Managing fisheries to conserve North Sea groundfish and benthic invertebrate species diversity

Concerns over man’s impact on the environment and ecosystems of the world have resulted in a shift in emphasis in the management of marine natural resources. Consequently, an ecosystem approach to management (EAM) is in the process of being developed and implemented for the North Se

Assessing the sampling effort required to estimate a species diversity in the groundfish assemblages of the North Sea

Conserving and restoring biodiversity are key objectives for an ecosystem approach to management in the North Sea, but ecological quality objectives for the groundfish community instead concentrate on restoring size structure. Species richness and diversity estimates are strongly influenced by sampling effort. Failure to account for this has led to the belief that species richness and diversity indices are not adequate indicators of ‘state’ for the groundfish community. However, adherence to a standard procedure that is robust within respect to sampling effort influence should allow these metrics to perform a state indicator role. The Arrhenius power and Gleason semi-log species–area relationships are examined to determine whether they can provide modelled estimates of species richness at the ICES (International Council for the Exploration of the Sea) rectangle scale. Of these, the Gleason semi-log appears most reliable, particularly when a randomised aggregation process is followed. Aggregation of at least 20 trawl samples is required to provide empirically derived index values that are representative of the communities sampled, and therefore sensitive to drivers of change in these communities. However, given current groundfish survey sampling levels, combining 20 half-hour trawl samples to provide single estimates of species richness and diversity will require considerable aggregation over time and/or space. This can lead to estimates of a or local richness/ diversity becoming inflated through the inclusion of elements of ß or regional richness/diversity. For the North Sea groundfish assemblage, this occurs when the distance between the focal position and the location of the most distant sample exceeds 49 k

Taking account of catchability in groundfish survey trawls: implications for estimating demersal fish biomass

Groundfish surveys are a key component of current scientific data monitoring and data-collection activities undertaken in support of fisheries management. Recent requirements to develop and implement an ecosystem approach to management are placing increasing and varied demands on such datasets. Successfully incorporating ecosystem and environmental objectives within fisheries management will, for example, require greater understanding of foodweb trophodynamics, which in turn requires detailed information on the abundance and distribution of fish predators and prey species on spatial scales hitherto rarely considered. However, no trawl gear catches all the fish in its path, so density estimates provided by such trawl samples do not reflect true densities of fish. Catchability in a trawl gear is affected by many factors and varies both between species and between different sized conspecifics, and therefore has the capacity to confound our understanding of predator¿prey interactions and of the relative abundance of different species and size classes of fish at any point in time or space. To overcome such problems, estimates of the catchability of each size class of each species sampled in a given survey are required for trawl sample densities to be raised to the actual densities of fish present at each location sampled. Here, we present a method for estimating catchability coefficients for 1-cm size classes of fish species sampled by the Grande Overture Vertical trawl during the third quarter ICES International Bottom Trawl Survey. The catchability coefficients obtained are applied to survey data collected between 1998 and 2004 to examine annual variation in actual abundance of the demersal fish assemblage in the North Sea

Using MPAs to address regional-scale ecological objectives in the North Sea: modelling the effects of fishing effort displacement

The use of Marine Protected Areas (MPAs) to address regional-scale objectives as part of an ecosystem approach to management in the North Sea is examined. Ensuring that displacement of fishing activity does not negate the ecological benefits gained from MPAs is a major concern. Two scenarios are considered: using MPAs to safeguard important areas for groundfish species diversity and using them to reduce fishing impacts on benthic invertebrates. Appropriate MPAs were identified using benthic invertebrate and fish abundance data. Fishing effort redistribution was modelled using international landings and fishing effort data. Closing 7.7% of the North Sea to protect groundfish species diversity increased the fishing impact on benthic invertebrates. Closing 7.3% of the North Sea specifically to protect benthic invertebrates reduced fishing mortality by just 1.7¿3.8%, but when combined with appropriate reductions in total allowable catch (TAC), 16.2¿17.4% reductions in fishing mortality were achieved. MPAs on their own are unlikely to achieve significant regional-scale ecosystem benefits, because local gains are largely negated by fishing effort displacement into the remainder of the North Sea. However, in combination with appropriate TAC reductions, the effectiveness of MPAs may be enhanced

Selecting MPAs to conserve ground fish biodiversity: the consequences of failing to account for catchability in survey trawls

Fishing has affected North Sea ground fish species diversity. De. ning Marine Protected Areas (MPAs) to address this will rely on ground fish surveys. Species-specific catch efficiencies vary between trawl gears, and apparent species diversity distributions are influenced by the type of gear used in each survey. It may be that no single survey depicts actual diversity distributions. Two MPA scenarios designed to protect ground fish species diversity are described, the first based on unadjusted International Bottom Trawl Survey data and the second based on the same data adjusted to take account of catchability. Spatial overlap between these scenarios is low. Assuming that the adjusted data best describe the actual species diversity distribution, the level of diversity safeguarded by MPAs, based on unadjusted data, is determined. A fishing effort redistribution model is used to estimate the increase in fishing activity that is likely to occur in MPAs that take catchability into account, if closed areas based solely on the unadjusted ground fish data were implemented. Our results highlight the need to take survey-gear catchability into account when designating MPAs to address fish-species diversity issue

Mapping spatial variation in demersal fish species diversity and composition in the North Sea: accounting for species- and size-related catchability in survey trawls

The paper maps spatial patterns of groundfish species diversity. It considers how the catchability of different fish species in two different types of bottom trawls, the IBTS GOV and the 8-m beam trawl, influences the estimates of species diversity. Maps of groundfish species diversity derived from these two survey trawls are compared to determine the extent to which the maps of spatial variation in groundfish species diversity are influenced by gear type. Catchability-at-length coefficients were applied to the IBTS data to raise the observed catches to estimates of "actual" numbers of fish present in the path of the trawl, which are then used to produce maps of "actual" species diversity across the North Sea. Finally, these raised maps of "actual" groundfish species diversity are shown to be more explainable based on physical environmental parameters such as depth. We suggest that species diversity maps that take account of catchability provide more reliable information on which to base management decisions than "gear-biased" views. The implications for management are discussed, with particular emphasis on using closed areas to conserve marine biodiversit

Modelling the direct impact of bottom trawling on the North Sea fish community to derive estimates of fishing mortality for non-target fish species

This study introduces a spatially explicit model that combines abundance data for all the main fish species in the demersal North Sea fish community with international effort data and estimates of gear-, species-, and size-dependent catch efficiency to determine the mortality of non-target fish species caused by bottom trawl fisheries and its spatial variation. Where necessary information was lacking, assumptions were made, and a sensitivity analysis performed to examine the impact of these issues on model results. Model outcomes were validated using international landings and discard data for five target species: cod, haddock, whiting, sole, and plaice. This showed that depending on its configuration, the model could reproduce recorded landings and discards of these species reasonably well. This suggests that the model could be used to simulate rates of fishing mortality for non-target fish species, for which few data are currently available. Sensitivity analyses revealed that model outcomes were most strongly influenced by the estimates of gear catch efficiency and the extent to which the distributions of fishing effort and each species overlapped. Better data for these processes would enhance the contribution that this type of model could make in supporting an ecosystem approach to fisheries managemen

Modelling the direct impact of bottom trawling on the North Sea fish community to derive estimates of fishing mortality for non-target fish species

This study introduces a spatially explicit model that combines abundance data for all the main fish species in the demersal North Sea fish community with international effort data and estimates of gear-, species-, and size-dependent catch efficiency to determine the mortality of non-target fish species caused by bottom trawl fisheries and its spatial variation. Where necessary information was lacking, assumptions were made, and a sensitivity analysis performed to examine the impact of these issues on model results. Model outcomes were validated using international landings and discard data for five target species: cod, haddock, whiting, sole, and plaice. This showed that depending on its configuration, the model could reproduce recorded landings and discards of these species reasonably well. This suggests that the model could be used to simulate rates of fishing mortality for non-target fish species, for which few data are currently available. Sensitivity analyses revealed that model outcomes were most strongly influenced by the estimates of gear catch efficiency and the extent to which the distributions of fishing effort and each species overlapped. Better data for these processes would enhance the contribution that this type of model could make in supporting an ecosystem approach to fisheries managemen