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Metcalf, S. J., Pember, M. B., and Bellchambers, L. M. 2011. Identifying indicators of the effects of fishing using alternative models, uncertainty, and aggregation error. – ICES Journal of Marine Science, 68: 1417–1425. The identification of indicators of the indirect effects of fishing is often an issue for fisheries management, particularly if just commercial catch data are available. Complex, intermediate, and simplified qualitative models were produced for a fishery case study off Western Australia to identify potential indicators of ecosystem change attributable to western rock lobster (Panulirus cygnus) extraction and bait input. Models of intermediate complexity were used to identify indicators because they produced the least aggregation error. Structural uncertainty was considered through a series of structurally different intermediate models. These alternate models consistently predicted that extraction of rock lobster may positively impact small fish of low economic value, such as old wife (Enoplosus armatus), footballer sweep (Neatypus obliquus), and king wrasse (Coris auricularis). These small fish were therefore identified as potential indicators of the effects of rock lobster extraction. Small crustaceans (amphipods and isopods) also displayed positive impacts attributable to bait input from the rock lobster fishery and were identified as potential indicators of bait effects. Monitoring of these indicators may aid the detection of ecosystem change caused by the rock lobster fishery.

The size and age data and patterns of growth of three abundant, reef-dwelling and protogynous labrid species (Coris auricularis, Notolabrus parilus and Ophthalmolepis lineolata) in waters off Perth at c. 32° S and in the warmer waters of the Jurien Bay Marine Park (JBMP) at c. 30° S on the lower west coast of Australia are compared. Using data for the top 10% of values and a randomization procedure, the maximum total length (L(T) ) and mass of each species and the maximum age of the first two species were estimated to be significantly greater off Perth than in the JBMP (all P0.001) and the maximum ages of O. lineolata in the two localities did not differ significantly (P0.05). These latitudinal trends, thus, typically conform to those frequently exhibited by fish species and the predictions of the metabolic theory of ecology (MTE). While, in terms of mass, the instantaneous growth rates of each species were similar at both latitudes during early life, they were greater at the higher latitude throughout the remainder and thus much of life, which is broadly consistent with the MTE. When expressed in terms of L(T), however, instantaneous growth rates did not exhibit consistent latitudinal trends across all three species. The above trends with mass, together with those for reproductive variables, demonstrate that a greater amount of energy is directed into somatic growth and gonadal development by each of these species at the higher latitude. The consistency of the direction of the latitudinal trends for maximum body size and age and pattern of growth across all three species implies that each species is responding in a similar manner to differences between the environmental characteristics, such as temperature, at those two latitudes. The individual maximum L(T), mass and age and pattern of growth of O. lineolata at a higher and thus cooler latitude on the eastern Australian coast are consistent with the latitudinal trends exhibited by those characteristics for this species in the two western Australian localities. The implications of using mass rather than length as the indicator variable when comparing the maximum sizes of the three species and the trends exhibited by the instantaneous growth rates of those species at different latitudes are explored. Although growth curves fitted to both the L(T) and masses at age for the males of each species lay above those for their females, this would not have influenced the conclusions drawn from common curves for both sexes.