Conservationists desperately want to minimize the impact of overfishing on the oceans and their inhabitants. Doing so requires defining which species are most at risk, and so far, that has not been done on a global level. In terrestrial ecosystems, large-bodied species and top predators seem to be the most susceptible to human impacts; it has been assumed that the same holds true in the water. However, a recent report in PNAS indicates that this is not the case.
Pinsky et al. analyzed two independent fisheries databases, collectively describing 578 species over 60 years, to figure out which life-history traits might correlate with population collapse. They looked at a number of traits that had previously been shown to cause vulnerability to human activity in the seas, including large body size, late maturity, long lifespan, low fecundity, high parental involvement in offspring (large egg diameters), and high trophic levels (being at the top of the food chain).
Contrary to what they expected, they found that almost twice the percentage of smaller, low trophic-level fish stocks had collapsed compared to large top predators. They could not find a correlation between collapse and any of the life history traits they examined, in any combination.
Small fish that live near the surface tend to have rapid growth rates and are highly catchable, so are particularly at risk of being overfished. They can also have a "fast" life history strategy that led people to erroneously believe they were less vulnerable to overfishing. Perhaps this disparity with the situation on land is precisely because fisheries had been targeting these smaller fish, assuming that their populations were more resilient than they actually are.
The fast growth rates and short generation times might also render these short-lived species particularly sensitive to environmental variability. There are plenty of these examples. Summer flounder (Paralichthys dentatus) and Spanish mackerel (Scomberomorus maculatus) are two short-lived species with stocks that collapsed; herring (Clupea pallasii) is a small, fast growing species with a stock that collapsed. Winter flounder (Pseudopleuronectes americanus) and chub mackerel (Scomber japonicus) are other delicious species low in the food chain that also collapsed.
In his book Four Fish, Paul Greenberg laments that we have come close to depleting the oceans of edible wild fish, and will eventually limit our dining options to farmed tuna, cod, sea bass, and salmon. Hopefully, the acquisition and analysis of data on a global level such as reported here will impact fisheries management to the point where we can continue to enjoy the ocean’s bounty without completely destroying it.