Development of sustainable fisheries management and monitoring for sensitive soft-bottom habitats and species in the Kattegat

Abstract

The Kattegat is home to an intensive bottom trawl fishery for Norway lobsters (Nephrops norvegicus), and although the fishery on the target species is sustainable, intensive bottom trawling is known to significantly impact sensitive seabed habitats and species. In this project, we investigated how bottom trawling for N. norvegicus affects the seabed fauna and the demersal fish assemblages in the Kattegat by sampling selected stations with bottom grabs and trawls, by recording the fauna on the seabed with a towed video camera, and by counting and identifying the species attracted to baited stereo-video cameras. The sampling stations were primarily located in the deeper, soft sediment areas where N. norvegicus is abundant, but samples were also collected in shallower areas where sensitive species had been identified in previous investigations. Prior to sampling, local fishers were interviewed to identify potential areas of N. norvegicus habitats that were ‘de facto’ closed to fishing due to the presence of obstacles on the sea bed, such as reefs and boulders. In some of these areas, side scan sonar was used to map the seabed. Results from previous investigations and maps of fishing intensity were used to select the sampling locations, in order to ensure that a bottom fishing pressure gradient ranging from zero to high intensity was represented in the sampling design. Four different sampling devices were deployed to compare their individual suitability to record different elements of the seabed and demersal fauna. The sediment grab sampler mostly contained invertebrate species that dwell in the sediment (infauna) and some of those which reside on the surface of the seabed (epifauna). The towed video camera primarily recorded the larger (megafauna) epifaunal species. These included epifaunal invertebrate species known to be sensitive to bottom trawling, such as sea pens (Virgularia mirabilis, Pennatula phosphorea), northern horse mussel (Modiolus modiolus), tube building crustaceans (Haploops spp.), and larger sea anemones (Pachycerianthus multiplicatus, Bolecera tuediae). The baited stereovideo cameras recorded scavenging invertebrates and fish species that were attracted to the bait. Most noteworthy was the large numbers of hagfish (Myxine glutinosa) recorded at all the deeper (>50 depth) stations. This species was rarely caught by the grab sampler and the bottom trawl deployed in the project. The bottom trawl caught a variety of other fish species as well as some of the larger, mobile, epifaunal, invertebrate species, which were not recorded by any of the other sampling devices. Given the differences in the species recorded at each station by the four sampling methods we conclude that adequate monitoring of the ecological impacts of bottom trawling requires the application of a number of different sampling devices to fully record the changes in the fauna affected by bottom trawling, such as shifts in density and species composition of benthic macrofauna, megafauna and demersal fish assemblages. It is therefore also necessary to apply different sampling devices to investigate the mitigating effects of management measures, such as areal closures. During the field survey we collected a total of 84 sediment grab samples across the Kattegat. After collection, each sample was passed through two different sieve sizes, to separate the macrofaunal community into small and large individuals. This size-separation was carried out because we expected large-bodied faunal benthos to be particularly sensitive to trawling disturbance and, therefore, have the potential to be effective indicators of trawling impacts on the seabed habitats. The macrofaunal data were analysed with fishing intensity data and six  additional environmental variables known to have a strong influence on benthic communities. The results describe the distribution of benthic communities across the study area, the distribution of pre-selected (a-priori) sensitive species, the effects of trawling pressure and other drivers on the composition of species and traits within the community, and the response of several ecological indicators and benthic life history traits to trawling. The pre-selected sensitive species were found to be relatively uncommon across the fishing grounds. Of these, the sea pen Virgularia mirabilis and sea anemones were only recorded at stations with low to intermediate trawling intensity, while Pennatula phosphorea, Modiolus modiolus and Haploops spp. were only recorded at a single station each. On the other hand, other species known to be sensitive to bottom trawling such as the bivalve Arctica islandica and several species of tubebuilding worms were more widely distributed. Analysis of the data showed a number of key life history traits were associated with high sensitivity to trawling. These included biological traits such as immobility (sessile life-habits), suspension feeding, and deposition of bioturbated material at the sediment surface. Further to this, large fauna with a lifespan of >10 years and a burrowing to depth of 6-10cm in the sediment were also significantly impacted by trawling. The sensitivity to trawling of common, large-bodied infauna and epifauna, such as bivalves and tube-building polychaetes, was strongly reflected in the performance of the ecological indicators. These results show that trawling remains a significant driver of macrofaunal density of individuals and biomass, even when differences in the local physical and hydrodynamic conditions are accounted for. Stakeholder meetings were held in order to engage the fishers in the project and in discussions about how trawling impacts on sensitive species and habitats can be managed. The results of the project were visualized by an interactive model of the Kattegat seabed, showing where sensitive species and habitats were found, and where the Nephrops fishery takes place. We conclude that areas closed to trawling primarily should be established where sensitive species and communities occur. Closing areas that are already heavily trawled cannot guarantee that sensitive species and communities will re-establish themselves in these areas, and may furthermore lead to fishing effort being diverted from these areas to others where the sensitive species are currently found.