Stock Assessment Process Research Articles

Influence on management advice of fishers acoustics—10 year review of blue grenadier monitoring

An industry-based acoustic observation program was developed and applied to the austral winter spawning blue grenadier fishery from 2002. Blue grenadier has the highest Total Allowable Catch (TAC) among fished stocks of Australia’s Southern and Eastern Scalefish and Shark Fishery (SESSF). High recruitment variability and stock size uncertainty motivated development of an industry-based cost-effective acoustic monitoring program in 2002 to optimise harvest while ensuring sustainability. The development of industry-based acoustic methods resulted in “routine” surveying being carried out by skippers on industry vessels since 2006. During this time an incentive based management approach was introduced by allocation of research quota to assist in the long term implementation and delivery of the biomass indicator. The survey strategy of the industry-based method differed significantly from chartered vessel based surveys; fishers were tasked with recording the maximum biomass during the spawning season. This monitoring method shifted the “burden of proof” to industry and reinforced the need for both local and broad scale surveys to estimate the stock size. Survey results have been used for management advice by reducing uncertainty in stock size, inclusion in the stock assessment model and informing spatial planning for the region. An associated outcome was the confidence by industry that fishers’ observations of the spawning biomass were recorded and used in the stock assessment process for management of the fishery.

Toward a new paradigm for growth modeling in fisheries stock assessments: Embracing plasticity and its consequences

Modeling of body growth forms an essential part of many fisheries stock assessments. Growth influences population dynamics through its effects on lifetime patterns of biomass production, natural and fishing mortality, and reproductive output. By relating size to age, growth models also enable the use of size-based data in age-based stock assessments. Growth patterns are commonly assumed to be constant (time-invariant) or at best subject to inconsequential process noise. However, fish growth is inherently plastic, often responding strongly to environmental conditions such as temperature and food availability. In wild fish stocks, this results in median temporal variation of around 15% in length-at-age and 40% in weight-at-age, with extremes of 20% (length) and 60% (weight). Plasticity mediates environmental forcing and density-dependence in growth, both of which can have important implications for stock assessment and management. Failing to account for such effects can lead to substantial deviations (often more than 30%) in reconstructed stock dynamics, projections and reference points. The nature and magnitude of such deviations depends not only on the statistical adequacy of the growth model but on how growth information is used in the stock assessment process and on the management options being evaluated. In addition to having direct assessment and management consequences, plasticity provides a unified conceptual framework for interpreting various disparate and at times, confusing patterns of fish growth. Therefore, I conclude that the “constant growth” paradigm of fisheries stock assessments should be replaced with a paradigm that embraces growth plasticity and its consequences.

Age and growth of four spot megrim (Lepidorhombus boscii) in northern Iberian waters corroborated by cohort tracking

Age and growth are key biological aspects for the age-structured assessment of exploited fish populations. Confirming the consistency of fish age interpretation by validation/corroboration studies is essential for providing accurate age estimates to the stock assessment process. The Atlantic Iberian stock of four-spot-megrim (Lepidorhombus boscii), an important exploited demersal fish, is annually assessed by age-structured models in ICES, and specimens are routinely aged by IEO expert readers for this purpose. A total of 11,741 otoliths of this stock were aged from 23 annual groundfish surveys carried out in Cantabrian Sea and Galician waters (ICES Div. VIIIc, IXa) and following internationally standardized protocols. Annual age-length-keys were built using the respective age estimates and applied to the length distribution of each survey, thus obtaining a matrix of abundance indices by age and year. Similar mean lengths are obtained for each age group along most of the time-series. The von Bertalanffy growth parameters for the time-series are estimated (L∞: 49.0; k: 0.13; t0: −1.13) and the results are compared with previous studies. The annual growth pattern estimated for L. boscii is here corroborated by tracking cohort abundance indices in the surveys. The current otolith age estimation criterion used and the resulting age estimates obtained by the age readers can be considered valid. Strong and weak cohorts are well tracked along most of their age classes, and the correlations between age abundance indices are statistically significant up to the age 6, which represent around 95% of the total number of this species caught in the surveys. The findings here presented on the age of L. boscii and its corroboration can help to reduce the uncertainty in the estimation of its demographic structure and hence, ultimately, it will be important for a better understanding of its relevance in the functioning of the benthic community of the shelf.

Updated Maturity Estimates for Flatfishes (Pleuronectidae) in the Eastern Bering Sea, with Implications for Fisheries Management

AbstractFemale Yellowfin Sole Limanda aspera, Alaska Plaice Pleuronectes quadrituberculatus, and Flathead Sole Hippoglossoides elassodon were collected from the eastern Bering Sea during known prespawning, spawning, and postspawning periods in 2012 and 2013, and their ovaries and otoliths were sampled for use in histological analysis to update historical maturity estimates. For fisheries management, new information on maturity at age can lead to possible changes in estimated reproduction potential (measured as female spawning stock biomass [SSB]) and values of fishing mortality reference points. Our analysis indicated that Yellowfin Sole currently mature at an age similar to that estimated in a study conducted 20 years ago. An evaluation of impacts on the stock assessment indicated that updated estimates of Yellowfin Sole SSB were over 7% higher, but the reference points only changed slightly. The first histologically derived maturity estimates for Alaska Plaice were close to the anatomically derived estimates (visual assessments from 1987), resulting in a marginal decrease (5%) in SSB, but changes in reference points were near 10%. Based on the new maturity estimates for Flathead Sole, SSB estimates increased by 7% compared with estimates currently used in the stock assessment, which relied on maturity data collected in 1999 and 2000. The change in Flathead Sole SSB was concomitant with changes of 16–18% in fishing mortality reference points. Our results indicated minimal differences from historical maturity estimates after re‐examination, but in some cases those differences led to relatively large changes in the respective reference points, underscoring the reference points' sensitivity to changes in maturity. Incorporation of these new maturity estimates into the stock assessment process provides valuable updated information for fisheries managers. However, a more comprehensive sampling program is needed to investigate the spatial and temporal aspects of reproduction for each species.Received November 13, 2014; accepted September 3, 2015

Model averaging to streamline the stock assessment process

Abstract The current fish stock assessment process in Europe can be very resource- and time-intensive. The scientists involved require a very particular set of skills, acquired over their career, drawing from biology, ecology, statistics, mathematical modelling, oceanography, fishery policy, and computing. There is a particular focus on producing a single “best” stock assessment model, but as fishery science advances, there are clear needs to address a range of hypotheses and uncertainties, from large-scale issues such as climate change to specific ones, such as high observation error on young hake. Key to our discussion is the use of the assessment for all frameworks to translate hypotheses into models. We propose a change to the current stock assessment procedure, driven by the use of model averaging to address a range of plausible hypotheses, where increased collaboration between the varied disciplines within fishery science will result in more robust advice.

Thermal habitat index of many northwest Atlantic temperate species stays neutral under warming projected for 2030 but changes radically by 2060.

Global scale forecasts of range shifts in response to global warming have provided vital insight into predicted species redistribution. We build on that insight by examining whether local warming will affect habitat on spatiotemporal scales relevant to regional agencies. We used generalized additive models to quantify the realized habitat of 46 temperate/boreal marine species using 41+ years of survey data from 35°N–48°N in the Northwest Atlantic. We then estimated change in a “realized thermal habitat index” under short-term (2030) and long-term (2060) warming scenarios. Under the 2030 scenario, ∼10% of species will lose realized thermal habitat at the national scale (USA and Canada) but planktivores are expected to lose significantly in both countries which may result in indirect changes in their predators’ distribution. In contrast, by 2060 in Canada, the realized habitat of 76% of species will change (55% will lose, 21% will gain) while in the USA, the realized habitat of 85% of species will change (65% will lose, 20% will gain). If all else were held constant, the ecosystem is projected to change radically based on thermal habitat alone. The magnitude of the 2060 warming projection (∼1.5–3°C) was observed in 2012 affirming that research is needed on effects of extreme “weather” in addition to increasing mean temperature. Our approach can be used to aggregate at smaller spatial scales where temperate/boreal species are hypothesized to have a greater loss at ∼40°N. The uncertainty associated with climate change forecasts is large, yet resource management agencies still have to address climate change. How? Since many fishery agencies do not plan beyond 5 years, a logical way forward is to incorporate a “realized thermal habitat index” into the stock assessment process. Over time, decisions would be influenced by the amount of suitable thermal habitat, in concert with gradual or extreme warming.

The Baltic Sea: Estimates of total fisheries removals 1950–2007

The International Council for the Exploration of the Sea (ICES) reports to the general public fisheries ‘landings’ not ‘total removals’ for north-east Atlantic waters. Thus, public data do not include all anthropogenic removals, including discards. We reconstructed total removals for the nine countries (Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia and Sweden) fishing in the Baltic Sea by estimating their unreported removals. Estimated total removals of 53.5 million tonnes are 30% higher than the 41.3 million tonnes ICES reports publicly for 1950–2007. During 2000–2007, total removals are 35% higher than reported data suggest. The major components of unreported removals since 2000 were unreported landings, discards and recreational removals. Unreported landings were dominated by Poland (47%), Sweden (12%), Denmark (10%) and Latvia (8%), while discarding was dominated by Poland (21%), Denmark (19%), Finland (15%) and Sweden (14%). Finland (32%) and Sweden (30%) contributed the most to recreational removals. While ICES includes some estimates of unreported landings and discards in their stock assessment process, the lack of comprehensive and transparent public reporting on a society-owned resource presents a hindrance to accountability and ecosystem-based management.

Maximizing profits and conserving stocks in the Australian Northern Prawn Fishery

The Australian Northern Prawn Fishery (NPF) is one of the few that has adopted a dynamic version of a ‘maximum economic yield’ (MEY) target, and, on this basis, the fishery is undergoing a process of substantial stock rebuilding. This study details the bioeconomic model used to provide scientific management advice for the NPF, in terms of the amount of allowable total gear length in the fishery, for both the MEY target and the path to MEY. It combines the stock assessment process for two species of tiger prawns with a specification for discounted economic profits, where the harvest function in the profit equation is stock‐dependent. Results for the NPF show a substantial ‘stock effect’, indicating the importance of conserving fish stocks for profitability. MEY thus occurs at a stock size that is larger than that at maximum sustainable yield, leading to a ‘win‐win’ situation for both the industry (added profitability) and the environment (larger fish stocks and lower impact on the ecosystem). Sensitivity results emphasize this effect by showing that the MEY target is much more sensitive to changes in the price of prawns and the cost of fuel, and far less so to the rate of discount.

Red snapper management in the Gulf of Mexico: science- or faith-based?

The most controversial fishery in U.S. waters of the Gulf of Mexico (Gulf) is for northern red snapper Lutjanus campechanus, which collapsed in the late 1980s when stock biomass became too low to be fished commercially in the eastern Gulf. Red snapper management began in 1989; the stock is now showing signs of recovery. The Gulf of Mexico Fishery Management Council has been slow to sufficiently reduce catches of the directed fisheries to rebuild the stock in a timely fashion, although compliance with the Magnuson-Stevens Reauthorization Act of 2006 (MSRA) required substantial cuts in the harvest of red snapper beginning in 2007. In our opinion, this could have been avoided if conservative management practices had been adopted earlier. We believe that ‘faith-based fisheries’ arguments have been used to defer effective management of red snapper in the Gulf, which in turn has strained the relationship between science, management, and stakeholders there. We provide a simple empirical argument and alternate interpretations of a recently published perspective on the historical fishery of red snapper in the Gulf to conclude that the preponderance of evidence used in the agency stock assessment process, and the simple arguments made here, do not support the perspective that the red snapper stock has increased in size sufficiently to defer compliance with the MSRA.

Development of long-term larval indices for Atlantic herring (Clupea harengus) on the northeast US continental shelf

Abstract Richardson, D. E., Hare, J. A., Overholtz, W. J., and Johnson, D. L. 2010. Development of long-term larval indices for Atlantic herring (Clupea harengus) on the northeast US continental shelf. – ICES Journal of Marine Science, 67: 617–627. The incorporation of data from ichthyoplankton programmes into the stock assessment process has lagged far behind the use of data from comparable adult-monitoring programmes. This can in part be attributed to a mismatch between established analytical approaches to larval-index development and the inconsistencies in sampling for many long-term ichthyoplankton datasets. Along the northeast US continental shelf, ichthyoplankton surveys have been carried out by the Northeast Fisheries Science Center (NMFS/NOAA) multiple times annually since 1971, but the spatial and seasonal allocation of sampling has varied substantially. Here, we present a non-linear least-squares approach to larval-index development. We use the age structure and abundance of larvae on each survey to derive the larval index, along with parameters describing the survival of larvae and the seasonal cycle of hatching. Application to Atlantic herring (Clupea harengus) revealed a large drop in the index around 1976, an increase in the index through the late 1980s, and another large drop in 2004. This index was correlated with the stock assessment estimate of Atlantic herring spawning-stock biomass from 1971 to 2003, but differed substantially during 2004. Our results demonstrate that our approach to larval-index development is flexible to the inconsistencies in sampling effort.

Demographic and risk analyses applied to management and conservation of the blue shark (Prionace glauca) in the North Atlantic Ocean

Management and conservation of the North Atlantic blue shark are handicapped by a fishery-data-limited stock-assessment process. The status of this stock is ambiguous at a time when catch appears to be on the rise and new trade and exploitation patterns are emerging. This research provides fishery-independent demographic and risk analysis results for use in conservation and management. An age-structured matrix population model in which the vital rates are stochastic was constructed. A mean finite rate of population increase (λ) of 1.23 year–1 and a mean population doubling time (t2) of 3.08 years suggests that the blue shark is one of the most productive shark species. However, this concept of high productivity could be misleading because an elasticity analysis shows a strong dependence of the population growth rate on the survival of juveniles (0–4 years). An analysis of the risk that the harvested population will decline to levels below an assumed threshold of 50% of pre-exploited levels was conducted. The risk analysis is proposed as a supplement to the data-limited stock assessment, to better evaluate the probability that a given management strategy will put the population at risk of decline.

Using survey data independently from commercial data in stock assessment: an example using haddock in ICES Division VIa

Abstract Currently standard fish stock biomass estimates are based most directly on commercial catch-at-age data. The main contribution made by research-vessel trawl survey data to the stock assessment process is to “tune” trends in the commercial data and provide estimates of incoming year-class strength. In this process much of the information contained with the survey data (e.g. spatial detail) is lost because the data are first aggregated into numbers-at-age indices for given areas. Another problem is that increasingly restrictive total allowable catches (TACs) imposed on the fishing industry have led to what is suspected to be widespread misreporting, i.e. the scientists do not know how many fish have been landed. This leads to negative biases in the catch data, low stock abundance estimates by scientists, even lower TACs, followed by even more misreporting. One potential way to escape this downward spiral is to explore scientific trawl survey data in more detail since trawl surveys are more straightforward to regulate. Traditionally, there has been resistance to this idea since, in comparison to commercial catch-at-age data, trawl survey data are very sparse in space and time. In this study, the potential for using trawl survey data independently in stock assessments is explored for the case of ICES Area VIa haddock, using two different tools. Findings suggest that it is possible to get qualitatively useful information from trawl survey data alone as well as quantitative, spatially resolved, estimates of fish abundance by making simple swept-area assumptions. In addition, interesting differences between survey and commercial data are highlighted by the study. The mean age of fish reported by the commercial fleet, for example, is higher than that reflected by the survey data, while fishing mortality estimates tend to be higher when estimated from survey data alone.

A complex quota-managed fishery: science and management in Australia's South East Fishery. Introduction and overview

The stock assessment process in the South East Fishery involves scientists, industry, fishery managers, economists and non-government organizations. A comparison with such processes in other countries, where stakeholder involvement ranges from government scientists only to involvement of scientists, industry and conservation non-government organizations, suggests that Australia is the only country in which fishery managers are active and integral stock-assessment participants. In Australia, as in several other countries, the form of advice is comparative (consequences of alternative decisions) rather than prescriptive. Although all approaches have advantages and disadvantages, the South East Fishery process has advantages that appear to clearly outweigh the disadvantages. The advantages include better communication among interest groups, improved ownership of and hence support for outcomes and better interactions among groups. The disadvantages include the cost of the process, lack of consistency among assessments, vulnerability of scientists and the frustrations of industry.

History of Australia's South East Fishery: a scientist's perspective

The South East Fishery is one of Australia’s oldest fisheries. Early research on this trawl fishery centred on tiger flathead, the major target species. In the 1970s, the Federal Government actively encouraged fishery development and funded several trawl surveys. Profitable catch rates and optimistic assessments of resource size caused a rapid expansion of the trawl fleet. Separate jurisdictions hampered strategic approaches to SEF-wide research until 1978. Most SEF research in the mid 1960s to mid 1980s was conducted by State fisheries agencies, but federal involvement increased thereafter. A management shift in 1992 to Individual Transferable Quotas saw stock assessment become the major research priority. Industry involvement in the stock assessment process has increased markedly over the past decade and communication between scientists, managers and industry has improved. Predictive models are still limited by poor biological data for most quota species. Stock assessment uncertainty necessitates the increasing use of risk assessments and management strategy evaluation. Assessment scientists are frustrated by limited resources for research and the perceived reluctance of managers to adopt more precautionary approaches to uncertainty. Since ITQ management, fishing effort has risen significantly and concerns have been expressed about the stock status of several SEF species. Recent federal environmental legislation is now directing more attention towards effects of fishing and resource sustainability issues.

Approach and process for stock assessment in the South East Fishery, Australia: a perspective

The stock assessment process in the South East Fishery involves scientists, industry, fishery managers, economists and non-government organizations. A comparison with such processes in other countries, where stakeholder involvement ranges from government scientists only to involvement of scientists, industry and conservation non-government organizations, suggests that Australia is the only country in which fishery managers are active and integral stock-assessment participants. In Australia, as in several other countries, the form of advice is comparative (consequences of alternative decisions) rather than prescriptive. Although all approaches have advantages and disadvantages, the South East Fishery process has advantages that appear to clearly outweigh the disadvantages. The advantages include better communication among interest groups, improved ownership of and hence support for outcomes and better interactions among groups. The disadvantages include the cost of the process, lack of consistency among assessments, vulnerability of scientists and the frustrations of industry.

Incorporating experimentally derived estimates of survey trawl efficiency into the stock assessment process: a discussion

The efficiency (Q) of survey trawls can now be estimated using a variety of experimental techniques. In this paper, we review these techniques, then consider how the estimates of Q might best be used in stock assessment. Although catch-at-age models such as tuned VPA and Stock Synthesis can estimate the catchability (q) of a survey trawl when the required time series of catch and survey catch per unit effort (c.p.u.e.) data are sufficiently informative, if the time series are short, q estimates may be quite imprecise. In such cases, q in a catch-at-age model might be fixed at a value derived from an experimental estimate of Q. Furthermore, data series are sometimes so deficient that catch-at-age methods can not be used and fishery managers must turn to swept-area estimates of biomass. In these cases, experimentally estimated values of Q may allow less biased estimates of biomass than those now derived under the assumption of Q=1.0.

Application of the bomb radiocarbon chronometer to the validation of redfish Centroberyx affinis age

Validation of methods used to estimate fish age is a critical element of the fish stock assessment process. Despite the importance of validation, few procedures are available that provide unbiased estimates of true fish age and those methods that are available are seldom used. The majority of these methods are unlikely to provide an indication of the true age of individual fish, data that are best suited to the validation process. Accelerator mass spectrometry analyses of radiocarbon in selected regions of Centroberyx affinis otoliths were used to validate the age estimation method for this species. Radiocarbon data from the otoliths of C. affinis with presumed birth dates between 1955 and 1985 described the increase in ocean radiocarbon attributable to the atmospheric detonation of nuclear weapons in the 1950s and 1960s. The results confirm the longevity of C. affinis and demonstrate the effectiveness of the bomb radiocarbon chronometer for the validation of age-estimation methods.

Stock Assessment Problems in Atlantic Herring (Clupea harengus) in the Northwest Atlantic

Total annual catches of Atlantic herring (Clupea harengus) in NAFO statistical areas 3–6 have declined steadily since 1968, and have not rebounded after management measures were implemented in the late 1970's (unlike demersal species in the area). The accuracy of the biomass estimates generated by sequential population analysis is evaluated by a retrospective analysis for 3 of the 10 management units for herring fisheries. In each case, the stock assessments have tended to seriously underestimate the declines in stock abundance and overestimate the upswings. Fishing mortalities have exceeded F0.1, but they have rarely exceeded Fmax, the level below which growth overfishing is prevented. Three assumptions in the stock assessment process are evaluated in relation to the 4WX management unit: (1) the various management units are in fact self-sustaining populations; (2) catchability (i.e. fishing mortality per unit of effort) is constant and independent of stock size; and (3) the population is in the stable age distribution. We conclude that at least part of the management failures are due to invalid assumptions in the stock assessment models. An approach that has prevented growth overfishing may well have caused recruitment overfishing. Atlantic herring should be managed on principles suited to salmon rather than those applied to groundfish.