Estimates Of Fishing Mortality Research Articles

An integrated catch-at-age model for analyzing the variability in biomass of Pacific sardine (Sardinops sagax) from the Gulf of California, Mexico

Knowledge of the population dynamics and changes in the abundance of Pacific sardine fishery is relevant for fishery management. In this study, an integrated catch-at-age model (ASAP) was performed on 47 fishing seasons (1971−2018) ofSardinops sagaxin the Gulf of California, including five indices of relative abundance (survey catch rate, acoustic surveys, egg and larvae, spawning probability, and bird diet index). The ASAP model showed the importance of age group 0 (recruits); this is the most important age group in the Pacific sardine fishery, and its abundance has consequences for the variability of the total abundance of the species. In this study, the spawning and acoustic indices denoted the changes in abundance of Pacific sardine population better than the survey catch rate and bird diet ratio. Historically, the total biomass is estimated to have varied from 461 thousand to 4.82 million t. In the 1970s and 1980s, a peak biomass close to three million tons was reached, and in the second peak from the mid-1990s to the late 2000s, the total biomass varied between 3.26 and 4.82 million t. When the spawning biomass was at least 750 x 103t, the recruitment (age group 0) showed high values, promoting high catches in the subsequent years. The annual fishing mortality estimates did not show a rise related to the periods of high harvestable biomass; therefore, the catch records were low in comparison to the harvestable biomass estimated from ASAP, and the annual fishing mortalities were less than 0.22 during periods of high abundance. The changes in biomass and the variability observed in the Pacific sardine fishery were validated for the model diagnostics; the retrospective patterns did not identify poor fits to data, therefore the robustness of the model can be used for management purposes. According to the Kobe phase plot, the Pacific sardine fishery in the Gulf of California has been fully exploited with an apparent stable level of harvest.

How consistent is the advice from stock assessments? Empirical estimates of inter‐assessment bias and uncertainty for marine fish and invertebrate stocks

AbstractFishery management frequently involves precautionary buffering for scientific uncertainty. For example, a precautionary buffer that scales with scientific uncertainty is used to calculate the acceptable biological catch downward from the overfishing limit in the US federal fishery management system. However, there is little empirical guidance to suggest how large buffers for scientific uncertainty should be. One important component of uncertainty is variation among different assessments of the same stock in estimates of management‐relevant quantities. We analysed commercially exploited marine fish and invertebrate stocks around the world and developed Bayesian hierarchical models to quantify inter‐assessment variation in terminal year biomass and fishing mortality estimates, reference points, relative biomass and fishing mortality estimates, and overfishing limits. There was little evidence of inter‐assessment bias; stock assessment estimates in the terminal year of the assessment were not consistently higher or lower than estimates of the same quantities in future years. However, there was a tendency for extreme values from the terminal year to be pulled closer to the mean in future years. Inter‐assessment variation in all estimates differed across regions, and a longer inter‐assessment interval generally resulted in greater variation. Inter‐assessment uncertainty was greatest for estimates of the overfishing limit, with coefficients of variation ranging from 17% in Europe (non‐EU) to 107% for Pacific Ocean pelagic stocks. Because inter‐assessment variation is only one component of scientific uncertainty, we suggest that these uncertainty estimates may provide a basis for determining the minimum size of precautionary buffers.

Geographical variation in age and growth of the endemic Australian sciaenid Atractoscion atelodus.

Growth rates and other age-related population characteristics are essential parameters underpinning management of a stock. This is the first study to estimate length-at-age of Atractoscion atelodus (family Sciaenidae) in New South Wales (NSW), despite the species being exploited since the 1940s. The aim of the current study was to quantify the age-based biological characteristics of A. atelodus and in particular: (a) validate the use of sagittal otoliths to quantify age; (b) estimate the growth rates and longevity of males and females; (c) examine geographical variation in age and growth; (d) document the age composition in commercial landings and (e) estimate mortality rates. A. atelodus is relatively fast growing, reaching 40 cm fork length (LF ) in the first year of life and living to at least 14 years. Growth was significantly different between sexes, with modelled asymptotic lengths of females (L∞ =84.6 cm LF ) greater than males (L∞ =69.0 cm LF ). Growth rates were also significantly different between northern and southern populations. Fish sampled from the southern region were younger and faster growing than those sampled in the northern region, supporting the counter-gradient growth theory and the influence of upwelling providing greater nutrient and food availability. The commercial fishery was predominantly based on young fish <3 years. with few (c. 5%) greater than 5 years. Fishing mortality estimates (F =0.42) were similar to natural mortality estimates (M =0.44). The age-based parameters estimated in the present study suggest that A. atelodus should be relatively resilient to fishing; nonetheless, the sizes and ages in landings are indicative of a stock heavily fished. The fishery is experiencing truncated age distributions and appears to be largely recruitment driven, increasing susceptibility to overexploitation. Determining geographical differences in growth rates of a population has important implications when considering impacts of anthropogenic drivers such as global warming and overexploitation and is important to determine in managing exploited fish populations.

Estimating mortality for the assessment of a small-scale fishery: Lane snapper (Lutjanus synagris) in Honduras

Gillnets are very common in small-scale fisheries. However, data obtained using gillnets are biased because of their size selectivity, making accurate estimation of mortality difficult. This study applied various methods to estimate total, natural, and fishing mortality, using data for Lane Snapper (Lutjanus synagris) collected in Tela, Honduras. Total mortality was estimated using a catch-curve analysis with and without accounting for size-selectivity using fitted selectivity curves, the Chapman and Robson method, and Heincke’s method. Gillnet selectivity of Lane Snapper was best characterized by a skewed normal selectivity curve. The catch-curve method in the absence of correction for gear selectivity produced greater total mortality estimates than the Chapman and Robson and Heincke’s methods, with substantial variation among the three methods. Natural mortality estimates were consistent among five methods: Lorenzen, Peterson and Wroblewski, Sekharan, “FishLife” R package and Pauly’s method. Consequently, variation among fishing mortality estimates was primarily the result of variation in estimates of total mortality. Our results suggest that accounting for gear selectivity is critically important when assessing and managing small-scale fisheries.

Influence of Growth and Recruitment Parameters in the Assessment and Management Variables of the Yellow Squat Lobster (Cervimunida johni)

Fitting length data in age-structured stock assessment is a common method for evaluating hard-to-age animals, such as crustaceans. Growth specification and the uncertainty in the stock recruitment relationship are key issues in length-based assessment models. We conducted sensitivity analyses to evaluate the impact of growth and recruitment parameters on the stock assessment and management variables of the yellow squat lobster (Cervimunida johni) caught off the Chilean coast. Nine different scenarios of the length at first capture ( L a = 1 ) and the coefficient of variation at age ( c v a ) were tested for six combinations of values for the steepness parameter (h) and the recruitment variance ( σ R 2 ). We also investigated the reliability of these estimates using an operating model. Our findings indicate that the parameter related to growth, L a = 1 , has the greatest impact on the assessment and management variables of this fishery resource, with c v a having a lesser effect. Recruitment and fishing mortality estimates were the main variables affected. Parameters h and σ R 2 did not profoundly impact the variables assessed. In addition, L a = 1 was the most biased estimated parameter. We discuss that the high influence of growth parameters is related to model structure, and thus implications for determination of the status of yellow squat lobster should be addressed in the future. We recommended developing simulation protocols for the selection of growth parameters when using an age-structured model with length observations, and we believe that our findings are relevant for all Chilean fisheries with a similar stock assessment framework.

Estimates of Red Drum Mortality via Acoustic Telemetry

AbstractSubadult (age &lt; 3) Red Drum Sciaenops ocellatus support a valuable recreational fishery, and mortality estimates for young Red Drum are needed for proper management. To obtain these estimates, age‐1 and age‐2 Red Drum were implanted with acoustic transmitters and external Floy tags in two coastal Alabama rivers (Fowl and Dog rivers). Fates of tagged fish were inferred from stationary receiver detections and active relocations over 1 year. These fates were used in a Bayesian multistate model to estimate instantaneous monthly and annual mortality and emigration rates for each river and overall from both rivers. Instantaneous monthly fishing mortality (F) ranged from 0.001 to 0.112 (annual F = 0.414) in Dog River, from 0.001 to 0.126 in Fowl River (annual F = 0.309), and was 0.001–0.054 (annual F = 0.337) overall. Instantaneous monthly natural mortality (M) ranged from 0.001 to 0.002 (annual M = 0.069) in Dog River, from 0.001 to 0.036 (annual M = 0.178) in Fowl River, and from 0.001 to 0.017 (annual M = 0.090) overall. The overall annual estimate of instantaneous total mortality (Z) was 0.435. The median escapement percentage was estimated at 36.3% (95% posterior credible interval = 19.5–56.0%) using M and Z from the overall model. Unfortunately, the error on this estimate was large and inconclusive as to whether the 30% escapement goal for juvenile Red Drum to the adult population from Dog and Fowl rivers is being met. Monthly residency estimates were typically greater than 0.90, and overall annual residency was estimated at 0.716. Fishing mortality estimates from the current study are higher than recent catch curve estimates that did not include young Red Drum. These results demonstrate that young Red Drum need to be accounted for when generating mortality estimates and provide needed data for the Red Drum recreational fishery.

Parameter estimation using randomized phases in an integrated assessment model for Antarctic krill.

An integrated model assessing the status and productivity of Antarctic krill (Euphausia superba, hereafter krill) was configured to estimate different subsets of 118 potentially estimable parameters in alternative configurations. We fixed the parameters that were not estimated in any given configuration at pre-specified values. The model was fitted to over forty years of fisheries and survey data for krill in Subarea 48.1, a statistical reporting area around the Antarctic Peninsula used by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). The number of estimated parameters was gradually increased across model configurations. Configurations that estimated more parameters fitted the data better, but the order in which the parameters were estimated became more important in finding the best fit. Twenty-two configurations estimating from 48 to 107 parameters were able to obtain an invertible Hessian matrix that was subsequently used to estimate parameter uncertainty. Parameter uncertainties calculated using asymptotic approximation around the maximum likelihood estimates were often larger than uncertainties based on Markov chain Monte Carlo sampling for the same parameters. Diagnostics applied to MCMC samples in the best model of each configuration that obtained an invertible Hessian indicated that the most highly parameterized configurations did not reach stationary distributions. A 96-parameter configuration was the best fitting model of those that passed the MCMC diagnostics. The ΔAIC and ΔBIC scores indicated essentially no support relative to the best model for the alternative models that also passed MCMC diagnostics. Simulated data using the configurations as operating models showed that while all configurations passed "self-tests" for spawning biomass and recruitment, there was a small negative bias due to model penalties in the fishing mortality estimates for years with the highest fishing mortalities. "Cross-tests" of configurations that estimated different parameters often differed from the operating model values.

Towards a spatial integrated stock assessment model for European hake northern stock

European hake (Merluccius merluccius) is a key species in the management of several mixed fisheries in the north-east Atlantic where regional spatial management plans are being set up. The wide spatial distribution of hake and the lack of knowledge of some processes involved in its spatial dynamics could hamper the assessment and management of hake.To help understand the processes, we implemented a spatial version of the Stock Synthesis stock assessment model for northern hake, including available data on this stock and the associated fisheries. For the 1978–2012 time series, the model distinguished the Bay of Biscay, the Celtic Sea and a northern area aggregating the West of Scotland and the North Sea. The model considered an age-structured population for each age class and area, a seasonal migration, global recruitment allocated to areas and fishing fleets for each area. Parameters for stock and exploitation spatio-temporal dynamics were estimated by likelihood maximization for each quarter and area.The comparison of spatial and non spatial models goodness of fit showed they were close, although the fit to abundance indices slightly improved with the spatial model.Fishing mortalities, spawning stock biomasses and recruitments were slightly different from the current assessment model estimates, essentially at the beginning and the end of the time series. On average over the time series, fishing mortality estimates relative to the abundance were higher in the Bay of Biscay than elsewhere, spawning stock biomass estimates were higher in the Celtic Sea than elsewhere, and recruitment occurred mainly in the Bay of Biscay. Finally, the final spatial model estimated a strong movement from the northern area to the Celtic Sea. The final spatial model is a step towards a spatial assessment and management of the stock.

Grey seal predation mortality on three depleted stocks in the West of Scotland: What are the implications for stock assessments?

The decrease in groundfish stocks in the North Atlantic since the mid-1900s coupled with increases in grey seal (Halichoerus grypus) populations is responsible for an enduring controversy between fishers and conservationists regarding the role seals have played in stock declines. We used a Bayesian state-space model to investigate stock trends in the presence of grey seals and associated maximum sustainable yield (MSY) reference points in the West of Scotland. This study provides new estimates of seal predation mortality on haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) and updates the estimates for cod (Gadus morhua), which together form the traditional main components of the mixed demersal fishery in this area. Grey seal predation mortality is greatest on cod, resulting in estimates of total natural mortality higher than those used in the current ICES assessments. Seal predation mortality is low for haddock and whiting. Considering seal predation in stock assessments changes the scale of biomass and fishing mortality estimates for the three stocks. The estimates of F0.1and FMSYare sensitive to seal predation for cod and whiting but not for haddock. In all cases, MSY decreases with increased seal predation.

Statistical Catch‐Free Age‐Structured Assessment to Estimate Year‐Class Strength for Inland Recreational Fisheries: A Largemouth Bass Example

AbstractStatistical catch‐at‐age analysis (SCAA) is commonly used to estimate recruitment and mortality for large‐scale, economically important fish stocks, but fishery‐dependent catch‐at‐age data are often unavailable for smaller‐scale inland fisheries, thus limiting the use of SCAA. In these systems, a catch‐free approach to age‐structure modeling may be useful for estimating recruitment and mortality rates solely from a time series of survey age composition and catch rate data by estimating recruitment as a relative index of year‐class strength (Rt). Here, we demonstrate a statistical catch‐free age‐structured assessment (SCFASA) model for small‐scale inland fish stocks as an alternative to traditional catch curve approaches by using Largemouth Bass Micropterus salmoides as an example. We fitted the SCFASA to fishery‐independent survey age composition and catch‐per‐effort data from Largemouth Bass stocks in three Alabama reservoirs to estimate Rt, annual fishing mortality rates, and age‐specific electrofishing survey vulnerability; we compared Rt to catch curve residual estimates and performed a sensitivity analysis under a variety of model assumptions. Model estimates of Rt and temporal trends in fishing mortality were robust to changes in assumptions about the effective sample size of multinomial age samples, natural mortality rate, and temporal variability in fishing mortality. However, the magnitude of instantaneous fishing mortality rates at two of the three reservoirs was somewhat higher than empirical estimates for similar stocks and was sensitive to model assumptions. Our application of the SCFASA to survey data sets routinely collected by management agencies for Largemouth Bass potentially provides a comprehensive approach to estimating Rt, mortality, and sampling vulnerability for these stocks without increasing sampling effort. Future investigations of SCFASA's utility for small‐scale inland stocks should address the potential for bias in the magnitude of fishing mortality estimates due to incorrectly specified vulnerability schedules.Received October 5, 2015; accepted September 19, 2016 Published online December 21, 2016

Estimating Regional Fishing Mortality for Freshwater Systems: a Florida Largemouth Bass Example

AbstractSpecies‐specific harvest regulations in recreational fisheries are commonly applied regionally to protect stocks from overharvest and satisfy a diverse set of anglers. While setting regulations is a complex task and may incorporate the best available social and biological information, fisheries managers commonly obtain directed fishing mortality estimates within a single lake and then assume similar rates among other systems when setting regional harvest regulations. Thus, there is a need to assess regional levels of fishing mortality for informed use of regionally applied regulations. We implemented a practical method for assessing catch and harvest for a recreational fishery across a broad spatial region. We used a passive tag‐reward study design and a regional management regulation area for Florida Largemouth Bass Micropterus salmoides floridanus in central Florida as our case study. The estimated fishing mortality rate included both harvest and deaths due to catch and release. We found overall regional fishing mortality for Florida Largemouth Bass in central Florida was relatively low. From the 247 dart tags returned, the mean annual instantaneous total fishing mortality rate was 0.11 (95% credible interval = 0.08–0.15). We also found fishing mortality rates did not vary with lake size or fish total length. Our study design did not provide mortality estimates for any specific lake due to a low number of tagged fish per lake, but the method could be used to elucidate the effectiveness of regulations that are applied at a regional scale.Received November 20, 2014; accepted March 31, 2015

Recent experience with effort management in Europe: Implications for mixed fisheries

This study addresses a common assumption in fisheries science: that partial fishing mortality is directly proportional to fishing effort. It is important to challenge this a priori sensible assumption, as it is also built into many models and tools used by the International Council for the Exploration of the Sea (ICES) to help provide advice and conserve fish stocks. Here we test this assumption at scales relevant to management using catch and effort data from the EC Scientific, Technical and Economic Committee for Fisheries (STECF), together with mean fishing mortality estimates from ICES. We focus on the eight predominant fishing gear types in the North Sea, and for each gear, test how effort is related to partial fishing mortality exerted by each gear on their main target species. For many gear types, the relationships found were not inconsistent with the assumption being tested, although in many cases there was also no significant pattern between fishing mortality and effort. Fishing mortality-effort relationships can be complex, as exemplified by the frequent lack of significant relationships in mixed demersal fisheries. The relationship between fishing mortality and effort can also vary over time, particularly in response to changes in catchability, producing patterns that can be difficult to uncover. These factors may significantly affect the impact management strategies may have on stocks and fleets. As tools are developed to provide mixed fisheries advice, we encourage efforts that help understand the implications of assumptions made for management strategies.

Simulated mark-recovery for spatial assessment of a spiny lobster (Panulirus argus) fishery

Marine reserves are becoming widely implemented along with conventional fisheries controls as integrated approaches to fisheries management. The restricted spatial distribution of fishing effort, relative to the spatial distribution of fish stocks that may be partially protected by marine reserves, often necessitates spatial considerations in the design of monitoring and stock assessment. Simulation modeling was used to evaluate whether a mark-recovery design could be used to accurately estimate fishing mortality rates without information about movement rates being available to the assessment procedure. A spatially-explicit individual-based simulation was developed with environmental characteristics of Glover's Reef Marine Reserve, Belize and with biological characteristics of a fished population of Caribbean spiny lobster (Panulirus argus). Accuracy of fishing mortality estimates depended on whether these estimates were calculated for the fished area only or for the entire stock. Stock-wide fishing mortality estimates could usually be obtained that were robust to uncertainty about dispersive movement. We discuss results in the context of managing fisheries based on the status of fished areas alone or on the entire stock and discuss the necessity for information about fish movement for accurate assessment of stocks managed using marine reserves.

Implications of stock recovery for a neighbouring management unit: experience from the Baltic cod

Cod in the Baltic Sea is assessed and managed as two separate stocks, i.e. eastern and western Baltic cod. The eastern Baltic cod has recently started to recover after several decades of severe depletion. In the present study, we suggest that the recovery of the eastern Baltic cod population has also substantially increased cod abundance in a specific area of the adjacent western Baltic management unit. This is investigated through long time-series of spatially resolved stock assessment data supplemented by genetic analyses of origin of the cod currently found in the transition area between the two populations. Due to immigrating cod from the east, there are currently large spatial differences in cod abundance and mean weight in the western Baltic management unit that raise new management concerns. First, the high abundance of cod of eastern origin found in the western Baltic management unit can mask the relatively poor state of the western Baltic cod population. Second, the current fishing mortality estimates for the entire western Baltic management unit, used as basis for fisheries management, are difficult to interpret as these are highly influenced by mixing of biological populations and the spatial distribution of fisheries.

Rebuilding fish communities: the ghost of fisheries past and the virtue of patience

The ecosystem approach to management requires the status of individual species to be considered in a community context. We conducted a comparative ecosystem analysis of the Georges Bank and North Sea fish communities to determine the extent to which biological diversity is restored when fishing pressure is reduced. First, fishing mortality estimates were combined to quantify the community-level intensity and selectivity of fishing pressure. Second, standardized bottom-trawl survey data were used to investigate the temporal trends in community metrics. Third, a size-based, multispecies model (LeMans) was simulated to test the response of community metrics to both hypothetical and observed changes in fishing pressure in the two communities. These temperate North Atlantic fish communities have much in common, including a history of overfishing. In recent decades fishing pressure has been reduced, and some species have started to rebuild. The Georges Bank fishery has been more selective, and fishing pressure was reduced sooner. The two communities have similar levels of size diversity and biomass per unit area, but fundamentally different community structure. The North Sea is dominated by smaller species and has lower evenness than Georges Bank. These fundamental differences in community structure are not explained by recent fishing patterns. The multispecies model was able to predict the observed changes in community metrics better on Georges Bank, where rebuilding is more apparent than in the North Sea. Model simulations predicted hysteresis in rebuilding community metrics toward their unfished levels, particularly in the North Sea. Species in the community rebuild at different rates, with smaller prey species outpacing their large predators and overshooting their pre-exploitation abundances. This indirect effect of predator release delays the rebuilding of community structure and biodiversity. Therefore community rebuilding is not just the sum of single-species rebuilding plans. Management strategies that account for interspecific interactions will be needed to restore biodiversity and community structure.

Short-term post-release mortality of skates (family Rajidae) discarded in a western North Atlantic commercial otter trawl fishery

Due to market and regulatory factors, Rajidae skates are routinely discarded by commercial otter trawlers in the western North Atlantic. Accounting for post-release mortality is therefore essential to total fishing mortality estimates, stock status and management of this group of fishes. However, despite a presumed species-specific range in tolerance, few studies have investigated the short-term post-release mortality among skates indigenous to the western North Atlantic following capture by mobile fishing gears, and never in the Gulf of Maine. This study addresses this shortfall for the prohibited thorny skate, Amblyraja radiate and smooth skate, Malacoraja senta, and the targeted winter skate, Leucoraja ocellata, and little skate, Leucoraja erinacea. Of 1288 skates evaluated, negligible immediate mortality was observed at the time of capture, even in relation to the largest catches and/or most prolonged tows. However, injury frequency was moderate, with highest levels in the smooth (60%) and thorny (52%) skates. Aside from the smooth skate (59%), 72h mortality rates were low overall (19% across all species when accounting tow durations indicative of the fishery), with the winter skate (8%) exhibiting the lowest levels. Logistic regression modeling revealed tow duration as the most universal predictor of condition and 72h mortality, while catch biomass, sex, temperature changes, and animal size also held influence in certain species. Although in general the studied species appear more resilient to trawl capture and handling than previously estimated, interspecific differences must be accounted for when managing this group.

Using electronic tag data to improve mortality and movement estimates in a tag-based spatial fisheries assessment model

Despite increased deployment of archival tags on exploited fish species, analytical methods for including archival tag data in fishery assessment models are lacking. We present a method for integrating archival tag data into a spatial tag–recapture model for estimating natural mortality, fishing mortality, abundance, and movement. Archival tags provide important information on fish movement not available from conventional tags that facilitates separation of movement from mortality estimates. Using simulations, we evaluate the benefit of including archival tag data in the model using two model formulations: one with a general spatial structure and one with movement and fishery dynamics based on juvenile southern bluefin tuna (SBT; Thunnus maccoyii ). If fish are not tagged in all regions and time periods, then including archival tag data can substantially improve the precision of the fishing mortality and movement estimates. For example, with the general spatial structure and specific scenario presented, standard errors of the fishing mortality estimates decreased by an average of 34% (21%) when 25 archival tags were released in addition to 500 (2500) conventional tags in each region and period of tagging, respectively. Furthermore, with the SBT spatial structure, archival tag data were necessary for all parameters to be estimable.

SPAM (Sex-Structured Pandalus Assessment Model): a stock assessment model for Pandalus stocks

Despite the economic importance of Pandalus shrimp fisheries, few analytical tools have been developed to assess their stocks, and traditional stock assessment models are not appropriate because of biological specificities of Pandalus species. In this context, we propose SPAM (Sex-Structured Pandalus Assessment Model), a model dedicated to protandric hermaphrodite pandalids stock assessment. Pandalids are difficult to assess because the cues affecting sex change, size at recruitment, and mortality variability are not well understood or characterized. The novel structure of the model makes it possible to adequately describe variability in natural mortality by stage and in time, as well as variability in size at sex change and recruitment. The model provides traditional stock assessment outputs, such as fishing mortality estimates and numbers of individuals, and provides in addition yearly natural mortality estimates. The model is applied to the exploited shrimp stock of Pandalus borealis in Sept-Îles (Québec, Canada) as an illustrative example of the utility of the approach.

Size-Selectivity of the Northwest Atlantic Sea Scallop (Placopecten magellanicus) Dredge

A size-selectivity curve was constructed to characterize the performance of the New Bedford style Atlantic sea scallop (Placopecten magellanicus, Gmelin 1791) dredge when it is configured to meet the requirements of Amendment #10 to the Sea Scallop Fishery Management Plan. The curve was generated using the SELECT model on catch-at-length data, obtained by simultaneously towing a New Bedford style dredge and a nonselective National Marine Fisheries Service sea scallop survey dredge from commercial scallop vessels. Data were collected during three cruises in the Northwest Atlantic between 2005 and 2006. The resultant selectivity curve yielded a 50% retention length of 100.1 mm, a selection range of 23.6 mm, and a value of 0.77 for the efficiency of the commercial dredge relative to the survey dredge. A length of 100.1 mm corresponds to a meat weight of 17.2 g in Georges Bank and 16.8 g in the mid-Atlantic. These results can assist fisheries managers with stock assessments, fishing mortality estimates, and the interpretation of catch data from resource surveys. Additionally, the curve can be used as a baseline to evaluate the effect of future changes to sea scallop dredge configuration.

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.