When Evolution Bites Back

New Scientist had a really interesting article about how humans are driving natural selection in a number of areas, and how mechanisms that were put in place to help the survival of species is actually having the opposite effect due to the action of evolution .

The example that caught my eye is the one of only catching fish larger than a certain size, the idea being that by only taking the more mature adult fish you are leaving more fish to reproduce and to keep the population sustainable.

The problem comes when you think of what that means from an evolutionary point of view.

In that scenario it is an evolutionary advantage to be a fish that stays smaller than the allowed catch size, and conversely a disadvantage to grow big. Now both data from fisheries and experimental data has shown that this policy does indeed result in the sizes of fish decreasing over time. In one lab experiment on Menidia Menidia (the Atlantic Silverside) they found that it rapidly evolved into a size that was outside of the catch regime. So when they took large fish they evolved into a smaller size and when they took small fish they evolved into ones that grew bigger.

One solution that’s been posited is catching only medium size fish, the hope being that this will result in selection of fish that grow rapidly from small to large. Of course, if you catch too many then you could end up with a situation like on the Grand Banks where the cod fishery collapsed through overfishing, and even after 13 years of no cod being taken at all they have not bounced back (the suspicion being that this has tipped the balance against code in that ecosystem now).

Of course, designing a net that will only catch fish between certain sizes will be a headache, but then humans have gotten ourselves into this mess in the first place..