Extinction of marine renewable resources: a demographic analysis

A simple (deterministic) population dynamics model is used to examine the l evel of fishing mortality at which a population is expected to be driven to extinction (F-crash). Values for F-crash are determined for six marine fis h species (Cape hake, blue grenadier, blue warehou, school shark, gummy sha rk, and orange roughy) subject to commercial harvest. The sensitivity of th e value of F-crash to changing the selectivity pattern of the fishery and t he relationship between the size of the reproductive component of the popul ation and subsequent births to allow for depensatory effects is examined. F -crash is greatest for highly productive species and when the fishery does not target immature animals. The ratio of F-crash to the fishing mortality at which maximum sustainable yield is achieved, F-crash is a decreasing fun ction of the productivity of the population. The possibility of depensation has little impact on F-MSY but can substantially reduce the ratio F-crash/ F-MSY. A series of stochastic simulations is conducted to assess the probab ility of detecting that the extent of fishing exceeds F-crash using the cur rent IUCN A criterion. The results of these simulations are also used to de termine the probability that the IUCN A criteria will be triggered when fis hing takes place at F-MSY. The results indicate that there are substantial probabilities of incorrectly identifying species being harvested at F-MSY a s being threatened during the 'fishing down' phase and also of not identify ing species actually at risk of extinction if fishing mortality is not redu ced.