An important consideration in assessing the impacts of fishing on seabed habitats is to understand the functional links (as trophic interactions) between populations of demersal fish species and potential benthic invertebrate prey (food) which live on or in the seabed. The type of sea-bed fauna has been shown to respond to both natural variation in habitat conditions and in response to different levels of fishing pressure. The extent to which different commercial fish species will depend on specific combinations of habitat type and fishing disturbance to feed will likely be species specific. It has been suggested that positive changes in growth rates of different demersal fish species are not only related to density-dependent processes, but may also be dependent on increased bottom-trawl disturbance and eutrophication (Millner and Whiting 1996, Rijnsdorp and van Leeuwen 1996, Shephard et al., 2010). However, different trawling and habitat specific responses in relation to fish feeding in different size classes of fish has not been investigated before. The aim1 of this study is therefore to examine such relationships using biological traits analysis (BTA) through a selected quantification of demersal fish stomach contents and habitat fauna using grab and epi-benthic trawl data previously analysed as part of BENTHIS deliverable D3.4.
The present study addresses two important questions, namely; i. what type of sea-bed habitats serve as important feeding areas for different species of demersal fish, and ii. what, if any, differences do we observe in habitat preference and diet associated with different size classes of demersal fish, both within and between different fish species. Central to answering these two questions is the need to ensure that the sea-bed habitat characteristics are assessed and described at a scale appropriate to the operation of the fisheries and the assessment of demersal fish stocks.
The study found strong associations between community trait composition and prey consumed by plaice of all sizes under fished conditions for a shallow sand – muddy sand habitat located in the eastern North Sea and Dogger Bank. This result suggests that there is some positive association between fishing and the presence of plaice. By contrast, Long rough dab, haddock, cod and whiting did not appear to target fauna that was abundant in the environment within any one of the habitat clusters under either fished or unfished conditions. These species may therefore be less affected by changes in fishing pressure on a wide range of habitats than those species (such as sole and plaice) which favour living in closer association with the benthic environment.
Secondary production was found to be highest within shallow sand – muddy sand habitat although not significantly so. Bolam et al., 2010 and 2014 indicated that production is indeed affected by bottom disturbance, but as that affects substrate type it is not possible to know whether this is a direct or indirect relationship with fishing pressure.