When a trawler trundles its fishing gear along the bottom, its target fish are affected, the substrate is disturbed and every species in the ecosystem will be involved. 23% of all fish in the ocean are caught in this way, usually on a soft-bottomed habitat, along one of the continental shelves. The physical impact of the gear is one major impediment to any sustainable practice, creating the incentive for many modifications, many of which are still being designed.
The species most affected by the trawl net are large bivalves and crustacean, while some annelids are relatively unaffected. Flatfish that are directly dependent on benthic invertebrates (benthos) could benefit from any shift in the habitat caused by the trawl. Certainly plaice and sole do increase their growth rate when high intensities of trawling are conducted. The substrate might be causing this effect itself, however, so this paper's modelling exercises were prepared.
P. Daniel van Denderen, Tobias van Kooten and Adriaan D. Rijnsdorp of the Wageningen Institute for Marine Resources and Ecosystem Studies and Wageningen University in the Netherlands carried out the research for their paper in the Proceedings of the Royal Society B today. They entitle it, "When does fishing lead to more fish? Community consequences of bottom trawl fisheries in demersal food webs."
What the authors discovered is that the long-term effects of trawling increase fish and probably benthos mortality. In one form of bottom-up community(see later), fish predation doesn't affect the benthos community to any great degree, but the trawling does and also increases fish biomass. What could be done next is to consider the edibility of the benthos to fish, but it seems that the models don't produce any different results if the attractiveness of these invertebrates is varied. Other models have found the same result. Either the reduced fish predation or the resultant competition somehow increase the biomass of the invertebrates on the bottom, especially annelids, although it has been impossible to find out which of these two factors cause the polychaetes and other annelids to increase. Large bivalves and several other bottom species are a different kettle of fish. They are definitely reduced in numbers by trawling. Technical changes in trawling are essential to prevent such high mortality, unless the trawlers are prevented from fishing altogether. Fish abundance and yield is shown to be reduced when these measures are taken however. In the North Sea, it's been assumed that primary productivity can increase when the bottom is disturbed or "discards" could help scavengers to increase their particular biomass.
What the authors conclude is that the state of the ecosystem is crucial. What they refer to as a "top-down" system is more prevalent in some areas such as the Irish Sea, where fish are still quite numerous and control the prey numbers. In bottom-up systems, the fish have often been exploited by fisheries, as in the southern North Sea. The invertebrates are not predated as much and their nutrient resources seem to determine how the ecosystems operate.
That means that the positive effect of trawling on some benthic invertebrates would only apply at high trawling frequencies. Change in fishing gear in such areas would be necessary to prevent depletion of the rest of the benthos - the large bivalves that are decimated by trawls. Then we would see if the dream was true, more fishing produces more fish, but only in these specialised and over-fished circumstances!
Publication of the paper is in the Proceedings of the Royal Society B: Biological Sciences.