An extension to the simple implementation of the "River Model" to estimate the impact of fishing on the amount of anchovy available to West Coast penguin colonies which takes account of within season variability in recruitment

Abstract

The assumption of an earlier analysis of a constant flow of anchovy recruits along the West Coast during the winter months is replaced by introducing variability at a monthly scale. The extent of this variability is based on the monthly variability of anchovy catches, and the correlation of this variability with a series of hydroacoustic biomass surveys that took place around Robben Island during the winter months of 2009. The results are qualitatively unchanged from those obtained from the model before the addition of these stochastic elements: fishing decreased the density of anchovy that would otherwise have been available to penguins by appreciably less after 2000 than before. The introduction of stochasticity increases the extent of this reduction since 2000 from at most about 10% to at most about 20%.