Effects of bottom trawling on fish foraging and feeding

Abstract

The effects of bottom trawling on benthic invertebrates include reductions of biomass, diversity and body size. These changes may negatively affect prey availability for demersal fishes, potentially leading to reduced food intake, body condition and yield of fishes in chronically trawled areas. Here, the effect of trawling on the prey availability and diet of two commercially important flatfish species, plaice (Pleuronectes platessa) and dab (Limanda limanda), was investigated over a trawling intensity gradient in the Irish Sea. Previous work in this area has shown that trawling negatively affects the condition of plaice but not of dab. This study showed that reductions in local prey availability did not result in reduced feeding of fish. As trawling frequency increased, both fish and prey biomass declined, such that the ratio of fish to prey remained unchanged. Consequently, even at frequently trawled sites with low prey biomass, both plaice and dab maintained constant levels of stomach fullness and gut energy contents. However, dietary shifts in plaice towards energy-poor prey items were evident when prey species were analysed individually. This, together with a potential decrease in foraging efficiency due to low prey densities, was seen as the most plausible cause for the reduced body condition observed. Understanding the relationship between trawling, benthic impacts, fish foraging and resultant body condition is an important step in designing successful mitigation measures for future management strategies in bottom trawl fisheries.

Trawling disturbance effects on the trophic ecology of two co-generic Astropectinid species

“…In relation to trawl intensity the two species showed a highly opportunistic feeding strategy and co-occurred at all sites by showing differences in their diet preferences (Ribi et al, 1977) and shifting in size-frequency distributions. Our results support the hypothesis that bottom trawling affects the fitness of benthivorous organisms by altering the food availability and generating a positive response of facultative scavengers to carrion (Demestre et al, 2000;Link, 2004;Dell et al, 2013;Johnson et al, 2015).…”

“…Here, we tested the hypothesis that bottom trawling affects the diet of benthivorous organisms by altering food availability and generating a positive response of facultative scavengers to carrion. Benthic scavengers feeding on the benthic community that results from the effects of trawling might represent costeffective samplers, hence diet studies may provide insights into the trophic consequences of such community changes for other components of the ecosystem (Link, 2004;Dell et al, 2013;Johnson et al, 2015).…”

Stable isotopes reveal the effect of trawl fisheries on the diet of commercially exploited species

“…Bottom trawl fisheries affect populations of commercial species, such as flatfish and crustaceans, directly by harvesting, or indirectly through physical habitat changes induced by mechanical trawl equipment leading to population changes in benthic invertebrates on which these benthivorous species feed 3–5 . So far most studies on bottom trawling have examined these direct and indirect effects separately.…”

Indirect effects of bottom fishing on the productivity of marine fish

“…; Johnson et al . ). Modelling studies of the effects of bottom trawling on fish feeding (Hiddink et al .…”

“…The most insightful study to date is that of Johnson et al . () who showed that flatfish were able to maintain their stomach fullness despite the reduction in local prey biomass and abundance. However, these fish had reduced body condition, which is most likely explained by the additional energetic costs of searching for food in a reduced prey field.…”