Project Title: Interaction between Australian sea lions and the demersal gillnet fisheries in Western Australia.
Chief Investigator: Dr Richard Campbell
This study suggests that ASLs forage throughout the continental shelf waters from shallow nearshore environments to the continental shelf edge. Tracking of individual sea lions has shown considerable differences between colonies and among individuals within the same cohort at the same colony. Preferences for directional foraging travel were consistent among age groups at one colony, Beagle Is, in particular. This study adds to the weight of data showing that ASL are very individualistic foragers, and that they show faithfulness to particular foraging sites over short time periods (4-6 weeks, Goldsworthy et al. 2007). In general, animals from colonies on the west coast of WA were predominantly shallow water foragers with restricted foraging ranges. Sea lions from the one offshore site on the west coast, Abrolhos Is, showed the most restricted foraging ranges and diving behaviour. Sea lions tagged at the one offshore site on the south coast, Investigator Is, showed the most extensive foraging ranges and were the deepest divers studied. Whilst there were considerable differences in the bathymetry adjacent to these two sites, this was a remarkable difference. Animals tagged at nearshore sites on the south coast displayed intermediate foraging ranges and there was some evidence to suggest that animals were either foraging in shallow (0-20m) or deeper water.
It is presumed that these differences in foraging strategies among and within colonies is driven by available habitat, patchiness of prey distribution and possible prey preferences among animals. The limited spatial overlap of foraging areas among animals from the same colony may be an adaptation to reduce resource competition among individuals and maximize individual reproductive success through an optimal foraging strategy. This pattern has many implications for the assessment of spatial overlap and incidental mortality with the demersal gillnet fisheries and for the assessment of ecosystem impacts with human activities.
The model of ASL spatial foraging patterns demonstrates the broad use of the continental shelf waters by the entire population. The greatest concentration of foraging effort is on the central-west coast where there are three relatively large populations with restricted foraging ranges. Foraging effort on the south coast is more broadly distributed due to the extensive foraging ranges of sea lions from these colonies. The area of greatest foraging effort is around the easternmost group of breeding colonies which produce approximately 10% of the pup numbers in WA.
The model is based on central place foraging from breeding colonies and does not incorporate spatially discrete haulout populations such as those present in the Perth metropolitan area, and does not take into account the dispersal of animals away from the breeding colonies outside of the breeding season. During several visits to the breeding colony of Kimberley Island in the breeding season interval, there were very few animals observed (2-10), when during the breeding season there are upwards of 100 animals present (Gales et al. 1994). Based on the limited spatial foraging data, adult females used many other islands in the area, up to 40 kilometres away, as haulouts during the breeding season. It is assumed that animals from all age groups disperse to many of the islands in the area outside of the breeding season and visits to come of these islands confirmed the presence of mother-pup pairs and juvenile animals. This will act to distribute the foraging effort even wider than the model suggests for colonies that show widespread dispersal. This pattern of dispersal was not evident at all breeding colonies visited outside of the breeding season. In most cases, sea lions of nearly all age groups remain resident at the majority of breeding colonies visited throughout the breeding cycle. It is thought that the proximity of Kimberley Island to a large number of suitable islands may facilitate dispersal of animals of all age groups. It is expected that similar patterns of dispersal occur for breeding colonies within the Recherche Archipelago, but are not expected at most other sites. Observations of consistent numbers of animals representative of all age groups at Investigator Island and Red Islet both within and outside of the breeding season show that there is limited dispersal, most likely due to the lack of suitable island sites in the vicinity.
This pattern of dispersal within the Recherche Archipelago may also be driven by limited resource availability and could represent an optimal foraging strategy to exploit prey resources in a variety of habitats throughout the 17.5 month breeding cycle. Long term satellite tagging of individuals may provide some answers to these questions and discern patterns over broad time periods. These extended foraging patterns will also have a bearing on the likely interaction rate with demersal fishing activities.
Australian sea lions display very distinct and individualistic foraging patterns, which may be related to habitat variation, patch quality and prey preferences. There are also regional and ontogenetic patterns evident in the size of the foraging range and depths to which animals can dive. This makes the construction of a generalized foraging model for this species difficult. There appears to be almost complete spatial overlap of demersal gillnetting activity and ASL foraging areas. The threat of demersal gillnetting activity to ASL has been markedly reduced over the past 25 years due to the reduction in fishing effort, but populations of ASL may still be under threat due to the lag effects of this historical effort and contemporary chronic low levels of incidental mortality. ASL populations in WA are particularly vulnerable to very low levels of additional mortality and a better understanding of the real rate of bycatch in demersal gillnetting is required to accurately determine the threat. Estimates of the incidental mortality rate in a number of areas from an independent observer programme would provide very valuable information to fully assess the risk to this threatened species. A more refined spatial scale of the fishing effort is required to better understand the issue and further refinement of the sea lion foraging model is also needed, especially if mitigative action (i.e spatial closures) was proposed and its potential effect to be quantified. This is a priority action in understanding the interaction between ASL and the gillnet fisheries. Alternative models of determining the interaction rate would also be valuable. There is still some considerable uncertainty in the PVA model of ASL demography, especially in the understanding of underlying population trajectories and the possibility of density-dependent rates of demographic traits.