The effect of modified and lightweight fishing gears on benthic macrofauna

Fishing with bottom-towed gears represents the largest anthropogenic impact to benthic habitats. In order to mitigate further degradation, fisheries manager are adopting an Ecosystems Approach to Fisheries Management, in which the impact of trawling on the seabed and associated biota is taken into consideration. The magnitude of benthic impact from a single pass of a fishing gear can be attributed, in part, to the correlation between gear penetration depth and benthic faunal depletion rates. As such, bottom-towed gears that penetrate less deeply into the sediment are thought to cause less benthic mortality. The use of lighter-weight bottom-towed fishing gears has therefore been suggested as a potential solution to reduce benthic impact from conventional fishing gears, whilst retaining an active fishery. Thus, gear modifications have been developed, and interest into lightweight conventional gears has been stimulated. Some modified fishing gears have been adopted into Danish fisheries. However, quantification of the effects that modified and light weight gears have on benthic macrofauna is rather limited. Therefore, in order to support whether gear modifications have the potential to reduce benthic habitat damage, an assessment of their impact to benthic macrofauna is necessary. Here we examine the effect of three lightweight or modified bottom-towed fishing gears on benthic macrofauna. The results are presented in three research papers, which form the basis of this thesis.

Research Paper 1: In Danish inshore waters, blue mussels (Mytilus edulis) are fished from wild stocks using a lightweight mussel dredge. In this chapter, we quantified the instantaneous impact of mussel dredging directly in the dredge track, the area adjacent to the track, as well as the short-term recovery after 4 months. Density and species richness was significantly reduced in the dredge track, and density significantly differed in the adjacent area. After 4 months, density and species richness remained affected, but signs of recovery may be indicated. No effect of dredging was observed on biomass in the dredge track, the adjacent area or over time.

Research Paper 2: In nearshore open coast, the Danish seine is used to catch mixed demersal fish species. The Danish seine is thought to impose very little physical impact to the seabed, as the footprint consist mainly (99%) of long ropes used to herd fish into the net, and only 1% from the ground gear and net. In this chapter, we quantified the effect of the two gear components of the Danish seine on benthic macrofauna. The impact on benthic fauna was examined after a single and multiple hauls of the Danish seine. We observed little effect of either the ropes or ground gear on benthic macrofaunal indicators. However, for an individual taxa, the tube-building horseshoe worm, Phoronis spp, density was reduced by ~90% after multiple hauls of the Danish seine ropes.

Research Paper 3: On an offshore sandbank, the Dogger Bank, one of Denmark’s largest fisheries targets sandeel (Ammodytes sp.) using otter trawls. In an effort to reduce the benthic impact, a modified sandeel otter trawl (SOT) was developed and tested. In this Chapter, we compared the benthic impact of conventional SOT with the modified SOT. The conventional SOT resulted in a greater reduction of benthic faunal biomass, specifically of the large-bodied fauna, living at the sediment surface, and also reduced the overal footprint by 33%. From these results, it would appear that by switching to the modified gear, the benthic impact of the sandeel fishery could be reduced.

Despite the relatively large losses in some components of the community, the modified gears generally exhibited small effects to seabed macrofauna and benefits to seabed habitats could be expected from replacing conventional gears with these lighter weight modified gears.