Impacts of trawl door scouring on infaunal bivalves: results of a physical trawl door model/dense sand interaction

Demersal fishing alters seabed habitats and affects the structure and functioning of benthic invertebrate communities. At a critical level of disturbance, such communities may approach an equilibrium disturbed state in which a further increase in disturbance has little additional impact. Such arguments have been used to suggest that an ecosystem approach to fisheries management (EAFM) should protect lightly fished areas and deflect fishing activity into areas that are already intensively fished. In this study, the effects of variation in fishing disturbance on the secondary production, species diversity, abundance, biomass, and community structure of benthic infauna were examined in a region of the German Bight (North Sea) that has been intensively trawled for decades. Variation in fishing disturbance across the study area was determined using automated position registration and vessel monitoring through satellite. Even in such a heavily fished area, linear regression analyses revealed that biomass, species richness, and production decreased significantly with increasing fishing intensity. Although redundancy analyses (RDA) showed that sediment characteristics were influential in determining the structure of the infauna community, partial RDA revealed that fishing continued to have an impact on community structure in terms of biomass. These results suggest that, in implementing an EAFM, managers will need to consider the possibility that, even in areas with high chronic fishing disturbance, further increases in fishing activity may still cause additional damage to benthic invertebrate communities.

Section: Discussionmentioning

confidence: 99%

Effects of fishing disturbance on benthic communities and secondary production within an intensively fished area

Reiss¹,

Greenstreet²,

Sieben³

et al. 2009

“…Trawling leads to mortality in benthic invertebrates and fishes, but the rates may be lower for the smallest individuals within and among species, as small organisms may be pushed aside by the pressure wave in front of the trawl (Gilkinson et al 1998), or escape from the trawl through the mesh. Larger individuals usually suffer higher mortality when they are in the path of the trawl (Bergman & van Santbrink 2000).…”

Section: Methodsmentioning

confidence: 99%

Fishing effects on diversity, size and community structure of the benthic invertebrate and fish megafauna on the Bay of Biscay coast of France

Blanchard¹,

Le-Loc’h²,

Hily³

et al. 2004

Within the framework of the 'dynamic equilibrium model', we tested the hypothesis that fishing causes major disturbance to benthic communities on deeper bottoms of the continental shelf. Benthic megafauna on the Bay of Biscay coast of France was sampled on fishing grounds subject to various conditions of exploitation. Samples were taken at around 100 m depth to avoid strong natural disturbances. Species diversity and the largest body mass class of invertebrates were smaller in strongly exploited areas than in moderately exploited ones. Biomass size spectra in strongly exploited areas were characterised by a comparatively large biomass of small invertebrates, hence the K-dominance curve of abundance was either above the biomass curve, or the curves intersected. In moderately exploited areas, the K-dominance biomass curves were above the abundance curves; the dominant species were a commercial species and a benthic species sensitive to the physical effects of the fishing gears. In the heavily exploited areas, the dominant species were opportunistic carnivorous species of minor or no commercial interest. There were no dominant fish species in the most strongly exploited areas, while 2 fish species were found in the moderately exploited areas. No fragile species were found in the most exploited areas, whereas 6 fragile species were found in the moderately exploited ones. It is concluded that the differences between the most strongly exploited and the moderately exploited areas were consistent with the hypothesis.

“…Bottom trawls, for example, crush benthic invertebrates in the path of the net and lead to the mortality of invertebrates that are caught in the net and subsequently discarded because they have little or no commercial value (Jennings & Kaiser 1998, Lindeboom & de Groot 1998, Hall 1999, Kaiser & de Groot 2000. The mortality suffered by larger benthic species is generally greater than that of smaller species (Gilkinson et al 1998, Lindeboom & de Groot 1998, and larger species, with lower intrinsic rates of natural increase (Brey 1999), are more likely to decline in abundance at a given rate of mortality. Since larger species are often those at higher trophic levels (because predators are usually larger than their prey; Cohen et al 1993), the reductions in abundance of larger species might be expected to reduce the mean trophic level of the community.…”

Section: Introductionmentioning

confidence: 99%

Impacts of trawling disturbance on the trophic structure of benthic invertebrate communities

Jennings¹,

Pinnegar²,

Polunin³

et al. 2001

188

112

Bottom trawling causes chronic and widespread disturbance to the seabed in shallow shelf seas and could lead to changes in the trophic structure and function of benthic communities, with important implications for the processing of primary production and the wider functioning of the marine ecosystem. We studied the effects of bottom trawling on the trophic structure of infaunal and epifaunal benthic communities in 2 regions (Silver Pit and Hills) of the central North Sea. Within each region, we quantified long-term (over 5 yr) differences in trawling disturbance at a series of sites (using sightings data from fishery protection flights), and related this to differences in the biomass and trophic structure of the benthic community. There were 27-and 10-fold differences in levels of beam trawl disturbance among the Silver Pit and Hills sites respectively, and we estimated that the frequency with which the entire area of the sites was trawled ranged from 0.2 to 6.5 times yr -1 in the Silver Pit and 0.2 to 2.3 times yr -1 in the Hills. The impacts of fishing were most pronounced in the Silver Pit region, where the range of trawling disturbance was greater. Infaunal and epifaunal biomass decreased significantly with trawling disturbance. Within the infauna, there were highly significant decreases in the biomass of bivalves and spatangoids (burrowing sea-urchins) but no significant change in polychaetes. Relationships between trophic level (estimated using nitrogen stable isotope composition, δ 15 N) and body mass (as log 2 size classes) were rarely significant, implying that the larger individuals in this community did not consistently prey on the smaller ones. For epifauna, the relationships were significant, but the slopes or intercepts of the fitted linear regressions were not significantly related to trawling disturbance. Moreover, mean δ 15 N of the sampled infaunal and epifaunal communities were remarkably consistent across sites and not significantly related to trawling disturbance. Our results suggest that chronic trawling disturbance led to dramatic reductions in the biomass of infauna and epifauna, but these reductions were not reflected in changes to the mean trophic level of the community, or the relationships between the trophic levels of different sizes of epifauna. The trophic structure of intensively trawled benthic invertebrate communities may be a robust feature of this marine ecosystem, thus ensuring the efficient processing of production within those animals that have sufficiently high intrinsic rates of population increase to withstand the levels of mortality imposed by trawling.