Ecosystem effects of bottom trawl fishing

cum laude graduation

P. Daniël van Denderen – Ecosystem effects of bottom trawl fishing.

Thesis summary.

Bottom trawls target demersal fish and benthic invertebrate organisms by dragging a net, or other collection device, over the seabed. Bottom trawl fishing is responsible for about one fourth of the global capture fisheries production. The effects of trawling on the seabed and the benthic ecosystem have spurred numerous serious concerns starting in the 13th century, up to today. As a result, bottom trawling has a notoriously bad reputation and due to the side effects it is thought, it will be hard to arrange into an ecosystem-based fisheries management framework. In this thesis, I examined the ecosystem effects of bottom trawl fishing. I focussed on the direct and indirect effects of trawling on both benthos and benthivorous fish and how these effects translate into changes in the fish stock, the demersal food web and the composition and function of benthic communities.

The first research chapter of this thesis (chapter 2) describes the interactions between fisheries, fish and benthos and how these affect the impact of trawling on the benthic ecosystem, the amount of target fish food, fish production and fisheries yield. When the direct negative effects of trawling (mortality) on benthos are considered in a food-web perspective, the ultimate effects of trawling on both fish and benthos are strongly mediated by the relative importance of top-down and bottom-up processes in the benthic ecosystem. When benthos competes for the same food, and when the seafloor is trawled, the species which are not so sensitive can use more of the available food, left by their more sensitive competitors. If these less sensitive benthos is a particularly good food source for fish, the benefits can even percolate through to fish, so that the production of fish is stimulated by the negative effect of trawling on benthos. Alternatively, when sensitive benthos is the most profitable food source, trawling leads to a lower production of fish compared with a fishery without impact on benthos.

The trophic interactions between fish and benthos also have important implications for the effectivity of marine protected areas (MPAs) to conserve the benthic ecosystem, fish populations, fisheries yield and the trophic structure of the community, as discussed in chapter 3. MPAs may potentially be used simultaneously to protect trawled target fish and the benthic ecosystem. However, their potential for the fisheries yield is not as well established as regular effort limitations. Chapter 4 of this thesis shows that MPAs, compared to regular effort limitations, can become the best management measure for the fisheries yield when the objective is not only to maximize yield but also to protect benthos vulnerable to bottom fishing. Only when benthic competitive interactions strongly reduce fish prey production in absence of fishing or when fish have limited mobility, there will be lower fisheries yields when benthos is protected with MPAs.

Chapter 5 of this thesis describes the interaction between trawl disturbance intensity and primary production for the Dutch beam trawl fishery fleet that targets plaice (Pleuronectes platessa) and sole (Solea solea) in the Dutch part of the North Sea. It shows that a large part of the spatial variation in trawling intensity, up to 60%, is explained by a positive relationship with primary productivity and, to a lesser extent, sediment grain size and depth. The positive relationship between productivity and trawling intensity may be the result of an accumulation of plaice and sole in highly productive areas, attracted to a high availability of benthic fish food. The findings highlight how environmental conditions mediate bottom trawl fishing intensity and as such the exposure of benthos to trawling. The environmental conditions not only determine how much benthos is exposed to trawling, but they also directly affect benthic species richness and biomass. This leads to habitat-specific effects of bottom trawling on benthic diversity.

Within the spatial boundaries of the Dutch part of the North Sea, there is also large temporal variation in exposure to trawl impact. Chapter 6 of this thesis presents these temporal patterns for the Dutch beam trawl fishery fleet for a period of 10 years. It shows that trawl fishing is aggregated in time and displays clear seasonality, related to the behaviour of the fleet and migration patterns of target species. The temporal patterns are of relevance to our understanding of trawling impact on the seabed community, because repeated trawling in a short period, alternated with longer undisturbed periods, have a different effect on invertebrate benthos than single trawling events which are randomly spaced in time. It is also important because benthos vulnerability to trawling may vary seasonally due to changes in population structure and behaviour. .

Finally, chapter 7 of this thesis supports the hypothesis that natural and trawl disturbance affect benthic communities in similar ways. Both sources of disturbance cause declines in long-lived, hard-bodied, and suspension-feeding organisms. No effects of trawling are found in areas with high natural disturbance, while in three out of five areas with low natural disturbance, responses to trawling disturbance are detected. In these areas, trawling results in community compositions comparable with those in areas subject to high natural disturbance. Together, these observations provide strong evidence for the similarity of the community states induced by trawling and natural disturbance.

Overall, this thesis shows how a mechanistic understanding of the factors that determine interactions between fisheries, benthivorous target fish and benthos is a prerequisite for successful management of trawled fish stocks and conservation of the benthic ecosystem. It is this type of information that is needed to shift the ecosystem approach to fisheries management from a policy objective towards an effective instrument for successful management of trawled fish stocks and the conservation of the benthic ecosystem.