Changing fishery discard practices has cascading effect on ecosystems

When humans go fishing, we don’t catch only the type of fish we want. Vessels tend to haul in not only the cod or haddock or herring but also many other unwanted species; without a market for those also-rans, over the side they go. The discard rate for some fisheries can be as high as 40 percent (demersal fish such as cod, plaice, and whiting), and this represents a “food subsidy” to other wildlife. What happens when you take away the subsidy?

Researchers from the University of Strathclyde in Scotland did some computer modeling to figure it out. The answer, unsurprisingly: It’s complicated. Changing how fishing vessels discard parts of their catch does have cascading effects, sometimes from sea birds and large mammals on down and sometimes from phytoplankton on up. It depends on the type of policy change one considers, and on the state of that ecosystem in which the policy change occurs.

The two main policy shifts the researchers considered were labeled a “discards-landed” scenario, where landing quotas are inflated so discard fish are simply not discarded, and an “improved selectivity” scenario where fishing practices are adjusted so that the unwanted fish are never caught in the first place. They conducted these models based on the continental shelf of the North Sea, an area the authors say is a “prime example of a heavily exploited continental shelf ecosystem.”

The model showed that the discards-landed scenario results in a bottom-up cascade of effects, with the end result being a penalty to sea birds and mammals at the top of the food chain and little to no benefit with regard to fish stocks. So, everyone loses, essentially. The improved selectivity scenario, meanwhile, does show some benefit, with a top-down cascade ultimately benefiting sea birds, mammals, and most fish stocks; however, this only occurs when a system is already “heavily exploited.” In a lightly exploited area, the opposite appears true.

Sometimes the change in harvest rates affects the total number of pelagic or demersal fish, leading to more or less food for sea birds and mammals. But with more or less fish, that means more or less plankton availability, which changes the ecosystem for everybody. Again: It’s complicated.

The authors do note that the model they used has relatively “coarse” resolution and can’t distinguish effects based on specific species. They also didn’t try to examine what might happen when fish offal is discarded at sea; viscera can make up 15 percent of catch weight at times, and sending it overboard wouldn’t necessarily violate rules against discards. And there would be plenty of takers for some tasty fish viscera.

But these modeling results, as coarse as they may be, do suggest better fisheries management policies are out there; this isn’t pure academic exercise either, with the European Union moving last year to incrementally reduce and eventually outlaw discards in some fisheries. How exactly they do this should probably be based on studies like this one.

“Inflating landing quotas to accommodate the entire catch is a meagre solution with few conservation benefits,” they conclude. “On the other hand, the effective reductions in the harvest rates resulting from changes in fishing practices to eliminate the capture of unwanted fish can deliver conservation benefits, especially in heavily exploited systems.” – Dave Levitan | May 27 2014

Source: Heath MR, Cook RM, Cameron AI, et al (2014). Cascading ecological effects of eliminating fishery discards, Nature Communications. DOI: 10.1038/ncomms4893.

Image:, ostill