To what extent fishing effort is affecting trophic level of the demersal communities at small scale? Preliminary results of a new food web indicator

Due to the complexity of benthic and demersal ecosystems, to be really helpful, a food web indicator should summarize, in a single number, a variety of complex processes that are otherwise hard to apprehend. The present work assesses the direct impact of bottom fishing gears on the trophic level (TL) of benthic and demersal communities dwelling in the continental shelf of the southern Bay of Biscay. Using a spatio-temporal approach at small scale, we tackle two main objectives: 1) to evaluate temporal trends in mean TL at community scale per sampling rectangle (5 x 5 miles grid resolution), and 2) to assess to what extent these trends are related to fishing effort. Two types of data are involved in the analyses: 1) TL data (mean trophic level by sampling rectangle), and 2) VMS data (number of fishing days by sampling rectangle). TL data were calculated using stomach contents sampled for demersal fish species during the Demersales surveys (IBTS), carried out every autumn in the southern Bay of Biscay since 1983, and combined with data from Fishbase and local references for those species which lacked empirical data. We only considered species that appeared in a minimum of 5 years and were consistently well identified along the whole historical series, in order to keep consistency in the community spatiotemporal sampling. Preliminary results point to a significant increase in TL in most of the sampling rectangles, suggesting that the trend observed globally is spatially homogeneous. A clear relationship between fishing effort and TL was also identified, particularly in those areas where fishing effort (mainly otter trawl fisheries) has been decreasing in recent years.

Peer reviewed