Size-selective cod predation: impact on prey, implications for pelagic fisheries

Targeting the same prey makes competition between humans and predators unavoidable. Predators often predate on a limited size range of prey, which may or may not overlap with size ranges targeted by fisheries. When they do overlap, the effect of competition over that prey is immediate, as the predator removes prey which are at the same time suitable for the fishery. However, if the predator consumes the same prey species as the fishery, but targets smaller prey sizes, this predation on smaller sizes may result in a potential loss of future, rather than current, fishing opportunities. That is, the effect of competition will be delayed, as predator consumption would affect the availability of fish to a fishery later than when the predation occurs. The comparison between lengths of prey targeted by predators and fisheries is generally lacking in previous studies.

We evaluated how size-selective cod predation influences the dynamics of sprat and herring in the Baltic Sea, as well as the competition with pelagic fisheries through immediate and delayed effects. We found a large overlap (30-60 %) between prey lengths targeted by cod and fisheries, which was largest in the 1974-1988 when cod had higher abundance and was larger in size. Cod generally consumes herring and sprat, which are smaller than those caught by the fisheries. This caused a delayed effect on prey biomass available for the fisheries, since smaller individuals that could have grown into harvestable sizes were consumed by cod. Estimated prey biomass unavailable for fisheries due to a delayed effect of cod consumption was estimated to be often similar to the biomass consumed in a size range suitable for the fisheries, essentially doubling the total potential effect of cod consumption.

Keywords: multi-species model, Baltic Sea, size-selective predation, competition with fisheries, mortality.