Gillnet Surveys Research Articles

Prey Composition of Great Crested Grebe (Podiceps cristatus) and Red-Throated Diver (Gavia stellata) in Freshwater Reservoirs during Migration

The diet composition and prey length of the Great Crested Grebe (Podiceps cristatus) and the Red-throated Diver (Gavia stellata) co-occurring in late autumn in two reservoirs was studied. Their diet was analyzed based on carcasses from incidental mortality of individuals in fishery gill net surveys in two different freshwater reservoirs (one deep, one shallow) in southern Poland during autumn migration (October - December). Five fish species were identified in the diet of the Great Crested Grebe and nine fish species were identified in the diet of the Red-throated Diver. The proportion of a particular fish species in diet was different between the studied bird species. The ruffe (Gymnocephalus cernua) was the most common prey in the diet of the Great Crested Grebe at both reservoirs. The diet of Red-throated Diver was dominated by roach (Rutilus rutilus) at the deep reservoir and the zander (Sander lucioperca) at the shallow one. The Red-throated Diver consumed larger fish than the Great Crested Grebe at both reservoirs.

Using environmental DNA for biomonitoring of freshwater fish communities: Comparison with established gillnet surveys in a boreal hydroelectric impoundment

AbstractAccurate data characterizing species distribution and abundance are critical for conservation and management of aquatic resources. Inventory methods, such as gillnet surveys, are widely used to estimate distribution and abundance of fish. However, gillnet surveys can be costly in terms of material and human resources, may cause unwanted mortality in the fish communities being studied, and is subject to size and species selection bias. Detecting allochthonous DNA released by species in their environment (i.e., environmental DNA, hereafter eDNA) could be used as a noninvasive and less costly alternative. In this study, we directly compare eDNA metabarcoding and gillnets for monitoring freshwater fish communities in terms of species richness and relative species abundance. Metabarcoding was performed with the 12S Mifish primers. We also used species‐specific quantitative PCR (qPCR) for the most abundant species, the walleye (Sander vitreus), to compare estimated relative abundance with metabarcoding and gillnet captures. Water sample collection, prior to gillnet assessment, was performed on 17 sites in the hydroelectric impoundment of the Rupert River (James Bay, Canada), comparing two water filtration methods. After controlling for amplification biases and repeatability, we show that fish communities’ complexity is better represented using eDNA metabarcoding than previously recorded gillnet data and that metabarcoding read count correlates with qPCR (r = 0.78, p < .001) in reflecting walleye abundance. Finally, based on partial redundancy analysis, we identified alpha chlorophyll, pH, and dissolved oxygen as environmental variable candidates that may influence differences in fish relative abundance between metabarcoding and gillnets. Altogether, our study demonstrates that the proposed eDNA metabarcoding method can be used as an efficient alternative or complementary technique adapted to the biomonitoring of the fish communities in boreal aquatic ecosystems.

Identification and Delineation of Essential Habitat for Elasmobranchs in Estuaries on the Texas Coast

Essential habitat is required for a given species to complete its lifecycle and includes habitat for breeding, feeding, and growth. Estuaries provide essential habitat for many fishes, including elasmobranchs, particularly as nurseries. Shore-based, gillnet surveys have documented ten elasmobranch species (sharks and batoids) inhabiting Texas estuaries; however, limitations in the methodology, spatial, and temporal scope employed mean the role of these estuaries as elasmobranch nurseries remains uncertain. Therefore, an alternative sampling technique was implemented to survey open water locations in two Texas estuaries during the peak time of putative elasmobranch nursery usage (May to October). Ten species of elasmobranch were observed, including a shark, Carcharhinus porosus, and batoid species, Hypanus americanus, that were not documented in previous surveys. The most commonly encountered species, Carcharhinus brevipinna, comprised 45% of individuals caught, in contrast to previous surveys of these estuaries, in which the species was less than 1% of the total elasmobranch catch. The results revealed a distinction between sharks and batoids in terms of the life history stages present, as well as the depths and distances from the tidal inlet of areas in which they were encountered. Young-of-the-year and small juvenile Carcharhinus brevipinna and Sphyrna lewini were observed across multiple years and in multiple months within years at one site, suggesting that one of the sampled estuaries functions as a nursery. Although putative nurseries for these species have been identified elsewhere in the world, estuaries serving as nursery grounds in the western Gulf of Mexico are not well described.

Gillnet catchability of Walleye (Sander vitreus): comparison of North American and provincial standards

Multi-mesh index gillnet surveys are widely employed for the assessment of fish populations, including Walleye (Sander vitreus), an important commercial and recreational species in North America. In this paper, we estimate gillnet catchability of Walleye for two standard sampling protocols: the North American (NA) and the Fall Walleye Index Netting (FWIN), allowing the conversion of Catch Per Unit Effort (CPUE) into absolute estimates of fish density or abundance. We combined mark-recapture studies from 14 lakes in Ontario and Quebec, 11 of which were concurrently sampled using NA and FWIN methods, totalling 53 recapture surveys from 1996 to 2012. To account for lake-specific influences on catchability, we analyzed the data with Generalized Linear Mixed Models (GLMMs) using lake as a random factor and a model-averaging approach to calculate mean estimates and variances. Catchability for Walleye ≥ 350mm total length was 1.04 ha/gang for the FWIN, meaning each fish caught in a FWIN net gang (1.8 x 61 m) represents roughly one fish per hectare. Catchability for one gang of NA net (1.8 x 24.8 m) was 0.52 ha/gang, which is 20% higher than FWIN catchability after accounting for net area. Two important sources of variation in catchability were: (i) lake-specific differences and (ii) seasonal changes within the NA protocol. The mechanisms underlying such variation are still not clear and will require additional studies covering a broader geographic range to reveal potential environment-catchability relationships. We provide recommendations for estimating Walleye density and confidence intervals, and for how to combine our catchability estimates with size-selectivity curves to predict density at any specific size interval.

Species succession and the development of a lacustrine fish community in an ephemeral lake.

Here, we present a gillnet survey of Lake Liambezi a 370 km2 shallow ephemeral floodplain lake situated in north-eastern Namibia, which is fed irregularly by the upper Zambezi and Kwando Rivers during years of high flooding. The lake dried up in 1985 and, with the exception of sporadic minor annual inundation events, remained dry until 2007. We describe the temporal succession of fish species over an 8 year period from initial inundation 2007 to maturation in 2014. The succession of the fish community did not follow the typical pattern of opportunistic strategists during colonisation, to periodic strategists that are eventually succeeded by equilibrium strategists. Instead, the evolution of the fish community was characterised by three distinct phases. The first phase involved the inundation and colonisation of the lake in 2007, followed by its decline until the floods that filled the lake in 2009. During this phase the lake was colonised by fishes from the adjacent upper Zambezi and Chobe River floodplains. Fish communities predominantly comprised floodplain specialists including the barbs Enteromius paludinosus and Enteromius poechii, the mormyrid Marcusenius altisambesi and catfishes Schilbe intermedius and Clarias gariepinus. The filling of the lake in the March 2009 floods marked the beginning of the second, successional phase. The barbs declined in abundance and the alestid Rhabdalestes maunensis underwent explosive population growth between 2009 and 2010, but populations crashed equally rapidly and were replaced by Brycinus lateralis which, together with S. intermedius went on to dominate the fish community 2011-2014. Larger, slower growing tilapiine cichlids increased steadily in abundance and became the dominant components in a 2700 t y-1 artisanal fishery that developed on the lake. The fish community in the ephemeral Lake Liambezi is clearly influenced by numerous factors including connectivity, lake level fluctuations, competition and the effects of fishing, which may disrupt typical succession processes in floodplain ecosystems.

Resurgence of Cisco (Coregonus artedi) in Lake Michigan

In recent decades, many factors that were linked with the decline of Great Lakes cisco (Coregonus artedi) populations have subsided. The goal of this study was to investigate where cisco exist in Lake Michigan and evaluate evidence for recovery including when, where, and to what extent it is occurring. We evaluated datasets from several independent monitoring efforts that did and did not target cisco. We also evaluated trends in commercial and recreational catches of cisco. Across these datasets, there was strong evidence of a sustained recovery of cisco stocks that began in Lake Michigan in the mid-2000s. Fall gill net surveys and commercial fisheries provided reasonable indications of a population recovery in the northeast by 2011. Further south, Ludington Pump Storage barrier net monitoring also recorded increasing numbers of cisco starting in 2011. Recreational harvest estimates were valuable in evaluating spatial distributions but were less valuable as an early signal of abundance shifts. Measures of the recreational harvest of cisco most notably increased in 2014. The highest catch rates and harvest occurred in Grand Traverse Bay and northern Lake Michigan as evidenced by recreational, commercial, and fall netting surveys. Observations of cisco are expanding and have increased in intensity along the eastern shore of Lake Michigan south to Muskegon in both fishery dependent and independent surveys. The similarity in trends from all data sources indicate that cisco abundance has increased, and their range within the basin continues to expand.

Long-Term Data Suggest Potential Interactions of Introduced Walleye and Smallmouth Bass on Native Sauger in Four Missouri River Impoundments

Abstract Sauger Sander canadensis, Walleye Sander vitreus, and Smallmouth Bass Micropterus dolomieu, are important sportfish in the four main stem Missouri River reservoirs in South Dakota: Lakes Oahe, Sharpe, Francis Case, and Lewis and Clark. However, native Sauger populations, once assumed to be stable, may be in decline. To identify temporal trends and potential interspecific mechanisms affecting Sauger populations, we examined their long-term abundance trends in standard gillnet surveys and angler catch and harvest trends in long-term creel surveys. We also used a robust design occupancy model to examine changes in within-lake distribution of this species. There are concerns regarding the effects of Walleye and Smallmouth Bass on Sauger, so we also described the population trends of these potential competitors. Standard gillnet surveys indicated declining abundance of both Sauger and Walleye in Lakes Oahe and Sharpe. Sauger abundance has trended down in Lewis and Clark, but upward in Francis Case. Conversely, Walleye abundance trends declined in Francis Case and increased in Lewis and Clark. Occupancy (ψ̂) of Sauger declined in all four reservoirs, indicating a contracting distribution throughout the reservoirs. Walleye occupancy remained ∼1.0. Smallmouth Bass occupancy increased in the three reservoirs with sufficient data for analysis, excluding Lewis and Clark Lake. Smallmouth Bass exhibited steady increases in angler catch and harvest, as well as abundance in long-term gillnet surveys, suggesting expanding and increasing populations. Habitat alteration is hypothesized to be a major driver of the Sauger occupancy and abundance declines. However, Walleye and Smallmouth Bass interactions could also be contributing to observed declines of native Sauger.

A Bayesian spatiotemporal approach to inform management unit appropriateness

One prerequisite for sustainable fisheries management is to match management actions with biological processes. Stocks are fundamental units for fisheries management. Understanding the spatial structure of fish stocks is critical for conducting defensible stock assessments, applying efficient management strategies, and ensuring the sustainability of fish stocks. Yellow perch (Perca flavescens) is an important fishery in the Great Lakes. The appropriateness of its management units (MUs) has been identified as of high concern by the Great Lakes Fisheries Commission. Here we established integrated nested Laplace approximations and stochastic partial differential equations as two powerful tools for modeling spatiotemporal patterns of fish relative biomass. These fast computational approaches were applied to fit a Bayesian hierarchical hurdle model to occurrence and positive mass of yellow perch caught in gill-net surveys. Yellow perch relative biomass index has clear temporal variation and spatial heterogeneity, with the two middle MUs for yellow perch within Lake Erie merging together. The method explicitly models the spatiotemporal correlation structure inherent in biomass survey data at a reasonable computational cost, and the estimated spatiotemporal correlation informs stock structure.

Evaluating catchability in a large-scale gillnet survey using hydroacoustics: Making the case for coupled surveys

Abundance estimates facilitate successful fisheries management. Fisheries agencies often monitor abundance through fishery independent standardized protocols generating relative measures such as catch-per-unit-effort (CPUE), where CPUE is assumed proportional to true abundance. Unfortunately, this assumption is rarely met as fish behavior and environmental conditions influence catchability and sample gear efficiency. We used paired gillnet and hydroacoustic samples and a catchability equation (Ui = qNiβ) to assess the correspondence between gillnet CPUE (Ui) and hydroacoustic abundance estimates (Ni). We found that gill nets were hyperstable (i.e., β < 1) and efficiency declined along environmental gradients. These gradients, such as increased depths, and decreased turbidity and water temperatures, likely influenced fish behavior, and encounter and gear saturation rates. As a result, catchability declined with increasing abundance qacross survey regions. Finally, simulations showed that catchability gradients and variable migratory patterns can contribute to annual variation in CPUE indices regardless of changes in abundance. Surveys plagued by varying catchability could benefit from coupling with hydroacoustics, a sample gear less subject to gear efficiency and catchability issues.

Increased Abundance and Nursery Habitat Use of the Bull Shark (Carcharhinus leucas) in Response to a Changing Environment in a Warm-Temperate Estuary

A general northward shift in marine species distributions has been observed in the western North Atlantic Ocean, which may have significant ecological consequences. Large coastal sharks can have wide migratory distributions but show fidelity to specific nursery habitats. Here we show evidence for nursery range expansion into Pamlico Sound, North Carolina by a marine apex predator, the Bull Shark (Carcharhinus leucas). Previous assessments have shown little to no use of estuarine North Carolina waters as nursery habitat by Bull Sharks from 1965–2011. Juvenile sharks were rarely captured in a fishery-independent gillnet survey conducted by the North Carolina Division of Marine Fisheries (NCDMF) from 2003–2011, but were present every year from 2011–2016. Juvenile Bull Shark presence in the Sound was strongly related to early summer temperatures and late summer salinities, which have increased in the estuary over the 13 survey years, and further evidence for increasing water temperatures in Pamlico Sound was found in a 45-year data set for the NCDMF estuarine trawl survey. These results suggest that increasing water temperature and salinity have allowed Bull Sharks to expand their nursery habitat. This shift will have unknown, but potentially strong, impacts on both the local ecosystem and interactions with humans.

Gillnet data enhance performance of rockfishes habitat suitability index model derived from bottom-trawl survey data: A case study with Sebasticus marmoratus

Bottom-trawl surveys are widely used to generate data for developing habitat suitability models for rockfish species. However, bottom trawl selectivity varies with substrate type and can lead to biased and unreliable model outputs. Bottom-set gillnets provide more efficient and reliable data in untrawlable substrates, and fine-tuning bottom-trawl survey indices with supplementary gillnet survey data could enhance performance of rockfish habitat models. We applied this approach with bottom trawl and gillnet data for Sebasticus marmoratus in Ma’an Archipelago, China. Abundance indices and environmental data collected by the gillnet surveys were used to tune the bottom trawl survey data for empirical habitat suitability modeling of S. marmoratus. Under cross-validation, the habitat model based on tuned trawl data generated ecologically reliable suitability indices (SIs) and improved habitat suitability prediction for Sebasticus marmoratus compared to the habitat model developed using untuned trawl data. The data-tuning method developed in this study can be used to enhance the performance of habitat models of other demersal species whose habitats are not adequately sampled by bottom-trawl surveys.

Community structure of elasmobranchs in estuaries along the northwest Gulf of Mexico

Estuaries promote high levels of productivity and biodiversity by providing habitat for many biological communities due to their wide range of environmental conditions. Estuarine systems serve as nurseries, areas for parturition, and feeding grounds for elasmobranchs. However, estuaries face an array of anthropogenic pressures, including overfishing, altered flow regimes, pollution, and habitat destruction. Given the vulnerability of estuarine ecosystems, observing long-term changes in community structure is essential to understanding the effects of anthropogenic stressors. Elasmobranch community structure was analyzed among eight estuaries in the northwest Gulf of Mexico to evaluate spatial and temporal variability in species abundance and diversity using bi-annual fisheries independent gillnet survey data over three decades (1985–2014). Ten species comprised 99.4% of elasmobranchs caught which included 35.3% bull sharks (Carcharhinus leucas), 18.1% bonnetheads (Sphyrna tiburo), 17.0% cownose rays (Rhinoptera bonasus), 13.4% blacktip sharks (Carcharhinus limbatus), 5.9% Atlantic stingrays (Dasyatis sabina), 3.1% Atlantic sharpnose sharks (Rhizoprionodon terraenovae), 2.7% spinner sharks (Carcharhinus brevipinna), 2.1% scalloped hammerheads (Sphyrna lewini), 1.7% finetooth sharks (Carcharhinus isodon), and 0.7% lemon sharks (Negaprion brevirostris). During the study period, elasmobranch community structure changed among estuaries and among decades. Bull sharks, bonnetheads, cownose rays, blacktip sharks, and spinner sharks all increased in abundance during the study period, whereas finetooth sharks and lemon sharks decreased over time. Higher latitude estuaries were dominated by bull sharks while lower latitude estuaries were dominated by cownose rays. Salinity was the most important environmental variable in predicting individual elasmobranch species abundance (deviance explained: 14.4 ± 6.5 SD), while temperature and depth also played a role in shaping community structure. Diversity was greatest in mid-latitudinal estuaries with spatially and temporally dynamic salinity regimes. As environmental change and human impacts persist across much of the world, understanding environmental drivers of community structure using long-term datasets will provide insight to how these changes influence coastal ecosystems, and enable more comprehensive and scale-independent models to be developed for the management and conservation of coastal ecotones.

Long-term change in the distribution of Japanese sardine in the Sea of Japan during population fluctuations

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 593:141-154 (2018) - DOI: https://doi.org/10.3354/meps12491 Long-term change in the distribution of Japanese sardine in the Sea of Japan during population fluctuations Soyoka Muko1,*, Seiji Ohshimo1, Hiroyuki Kurota1, Tohya Yasuda1, Masa-Aki Fukuwaka1,2 1Seikai National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Nagasaki 851-2213, Japan 2Present address: Hokkaido National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Sapporo 062-0922, Japan *Corresponding author: symuko@gmail.com ABSTRACT: The distribution of small pelagic fish species changes in association with fluctuating stock biomass. We analyzed the spatio-temporal dynamics of Japanese sardine Sardinops melanostictus inhabiting the Sea of Japan during periods of stock biomass fluctuation using generalized additive models (GAMs). Logbook data from drift gill net surveys and large- and medium-type purse seine fisheries, conducted from 1970 to 1999, were used. A 2-step data analysis was performed: (1) for the presence or absence of sardine, and (2) for the catch per unit effort (CPUE) where fish were present. The explanatory variables in the GAMs were year, longitude/latitude, sea surface temperature, and fishing method. The best model based on Akaike’s information criterion showed that the likelihood of high sardine presence was limited to areas around the Oki and Tsushima Islands in the early 1970s. However, these areas expanded south along the coast and offshore to the north until the mid-1980s. Subsequently, sardine presence contracted to the initial distribution areas during the mid-1990s. Over the 30 yr period, the spatial distribution of the CPUE changed minimally for both fishing methods. Compared with the trajectory of stock biomass, the potential distribution area began expanding during the initial phase of population expansion, and started contracting before biomass peaked. There was a time lag between the peaks of distribution area and biomass. In conclusion, we discuss how changes to oceanographic conditions in response to a climate regime shift may influence the distribution of Japanese sardine in the Sea of Japan. KEY WORDS: Japanese sardine · Sardinops melanostictus · Spatio-temporal dynamics · Generalized additive model Full text in pdf format Supplementary material PreviousNextCite this article as: Muko S, Ohshimo S, Kurota H, Yasuda T, Fukuwaka MA (2018) Long-term change in the distribution of Japanese sardine in the Sea of Japan during population fluctuations. Mar Ecol Prog Ser 593:141-154. https://doi.org/10.3354/meps12491 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 593. Online publication date: April 12, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Relative importance of fishing and natural mortality for spotted seatrout (Cynoscion nebulosus) estimated from a tag-return model and corroborated with survey data

Abstract The spotted seatrout (Cynoscion nebulosus) is one of the most economically important sportfish in the U.S. South Atlantic and Gulf of Mexico, including at its northern distributional extent in North Carolina and Virginia. The recent stock assessment for this region used an assumed fixed rate of natural mortality (M), obtained from a general life-history relationship based on weight. However, biased estimates of fishing mortality (F) could result if the life-history proxy failed to capture either the magnitude or temporal variation in M. Data from the first comprehensive tag-return study of spotted seatrout in this region were used in a Bayesian statistical modeling framework to estimate F and M. Both laboratory and field studies, including high-reward and double tagging, were conducted to obtain estimates of auxiliary parameters (i.e., tag-reporting rate, tag loss, and tagging mortality) necessary for the tag-return model. There was no measured mortality associated with tagging, but reporting rate and loss of internal anchor tags limited returns in this study. From 2008 to 2012, tag-return model estimates of bimonthly instantaneous mortality rates ranged from 0.003 to 0.067 2-mo−1 for F and from 0.002 to 2.850 2-mo−1 for M. Annual estimates of F were much lower than M for the three years studied, and annual M-estimates were higher than those used for spotted seatrout in this region’s recent stock assessment. Bimonthly estimates of total mortality rate (Z) from tag-return data were similar to bimonthly estimates of Z from an independent analysis of concurrent gill net survey data, which corroborates the variability and magnitude of mortality estimates determined from tagging. A strong seasonal influence (i.e., winter severity) on annual loss of spotted seatrout was observed, suggesting that future assessments and management measures for this stock would be improved by explicitly accounting for temporal variation in M in models of fishery population dynamics.

Habitat supply for Yellow Perch (Actinopterygii, Percidae) varies with space, time, and life stage in Lake Erie

Exploring species-environment relationships to identify essential habitat areas is recognized as an effective way for resource conservation and management. Because of ecological and economic importance of Yellow Perch (Perca flavescens) in Lake Erie, it is essential to develop appropriate habitat models to analyze habitat supply of this species. Empirical species-environment models for two age-groups (juveniles and adults) at two depth strata were developed based on a long-term (27 years) gillnet survey data. Models incorporated interactive terms indicate a high degree of prediction accuracy and satisfied the assumption of residual independence. As a benthivorous species, high-quality habitat is mainly distributed in the bottom waters. Suitable habitat for adults is also found in the mid-waters in the central and east basins. Weighted suitability index per unit area differs significantly among years, basins, and vertical strata. Currently, quota allocation among jurisdictions is determined by using GIS applications of jurisdictional surface area of waters within each management unit. The outputs from our study can be used to revise the current quota allocation schemes and habitat-based quota allocation protocol will provide more scientific and rational total allowable catch for each jurisdiction.

Disentangling the effects of a century of eutrophication and climate warming on freshwater lake fish assemblages.

Eutrophication and climate warming are profoundly affecting fish in many freshwater lakes. Understanding the specific effects of these stressors is critical for development of effective adaptation and remediation strategies for conserving fish populations in a changing environment. Ecological niche models that incorporated the individual effects of nutrient concentration and climate were developed for 25 species of fish sampled in standard gillnet surveys from 1,577 Minnesota lakes. Lake phosphorus concentrations and climates were hindcasted to a pre-disturbance period of 1896–1925 using existing land use models and historical temperature data. Then historical fish assemblages were reconstructed using the ecological niche models. Substantial changes were noted when reconstructed fish assemblages were compared to those from the contemporary period (1981–2010). Disentangling the sometimes opposing, sometimes compounding, effects of eutrophication and climate warming was critical for understanding changes in fish assemblages. Reconstructed abundances of eutrophication-tolerant, warmwater taxa increased in prairie lakes that experienced significant eutrophication and climate warming. Eutrophication-intolerant, warmwater taxa abundance increased in forest lakes where primarily climate warming was the stressor. Coolwater fish declined in abundance in both ecoregions. Large changes in modeled abundance occurred when the effects of both climate and eutrophication operated in the same direction for some species. Conversely, the effects of climate warming and eutrophication operated in opposing directions for other species and dampened net changes in abundance. Quantifying the specific effects of climate and eutrophication will allow water resource managers to better understand how lakes have changed and provide expectations for sustainable fish assemblages in the future.

Gill net catch composition and catch per unit effort in Flag Boshielo Dam, Limpopo Province, South Africa

Gill net surveys were conducted in 2013 to determine species composition and fisheries potential of Flag Boshielo Dam. Species contributing the most towards total biomass were Labeo rosae (40%), Oreochromis mossambicus (15%), Schilbe intermedius (10%) and Labeobarbus marequensis (9.8%). Catch per unit effort for gill nets set at night (4.4 ± 0.6 kg·100 m-net −1 ·hr −1 ) was significantly higher ( P < 0.05) than for those set during the day (0.9 ± 0.1 kg·100 m-net −1 ·hr −1 ). Total fish biomass captured in 30, 50, 70, 90 and 110 mm mesh sized nets was 3.1, 31.5, 43.5, 23.5 and 16.1 kg, respectively. Catch in gillnets with mesh sizes ≥ 70 mm was dominated by L. rosae comprising 60% of the catch in the 70 mm mesh; L. rosae (40%) and O. mossambicus (36%) in the 90 mm mesh; and O. mossambicus (40%) and Clarias gariepinus (40%) in the 110 mm mesh. If a small-scale fishery were to be initiated, it is recommended that mesh sizes should exceed 70 mm and that further research on the biology and ecology of the main target species and of the current utilisation of the fishery be conducted to guide sustainable utilisation. Keywords: Arabie Dam, CPUE, fish composition, net selectivity, inland fisheries

Characterizing and predicting the distribution of Baltic Sea flounder (Platichthys flesus) during the spawning season

Identification of essential fish habitats (EFH), such as spawning habitats, is important for nature conservation, sustainable fisheries management and marine spatial planning. Two sympatric flounder (Platichthys flesus) ecotypes are present in the Baltic Sea, pelagic and demersal spawning flounder, both displaying ecological and physiological adaptations to the low-salinity environment of this young inland sea. In this study we have addressed three main research questions: 1) What environmental conditions characterize the spatial distribution and abundance of adult flounder during the spawning season? 2) What are the main factors defining the habitats of the two flounder ecotypes during the spawning season? 3) Where are the potential spawning areas of flounder? We modelled catch per unit of effort (CPUE) of flounder from gillnet surveys conducted over the southern and central Baltic Sea in the spring of 2014 and 2015 using generalized additive models. A general model included all the stations fished during the survey while two other models, one for the demersal and one for the pelagic spawning flounder, included only the stations where each flounder ecotype should dominate. The general model captured distinct ecotype-specific signals as it identified dual salinity and water depth responses. The model for the demersal spawning flounder revealed a negative relation with the abundance of round goby (Neogobius melanostomus) and a positive relation with Secchi depth and cod abundance. Vegetation and substrate did not play an important role in the choice of habitat for the demersal ecotype. The model for the pelagic spawning flounder showed a negative relation with temperature and bottom current and a positive relation with salinity. Spatial predictions of potential spawning areas of flounder showed a decrease in habitat availability for the pelagic spawning flounder over the last 20years in the central part of the Baltic Sea, which may explain part of the observed changes in populations' biomass. We conclude that spatiotemporal modelling of habitat availability can improve our understanding of fish stock dynamics and may provide necessary biological knowledge for the development of marine spatial plans.

Spatial distribution analysis of the North Pacific spiny dogfish, Squalus suckleyi, in the North Pacific using generalized additive models

AbstractThe North Pacific spiny dogfish (SPD), Squalus suckleyi, is a commercially exploited shark species that plays an important role in the ecosystem. To elucidate the distribution of the SPD in the North Pacific and to evaluate the effects of sea surface temperature (SST) and prey availability on its distribution, we estimated the probability of SPD presence using a generalized additive model with a binomial error distribution from SPD presence/absence data on 14,824 operations in fishery‐independent gillnet surveys between 1972 and 2011. The habitat model was structured in the east and west to reflect differences in the North Pacific oceanic environments. In the east, a higher probability of SPD presence was identified along the coast from the eastern Gulf of Alaska to Queen Charlotte Sound. In the west, it was identified around northern Japan. The estimated distribution was continuous between the two areas, whereas the probability of SPD presence was relatively low. Although the probability of SPD presence was higher at SSTs between 6°C and 12°C, the SST at the peak probability of SPD presence differed between the west and east. The prey species, Japanese sardine, Sardinops melanostictus, and walleye pollock, Gadus chalcogrammus, in the west and boreal clubhook squid, Onychoteuthis borealijaponica, in the east significantly affected the probability of SPD presence, which was higher if the prey species co‐existed with SPD. Therefore, SPD might adapt their distribution to that of available prey species. SPD stock assessment and management in these two important areas are required for its sustainable utilization.

Factors shaping the co-occurrence of two juvenile shark species along the Texas Gulf Coast

As top predators, sharks play important ecological roles in coastal marine ecosystems. Yet, environmental changes in many coastal regions are likely altering the composition of ecological communities, the interspecific interactions sharks have within coastal waters, and subsequently the ecological roles sharks play within these ecosystems. As such, understanding interactions among sharks, and how extrinsic factors shape these interactions, is important for predicting the consequences of future human impacts and environmental changes. Elucidating the contexts under which species co-occur and the implications of co-occurrence is an important step in developing such a predictive framework. Blacktip shark (Carcharhinus limbatus) and bull shark (Carcharhinus leucas) concentrations were quantified using long-term coastal gill net survey data across five bay systems in Texas. Relationships between co-occurrence, and environmental factors and shark sizes were examined within and across species. Co-occurrence of blacktip sharks and bull sharks varied spatially and temporally, with a significant increase in interspecific co-occurrence from the 1970s to 2010s, and a significant decrease in bull shark concentrations through time. Changes in environmental conditions, specifically increasing salinities, may have been responsible for increased blacktip and bull shark co-occurrence, and potential interspecific competition, which in turn may have led to decreased bull shark concentrations to reduce intraspecific competition. However, more refined questions are needed before predictive frameworks can be developed concerning the contexts under which co-occurrence is prevalent. Quantifying resource use among coastal sharks will help elucidate the drivers and implications of co-occurrence, and the potential for competitive interactions within and across species.