Destabilized fish stocks

Abstract

Fishing of natural populations increases the variability of fish abundance. A unique data set from the southern California Current has allowed an evaluation of three hypotheses for why that should be so. Ecologists have long suspected that the reason why the abundance of harvested fish stocks fluctuates more than that of unharvested stocks is linked to the fact that they are harvested. Three main hypotheses have been put forward in explanation. The first is that variable fishing pressure itself directly increases stock variability. The other two hypotheses relate to the age truncation effect: either the loss of mature fish makes the more juvenile population more susceptible to environmental change, or it causes unstable population dynamics by altering factors such as intrinsic growth rates. A 50-year record of larval fish populations in the California Current fisheries has been used to distinguish between these possibilities. There was no evidence for the first hypothesis and little support for the second. It is the third option that finds support: fishing increases the dynamic instability of populations and can lead to booms and busts followed by systematic declines in stock levels. This means that, without harvest policies to restrict stock depletion, many economically important fisheries can be expected to collapse.