Effects of the Leeuwin Current on the distribution of carnivorous Macrozooplankton in the shelf waters off southern Western Australia

Plankton samples were collected using 500-μm nets in the surface waters (up to a depth of 70 m) of the continental shelf in three regions (Albany, Bremer Bay and Esperance) off southern Western Australia in July 1992 (winter) and January 1993 (summer). The carnivorous zooplankton of these samples were characterized by low species richness and high variability in abundance. Abundances of most of the major taxa collected differed significantly between seasons and many also differed between regions. Siphonophores and chaetognaths dominated the carnivorous plankton, with lower abundances of hydromedusae and raptorial copepods. The most abundant siphonophores wereChelophyes appendiculataandEudoxoides spiralisin summer and winter, respectively. The most abundant chaetognaths wereSagitta minimain summer, withPterosagitta dracoandSagitta enflatadominant during winter. Overall, the numbers of species of both siphonophores and chaetognaths were highest during winter. At this time, there was also a trend for decreasing numbers of species in an easterly direction between Albany and Esperance, which was probably due to the presence of subtropical species entrained within the warm Leeuwin Current, which was flowing east along the continental shelf during winter. During summer, when the current was not present in this region, there was an even spread of fewer species along the coast

Spatial and seasonal distribution of eggs and larvae of sandy sprat, Hyperlophus vittatus (Clupeidae), off south-western Australia

Ichthyoplankton surveys were employed to determine the distribution and spawning season of Hyperlophus vittatus off south-western Australia. Eggs and larvae of H. vittatus were sampled with 500-μm-mesh bongo-nets monthly during 1992, and less regularly during 1993, close to the beach and at 5.5 and 11 km offshore in four areas within the region of the fishery. The spatio-temporal distribution and abundance of eggs indicates that H. vittatus spawns in nearshore marine waters from May to September, with a peak in June and July. Larvae were rarer and less abundant than the eggs and therefore were less reliable indicators of spawning areas and season. Samples taken along transects across the continental shelf in July of both 1993 and 1994 indicated that H. vittatus did not spawn further than 14 km from the coast. Samples taken in July 1994 just beyond the surf zone at beaches, and at corresponding sites 5.5 km offshore, at 3.7-km intervals along 150 km of coastline indicated that H. vittatus spawns throughout the distribution of the fished stock off south-westem Australia

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

The links between functionally distinct adult assemblages of Sardinops sagax : larval advection across management boundaries

Otolith increment data were used to age larval Sardinops sagax collected from shelf waters between Kangaroo Island in South Australia (SA) and Albany in Western Australia (WA). Distributions of both ages and hatch-dates were compared between six regions across this broad area to examine the hypothesis that larvae arising from spawning sardines in WA may be advected by the Leeuwin Current to the central coast of SA prior to metamorphosis. Current and wind data were analysed to estimate potential rates of passive transport. It appears likely that larvae arising from commercially exploited S. sagax populations in WA can be passively transported close to the region of the S. sagax fishery on the central coast of SA prior to metamorphosis. The potential for links between distant (\u3e1000 km) management units requires reconsideration of the scale over which the population dynamics of southern Australian S. sagax operate. This study provides an example of how information on larval dynamics, including the influences of oceanographic factors, gained from a relatively short period of field work (1 month) can significantly increase our understanding of population dynamics of exploited fish and subsequently have direct relevance to resource management

Integrated fisheries management in Western Australia - a significant challenge for fisheries scientists

The advent of the 21st century has seen the Department of Fisheries (DoF), Government of Western Australia (WA) embark on an ambitious initiative of integrated fisheries management (IFM) within the broad context of the principles of ecologically sustainable development (ESD). This initiative consolidates the outcome of earlier important initiatives such as the freeze on the issue of commercial fishing boat licences (FBLs) in 1983, and more recently the development of regionally based management strategies for recreational fishing, the formal management of charter fishing, and the recognition of the importance of indigenous fishing. An initial challenge for finfish scientists was the development of a means of prioritizing the expenditure of research effort directed at the important task of determining sustainable harvest levels for key “indicator” species within each Bioregion of the state. It is anticipated that at times, such determinations will need to be made in the absence of adequate data. The ongoing monitoring of the catch shares allocated to each sector also poses a significant challenge. Methods being developed to handle these challenges, and other important future needs identified as a consequence of embarking on this process, need to be addressed to ensure that the limited funds available for monitoring are well spent

A risk assessment and prioritisation approach to the selection of indicator species for the assessment of multi-species, multi-gear, multi-sector fishery resources

Assessing the stock status of mixed and/or multi-species fishery resources is challenging. This is especially true in highly diverse systems, where landed catches are small, but comprise many species. In these circumstances, whole-of-ecosystem management requires consideration of the impact of harvesting on a plethora of species. However, this is logistically infeasible and cost prohibitive. To overcome this issue, selected ‘indicator’ species are used to assess the risk to sustainability of all ‘like’ species susceptible to capture within a fishery resource. Indicator species are determined via information on their (1) inherent vulnerability, i.e. biological attributes; (2) risk to sustainability, i.e. stock status; and (3) management importance, i.e. commercial prominence, social and/or cultural amenity value of the resource. These attributes are used to determine an overall score for each species which is used to identify ‘indicator’ species. The risk status (i.e. current risk) of the indicator species then determines the risk-level for the biological sustainability of the entire fishery resource and thus the level of priority for management, monitoring, assessment and compliance. A range of fishery management regimes are amenable to the indicator species approach, including both effort limited fisheries (e.g. individually transferable effort systems) and output controlled fisheries (e.g. species-specific catch quotas). The indicator species approach has been used and refined for fisheries resources in Western Australia over two decades. This process is now widely understood and accepted by stakeholders, as it focuses fishery dependent- and/or independent-monitoring, biological sampling, stock assessment and compliance priorities, thereby optimising the use of available jurisdictional resources