Functionally distinct adult assemblages within a single breeding stock of the sardine, Sardinops sagax: management units within a management unit

Distribution of Sardinops eggs from seven surveys off southern Western Australia (WA) were assessed to determine if there were separate concentrations of spawning adults. Ten year series (1989–1998) of both age compositions and gonadosomatic indices (GSIs), obtained from routine sampling of commercial catches, were also analysed to aid assessment of stock structure of Sardinops from three regions on the southern WA coast. The patterns in the distribution of Sardinops eggs provided evidence for the existence of distinct centres of spawning that were joined by intervening areas of less spawning activity. Together with regional differences in mean GSI and independence of age compositions between regions in 5 of the 10 years examined, these results indicate that there is not wide-scale mixing of mature age classes between the three regions. As first vulnerability occurs at time of maturity, these non-mixing assemblages are termed functionally distinct adult assemblages (FDAAs) and may persist due to fitness-related ties to localised areas of higher habitat suitability in a region of the world with an oligotrophic pelagic ecosystem. Alternatively, given that the interrelationships between pre-recruit Sardinops across the three regions are poorly understood, it is also possible that behavioural mechanisms, such as natal homing, may be involved in maintaining distinctness of the FDAAs. Regardless of the cause of non-mixing, when fisheries exploit spatially limited but disjunct parts of a single breeding stock, the exploited portions of the stock may in some cases also be considered as distinct for the purposes of management.