluated the performance of the proposed HCR against the implementation error for fishing pressure and the observation error for fishing mortality. We conducted simulations using two stock recruitment relationships and three historic starting years to investigate the effects of rule initiation timing. The results indicate that this HCR could help to facilitate a recovery of the stock biomass and improve annual catches of the Japanese sardine. Especially when the observation error is large, the HCR is effective in maintaining stock biomass, increasing the average annual catch, and stabilizing the overall annual catch.

Analytical expressions of sustainable benchmarks generic for all two-stage-structured models of exploited fish populations

Of the types of fishery models that use time series of removals, fishing effort, and indices of abundance, two-stage-structured models are less prevalent, especially in developing the maximum sustainable yield (MSY) benchmarks, in part because of ignorance of how to calculate those benchmarks. To fill this gap, this study aims to derive analytical expressions for calculating MSY benchmarks associated with a continuous delay-differential model, given that the recruit stage relates to the spawning-stock biomass (SSB) according to a stock–recruit relationship (SRR) and that the SRR parameters are combined with the components of the composite (two-stage-structured or age-aggregated) yield per-recruit model CYPR14. The focus is on analytical expressions of the fishing mortality producing MSY ( FMSY), considering that the equilibrium yield comes from either the equilibrium SSB at the start of fishing regimes or the equilibrium average SSB during fishing regimes. The expressions obtained are versatile for all two-stage-structured models. Derivations employing the fished equilibrium average SSB are recommended for use because they produce precautionary MSY benchmarks and because they are consistent with their counterparts in contemporary age-structured stock assessment models.

Stage‐specific drivers of Pacific hake (Merluccius productus) recruitment in the California Current Ecosystem

AbstractUnderstanding environmental drivers of recruitment variability in marine fishes remains an important challenge in fish ecology and fisheries management. We developed a conceptual life‐history model for Pacific hake (Merluccius productus) along the west coast of the United States and Canada to generate stage‐specific and spatiotemporally‐specific hypotheses regarding the oceanographic and biological variables that likely influence their recruitment. Our model included seven life stages from pre‐spawning female conditioning through pelagic juvenile recruitment (age‐0 fish) for the coastal Pacific hake stock. Model‐estimated log recruitment deviations from the 2020 hake assessment were used as the dependent variable, with predictor variables drawn primarily from a regional ocean reanalysis for the California Current Ecosystem. Indices of prey and predator abundance were also included in our analysis, as were predictors of local‐ and basin‐scale climate. Five variables explained 59% of the recruitment variability not accounted for by the stock–recruitment relationship in the hake assessment. Recruitment deviations were negatively correlated with May–September eddy kinetic energy between 34.5° and 42.5°N, the North Pacific Current Bifurcation Index, and Pacific herring (Clupea pallasii) biomass during the spawner preconditioning stage, alongshore transport during the yolk‐sac larval stage, and the number of days between storm events during the first‐feeding larval stage. Other important predictors included upwelling strength during the preconditioning stage, the number of calm periods during the first‐feeding larval stage, and age‐1 hake predation on age‐0 pelagic juveniles. These findings suggest that multiple mechanisms affect Pacific hake survival across different life stages, leading to variability in population‐level recruitment.

Multi-decadal distribution changes of commercially important demersal species in the central-western Sea of Japan based on a multi-species spatiotemporal model

Understanding long-term changes in the distribution of commercially important species is a key first step for developing appropriate management plans. We estimated multi-decadal changes in the distributions of seven demersal fish species in the central-western Sea of Japan by fitting a multi-species spatiotemporal model to catch and effort data from a Japanese offshore trawl fishery from 1982 to 2020. We then identified any spatiotemporal patterns and discussed their relationship to oceanographic factors. Large distribution expansions were detected for Pacific cod (Gadus macrocephalus) and blackthroat seaperch (Doederleinia berycoides) in the southwestern region of the study area in the 2000s and 2010s. Combined with previous findings and an interannual trend of water temperature, it is thought that these changes could have been caused by expansions in the distribution of optimal thermal condition for stock-recruitment relationships. Species that did not experience large changes in distribution, such as cold-water flounders [the flathead flounder (Hippoglossoides dubius) and Korean flounder (Glyptocephalus stelleri)], mainly inhabit deep waters that are thought to be dominated by a colder and homogeneous water mass, meaning that their habitats were more likely to remain stable over time. Our results will contribute to improvement of adaptive stock managements for commercially important demersal species.

Assessment of Antarctic minke whales using statistical catch-at-age analysis (SCAA)

Statistical catch-at-age analysis (SCAA) is applied to data for Antarctic minke whales. The SCAA model is spatially-structured, can model multiple stocks of minke whales, and can utilise several data types for parameter estimation. The application to Antarctic minke whales considers two stocks (I and P) in five areas which cover Antarctic Areas III–E to VI–W. The parameters of the model (annual deviations about the stock-recruitment relationship, changes over time in carrying capacity, density-dependence parameters related to productivity and carrying capacity, and the parameters which determine growth by stock, age-specific natural mortality by stock, and vulnerability by area and ‘fleet’) are estimated by fitting the model to data on catches, catch-at-length, conditional age-at-length, and estimates of absolute and relative abundance. A reference case analysis is selected, and sensitivity explored using retrospective analyses and by varying the assumptions on which the reference case analysis is based. The reference case analysis is able to mimic all of the data sources adequately. Most of the analyses (reference and sensitivity) indicates that Antarctic minke whales in the assessed area increased from 1930 until the mid-1970s and have declined thereafter, with the extent of the decline greater for minke whales in Antarctic Areas III–E to V–W than for those further east. Natural mortality is consistently estimated to be higher for younger and older individuals than for individuals of intermediate age. Estimates of MSYR1+ (the exploitation rate on animals 1 and older at which sustainable yield is maximised) are presented, but are unreliable owing to the lack of contrast.