Can biomanipulation of the sea rescue a collapsed fishery?

European sprat (spratus spratus). Image credit, Hans Hillewaert.The cod stock in the Baltic Sea collapsed in the 1990s because of overfishing and climate change, and this once-valuable fishery has not yet recovered. Could intensified harvesting of sprat—a small fish that eats cod eggs and competes with young cod for planktonic food—be the solution to restore cod, as some people suggest?  

When Martin Lindegren and two colleagues probed this question in a new study set for publication in Ecological Applications, they found the answer to be a resounding 'no.' According to their research, it would be expensive and operationally impractical to dramatically increase the commercial fishing pressure on sprat. And, most important, it would do little to enhance the abundance of cod.  

This is one of the first studies to evaluate the potential for using the practice of biomanipulation in a marine environment. In freshwater systems, biomanipulation has been used to restore degraded lakes by fishing down planktivore species.

It has been suggested that a similar practice might help restore cod - a keystone predator in the Baltic marine ecosystem. The collapse of the species has reduced predatory pressure on sprat, leading to a population boom that has resulted in a negative feedback loop hindering cod recovery and locking the ecosystem in a degraded state.

The scientists used a statistical food-web model of the Baltic Sea ecosystem to test what would happen to cod if sprat biomass were reduced by 25, 50, and 75 percent over three years.

In the real world, this strategy would require adding 276 fishing vessels and 1,244 crew members to the existing fleet. However, even if this feat were accomplished, the model showed that only minor increases in cod biomass would result—not enough to bring the cod to a sustainable level.  

As an alternative, the scientists also used the food-web model to test the strategy of reducing fishing pressure on cod by 25, 50, and 75 percent. The model demonstrated that this direct approach to helping cod would be far more practical and effective. It would lead to substantial increases in cod biomass and provide a better chance of long-term persistence of the cod stock under climate change.  

-- Reviewed by Peter Taylor

Lindegren, M., Möllmann, C., & Hansson, L. (2010). Biomanipulation - a tool in Marine Ecosystem Management and Restoration? Ecological Applications DOI: 10.1890/09-0754