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Targeting long-lasting fishing efforts to species with low market value might prove cost-effective in combatting eutrophication of surface waters. This is suggested by researchers of Natural Resources Institute Finland, Finnish Environment Institute, University of Helsinki and VATT Institute for Economic Research.

Photo: Tuomas Heinonen / Rodeo.fi

Eutrophication and warming of surface waters favor cyprinid species such as bream and roach. Commercial fisheries targeting these species decreased substantially in the last decades of the 21st century. The annual commercial landings of bream from the Finnish marine area, for instance, were sixfold in 1950’s compared to the average between 1980 and 2010.

Harvesting cyprinids removes nutrients from waters under stress by nutrient flows from municipal waste waters and agricultural runoff among other sources.

“High cyprinid fish stocks maintain, even accelerate eutrophication. They feed on zooplankton and recycle nutrients back to the water column by reworking bottom sediments. Targeting fishing on these species would initially decrease, and later retain lower cyprinid fish stocks, thus improving water quality,” said principal research scientist Heikki Peltonen from the Finnish Environment Institute.

Promoting harvesting of low market value species has a long history in the eutrophication management toolbox, but with mixed results. Good results call for long-term fishing activities and deep understanding of the role of the targeted fish stocks in the water ecosystem. However, fishing efforts fade away quickly if market appreciation or permanent economic support is lacking.

“What is needed is steady action,” Heikki Peltonen thus reminded. “Extensive one-shot removal of cyprinid fish does not yield the desired result. When it comes to eutrophication management, we should look at annual sustainable yields, just like in commercial fisheries management. This also prevents unintentional detrimental ecosystem effects brought by abrupt changes in species composition.”

Abundant fish stocks

Since 2010 – first supported by the government and currently by the John Nurminen Foundation – the commercial landings of, for instance, bream are back in the levels of 1960’s. The existing stocks would likely allow for markedly higher, yet sustainable catch rates.

Should we then maintain intensive cyprinid fishing efforts, and will the ongoing commercialization efforts bring bream and roach back to Finns’ dining tables? Does fishing non-commercial species have a role in cost-effective eutrophication management mix of measures?

 “To answer these questions we developed a dynamic, bio-economic optimization model which allocates scarce resource to two alternative measures: reducing external loading from agriculture and fishing cyprinid fish. The effects of fisheries are twofold – fishing removes nutrients from water and the subsequent changes in long-term stock sizes have a further positive effect on water quality” said principal research scientist Antti Iho from the Natural Resources Institute Finland. “Our research shows that should the stock size alone affect water quality negatively, we should keep the cyprinid fish stock on a low level. If the cyprinid stock has no detrimental effect on water quality, setting cyprinid harvest close to maximum sustainable yield would provide the most efficient nutrient reduction.”

The model was applied on Mynälahti bay, South-West Finland, and the results indicated that permanent harvesting of cyprinid fish does have a role to play in cost-effective water protection. Due to complexities of water ecosystems the results should not be carelessly extended to other areas without careful considerations.

“On the other hand, restoring coastal fisheries to a state they have been for a very long time can’t be very hazardous,” noted Iho. “Ideally, cyprinid fisheries could be based on actual market demand, once a well operating supply chain is in place.”

The publication “The Role of Fisheries in Optimal Eutrophication Management” was published in Water Economics and Policy and it is downloadable on the journal pages. The research got financing from the AKVA program of the Finnish Academy.

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