Estimation of the fishery–krill–predator overlap

Work undertaken during the intersessional period has enabled better use of the krill fishery data, and a comparison between the three existing indices of fishery-krill-predator overlap used by CCAMLR (catch in the Critical Period and Distance [CPD], Agnew-Phegan index, and Realised Potential Overlap [RPO]) and the Schroeder index. The Agnew-Phegan and Schroeder indices were also compared at nine levels of spatio-temporal resolution and using normal, exponential and uniform foraging distributions.
Only catch in the CPD can presently be used to estimate overlap in areas other than Subarea 48.1. The catch in the CPD and the Agnew-Phegan index were closely correlated, as were the RPO and Schroeder index. All indices exhibited similar broad inter-annual trends over the period 1983 to 1998, indicating reduced overlap in Subarea 48.1 after 1990. The RPO and Schroeder index indicated a greater increase in the level of overlap in recent years (1995-98) than estimated with either the catch in the CPD or the Agnew-Phegan index. The type of foraging distribution and level of spatio-temporal resolution produced small difference in the values of the Agnew-Phegan and Schroeder indices, but had little influence on inter-annual trends. Increasing the level of resolution of the model, and in particular the spatial resolution, resulted in lower overlap between the krill fishery and the krill predators, and lower values of both the Agnew-Phegan and Schroeder indices.