Recent paper points to a need for improved ecosystem modeling
Fisheries managers in the U.S., and increasingly around the world, use stock assessments and scientific information about fish populations to set catch limits for fisheries. It is typically assumed that more adult fish means more reproduction, and thus more fish available for us to harvest. But the authors of a paper published last week by several prominent fisheries biologists found that the productivity of fish stocks can be nearly independent of the abundance of adults, and is influenced by other factors. They report that a simple relationship between population size and productivity is relatively uncommon (18% of 230 stocks examined), with many stocks showing more complex dependencies. The paper finds that, when setting catch limits, population “regimes” (either low or high productivity) can be as important to consider as the number of adult fish.
The stocks reviewed by the paper fluctuated due to low or high productivity, without egard for abundance, in 38% of the cases, and in 30% of the stocks the relationship between abundance and productivity was present but “irregular.” The authors suggest that a variety of factors, including the predominance of predators and prey, and environmental factors, can in some cases impact a stock even more than having a low population size. The study calls for increased modeling of these other factors, to inform managers making decisions about catch limits. Without taking into account factors other than abundance, limits may be set too low, which could limit fishing opportunity, or too high and could risk population collapse.
Although the authors say that the “one of the primary economic arguments for rebuilding overfished stocks would be negated” when productivity is not related to abundance, this is only true in just over a third of the cases they studied. The “promise of higher sustained yield” might not be possible in some stocks, even with harvest reductions, but there is still likely an ecological argument for taking a precautionary approach. The authors acknowledge that “there are often other reasons why larger stock sizes and low fishing pressure provide economic or ecological benefits” such as with fish species where there is not a lot known about the population, or in fisheries that target forage fish, which are important for the health of larger predators.
The study sheds some interesting new light on how productivity and abundance are related and makes the case for increased effort to improve fish models. Looking at the ecosystem as a whole, including predator-prey relationships, and climatic changes like warming ocean temperatures, will help fisheries managers make decisions that are better for fish populations, and the fisheries that depend on them.