Variation In Catch Rates Research Articles

Enhancing Buprestidae monitoring in Europe: Trap catches increase with a fluorescent yellow colour but not with the presence of decoys.

This study investigated the efficacy of various traps differing in colour (green or yellow), presence or absence of decoys (dead Agrilus planipennis) or design (commercial MULTz or multifunnel traps, and homemade bottle- or fan-traps) for monitoring European Buprestidae in deciduous forests and pear orchards. Over two years, we collected 2220 samples on a two-week basis from 382 traps across 46 sites in Belgium and France. None of the traps proved effective for monitoring Agrilus sinuatus in infested pear orchards (17 specimens captured in 2021, 0 in 2022). The decoys did not affect the catch rates whatever the trap model, colour, buprestid species or sex. The fluorescent yellow traps (MULTz and yellow fan-traps) tended to be more attractive than the green traps (green fan-traps and, to a lower extent, multifunnel green traps). Most Agrilus species showed similar patterns in mean trap catches, with the exception of Agrilus biguttatus, which had the largest catches in the green multifunnel traps. Finally, we observed a high variation in catch rates between localities: the site explained 64% of the catches variance, while the tree within the site and the type of trap explained only 6-8.5% each. In many sites, we captured very few specimens, despite the abundance of dying mature trees favourable to the development of Buprestidae. For the early detection of non-native Buprestidae, it therefore seems essential to maximise the number of monitoring sites. Due to their cost-effectiveness, lightweight design, and modularity, fan-traps emerged as promising tools for buprestid monitoring. The study's findings extend beyond European fauna, as a preliminary trial in Canada suggested that yellow fan-traps could also improve captures of non-European buprestid species and catch species of interest such as Agrilus bilineatus (a species on the EPPO A2 list of pests/pathogens recommended for regulation in the EU).

Characterizing gear-based exploitation patterns of artisanal tuna fisheries in the western Indian Ocean: A snapshot from Kenya

This study explores gear-based patterns in catch rates and catch composition of landings by artisanal tuna fishers along the Kenya coast based on data collected at eight landing sites from March 2014 to March 2020, representing 1960 fishing trips and 192 mt of fish. Tuna and tuna-like species constituted 55% and 38% respectively of the sampled catch by weight. Troll lines, ringnets and handlines were the most commonly used representing 52%, 18% and 12% of the sampled fishing trips. Ringnets were the most productive catching an average of 547 ± 99.3 kg of tuna per trip, while handlines caught the lowest (12.6 ± 1.4 kg trip −1). Longlines and drift gillnets caught the highest proportion of tuna representing 95% and 89% respectively of landings, while reef seines and monofilament gillnets caught the lowest. The mean catch per unit effort for tuna was 78.5 ± 7.7 (SE) kg trip −1, while that for tuna-like species was 35.2 ± 1.9 (SE) kg trip −1. Kawakawa (Euthynnus affinis), bigeye (Thunnus obesus), and yellowfin (Thunnus albacares) were the most common and most abundant representing 31.5%, 21.2%, and 20% of the tuna landings, respectively; while skipjack (Katsuwonus pelamis) constituted the lowest at 4%. Although tuna was landed year-round, species-specific variations in catch rates and size distribution were observed among the gear types and between seasons. Cluster analysis and non-metric ordination based on combinations of gear type and season defined four groupings with two gear types (ringnets and set gillnets) showing seasonal differentiation in the composition of tuna and tuna-like species. Majority of tuna were above the reported size at first maturity (L50) except for yellowfin and bigeye tuna. Overall, the findings provide new insights on Kenya’s artisanal tuna fishery addressing important information gaps on exploitation patterns of artisanal tuna fleets of the western Indian Ocean region.

The significance of the Sikunga Fish Protected Area towards fisheries conservation in the Zambezi River, Namibia

AbstractLarge cichlids of the genera Oreochromis, Tilapia, Serranochromis and Sargochromis are the backbone of the gillnet fishery of the Zambezi River, but Oreochromis spp. is currently over‐exploited and at risk of local extinction. As a result, the Sikunga Fish Protected Area was established in 2012 to restore a decline in fish stocks of the Zambezi River, but its efficiency has never been assessed. The aim of this study was to compare fish assemblages, abundance and mean sizes between a fish protected area (Sikunga FPA) and nonprotected areas; the Sikunga Buffer Zone and the Lisikili Fished Area on the Zambezi River. Monthly gillnet surveys, using commercial gillnets, were conducted from July to December 2020. Overall catch rate (CPUE) by weight of all species differed significantly among the sampling sites (Kruskal–Wallis H‐test, range χ2(2) = −27.95, p = 0.0001). The highest CPUE was recorded at Sikunga Fish Protected Area (2.85 kg/set ± 0.42 (SE)), followed by the Sikunga Buffer Zone (0.93 kg/set ± 0.16 (SE)) and the Lisikili Fished Area (0.61 kg/set ± 0.14 (SE)). Further analyses showed that the mean sizes of O. andersonii, Coptodon rendalli, Serranochromis and macrocephalus were significantly larger at the Sikunga Fish Protected Area than at the Lisikili Fished Area. A particularly interesting observation pertains to a lack of variation in catch rates of Serranochromis macrocephalus and mean sizes of Clarias gariepinus between the protected and nonprotected areas, signifying that fish protected areas can be species‐specific, and not all species in a water body will respond positively to protection. The overall results of this study support the innovation of fish protected areas as a fundamental tool in attaining sustainable management of fishery resources in inland waters of Namibia and other developing countries.

Seasonal ocean forecasts to improve predictions of Dungeness crab catch rates, co-developed with state and tribal fishery managers

Abstract The commercial Dungeness crab (Metacarcinus magister) fishery in Oregon and Washington (USA) is one of the most valuable fisheries in the region, but it experiences high interannual variability. These fluctuations have been attributed to environmental drivers on seasonal and annual timescales. In this study, researchers and state and tribal fisheries managers develop a statistical model for Dungeness crab catch per unit effort (CPUE) to help inform dynamic management decisions in Oregon and Washington. Fishing observations were matched to seasonally forecast and lagged ocean conditions from J-SCOPE, a regional forecast system (http://www.nanoos.org/products/j-scope/). Inclusion of dynamic and lagged ocean conditions improved model skill compared to simpler models, and the best model captured intraseasonal trends and interannual variability in catch rates, and spatial catch patterns. We also found that model skill relied on fishing behaviour, which varies interannually, highlighting the need for advanced fishing behaviour modelling to reduce uncertainty. The relationships between catch rates and ocean conditions may help elucidate environmental influences of catch variability. Forecast products were co-designed with managers to meet their needs for key decision points. Our results illustrate a seasonal forecasting approach for management of other highly productive, but also dynamic, invertebrates that increasingly contribute to global fisheries yield.

Bayesian prediction of fishery biological impacts from limited data: A deep-set buoy gear case study

Predicting the biological impacts of new or expanding fisheries presents challenges due to limited data, high variability in catch rates, and the often low frequency of bycatch events. These issues arose in the case of the West Coast deep-set buoy gear (DSBG) fleet, which the Pacific Fisheries Management Council recommended in 2019 for authorization as a legal gear type. DSBG selectively targets swordfish (Xiphias gladius) with infrequent bycatch of other species. Limited effort and incomplete observer coverage result in a data-limited context for estimating the impacts of a fully authorized and expanded fishery. Recently, data analysts have explored Bayesian estimation for modeling rare-event bycatch in a manner that incorporates uncertainty and enables updating as more data become available. Here, we apply a Bayesian methodology to an integrated dataset of DSBG observer and logbook records to estimate bycatch rates under several plausible scenarios of DSBG authorization. We estimate posterior distributions of catch rates for three species caught in DSBG Exempted Fishing Permit (EFP) trials, and incorporate bootstrap samples of vessel-level effort to calculate posterior predictive distributions of catch counts under alternative management regimes. We discuss how our results can inform policy decisions about a new fishery with limited data, and how to extend this approach to other federal environmental actions. This approach allows policymakers to compare biological impacts of management alternatives while considering the uncertainty inherent in the predictions, and to determine whether the range of potential impacts is likely to significantly alter the affected environment.

Historical reconstruction and social context of recreational fisheries: The Australian East Coast Barramundi

AbstractRecreational fishing in Australia makes important contributions to local economies and fisheries harvests. Historical evaluations of the cumulative effects of exploitation for most recreationally targeted Australian fish stocks remain unexplored, as do the social and cultural contexts of recreational fishing. Historical newspaper articles were used to derive quantitative and qualitative catch data on Queensland's east coast barramundi, Lates calcarifer (Bloch), a popular species targeted by recreational fishers. Decadal‐scale catch rates (1869–1952) were reconstructed, as was the development of the fishery since its earliest documentation. Models indicated a significant decline in catch rates, from an average of 5.20 kg /fishing trip (95% confidence interval, 3.82–7.09) in 1870–3.24 kg/fishing trip (95% CI 2.72–3.87) in 1952. Variability in catch rates was associated with a spatial and seasonal interaction effect, with average catch rates differing among regions and seasons. Qualitative analysis showed a northward expansion of the fishery along the east coast, coinciding with increasing number of locations fished, establishment of organised fishing clubs in the 1900s and concerns of overexploitation starting in 1909. Integration of quantitative and qualitative analysis revealed past socio‐cultural factors and trends in the barramundi fishery, while building confidence in the reliability of inferences made from archival data sets.

Bycatch Estimates From a Pacific Tuna Longline Fishery Provide a Baseline for Understanding the Long-Term Benefits of a Large, Blue Water Marine Sanctuary

Bycatch on pelagic tuna longlines has contributed to population declines in several far-ranging, oceanic species and presents a conservation challenge that area-based management tools are increasingly promoted to address. In January 2020 the Republic of Palau, concerned about the impacts of longline fishing in its waters, closed 80% of its exclusive economic zone to all extractive activities, reserving the remaining 20% for a domestic fishing zone (DFZ). One of a growing number of very large marine protected areas, the Palau National Marine Sanctuary (PNMS) spans ∼500,000 km2 and was established inter alia to allow for the recovery of fish stocks adversely impacted by tuna longline fleets. Given that the main tuna stocks targeted in the western Pacific are not overexploited, the benefits of protection potentially afforded by the sanctuary are likely greater for vulnerable bycatch species. Evaluations of the sanctuary’s performance require, in part, a baseline of historical catch rates and effort distribution in the distant-water fleet (DWF) and locally based fleet (LBF) operating in Palau prior to sanctuary implementation. We describe the fishing effort, catch rates, catch estimates and fishing mortality in Palau’s longline fishery based on logbook, observer and electronic monitoring data. We defined bycatch as any species, retained or discarded, other than targeted tunas. Between 2010 and 2020, 104.8 million hooks were deployed, catching over 2 million individuals from 117 taxa at an overall target:bycatch ratio of 1:1, with a retention rate of ∼62%. Pronounced differences in fishing strategies and spatial distribution of effort between fleets were associated with large variations in catch rates and composition. The LBF had a larger effect on populations of at-risk species relative to the DWF, with higher catch rates and magnitudes for several vulnerable species and higher observable fishing mortality rates (64% vs 50% in the DWF). The sanctuary reshaped Palau’s longline fishery, contracting the fishery’s area and capacity. The relocation of the DFZ eliminated the LBF and constrained the DWF to an area where the fleet’s total catch rates and those of a number of vulnerable species were historically lower relative to former fishing grounds now closed by the sanctuary. Our results highlight the importance of consistent bycatch monitoring and emphasize the need for regional area-based approaches for managing longline fisheries.

Fisheries benefits of a marine protected area with endogenous fishing efforts – A bioeconomic analysis

The study assesses the conservation and fisheries benefits of the Blue Bay Marine Park in Mauritius. It addresses the question - are the higher catch rates near the Park a result of population spillovers or of reduced fishing effort in those waters due to site-specific attributes? There is no data on catches and fishing effort prior to the reserve's establishment; a bioeconomic model is used to separate the effects of spillover and effort redistribution on catch rates in waters next to the Marine Park. The area's fish populations are replicated using a dynamic age-structured model with a Beverton-Holt recruitment function, while fishing effort is predicted using a random utility model and Random Parameter Logit estimation. The bioeconomic model is characterised by two-way feedback loops between fish stocks and the geographic redistribution of fishing effort. A comparison of fish population, biomass, and catch rates in the fisheries with and without effort redistribution, suggests that the reduced fishing effort in waters near the Park, rather than spillover, is driving the increase in observed catch rates. Travel distance, variation in catch rates, depth of water and area status (lagoon vs. off-lagoon) explain the relocation of fishing effort away from the adjacent of the marine park. The study shows how site-specific characteristics affecting fisher behaviour are important in the design of marine reserves.

Dynamics of marine predators off an oceanic island and implications for management of a preventative shark fishing program

Oceanic islands are productive ecosystems, and so have higher densities of many marine predators. We investigated the dynamics of elasmobranch and teleost predators in coastal waters off Reunion Island, Indian Ocean, using fisheries-independent data from a preventative shark fishing program from January 2014 to December 2019. We developed a moonlight index that calculates exact moonlight through incorporating lunar azimuth, elevation angle and island topography. We quantified spatial–temporal and environmental drivers of occurrence using zero-inflated mixed models and assessed species-specific catchability in the program. A consistent segregated pattern was observed with higher occurrence of all species at dusk and after-dusk associated with lower luminosity. Scalloped hammerhead sharks (Sphyrna lewini) and giant trevally (Caranx ignobilis) were found to patrol coastal waters earlier in the day than the other species. Tiger (Galeocerdo cuvier) and bull (Carcharhinus leucas) sharks showed high spatial segregation, potentially reducing competition. Teleost predators were found more frequently inside the coral reef environment of the Marine Protected Area but there was no clear pattern for sharks. Seasonality was observed for giant trevally, stingrays, bull sharks, and giant guitarfish (Rhynchobatus australiae), with higher presence during early winter periods related to turbidity, photosynthetically available radiation, and temperature. Inter-annual variation in catch rates suggested that juvenile tiger sharks might be replacing bull sharks in nearshore habitats, and the consequences for mitigation of shark bite hazard are discussed. Operational alternatives are proposed to enhance reducing the impacts of preventative shark fishing upon critically endangered species, improve their conservation and ensure local ecosystem balance.

Effects of 3D seismic surveying on snow crab fishery

Commercial Snow Crab (Chionoecetes opilio) harvesters believe marine noise from seismic surveys reduces commercial Snow Crab catch rates. Depending on the type of seismic survey used, animals living in a particular area could be exposed to loud noise (e.g. daily Sound Exposure Level (SEL) >165 dB re 1 μPa2·s) for periods ranging from hours (typical 2D survey) to months (detailed 3D survey). This field experiment applied a series of comparisons conducted within a Before-After-Control-Impact study design to investigate the effect of prolonged industrial 3D seismic exposure on the catch rates of Snow Crab over nine weeks in 2017 and five weeks in 2018. Changes in catch rates at 3D seismic surveying sites were inconsistent across years, with reduced catches in 2017 and increased catches in 2018. Catch rates were similar at experimental and control sites within two weeks after exposure, and the potential effect of seismic surveying was not measured at a distance of 30 km. The large variation in catch rates across small temporal and spatial scales coupled with the absence of notable mechanistic responses of Snow Crab in past studies to seismic in associated snow crab movement behavior, gene expression and physiology, we conclude that the observed differences owing to seismic surveying in our study design are likely a result of stochastic processes external to our manipulation.

Investigating food limitations in wild fisheries: the attendance and growth responses of fish at an anthropogenic feeding station within a temperate estuary

To investigate whether wild fish populations are food limited, this study explored whether the provision of supplementary food had a positive effect on the abundance, condition and growth characteristics of estuarine fish assemblages in New Zealand. Feed (7690 kg) was delivered from an anthropogenic feeding station over a 60-week period to a naturally occurring assemblage of wild fish. Yellow-eyed mullet (Aldrichetta forsteri) of juvenile, sub-adult and newly matured size classes were the dominant species actively foraging at the feeding station throughout its operation, whereas larger piscivorous species visited and foraged from the feed station over the summer period only. Other species were present in the wider area of Nelson Haven, but not at the feed station. No obvious changes in the condition factor of yellow-eyed mullet were observed across the period of monitoring, and no changes in their catchability were detected - although marked seasonal variation in catch rates was observed. Results from tag-recapture data identify that the length-based growth rates of yellow-eyed mullet recaptured near the feed station were higher than those of tagged individuals recaptured at a nearby comparison site, and were higher than the growth rates observed in natural populations of yellow-eyed mullet in the wider region. Shifts in the median size of fish, as observed by acoustic observations, agreed with the tag-recapture data. Although some of the results identified were not amenable to statistical analyses, the attendance of yellow-eyed mullet at the feeding station, in combination with the improved estimates of growth by most of the techniques employed, indicates that yellow-eyed mullet are food limited in their natural environment and that the growth performance of these fish can be positively affected by the increased availability of food.

Shifts in population size structure for a drying‐tolerant fish in response to extreme drought

AbstractFor freshwater systems, climate change‐induced alterations to drought regimes are a considerable threat to already threatened species. This is particularly poignant for kōwaro (or Canterbury mudfish, Neochanna burrowsius), a critically endangered fish largely restricted to drying‐prone waterways on the Canterbury Plains, New Zealand. By comparing three catchment‐wide surveys (2007, 2010, 2015) within the Waianiwaniwa Valley, we assessed the scale and magnitude of population change induced by 2 years of consecutive drought (2014–15), when compared to surveys during wetter conditions (2007, 2010). The droughts triggered a catchment‐wide switch from adult‐dominated populations to populations comprised of juveniles indicated by a significant reduction in median size (~95 mm during the wet to ~60 mm after drought). In comparison, population abundances were highly variable, indicated by no significant change in catch‐per‐unit‐effort. The large variation in catch rates and connection of median size to reproductive potential mean median size will be useful to measure in monitoring to infer potential changes to population resilience, particularly during extreme events. Furthermore, because N. burrowsius could be regarded as extremophile fish, already restricted to harsh habitats, they are likely to become increasingly threatened by climate change. Thus, tools that allow for insightful comparisons between populations, such as a population resilience framework based on both abundance and body size distribution, will be increasingly important for pragmatic decision‐making for targeted conservation measures.

Recreational charter fishery attributes and variation in key species catches and discards: Resource management considerations

Strategic long-term sampling programmes that deliver recreational catch, effort and species demographic data are required for the effective assessment and management of recreational fisheries and harvested organisms. This study used a spatially and temporally stratified observer programme to examine variation in the rates, quantities and lengths of retained and discarded catches of key species in a recreational charter fishery. Geographic region, but not season, significantly influenced catch rates of key demersal species, being driven by temporally persistent latitudinal clines in environmental conditions influencing species distributions. There was considerable trip-to-trip variation in catch rates that were attributed to localised differences in fishing operations, locations, environmental conditions and client preferences. Broad trends in retained and discarded catch rates were nevertheless, similar across different fishing effort standardisations (per-trip, per-hour, per-client, per-client/fished hour), demonstrating that the coarsest unit of effort could be used in fishery assessments. Discard rates of organisms were variable and driven by a combination of mandated legal lengths, individual client and operator preferences for particular species and sizes of organisms, and not due to attainment of catch quotas or high-grading. This study has identified important fishery attributes that require consideration in assessing charter fisheries and stocks of recreational fish species.

Environmental effects on the spatio‐temporal patterns of abundance and distribution of Sardina pilchardus and sardinella off the Mauritanian coast (North‐West Africa)

AbstractFrom 1998 to 2011, the effects of environmental conditions on the spatial and temporal trends of sardine and sardinella catch rates in the Mauritanian waters were investigated using generalized additive models. Two models were used: a global model and an oceanographic model. The global models explained more of the variability in catch rates (60.4% for sardine and 40% for sardinella) than the oceanographic models (42% for sardine and 32.4% for sardinella). Both species showed clear and inverse seasonal variations in abundances corresponding to their main spawning activities and the hydrologic seasons off the Mauritanian waters. Sardine prefer colder waters and seem to occupy the ‘gap’ in the northern part of the Mauritanian waters as soon as sardinella has left the area because of to lower water temperatures. Unlike sardinella, sardine showed a gradual southward extension between 2002 and 2009. The oceanographic model revealed that a high proportion of catch variability for the two species could be explained by environmental variables. The main environmental parameters explaining the variability are sea surface temperature (SST) and the upwelling index for sardinella, and the chlorophyll‐a (Chl‐a) concentration, the upwelling index and SST for sardine. The sardinella spatio‐temporal variations off Mauritania seem to be more controlled by thermal than productivity gradients, probably linked to the species physiological constraints (thermal tolerance) whereas sardine seems to be more controlled by an ‘optimal upwelling and temperature’ windows. The results presented herein may be useful for understanding the movement of these species along the Mauritanian coast and hence their management under a changing climate.

Determinants of angling catch of northern pike (Esox lucius) as revealed by a controlled whole-lake catch-and-release angling experiment—The role of abiotic and biotic factors, spatial encounters and lure type

Studies on catches of anglers usually rely on observational data and are thus uncontrolled with respect to angler skill, bait/lure choice and site choice. We performed a controlled fishing experiment targeting northern pike (Esox lucius) in a small (25ha), weakly eutrophic natural lake situated about 80km northeast of Berlin (Germany) to understand abiotic, biotic and gear-related factors that relate to catch rates and size of pike captured by angling. The experiment was conducted over two one-week long fishing campaigns where boat-based anglers randomly sampled across 30 pre-determined sites. Sites were systematically exposed to two standardized lure types (soft plastic shad or spoon). We found the catch rates of pike per 15min to be significantly higher in shallow water and when soft plastic lures were used compared to deeper water and when spoons were used. Catch rates significantly dropped over the course of seven days, suggesting either learning or other reasons moving pike from vulnerable to invulnerable pools (e.g., due to stress caused by capture, sampling and release). Catch rates also varied by season and across anglers and sites as random effects. The variation in size of pike captured exhibited greater stochasticity than variation in catch rate. There was no lure effect on the size of the pike captured, but we found a seasonal effect and a day effect, suggesting larger fish were captured first. Pike captured in sublittoral areas were significantly smaller than those captured in other habitats. Overall, our study documented a novel effect of lure type on the catch rates of pike, but the explanatory power of the predictors was only moderate. Therefore, our results support the idea that the best fishing ingredients are investing time and maximizing encounter probabilities through habitat choice, with only moderate additional effects to be expected from attention to abiotic conditions, day time and choice of type of artificial lure.

Evaluating potential sources of variation in Chironomidae catch rates on sticky traps

Sticky traps are a convenient tool for assessing adult aquatic insect population dynamics, but there are many practical questions about how trap sampling artefacts may affect observed results. Utilising study sites on the Colorado River and two smaller streams in northern Arizona, USA, we evaluated whether catch rates and sex ratios of Chironomidae, a ubiquitous aquatic insect, were affected by spraying traps with insecticide, placing traps at different heights above ground, and placing traps at different locations within a terrestrial habitat patch. We also evaluated temporal variation in Chironomidae counts monthly over a 9-month growing season. We found no significant variation in catch rates or sex ratios between traps treated versus untreated with insecticide, nor between traps placed at the upstream or downstream end of a terrestrial habitat patch. Traps placed near ground level did have significantly higher catch rates than traps placed at 1.5m, although sex ratios were similar across heights. Chironomidae abundance and sex ratios also varied from month-to-month and seemed to be related to climatic conditions. Our results inform future sticky trap studies by demonstrating that trap height, but not insecticide treatment or precise trap placement within a habitat patch, is an important source of variation influencing catch rates.

Diel effects on bottom-trawl survey catch rates of shallow- and deep-water Cape hakes Merluccius capensis and M. paradoxus off Namibia, using solar zenith angle

Diel patterns in survey trawl catches for the Cape hakes Merluccius capensis and M. paradoxus off Namibia were studied in order to examine the effect of diel bias on catchability, and its implication for survey abundance estimation and the consistency of the survey time-series. Catch rates (numbers per haul) by species and length from summer demersal biomass surveys conducted during the period 2002–2013 were used, together with a computation of the corresponding light-level data from which the solar zenith angles were obtained. Generalised additive models were fitted to assess the relationship between the catches and a number of explanatory variables. Significant covariates were zenith angle, depth and geographical position. The final models explained 78% and 59% of the variability in catch rates of M. capensis and M. paradoxus, respectively. For M. capensis, the response to zenith angle increased sharply for values above 100°, which represents the time between sunset and sunrise. For M. paradoxus there was a moderate increase in the response to zenith angle during the night. In cases where some fishing took place at night in shallow water, the survey results for M. capensis were more greatly affected than was the case for M. paradoxus, which is related to the different depth preference of the two species. Fishing in depths shallower than 400 m outside daylight hours should therefore be avoided in order to reduce bias and ensure consistency in abundance estimates from surveys.

Characterization of Western Painted Turtle Bycatch in Fyke Nets During Freshwater Fish Population Assessments

AbstractIn fisheries management, fish populations are assessed using various net types that invariably also capture nontarget species. Although the bycatch of turtles tends to be a common occurrence, data describing the bycatch of turtles during freshwater fish sampling are lacking. To improve the available knowledge base concerning the bycatch of turtles during fish sampling, we characterize the dynamics of western painted turtle Chrysemys picta bellii bycatch in unbaited modified fyke nets used in fish population sampling in northeastern South Dakota. We collected data from June to September during fish population assessments in 39 lakes and nine impoundments between 2007 and 2012. We characterize western painted turtle bycatch relating to water type (lake and impoundment) including catch rates (number of turtles/net night), size structure, and sex ratio. Catch rates were higher in impoundments than lakes. Total mean annual catch rates ranged from 1.07 to 3.28 for lakes and from 0.70 to 6.63 for impoundments and the variation among years was significant for both water types. We observed no annual variation in water surface area or mean depth, precluding either from explaining the variation in annual catch rates. We observed a significant relationship between mean depth and catch rate for lakes, but not impoundments. We observed no significant relationship relating surface area to catch rate for lakes or impoundments. Catch rates differed significantly from June to September for lakes but not impoundments. Annual variation in catch rates was best explained by the previous winter precipitation for both water types. The sex ratio was skewed toward males and differed significantly from June to September for lakes but not impoundments. The size structure was skewed toward large turtles. Understanding bycatch dynamics during fish population assessments is a critical first step to understanding the impact of biological sampling on nontarget species and may prove useful in minimizing future bycatch of western painted turtles.

Time series analyses reveal environmental and fisheries controls on Atlantic horse mackerel (Trachurus trachurus) catch rates

Time-series models (Dynamic factorial analyses and; Min/max autocorrelation factor analysis) were used to explore the relative influences of environmental variables and fishing pressure of trawl, seine and artisanal fleets on catch rates on Trachurus trachurus in ICES IXa sub-divisions (IXaCN-North coast; IXa- CS-South coast; IXaS-Algarve, South coast, Algarve). Fishing effort influenced catch rates in all areas with a 2 year lag and fishing pressure for each area was related to specific fleet sectors effort. In IXaCN, winter upwelling (spawning peak) and both summer northerly wind and wind magnitude (outside of the spawning peak) were strongly correlated with catch rates. In IXaCS summer/autumn westerly winds were related with catch rates. Northerly winds in spring, upwelling and SST (winter and autumn) were related with catch rates in IXaS-Algarve. For species with a long spawning season such as horse mackerel, seasonal analyses at broad regional scales can detract from a better understanding of variability in short term sub-stock catch rates. Favorable environmental conditions, even during seasons with low spawning activity can positively affect catch rates. Ignoring the role of regional oceanographic features on the spatial distribution of the sub-stocks when analysing variability in catch rates can lead to poor inferences about the productivity of the populations.

Geostatistical delta-generalized linear mixed models improve precision for estimated abundance indices for West Coast groundfishes

AbstractIndices of abundance are the bedrock for stock assessments or empirical management procedures used to manage fishery catches for fish populations worldwide, and are generally obtained by processing catch-rate data. Recent research suggests that geostatistical models can explain a substantial portion of variability in catch rates via the location of samples (i.e. whether located in high- or low-density habitats), and thus use available catch-rate data more efficiently than conventional “design-based” or stratified estimators. However, the generality of this conclusion is currently unknown because geostatistical models are computationally challenging to simulation-test and have not previously been evaluated using multiple species. We develop a new maximum likelihood estimator for geostatistical index standardization, which uses recent improvements in estimation for Gaussian random fields. We apply the model to data for 28 groundfish species off the U.S. West Coast and compare results to a previous “stratified” index standardization model, which accounts for spatial variation using post-stratification of available data. This demonstrates that the stratified model generates a relative index with 60% larger estimation intervals than the geostatistical model. We also apply both models to simulated data and demonstrate (i) that the geostatistical model has well-calibrated confidence intervals (they include the true value at approximately the nominal rate), (ii) that neither model on average under- or overestimates changes in abundance, and (iii) that the geostatistical model has on average 20% lower estimation errors than a stratified model. We therefore conclude that the geostatistical model uses survey data more efficiently than the stratified model, and therefore provides a more cost-efficient treatment for historical and ongoing fish sampling data.