Are mean size data adequate to monitor freshwater eel fisheries?

Abstract

As the biomass of a fish population is reduced by fishing, the mean size (length or weight) of individuals in the population can be expected to decrease. Thus, it should be possible, in principle, to detect changes in fishing pressure by monitoring mean size. A simulation experiment was carried out to see whether this might work in practice with New Zealand freshwater eel fisheries. Information on the dynamics of eel populations was used to construct a simulation model of a fishery with a range of exploitation scenarios. It was found that size data, by themselves, could be useful for detecting large long-term changes in stock status, but are not likely to be good indicators for use in year-to-year management. Natural variations in eel recruitment cause substantial year-to-year variations in the mean size of eels, even when fishing pressure is constant. This makes it difficult to detect changes in size caused by long-term changes in fishing pressure. Even without sampling error, biomass would need to drop by more than 40% (or more, if the drop is gradual) before this change could be reliably detected. The presence of typical levels of sampling error makes this detection even harder.