If you’ve ever had to throw a fish back because it was below the legal size, you are familiar with a principle that guides many recreational, and commercial, fisheries. “Keepers” are defined as fish above a certain length, and the fishery is managed to minimize catch of fish below this size. The reasoning is that small fish should be allowed to grow up and reproduce before they are caught and killed. This kind of principle has been around for a long time, and seems pretty commonsensical.
But as with many things in the natural world, human “common sense” can be misleading. As it turns out, many fish species get more fecund as they get older and larger. As she ages, a female’s egg output increases much faster than her size. For example, a 30 cm female herring produces around four times as many eggs as fish just 10 cm shorter. Catching one Big Old Fat Fecund Female (BOFFF) can therefore be as damaging to the population’s reproductive potential as catching 4 young ones. With this in mind, some managers have tried to implement “slot” fisheries, where medium sized fish are fair game, but the large and small ones are verboten.
In the new paper in the ICES Journal of Marine Science by Law, Plank, and Kolding, the authors ask what would happen if the number of fish caught in a given size class was proportional to the number of fish of that size actually in the water–that is, in proportion to the population’s natural size spectrum. To try to answer this question, they built a simple model of a fish population that lives alone in the ocean. It subsists on plankton when it is small, and on its younger species-mates when it grows larger. In the absence of fishing, this fish population reaches an equilibrium size spectrum with many more small fish than large ones, as expected.
The interesting results came when the authors simulated different fishing regimes, targeting large fish, small fish, medium fish, and finally all sizes of fish, in proportion to their abundance–in other words, lots of small fish, fewer medium fish, and fewer large fish still. Perhaps surprisingly, this last type of fishery produced the highest sustainable yields and caused the least disruption to the natural size spectrum of the population. The authors suggest that such a management scheme would be better than the usual approaches, since it produces higher yield, distorts the ecosystem less than other methods, and doesn’t require fishermen to discard fish that don’t fall in the legal size window.
I’m wouldn’t jump to change all our management plans based on these simulations (at least not yet). They are (deliberately) very simple, with only one species in the ecosystem, no seasons, an other simplifying assumptions. Still, their results are thought-provoking, and I don’t see any reason to write them off out of hand at this point. This is actually a topic I’ve wondered about myself on a couple of occasions. I’d be interested to see if these results generalize to a system with more species, or one where there are multiple target species. Definitely a research topic worth keeping an eye on.
Law, R., Plank, M., & Kolding, J. (2012). On balanced exploitation of marine ecosystems: results from dynamic size spectra ICES Journal of Marine Science, 69 (4), 602-614 DOI: 10.1093/icesjms/fss031