Stock Assessment Research Articles

Accounting intra-annual variability in mortality and recruitment in stock status evaluation of Parapenaeopsis stylifera fishery of north-eastern Arabian Sea under moderate data scenario

Length-based integrated mixed effect (LIME) approach was applied for the assessment of Parapenaeopsis stylifera stock, the most prominent penaeid resource along Gujarat coast. Month was selected as a temporal step size to improve estimate precision and capture intra-annual variation in population dynamics. Catch was provided as an additional input to create a moderate data scenario and reduce bias in estimates of reference indices. The values of key life history priors Linf, K, M, t0, Lm50, Lm95 were 160.59 mm total length (TL), 1.68 yr−1, 2.406 yr−1, −0.01 yr, 89.1 mm TL, 95.06 mm TL, respectively. The LWR was established as W = 0.0117*TL2.652. The estimated SPR ranged between 0.33 and 0.70. The monthly fishing mortality was estimated between 0.06 and 0.31. In 37.5% of the observed month, the fishing mortality was above optimum, which by and large coincides with the peak fishing months. The recruitment is continuous, with a coefficient of variation of only 18.5%. Based on these indices, the stock can be considered as healthy or sustainable. A continuous monitoring and collection of abundance data through experimental surveys are recommended as a future course of action.

Stock assessment and fishing management of green abalone (Haliotis fulgens) along the West Coast of Baja California Sur, Mexico

The green abalone fishery in Mexico has showed declines in the availability of the resource, but the current state of the population is unknown. The aim of this study was to analyze inter-annual variation in recruitment, mortality, total and vulnerable biomass, selectivity, and harvest rate of Haliotis fulgens. A size-structured model was used to assess changes in the population dynamics and management quantities from 2010 to 2019. The model describes the exploitation and growth of individuals in the population, and incorporates a stochastic growth matrix that defines the variation in the length increments of individual to individual. Key results indicated that the recruitment failures occurred in infrequent pulses of varying intensity. Moreover, a slight recovery of total biomass during 2017 (791 t), 2018 (1496 t), and 2019 (1274 t) was observed after a slight increase in recruitment and a decrease in harvest rate. The vulnerable biomass increased 44.38% from 2010 to 2019. The harvest rate revealed that individuals larger than the minimum legal size of 145 mm were under high fishing pressure. The stock assessment model highlights an adverse effect in the population; given the deteriorating conditions of the green abalone fishery, it is imperative to transition fully to a recovery strategy, including all the administrative zones in the assessment and management. The adoption of proactive strategies such as the establishment of reference points, no-fishing reserves, and the implementation of a farming system would facilitate the protection of critical abalone populations, contributing to their medium-term recovery, and would be an economic alternative for local fishers.

Good practices for surplus production models

Surplus production models (SPMs) have a long history in fisheries ecology and are important for the assessment and management of marine species and stocks. However, the implementation, application, and usage of these models vary across regions and case studies. Good practice guidelines can streamline modelling workflows, inform the acceptance or rejection of an assessment, and facilitate the derivation of management advice from accepted assessments. This paper discusses current practices in the application of SPMs and proposes good practice guidelines for their use in stock assessment. We complement our recommendations with results from a simulation study examining the performance of an age-based operating model and a SPM assessment model under 60 scenarios with various assumptions regarding data quantity, quality, and model priors. We provide specific good practice guidelines for two widely used state-space SPMs: SPiCT and JABBA. Finally, we discuss current limitations and suggest avenues for future developments for SPMs.

Modeling Geospatial Distribution of Peat Layer Thickness Using Machine Learning and Aerial Laser Scanning Data

Organic horizons including peat deposits are important terrestrial carbon pools, and various chemical, biological, and water exchange processes take place within them. Accurate information on the spatial distribution of organic soils and their properties is important for decision-making and land management. In this study, we present a machine learning approach for mapping the distribution of organic soils and determining the thickness of the peat layer using more than 24,000 peat layer thickness measurements obtained from field data, airborne laser scanning (ALS) data and various indices obtained from therein, as well as other cartographic materials. Our objectives encompassed two primary aims. Firstly, we endeavored to develop updated cartographic materials depicting the spatial distribution of peat layers. Secondly, we aimed to predict the depth of peat layers, thereby enhancing our understanding of soil organic carbon content. Continentality, a wet area map, latitude, a depth to water map with catchment area of 10 ha, and a digital elevation model were the most important covariates for the machine learning model. As a result, we obtained a map with three peat layer thickness classes, an overall classification accuracy of 0.88, and a kappa value of 0.74. This research contributes to a better understanding of organic soil dynamics and facilitates improved assessments of soil organic carbon stocks.

Built‐environment stocks in the context of a master‐planned city: A case study of Chandigarh, India

AbstractConstruction materials accumulate in the built environment forming material stocks (MS) of buildings and infrastructure, providing various services to society that result in a nexus of human development and environmental impact. Meanwhile, unprecedented urbanization in the Global South is set to put significant demand on the resources required to ensure adequate standards of living in new and existing urban areas. This is particularly important within India; however, no study has yet explored MS within cities in India or within master‐planned urban areas designed to accommodate urbanization and a high standard of living. The present study begins to fill these gaps and aims to investigate patterns of built‐environment MS accumulation in Chandigarh, an exceptionally quickly developed city master planned to ensure universally high standards of living through a unique urban form. We adopt a bottom‐up approach to quantify the residential building and road MS at the city and sub‐city scale. The results reveal that the master plan, while enabling high standards of living, has resulted in a relative accumulation of road‐to‐building stock that is significantly larger than in other cities. This is shown to be environmentally detrimental as future urban development is limited and promotes the demolition of existing stocks, whose composition severely limits their potential as secondary resources. The study therefore provides empirical evidence to support the integration of material stock assessments into urban planning and development to ensure resource‐efficient provisioning of key services.

Age, Growth, and Mortality of Pontic Shad, Alosa immaculata Bennett, 1835, in the Danube River, Romania

This study aimed to evaluate the growth, mortality parameters, and exploitation rate of Pontic shad, Alosa immaculata Benett, 1835, in the Danube River, Romania (km 169–197). The sampling collection started with the first signs of Pontic shad migration, on 4 March 2023, and continued weekly until the beginning of June 2023, when the last specimens were caught in the nets. The estimation of the growth, mortality parameters, and exploitation rate was done in FiSAT (FAO-ICLARM Stock Assessment Tools). The von Bertalanffy growth equation was estimated at L∞ = 36.75 cm, the growth coefficient was k = 0.68 year−1, and the theoretical initial age was t0 = −0.67 year−1. The total mortality rate (Z) estimated was 2.76 year−1, with a natural mortality rate (M) and fishing mortality rate (F) of 0.89 year−1 and 1.87 year−1, respectively. The Z/k ratio was found to be 4.11 and the exploitation rate (E) was estimated at 0.68 year−1, indicating the overexploitation of Alosa immaculata stocks. In conclusion, this study provides valuable insights into the population dynamics of Pontic shad in the Danube River, Romania. The assessments of the growth parameters, mortality rates, and exploitation rates highlight a level of overexploitation of Alosa immaculata stocks. These findings underscore the importance of applying effective fishery management strategies to ensure the sustainability and conservation of this valuable fish species in the Danube River ecosystem.

Stock assessment and reproductive biology of bluespotted seabream Pagrus caeruleostictus (family: Sparidae)

AbstractBluespotted seabream Pagrus caeruleostictus, a member of the family Sparidae, is one of the high‐priced seabream species in Ghana. Although the fish has been exploited in the Gulf of Guinea for over hundreds of years, there is scanty information on the state of the fish stock. Hence, we sought to investigate the exploitation status and aspects of the reproductive biology of the species by collecting monthly length and weight data for 18 months (February 2016 to July 2017). These data were fitted to growth and mortality models. The sexual maturity, fecundity and spawning seasons were also determined within the same period. The growth model of the fish revealed that the asymptotic length (L∞) and growth coefficient (K) values determined were 52.7 cm (total length) and 0.52 year−1, respectively. The mortality models also showed that the fishing mortality (F) of the fish was 2.20 year−1, whereas its natural mortality (M) was 0.83 year−1, with F/M > 0.5 indicating that the fish is over‐exploited. The lengths at first sexual maturity (Lm) of the males and females were 36.2 and 28.0 cm, respectively. Pagrus caeruleostictus, which was found to be highly fecund, had two spawning periods in a year, occurring in September and March, both with unimodal ova diameter frequency distributions. Based on these findings, we recommend that catch limits and other appropriate management measures (e.g. temporal and spatial closures) should be prescribed for the seabream fishery in the Gulf of Guinea to ensure the sustainability of the fishery.

Asymmetrical individual both left and right otoliths of Tuna Scombrini collected from the Java Sea

Otolith can be used as a natural marker of fish stock or anomalies between individuals within fish stock. This research aims to study the relationships between fish length and otolith length and confirm differences in otolith shape between species in the Scombrini tribe and Carangidae family (Decapterus ruselli). Sample used in our analysis were collected around July - September 2023 from three different location, Rastrelliger brachysoma (Lekok waters, Pasuruan), Scomber australasicus (Bali Strait waters) and Decapterus ruselli (Sendang Biru, South Malang). Total 96 specimens were identified based on morphological character as Rastrelliger brachysoma, Scomber australasicus and Decapterus ruselli. Each of following samples were cut horizontally at the frontal head to open the skull and remove sagitta otolith below the hindbrain using dissecting set, then cleaned and stored dry in microtube 1,5 ml separately for left and right to further documented using Celestron Digital Microscope Pro Gray USB 44308. Otolith morphometric data collected includes maximum length using TpsDig and weight using analytical balance. The relationship between Forked Length (FL) and otolith length, length-weight relationship of otolith was determined using simple linear regression on software Statistical Product and Service Solutions (SPSS). Paired sample t-test was applied to determine asymmetrical size of left and right otoliths. In this study, it was found that the shape of the otoliths between species has differences in the rostrum, antirostrum, ventral and dorsal parts. FL-otolith length relationship of S. australasicus, R. brachysoma and D. ruselli are Y=0,725+0,164X, r2= 0,519; Y=0,191+0,230X, r2= 0,911; Y=3,156+0,075X, r2=0,071 respectively. The LW sagitta relationship shows that there is a very strong relationship (P<0,05) between L and W of otoliths. The results of the Paired Sample T-test showed that there was no significant difference in the size of the left and right otoliths (Sig>0.05). Otolith shape is influenced by fish behaviour, diet and environment and it may be used as additional information to distinguish between tribe or even species. Otherwise, its asymmetric relationship and LW relationship may be potential new methods for stock discrimination and have other implications such as stock assessment, understanding natural asymmetry, and its potential applications in fisheries management and conservation.

Modeling potential consequences of climate‐driven sex reversal on Southern and Summer Flounder population dynamics

AbstractObjectiveWe investigate the interacting effects of fishery selectivity and sexual dimorphism by using the fisheries for Southern Flounder Paralichthys lethostigma and Summer Flounder P. dentatus as case studies. In both species, females are larger than males, and temperature‐sensitive sex determination can lead to an increased proportion of males at sexual differentiation in warmer temperatures.MethodsWe developed a size‐, age‐, and sex‐structured population dynamics model for each species to understand how temperature‐induced sex reversal will interact with harvesting to impact population productivity. We considered scenarios with realistic sex ratios and varying levels of fishing pressure.ResultOur models of these species show that population egg production, abundance, biomass, and catch will all be constrained if sex ratios at differentiation become more male‐biased. For both species, climate‐induced changes to the sex ratio in early‐life stages could have a greater impact on relative catch than fishing mortality. We demonstrate that the spawning potential ratio may not be sensitive to climate‐induced changes in the sex ratio unless we account for changes in the sex ratio at differentiation in the historic baseline reproductive potential.ConclusionThese findings underscore the importance of utilizing sex‐specific stock assessments for species with temperature‐sensitive sex determination mechanisms.

Evaluating impacts of environmental stress and bioactive chemicals on the North Carolina blue crab population: An individual‐based model

AbstractObjectiveRecent estimates of the North Carolina blue crab Callinectes sapidus stock found that the stock is overfished and overfishing is occurring. Threats outlined in the 2018 stock assessment include climate change and estrogenic endocrine‐disrupting chemicals (EEDCs). The objective of this study was to use an individual‐based modeling approach to simulate the long‐term effects of climate change and EEDCs on the North Carolina blue crab stock.MethodsTo do this, we built an individual‐based model that simulated the life history of individual blue crabs, including but not limited to growth, reproduction, and mortality. We then tested our population of blue crabs against 30 different combinations of temperature and EEDC scenarios over 50 years to determine the long‐term effects on the population.ResultOur simulations suggested that the North Carolina blue crab population may be relatively resilient to climate change‐related temperature shifts but that there may be significant impacts at the population level as summer temperatures become more extreme. Endocrine‐disrupting chemical effects resulted in an alternative stable state of lower catch or the total extinction of the population.ConclusionThese results suggest that management strategy changes may be necessary as temperatures become more extreme in the region. In addition, more research is necessary to fully understand the effects of EEDCs on blue crabs and other crustaceans at the individual and population level.

Estimating the growth parameters, exploitation rate, biomass and maximum sustainable yield of long whisker catfish Mystus gulio (Hamilton, 1822) in the coastal waters from southwestern Bangladesh

The research provides a comprehensive analysis of Mystus gulio including growth pattern, growth parameters, recruitment patterns, mortality rates, biomass, exploitation rate (E), and the estimation of maximum sustainable yield (MSY) within the southwestern coastal waters of Bangladesh. From January to December 2017, fishers provided around 1200 specimens. FAO-ICLARM Stock Assessment Tool and Excel-add-in-solver were used to assess stock status through length-frequency data. Indeed, the research findings indicated that the population of M. gulio displayed negative allometric for both individuals (b = 2.53 for male, b = 2.50 for female), as demonstrated by the calculated allometric coefficient value. Nonetheless, the population's dynamic characteristics revealed an asymptotic length (L∞) of 19.34 cm, 23.28 cm and growth coefficient (K) 0.94 year−1 and 0.81 year−1 for male and female M. gulio. The growth performance indexes (Ø') of 2.55 and 2.64 for male and female and maximum lifespan (tmax) 3.20 years and 3.70 years respectively. This study revealed that the slightly variations in the natural mortality rate (M) for both specimens at 1.55 year−1 and 1.59 year−1. The fishing mortality rate (F) 2.75 year−1 and 1.98 year−1and total mortality rate (Z) 4.30 year−1 and 3.57 year−1 for male and females, respectively. The maximum permissible exploitation rate (Emax = 0.421) was lower than the actual exploitation rate (E = 0.63). The MSY was calculated at 67.968 metric tons. Without a doubt, overfishing stands out as the most critical threat to the wild stock. Therefore, it is clear that the existing fishing approach was not efficiently managed the standing stock in a sustainable manner. The findings would be useful for established proper fishing regulations in coastal waters and the surrounding ecosystems.

Decadal changes in biomass and distribution of key fisheries species on Newfoundland's Grand Banks.

Canadian fisheries management has embraced the precautionary approach and the incorporation of ecosystem information into decision-making processes. Accurate estimation of fish stock biomass is crucial for ensuring sustainable exploitation of marine resources. Spatio-temporal models can provide improved indices of biomass as they capture spatial and temporal correlations in data and can account for environmental factors influencing biomass distributions. In this study, we developed a spatio-temporal generalized additive model (st-GAM) to investigate the relationships between bottom temperature, depth, and the biomass of three key fished species on The Grand Banks: snow crab (Chionoecetes opilio), yellowtail flounder (Limanda ferruginea), and Atlantic cod (Gadus morhua). Our findings revealed changes in the centre of gravity of Atlantic cod that could be related to a northern shift of the species within the Grand Banks or to a faster recovery of the 2J3KL stock. Atlantic cod also displayed hyperaggregation behaviour with the species showing a continuous distribution over the Grand Banks when biomass is high. These findings suggest a joint stock assessment between the 2J3KL and 3NO stocks would be advisable. However, barriers may need to be addressed to achieve collaboration between the two distinct regulatory bodies (i.e., DFO and NAFO) in charge of managing the stocks. Snow crab and yellowtail flounder centres of gravity have remained relatively constant over time. We also estimated novel indices of biomass, informed by environmental factors. Our study represents a step towards ecosystem-based fisheries management for the highly dynamic Grand Banks.

Characterisation of phenotypic stock diversity of the crescent perch, Terapon jarbua (Forsskål, 1775) along the Indian coast using morphology and otolith shape analysis

Crescent Perch, Terapon jarbua is one of the important food cum marine ornamental fish in India. The present study deals with the delineation of T. jarbua stocks along the Indian coast using the morphology of fish and otolith shape analysis. A total of 609 fish samples were collected from six major fishing harbours representing the Indian coast from February to May 2022. The conventional morphometry, truss network analysis and otolith shape indices revealed the morphological divergence for the fish among the stocks. However, meristic traits could not show the variations among the different stocks. The morphometric and otolith shape variability for the species revealed the existence of different phenotypic stocks along the Indian coast, which might be due to the ability of the T. jarbua to adapt to different environmental conditions. The study highlights the need for separate biological and stock assessment studies for developing effective management strategies.

Quantitative fishery assessment in tropical waters: Stock dynamics and strategy options

This paper explores the possible options for quantitative fishery stock assessment in tropicalwaters and is composed of four layers viz., concepts, methodologies, data and indicators.The concept promotes the fundamental theory that has been popular in research on stockdynamics, including the possibility of combining it with various doses of intrinsicallybiological information, from individual fish to regional stocks. After reviewing the concepts,tools and their evolution, an attempt has also been made to propose a few guidelines as thebest ways to approach stock assessment and formulate input control or output regulationstrategies in the most pragmatic yet scientific manner, thereby ensuring minimal impact onthe fishery, fishers and stocks in the medium term. Keywords:Bayesian approach, Capture fisheries, Fish stockassessment, Models

Assessment of marine fish stocks within India’s ExclusiveEconomic Zone: Status report 2022

India is one of the major fishing nations in the Indian Ocean, with commercial marine fishlandings in the country comprising of nearly 1000 species. A comprehensive and periodicassessment of the stocks is crucial for achieving the goals of sustainable fisheries harvestand management. The ICAR-Central Marine Fisheries Research Institute, Ministry ofAgriculture and Farmers’ Welfare, Govt. of India, is mandated to monitor and assess themarine fisheries resources in the Indian EEZ. In the present study, 135 stocks across 70species were assessed using monthly length-frequency data. The health status of the stockis projected based on a combination of indicators such as relative biomass or spawningstock biomass (B/Bmsy or SSB/SSB0) and relative fishing pressure (F/Fmsy) estimated usinga standard length-based dynamic pool model (i.e., Thompson and Bell Model). Based onthe comparison of estimated values of indicators against the thresholds, the stocks wereclassified as sustainable, overfishing, overfished and rebuilding. The study revealed that91.1% of the assessed fish stocks have a healthy biomass level (sustainable stocks andthose subjected to overfishing), 8.2% of stocks are overfished, whereas 0.7% are in theprocess of rebuilding. Biology-based, Healthy stocks, Length-frequency,Stock assessment, Sustainable, Tropical

Fishery, population dynamics and stock assessment of flat needlefish Ablennes hians (Valenciennes, 1846) from the south-eastern Arabian Sea off Kerala coast, India

The age, growth, mortality parameters and stock status of Ablennes hians(Valenciennes, 1846) were assessed by using length frequency, catch, and effort datacollected from the commercial catches along the coast of Kerala (south-eastern ArabianSea) during 2015-2017. The growth parameters, L∞, K and t0 were estimated at 133.9 cm TL,0.72 yr-1 and -0.0026 yr, respectively. The instantaneous natural mortality (M), total mortality(Z), fishing mortality (F) and current exploitation rate (Ecur) were estimated as 0.95 yr-1,2.78 yr-1, 1.83 yr-1 and 0.66, respectively. Length at capture (Lc50) was estimated at 77.58cm TL, which is greater than Lm50. Beverton and Holt yield per recruit analysis, indicated thatmaximum Y/R could be obtained at an exploitation rate (Emax) of 0.75, suggesting scope foran increase in exploitation. In this analysis, spawning stock biomass (SSB) was not takeninto consideration, which is essential for maintaining recruitment in the future. In the presentstudy, E0.1 (0.61) and Ecur (0.66) were nearly equal, indicating that the fishery is running atan economic reference point i.e., maximum economic yield (MEY). Thompson and Bellprediction model showed that at the current level of exploitation, the SSB was reduced by20.49% and hence, there is no scope for further increase in fishing efforts to maintain the SSBat a relatively safe and sustainable level. The analysis revealed the maximum sustainableyield (MSY) of this species as 90.26 t, which can be obtained by increasing the present levelof fishing effort by 1.3 times but would result in the depletion of the SSB to a critically lowlevel of 15.65%. Therefore, a precautionary safe level of exploitation (E0.1), estimated at 0.61for the species, is recommended. Keywords:Belonidae, Fishery management, Population dynamics,Spawning stock biomass, Stock status

Contribution and Prospect of Marine Fisheries in the Economy of Bangladesh and Sustainable Blue Economy Challenges: A Review

Marine resources of Bangladesh are abundant, both in terms of huge sea area and its biological richness. The marine water area supports a large number of commercially important fin fish, mollusks, crustaceans, and seaweeds, offering significant economic potential due to their diversified use and high demand in the international market. Therefore, the current review comprehensively examines both the contribution and potentials of marine fisheries resources to the economy, as well as the challenges involved in marine fisheries sectors in attaining a sustainable blue economy in Bangladesh. The study revealed that, the marine fisheries plays an important role in national economy and has huge prospect in employment generation and ensuring food security of Bangladesh. The expansion of mariculture and effective utilization of marine resources will open a new window for the economic development of Bangladesh. Marine fisheries production of Bangladesh has increased over the last two decades, but its relative share in total fisheries production has declined. Despite being rich in marine living resources, commercial mariculture has been developed only for shrimp, prawn, and mud crab. Challenges hindering mariculture expansion includes limited research, technological deficiencies, inadequate skilled workforce, underdeveloped domestic seafood markets, and financial constraints. Concerted efforts including exploring new fishing grounds, conducting stock assessments, implementing scientific management strategies, enacting a national marine fisheries policy, enforcing legislation, and establishing robust surveillance and monitoring mechanisms are required to foster a sustainable blue economy in Bangladesh.

Stock Assessment of the Commercial Small Pelagic Fishes in the Beibu Gulf, the South China Sea, 2006-2020.

Long-term variations in population structure, growth, mortality, exploitation rate, and recruitment pattern of two major commercial small pelagic fishes (CSPFs) (Decapterus maruadsi and Trachurus japonicus) are reported based on bottom trawl survey data collected during 2006-2020 in the Beibu Gulf, South China Sea. All individuals collected during each sampling quarter over a period of 15 years were subjected to laboratory-based analysis. In this study, the stock of D. maruadsi and T. japonicus inhabiting the Beibu Gulf was assessed using length-based methods (bootstrapped electronic length frequency analysis (ELEFAN)) to complete stock assessment in different fishery management periods (the division of fisheries management periods was based on China's input and output in the South China Sea offshore fisheries over 15 years, specifically divided into period I (2006-2010), period II (2011-2015), and period III (2016-2020)). The results showed that the mean body length, dominant body size, and estimated asymptotic length of two CSPFs decreased, whereas their growth coefficient decreased, indicating miniaturization and slower growth, respectively. Estimated exploitation rates and catching body length for two CSPFs indicated that both stocks in the Beibu Gulf were overexploited in period I and moderately exploited after 2011. These stocks were taking a good turn in status in period III, with the exploitation rate much lower than the initial period and reversing the downward trend in catching body length. Furthermore, the variations in the spawning season of the two CSPF stocks and their barely satisfactory expected yield indicated the complexity of the current fishery management in the Beibu Gulf. These results suggest that management measures to reduce fishing pressure may have a positive influence on the biological characteristics of those CSPFs in the Beibu Gulf; however, the stock structure already affected by overfishing will be a huge challenge for the conservation and restoration of fisheries resources in the future. Given that the current stocks of D. maruadsi and T. japonicus in the Beibu Gulf still have low first-capture body length (Lc) and high fishing mortality (F) (compared to F0.1), we identify a need to refine population structure by controlling fishing efforts and increasing catchable size, and more consideration should be given to the local fishery resource status in fisheries management.

Review of methodologies for detecting an observer effect in commercial fisheries data

Observers are deployed on commercial fishing trips to collect representative samples of discard rates. However, fishers may change their fishing habits when an observer is onboard (“observer effect”) or observer programs may over- or under-sample portions of the fleet (“deployment effect”). If the extent of these effects are substantial, observer data will not be representative of unobserved trips, potentially biasing the estimation of discards. This sampling bias can impact catch monitoring, stock assessments, and fishery management. The purpose of this study was to examine the power and error rate of several published methods for detecting an observer effect using a simulation of observer and deployment effects at varying sampling ratios (i.e., observer coverage) for several sample statistics. The simplest methods (t-test and F-test for difference of means and variances) provided an accurate although imprecise estimate of the observer effect size, but only when there were no deployment effects. A generalized linear mixed effects model (GLMM) was also not reliable for detecting small bias, but was not confounded by deployment effects and was relatively robust to changing coverage rates. The most complicated tests involved comparing differences in trip characteristics between subsequent trips for observed-unobserved and unobserved-unobserved pairs. These tests were able to detect smaller observer effects and were not confounded by deployment effects, but were unreliable at high coverage rates (>60%), producing both high false positive and false negative rates. Sensitivity tests also showed differing detection accuracy as the distribution of the metric of interest changed. Thus, the optimal test for detecting an observer effect will depend on the metric of interest, the coverage rate, and whether a deployment effect exists. An example from the New England groundfish fishery is provided to illustrate how conflicting results may be explained. Results should always be considered carefully when declaring that an observer effect is or is not occurring because of the sensitivity of the tests.

A tale of two species: Disaggregating mixed historical catches of two most common skates in the Northeast Pacific Ocean

Historical catch data represent a key source of information for fisheries stock assessment models. Historically, commercial fishery catch statistics were estimated from the portion of catch landed in ports. However, many species with relatively low economic value have not been recorded on a species basis but instead as a part of an aggregate category. Reconstructing component species catch from an aggregate category is a common challenge for fisheries stock assessment efforts around the world. Skates are one group of species with low economic value for which landed catch has not been commonly reported by species. In this paper, we present a novel approach to disaggregate the historical catch of the two most abundant skate species on the West Coast of the United States, longnose skate (Caliraja rhina) and big skate (Beringraja binoculata), landed within the aggregated skate category, in ports of Washington State. We used a combination of fishery-dependent and fishery-independent data sources to account for changes in the spatial extent of the fishery over time, and differences in the depth distribution of these two skate species. While developed to disentangle aggregate catch of longnose and big skates, the approach is not limited to skates on the West Coast of the United States, but can be adapted for any species which landings have been reported within an aggregated category elsewhere.