Fishery biomass trends of exploited fish populations in marine ecoregions, climatic zones and ocean basins

Highlights: • 1st global long-term fishery biomass trends evaluation of 1300 exploited marine populations. • Decline in average fishery biomass observed across oceans and climate zones. • Systemic wide-spread overfishing of the world's coastal and continental shelf water. Abstract: This contribution presents time series of the ‘fishery biomass’ of fish populations, defined as the weight (whole-body, wet weight) of the in-water part of a fishable population, i.e., that part of a population (also called ‘stock’) that is exposed to a certain fishing gear. Detailed data of this type are only available for a limited number of species that are targets of the fisheries in the waters of economically developed regions, such as Europe, the USA, Canada or Australia. However, similar fishery biomass assessments are generally lacking for developing countries, even for many of their most heavily fished species. Here, an estimation of the long-term fishery biomass trends of 1320 fish and invertebrate populations for 483 species exploited by fisheries in the 232 coastal Marine Ecoregions (MEs) around the world was undertaken. Fishery biomass trends were derived using the Bayesian CMSY stock assessment method applied to the global fisheries catch database for 1950–2014 as reconstructed by the Sea Around Us for every maritime fishing country in the world. Overall, the results suggest a consistent decline in the fishery biomass of exploited populations, in virtually all climatic zones and ocean basins in the world. The only zone with currently higher fishery biomass than in 1950 is the northern Pacific polar-boreal zone, likely due to environmental changes that occurred in the region positively affecting fish populations, combined with prudent management of the fisheries. For populations in MEs that are known to have highly questionable catch statistics, the results suggested smaller declines in fishery biomass than likely occurred in reality, implying that these results do not exaggerate declining trends in fishery biomass. This study used informative Bayesian priors to improve the trend analyses in areas where systematic stock assessments were conducted. The use of these independent assessments reduced the uncertainty associated with the findings of this study

Combining in-trawl video with observer coverage improves understanding of protected and vulnerable species by-catch in trawl fisheries

Assessments of incidental wildlife mortality resulting from fishing rarely account for unobserved by-catch. We assessed by-catch of protected and vulnerable wildlife species in an Australian trawl fishery by comparing in-trawl video footage with data collected by an on-board observer. Data were obtained from 44 commercial trawls with two different by-catch reduction devices (BRDs). Eighty-six individuals from six major taxa (dolphins, sharks, rays, sea snakes, turtles and sygnathids) were documented from video analysis, including the endangered scalloped hammerhead shark (Sphyrna lewini) and the critically endangered green sawfish (Pristis zijsron). On the basis of the 2008–2009 fishing effort of 4149 trawls and scaling from these results, we estimated the annual catch of protected and vulnerable species (± 1 s.e.) at 8109 ± 910 individuals. Only 34% of by-catch was expelled through the BRDs. Independent observer data for the 44 trawls showed that 77% of the landed by-catch from these taxa were dead when discarded. The results indicate that unaccounted by-catch in trawl fisheries can be substantial, and that current methods of recording by-catch on-board vessels are likely to underestimate total fishing mortality. We recommend gear modifications and their validation through dedicated observer coverage, combined with in-trawl video camera deployments to improve current approaches to by-catch mitigation

Similarities between line fishing and baited stereo-video estimations of length-frequency: Novel application of Kernel Density Estimates

Age structure data is essential for single species stock assessments but length-frequency data can provide complementary information. In south-western Australia, the majority of these data for exploited species are derived from line caught fish. However, baited remote underwater stereo-video systems (stereo-BRUVS) surveys have also been found to provide accurate length measurements. Given that line fishing tends to be biased towards larger fish, we predicted that, stereo-BRUVS would yield length-frequency data with a smaller mean length and skewed towards smaller fish than that collected by fisheries-independent line fishing. To assess the biases and selectivity of stereo-BRUVS and line fishing we compared the length-frequencies obtained for three commonly fished species, using a novel application of the Kernel Density Estimate (KDE) method and the established Kolmogorov–Smirnov (KS) test. The shape of the length-frequency distribution obtained for the labrid Choerodon rubescens by stereo-BRUVS and line fishing did not differ significantly, but, as predicted, the mean length estimated from stereo-BRUVS was 17% smaller. Contrary to our predictions, the mean length and shape of the length-frequency distribution for the epinephelid Epinephelides armatus did not differ significantly between line fishing and stereo-BRUVS. For the sparid Pagrus auratus, the length frequency distribution derived from the stereo-BRUVS method was bi-modal, while that from line fishing was uni-modal. However, the location of the first modal length class for P. auratus observed by each sampling method was similar. No differences were found between the results of the KS and KDE tests, however, KDE provided a data-driven method for approximating length-frequency data to a probability function and a useful way of describing and testing any differences between length-frequency samples. This study found the overall size selectivity of line fishing and stereo-BRUVS were unexpectedly similar

Water temperature and fish growth: otoliths predict growth patterns of a marine fish in a changing climate

Ecological modeling shows that even small, gradual changes in body size in a fish population can have large effects on natural mortality, biomass, and catch. However, efforts to model the impact of climate change on fish growth have been hampered by a lack of long-term (multidecadal) data needed to understand the effects of temperature on growth rates in natural environments. We used a combination of dendrochronology techniques and additive mixed-effects modeling to examine the sensitivity of growth in a long-lived (up to 70 years), endemic marine fish, the western blue groper (Achoerodus gouldii), to changes in water temperature. A multi-decadal biochronology (1952-2003) of growth was constructed from the otoliths of 56 fish collected off the southwestern coast of Western Australia, and we tested for correlations between the mean index chronology and a range of potential environmental drivers. The chronology was significantly correlated with sea surface temperature in the region, but common variance among individuals was low. This suggests that this species has been relatively insensitive to past variations in climate. Growth increment and age data were also used in an additive mixed model to predict otolith growth and body size later this century. Although growth was relatively insensitive to changes in temperature, the model results suggested that a fish aged 20 in 2099 would have an otolith about 10% larger and a body size about 5% larger than a fish aged 20 in 1977. Our study shows that species or populations regarded as relatively insensitive to climate change could still undergo significant changes in growth rate and body size that are likely to have important effects on the productivity and yield of fisheries

From sea ice to blubber:linking whale condition to krill abundance using historical whaling records

Krill (Euphausia superba) are fundamentally important in the Southern Ocean ecosystem, forming a critical food web link between primary producers and top predators. Krill abundance fluctuates with oceanographic conditions, most notably variation in winter sea ice, and is susceptible to environmental change. Although links between local krill availability and performance of land breeding, central place foragers are recognised, the effects of krill variability on baleen whales remain largely unclear because concurrent long-term data on whale condition and krill abundance do not exist. Here, we quantify links between whale body condition and krill abundance using a simple model that links krill abundance to sea ice extent. Body condition of humpback whales (Megaptera novaeangliae) caught in west Australian waters between 1947 and 1963 was estimated from oil yields in whaling records. Annual estimates of krill abundance in the Southern Ocean where those whales foraged (70°–130°E) were correlated significantly with contemporary annual winter sea ice extent. We hindcast sea ice extent for the whaling period from reconstructed temperature data and found that whale body condition was significantly correlated with hindcasted winter sea ice extent, supporting the hypothesis that variations in body condition were likely mediated by associated krill fluctuations. As humpback whales migrate and breed on finite energy stores accrued during summer foraging in the Antarctic, changes in sea ice and concomitant changes in krill abundance have long-term implications for their condition and reproductive success

Assessment of the potential impacts of trap usage and ghost fishing on the Northern Demersal Scalefish Fishery

Fish traps are the principal fishing gear used in the Northern Demersal Scalefish Fishery. These fish traps are left at sea (dumped) between trips and are occasionally lost. The present study quantified the number of fish caught by baited fish traps set on the seabed and left to fish over the short ( 3 hr, within a trip) to medium term (12 days, between trips). Traps continued to retain fish for the duration of the experiment, despite the fact that all bait was exhausted within 3 hr. The catch rate of the traps was not significantly different for a range of teleost species and species groups over the 12-day duration of the experiment, with some exceptions. Catches after the 12-day soak time were similar to those soak times of only a few hours for most species and species groups. Importantly, despite the ability of fish to enter and exit traps, traps set for several days continue to catch fish. Fisheries monitoring and management implications primarily include errors in catch rate estimates from unaccounted fishing effort (soak time) from dumped traps and/or secondarily unaccounted mortality from lost fishing gear, both of which result in increased uncertainty in stock assessments

ცნობის ფურცელი N2390

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Challenges of transferring models of fish abundance between coral reefs

Reliable abundance estimates for species are fundamental in ecology, fisheries, and conservation. Consequently, predictive models able to provide reliable estimates for un- or poorly-surveyed locations would prove a valuable tool for management. Based on commonly used environmental and physical predictors, we developed predictive models of total fish abundance and of abundance by fish family for ten representative taxonomic families for the Great Barrier Reef (GBR) using multiple temporal scenarios. We then tested if models developed for the GBR (reference system) could predict fish abundances at Ningaloo Reef (NR; target system), i.e., if these GBR models could be successfully transferred to NR. Models of abundance by fish family resulted in improved performance (e.g., 44.1%  0.05). High spatio-temporal variability of patterns in fish abundance at the family and population levels in both reef systems likely affected the transferability of these models. Inclusion of additional predictors with potential direct effects on abundance, such as local fishing effort or topographic complexity, may improve transferability of fish abundance models. However, observations of these local-scale predictors are often not available, and might thereby hinder studies on model transferability and its usefulness for conservation planning and management.Ana M.M. Sequeira, Camille Mellin, Hector M. Lozano-Montes, Jessica J. Meeuwig, Mathew A. Vanderklift, Michael D.E. Haywood … et al