Demersal Otter Trawl Research Articles

A stochastic model for estimating sustainable limits to wildlife mortality in a changing world.

Human‐caused mortality of wildlife is a pervasive threat to biodiversity. Assessing the population‐level impact of fisheries bycatch and other human‐caused mortality of wildlife has typically relied upon deterministic methods. However, population declines are often accelerated by stochastic factors that are not accounted for in such conventional methods. Building on the widely applied potential biological removal (PBR) equation, we devised a new population modeling approach for estimating sustainable limits to human‐caused mortality and applied it in a case study of bottlenose dolphins affected by capture in an Australian demersal otter trawl fishery. Our approach, termed sustainable anthropogenic mortality in stochastic environments (SAMSE), incorporates environmental and demographic stochasticity, including the dependency of offspring on their mothers. The SAMSE limit is the maximum number of individuals that can be removed without causing negative stochastic population growth. We calculated a PBR of 16.2 dolphins per year based on the best abundance estimate available. In contrast, the SAMSE model indicated that only 2.3–8.0 dolphins could be removed annually without causing a population decline in a stochastic environment. These results suggest that reported bycatch rates are unsustainable in the long term, unless reproductive rates are consistently higher than average. The difference between the deterministic PBR calculation and the SAMSE limits showed that deterministic approaches may underestimate the true impact of human‐caused mortality of wildlife. This highlights the importance of integrating stochasticity when evaluating the impact of bycatch or other human‐caused mortality on wildlife, such as hunting, lethal control measures, and wind turbine collisions. Although population viability analysis (PVA) has been used to evaluate the impact of human‐caused mortality, SAMSE represents a novel PVA framework that incorporates stochasticity for estimating acceptable levels of human‐caused mortality. It offers a broadly applicable, stochastic addition to the demographic toolbox to evaluate the impact of human‐caused mortality on wildlife.

Benchmarking global fisheries discards

Discarding by fisheries is one of the most wasteful human marine activities, yet we have few estimates of its scale. Reliable estimates of global discards are essential for sustainable fisheries management. Using United Nations Food and Agriculture Organization databases on country-specific landings, we estimated the discard rate and magnitude for global marine and estuarine capture fisheries using fishery-specific discard rates derived from direct observations and global gear-specific discard rates estimated within a Bayesian modelling framework. An estimated 9.1 million tonnes are discarded annually (95% uncertainty interval: 7–16 M t)—or 10.8% of the global catch (95% UI: 10–12%). Encouragingly, this is about half of the annual global discard rate estimated in the late 1980s. Trawl fisheries, especially demersal otter trawls, warrant intensified efforts to reduce discards. Periodic benchmarks of global discards are needed to assess the performance of reduction efforts.

Modelling trawling discards of the Alboran fisheries in the Mediterranean Sea

The ecosystem processes underlying the discard rates of the demersal otter-trawl fishery in the Alboran Sea, Western Mediterranean are investigated. We evaluate whether environmental and climatic variability, measured from operational oceanographic data, allow assessment of discarded species abundances. Monthly data collected by onboard fishery observers during 2011 and 2012 in the Northern Alboran Sea served as a case study of the trawling fishery’s dynamics. The study area was split into west and east zones to account for differences in the Atlantic and Mediterranean Sea water influences, for distinct geomorphology and for species composition of catches. Sixteen species divided in three groups were analysed: species without discards, those partially discarded and those completely discarded. Applying Gaussian Generalized Additive Models (GAM), we tested the relationships of standard oceanographic data products and climatic variability with discard abundances. These variables were sea surface temperature (SST) and chlorophyll a (chl-a) from satellite imagery, as indicators of environmental conditions, and the North Atlantic Oscillation index (NAO) as a climatic indicator. Models were applied for target and bycatch species partially discarded. These were the following species: gadoid like European hake (Merluccius merluccius), Blue Whiting (Micromesistius poutassou), Seabreams (Pagellus acarne, and Pagellus erythrinus), and Mackerels (Trachurus mediterraneus, Trachurus picturatus and Trachurus trachurus). Surface seawater temperature was a relevant variable in coastal and eastern, more haline Mediterranean waters. Climatic variability represented by NAO was useful for assessing discard rates in the more productive western zone. Chlorophyll a showed less relevance for discard prediction models, but it was more related in the coastal zones. Including oceanographic data products and climatic patterns in GAM models of fishery discards provides a new perspective that may improve our predictive capabilities for management decisions.

The bycatch composition of shrimp trawl fishery in the Oman coastal waters, the Arabian Sea

Background. Shrimps are one of the most valuable marine resources of Oman and shrimp fi sheries is greatly contributing to the national economy, and sustainability of traditional fi sheries villages. However, the shrimp trawl fi shery has one of the highest bycatch rate of any fi shery and cause negative impacts to marine populations and benthic habitats. This study provides the fi rst data on the c omposition and abundance of the bycatch in the main shrimp fi shing grounds distributed along the south-eastern coastline of Oman in the Arabian Sea. Materials and methods. Shrimp trawl survey was conducted monthly from January to December 2013 in seven sites using demersal otter trawl. Overall, 93 trawl catches were used for the analysis of the bycatch composition based on weight, number, and frequency of occurrence. Top ten species and top fi sh families, cephalopods, and crabs were defi ned in the bycatch. The bycatch compositions were compared in different regions using biodiversity indices and the cluster analysis. Results. Mean monthly bycatch during the shrimp trawling survey in the coastal waters of Oman in 2013 ranged from 22.2 to 191.5 kg · h –1 , in different months with an overall mean (±SE) of 58.7 ± 5.2 kg · h –1 (n = 93). The bycatch to catch ratio was estimated 8.9 : 1 for shrimp fi sheries. This ratio fl uctuated signifi cantly over the year with minimum values from April to September. The bycatch composition included 97 bony fi sh species from 36 families, 12 species from 7 elasmobranchs families and several species of cephalopods and crabs. Catfi shes (Ariidae), frogfi sh (Antennariidae), ponyfi shes (Leiognathidae), and rays (Dasyatidae and Gymnuridae), represented a majority of the bycatch. Conclusion. Shrimp fi shery in Oman is affecting mainly small-sized and slow-moving fi shes, and invertebrates with low economic value. However, the fi ne-meshed shrimp trawls also capture juveniles of commercial important fi shes and cause some ecological problems. The results from the study point out the necessity of management actions to develop a special policy on bycatch reduction in Oman fi sheries.

Effects of Demersal Otter Trawls on the Re-suspension of Copepod Resting Eggs and its Potential Effects on Recruitment

Resting eggs are important phases in the life strategy of many coastal and estuarine copepods. The egg mortality in the sediment layers where they are buried, as well as re-suspension from the sediment into the water column where eggs may hatch are factors that affect the success of this life strategy. Considering that fishing effort in some coastal areas is high and trawling leads to re-suspension of the bottom sediments, it is important to understand these effects on the biology of organisms that utilize sediment habitats in part of their life cycle. This study examined the re-suspension and the hatching success of copepod resting eggs in the wake of two different demersal fishing gear components (doors and discs) that are in contact with the seabed, in two areas off the coast of Scotland that are rarely worked by fishermen. Sediment cores were taken and analysed for resting eggs quantity and hatching performance and compared with samples taken in the water column right after re-suspension of sediment by the gear components. This study demonstrated for the first time that although eggs are re-suspended in the water column together with the sediment, providing them with the opportunity to hatch and recruit nauplii to the pelagic, egg viability was reduced by the passage of the gear components. This study also suggests that the viability is dependent on the gear component and accordingly potential effects must be considered for at this level.

Fish sampling in the marine-dominated Kariega Estuary, South Africa, using a demersal otter trawl: day/night effects

Very few demersal fish assemblage studies using otter trawls in estuaries have been done in South Africa and none of these have tested for the effects of diel period. This study aimed to test whether diel period affects otter trawl catches in the clear, permanently open Kariega Estuary in terms of both species composition and abundance. Fish were sampled by day and night from 30 October to 1 November 2007 at 15 sites using an otter trawl. More species were collected at night, when there were higher catch rates for all key species, with the exception of Rhabdosargus holubi, which was more abundant during the day. Overall assemblages collected by day and by night were similar, with six species accounting for over 90% of the catch in both day and night samples. Both day and night samples reflected differences related to region, with the highest mean CPUE during day and night sampling being recorded in the head region.

Spatial patterns in the retained catch composition of Irish demersal otter trawlers: High-resolution fisheries data as a management tool

High-resolution fisheries data from integrated logbook and Vessel Monitoring Systems (VMS) records have revealed a detailed spatial structure in the species composition of the retained catches of the Irish demersal otter trawl fleets. Hierarchical cluster analysis was used to define 8 clusters with relatively homogenous species compositions. These clusters formed 34 distinct spatial regions in the waters around Ireland. Identification of these regions can be useful for a number applications, including spatial stratification of commercial or survey data, defining and characterising fishing grounds for marine spatial planning, evaluation of closed areas and prediction of how fishing effort might be re-allocated following a closure. A case-study is presented that explores options to reduce cod (Gadus morhua) catches by implementing seasonal closures in two of the 34 regions. Cod are caught by demersal trawlers in a mixed fishery and the catches often exceed the quota, resulting in discarding of marketable fish. Two regions were identified that had relatively low effort and high cod landings. The effects of closing these regions during the first quarter of the year were explored. Cod catches were likely to be reduced by 8–22% while only 3–9% of the annual demersal otter trawl effort would be displaced. Whiting catches were also likely to be reduced, the change in catches of some other species depended on the assumed effort displacement.

The mobilisation of sediment by demersal otter trawls

The mobilisation of sediment by towed demersal fishing gears has been related to the release of nutrients, benthic infaunal mortality and the resuspension of phytoplankton cysts and copepod eggs. Hence, to understand the broader environmental and ecological implications of demersal fishing, it is important to be able to estimate accurately the amount of sediment put into the water column by towed gears.Experimental trials were carried out in the Moray Firth, Scotland, to measure the quantity of sediment remobilised by trawl gear components. It is demonstrated, for a given sediment type, that there is a relationship between the hydrodynamic drag of the gear element and the mass of sediment entrained behind it. A better understanding of this relationship and the hydrodynamic processes involved will lead to the development of accurate predictive models and aid the design of fishing gears of reduced impact.

The commercial fishery for ocean leatherjackets (Nelusetta ayraudi, Monacanthidae) in New South Wales, Australia

The ocean leatherjacket (Nelusetta ayraudi) has a long history of commercial exploitation in New South Wales, Australia. Records of reported landings indicate that substantial peaks of between 600 and 900 tonnes per annum occurred during the 1920s and again during the 1950s. These peaks were followed by large declines, which suggest that this species is vulnerable to over-exploitation. In recent years from 2000/01 to 2006/07, annual commercial landings of ocean leatherjackets using oceanic demersal fish traps and demersal otter trawl have increased from 134 to 430 tonnes. Between 2003 and 2005 ocean leatherjackets in commercial landings ranged approximately between 22 and 65 cm in total length. Ocean leatherjackets were fully recruited to the fishery at two years of age, with the majority of the catch (83%) aged either two or three years. The instantaneous total mortality rate was estimated from an age-based catch curve as 1.1. Natural mortality was estimated as approximately 0.5, based on a maximum age of 6 years. Yield per recruit indicated that under current levels of exploitation the yield per recruit would be maximized at a length at first harvest of 35 cm in total length.

Sampling estuarine fish and invertebrates using demersal otter trawls: Effects of net height, tow duration and diel period

Trawl surveys are done in many parts of the world but few studies have used pilot experiments to examine the effects of gear design and sampling practices on retained samples. We did two separate experiments in two south-eastern Australian estuaries to test the hypotheses that samples of fish and invertebrates taken using a demersal otter trawl would be affected by: (i) vertical net height (standard 0.8 m vs. high-rise 1.2 m) and tow duration (5 vs. 10 vs. 20 min); and (ii) diel period (day vs. night) and tow duration. Mean catch-per-unit-effort (standardised to numbers of individuals caught 5 min −1) was significantly larger in the high-rise net for some variables, but in most cases, CPUE was correlated proportionately between the two heights of net. There were no differences in the structure and composition of assemblages between net heights or among tow durations. The size-frequencies of fish differed between net heights and among tow durations, but there were no clear patterns in the proportions of fish caught across different size classes. Several species were caught mostly, or in larger numbers, at night and contributed to differences in assemblages of fish and invertebrates between diel periods. We stress the importance of using pilot studies to develop more reliable and cost-effective surveys and conclude that future sampling with our trawl should be done at night using the standard net and short (5 min) tows.

Spatial gradients of food consumption and production of juvenile fish in the lower River Elbe

Food consumption and net production of juveniles of the dominant fish species of the lower River Elbe were estimated based on quantitative values of abundance and biomass of fish. Juvenile fishes were caught with a framed gape stow net in the main channel and with a demersal otter trawl in the marginal regions. Fish larvae were sampled at each station using ring-nets. Diet analysis was carried out on 6 dominant fish species which were smelt Osmerus eperlanus, twaite shad Alosa fallax, herring Clupea harengus, flounder Pleuronectes flesus, ruffe Gymnocephalus cernuus and three-spined stickleback Gasterosteus aculeatus. The WINBERG (1956) bioenergetic model was used and verified through the models of ELLIOTT & PERSSON (1978) and EGGERS (1979) to calculate daily rations of juvenile fish. Net production of fish was calculated according to the equation of RICKER (1975). A total of 56 different food components were identified in the diet of the dominant fish species. Their main food items were copepods and mysids and to a lesser extent cladocerans, amphipods, shrimps and insects. The fish species were classified into two feeding guilds based on their food utilization. The juveniles of herring, twaite shad and smelt consumed higher amounts of fishes, whereas detritus was an important prey item of three-spined stickleback, ruffe and flounder. Significant spatial differences in food composition of juvenile fish occurred along the lower River Elbe. The importance of the main prey organism, the calanoid copepod Eurytemora affinis, increased upstream. Total annual consumption and production of fish increased upstream. Highest annual production values of more than 200 kg ha -l were estimated for age group 0 fish in the sub- and intertidal freshwater habitats of the southern marginal areas. In the main channel, consumption and production values of age group 1 were higher compared to the marginal areas.

Effects of otter trawling on macrobenthos and management of demersal scalefish fisheries on the continental shelf of north-western Australia

The effects of two types of otter trawl on macrobenthos (mainly sponges, soft corals, and gorgonians) were measured in an experiment involving repeated trawling of a marked area interspersed with video transects to estimate density of benthos. The gears tested were a demersal otter trawl and a semi-pelagic trawl fished approximately 15 cm above the seabed. Fishing with the semi-pelagic trawl had no measurable effect, whereas the standard demersal trawl reduced benthos density by 15.5% on each tow through the site. Only 4% of the benthos detached was actually retained in the net. Comparison with other studies indicates that macrobenthos mortality can vary greatly depending on how an otter trawl is rigged. The experimental estimate of one-pass mortality was combined with the frequency and distribution of commercial trawling to estimate patterns of annual mortality of macrobenthos in 6-min square blocks throughout the area where the fishery operates. The management response to the problem of benthos mortality in the trawl fishery has been to limit trawling for scalefish to a small proportion of the area of the continental shelf and to control the level and distribution of trawling effort. 2000 International Council for the Exploration of the Sea

Comparison of Fish Assemblages Sampled by a Shrimp Trawl and a Fish Trawl in St Vincent Bay, New Caledonia

The spatial and temporal patterns of variation of a tropical soft-bottom fish community sampled by a shrimp trawl and a fish trawl were compared at two sites, North and South Bay, of St Vincent Bay, New Caledonia. Results indicated that the species richness and particularly the species composition were related to the type of trawl. However, the overall density and biomass, species diversity, and evenness did not vary significantly as a function of gear type. The same species dominated the catches of the two trawl types, but differences appeared between density, biomass and mean weight of particular species, calculated from the shrimp and the fish trawl catches. The spatial variations of the community structure were comparable between the two gear types, though the composition of the species assemblages were not the same. Differences appeared for the temporal structures. These differences were insignificant in one of the sampled sites (North Bay), but more important in the other site (South Bay), where species diversity, size range of the individuals, and shape diversity of fishes were more important. However, the major differences between the community structures remained the same, independent of sampling gear type. These results validate studies comparing spatial and temporal patterns of variations in general characteristics (i.e. number of species per haul, density, biomass, species diversity and evenness) of tropical soft-bottom fish communities determined by shrimp and fish demersal otter trawls. This also validates the determination of species assemblages especially spatial assemblages. However, important problems may arise when more particular points are studied, such as species composition of the communities, determination of the characteristic species of the assemblages, or study of a particular species.