Changes in the North Sea fish community: evidence of indirect effects of fishing?

Abstract

We investigate changes in the North Sea fish community with particular reference to possible indirect effects of fishing, mediated through the ecosystem. In the past, long-term changes in the slope of size spectra of research vessel catches have been related to changes in fishing effort, but such changes may simply reflect the cumulative, direct effects of fishing through selective removal of large individuals. If there is resilience in a fish community towards fishing, we may expect increases in specific components, for instance as a consequence of an associated reduction in predation and/or competition. We show on the basis of three long-term trawl surveys that abundance of small fish (all species) as well as abundance of demersal species with a low maximum length (Lmax) have steadily and significantly increased in absolute numbers over large parts of the North Sea during the last 30 years. Taking average fishing mortality of assessed commercial species as an index of exploitation rate of the fish community, it appears that fishing effort reached its maximum in the mid-1980s and has declined slightly since. If the observed changes in the community are caused by indirect effects of fishing, there must be a considerable delay in response time, because the observed changes generally proceed up to recent years, although both size and Lmax spectra suggest some levelling off, or even recovery in one of the surveys. Indeed, significant correlations between all community metrics and exploitation rate were obtained only if time lags R 6 years were introduced.

Effect of intra‐specific competition, surface chlorophyll and fishing on spatial variation of gadoid's body condition

“…However, positive effects of low levels of fishing pressure on body condition by removal of intraspecific competitors (Giacalone et al 2010) or predators (Walsh et al 2012) were still very scarce. We here provide evidence of a regional positive influence on condition at intermediate levels of fishing impact for juveniles of M. merluccius and P. blennoides, suggesting the positive effect of intermediate levels of fishinginduced disturbance on fish condition (Daan et al 2005, Hiddink et al 2008, 2011, Giacalone et al 2010. At these ontogenetic stages, these species are increasing their body condition by gaining weight (Fig.…”

“…Conversely, positive effects of low levels of trawling have been either hypothesized (e.g., Rijnsdorp et al 1991, Jennings et al 2001, Daan et al 2005 or proved (Hiddink et al 2008, Giacalone et al 2010, Walsh et al 2012, mostly due to removal of the larger specimens and leading to three non-exclusive processes: (1) lower predation rates, resulting in higher benthic prey availability, (2) reduced competition among benthic fauna over food and space, which leads to increases in the production of the small benthos that fish feed on, and (3) proliferation of smaller, more productive benthic species (''farming the sea''). However, positive effects of low levels of fishing pressure on body condition by removal of intraspecific competitors (Giacalone et al 2010) or predators (Walsh et al 2012) were still very scarce.…”

Size-selective fishing drives species composition in the Celtic Sea

“…the removal of large fish. However, it is also sensitive to changes in small fish biomass, such as those associated with predation release, and is now understood to integrate indirect trophic processes operating at multiyear scales (Daan et al, 2005;Greenstreet et al, 2011). The indicator also seems to respond to spatial heterogeneity in fishing effort (Shephard et al, 2011b).…”

“…Clearly, as well as registering a change in large fish abundance, the LFI will decline if the abundance of small individuals increases. This can be a consequence of predation and competitive release resulting from fishing on large-bodied fish predators (Daan et al, 2005;Greenstreet et al, 2011). The LFI is sensitive to both changes in intraspecific size structure and changes in the abundance of species of different size.…”

Methods for the Study of Marine Biodiversity

“…Although originally designed to provide fisheries independent information for fish stock assessment and management, they are now increasingly being used to analyse trends in the abundance, distribution and diversity of both commercial and non-commercial species of fish and epibenthos (e.g., Bianchi et al 2000;Shackell and Frank 2003;Daan et al 2005;Perry et al 2005;Atkinson et al 2011). …”