SEAwise Report on the spatiotemporal benthic effects of fishing on benthic habitats relative to suggested threshold levels, both with respect to area impacted and impact intensity
The SEAwise project works to deliver a fully operational tool that will allow fishers, managers, and policy makers to easily apply Ecosystem Based Fisheries Management (EBFM) in their own fisheries. This report gives an overview of the progress made to develop spatio–(temporal) assessments of fishing impact on benthic habitats in EU-waters and evaluating changes in benthic ecosystem functioning due to fishing. The assessments builds on the ICES FBIT (Fishery Benthic impact and trade-off) approach, determining relative benthic state (RBS) from the relationship between fishery footprint and the sensitivity of the habitats. The sensitivity of the habitats is determined by the lifespan of the benthic communities in undisturbed conditions, where a habitat with a higher fraction of longer living species is more sensitive compared to habitats with a higher fraction of short living species. The FBIT assessment approach is applied for the SEAWISE regional case areas (North Sea, Baltic Sea, Adriatic and Ionian Sea (Greek and Italian waters), Western Waters (Bay of Biscay, Celtic, Irish Sea). The regional assessments differ slightly in the type of data used (benthos data of trawls [epibenthic fraction] or grabs/cores [macrobenthic fraction]), model settings for determining sensitivity, fishery pressure data and generated output. Some of the assessments (e.g. Western Waters) are still explorative and describes development of the assessment, based on different data sources, instead of reaching an integrated, publishable assessment. Minding the different states of the assessments, summary of mean RBS range, min RBS and extent of habitats below 0.8 threshold are produced for all regions. Most habitats have a mean RBS value of more than 0.8, but the RBS range on grid cell level varies between <0.1 and 1, indicating there are locations within those regional areas with high fishery benthic impact. A clear threshold for RBS, determining when the habitat status is adversely affected, has yet to be determined, but to highlight differences between the regions, the extent of habitats with RBS below 0.8 is used as an example. Using this threshold, only a negligible surface (26km²) in the Danish part of Kattegat is below this value, whereas the corresponding extent is 30% for the North Sea. For the Western waters, such estimates are not yet available. In the Adriatic, approximately 59% of the habitat surface between 0-200m has an RBS value below 0.8. In the Ionian Sea (0-200m), around 27% in the Western part and 0% in the Eastern part fails the threshold of 0.8. These values give a risk-based indication of the possible impact on the benthic biomass under a certain fishery footprint. It should be noted that the assessment does not reflect the “realised” health of the benthic community within a habitat.
The approach generated similar type of outcome across the case study regions, which is rather unique in the benthic indicator world, as a wide variety of indicators, adapted to local conditions or pressures, exist and are seldomly widely applicable. Uniformity in the approach is helpful for comparing fishery management actions across areas. A model based tool can evaluate spatial fisheries management measures that minimise ecological (affecting RBS) and socio-economic (fishery activity) trade-offs providing information to spatially manage fisheries. It should be noted that this exercise is still conceptual and not yet applied on case study scale. To investigate this, some case studies has executed analyses to show how a change in spatial fishing intensity will change the RBS within their area (or proportion of habitat area above a certain RBS limit). The approach also showcase areas with a higher potential of presence of longer living species, based on the modelled sensitivity layer.
To manage seafloor integrity (e.g. MSFD), a reflection of changes in ecosystem functioning is also important. Bottom fishing will by means of depleting fauna and changing the species composition, result in alterations in the functional effect traits (bioturbation, bio-irrigation) of a community, which in turn may have broad implications for the overall ecosystem performance. With benthic and biogeochemical data in a study area in the Celtic Sea, the relation between macrobenthos traits and biogeochemical (bgc) metrics under different fishery pressure was investigated. The functional trait – bgc metric relationships observed in untrawled sediments, were altered under trawled conditions, but this is context dependent (e.g. sediment). It could be helpful for strengthening the effect of fishing on ecosystem functioning, by having benthic indicators integrating more biological traits in there fishing impact assessments.
Shaping ecosystem based fisheries management
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