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Wednesday, 4 September 2013

When does fishing lead to more fish? Community consequences of bottom trawl fisheries in demersal food webs

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!

Article courtesy of the Earth Times:

This the abstract taken from the the Royal Society.

"Bottom trawls are a globally used fishing gear that physically disturb the seabed and kill non-target organisms, including those that are food for the targeted fish species. There are indications that ensuing changes to the benthic invertebrate community may increase the availability of food and promote growth and even fisheries yield of target fish species. If and how this occurs is the subject of ongoing debate, with evidence both in favour and against. We model the effects of trawling on a simple ecosystem of benthivorous fish and two food populations (benthos), susceptible and resistant to trawling. We show that the ecosystem response to trawling depends on whether the abundance of benthos is top-down or bottom-up controlled. Fishing may result in higher fish abundance, higher (maximum sustainable) yield and increased persistence of fish when the benthos which is the best-quality fish food is also more resistant to trawling. These positive effects occur in bottom-up controlled systems and systems with limited impact of fish feeding on benthos, resembling bottom-up control. Fishing leads to lower yields and fish persistence in all configurations where susceptible benthos are more profitable prey. Our results highlight the importance of mechanistic ecosystem knowledge as a requirement for successful management."