Regrowth of pilchard (Sardinops sagax) stocks off southern WA following the mass mortality event of 1998/1999
Department of Primary Industries and Regional Development (DPIRD) WA
There is a need to assess the regrowth of pilchard stocks in southern Australia because of (1) the socioeconomic problems associated with fishery declines and the associated need to provide sufficiently detailed information so that management can proceed with the appropriate balance between sustaining the fishery and allowing recovery of the stocks, and (2) the potential for causes of wide ranging ecosystem affects to go unrecognized if there is no baseline data (i.e. abundance) for this key pelagic species. In order to provide a time-series of the regrowth of S. sagax stocks in WA the biomass size of each adult assemblage needs to be assessed over the next few years. Because the ecosystem-based goal of this project is to provide a baseline of pilchard abundance which may relate to other species, the longer life spans, slower growth etc. typical of these higher trophic levels necessitates a long time series of surveys. Changes in biomass of the four pilchard assemblages should be assessed for a period of at least five years. Although this is a relatively short period of time over which to examine the recovery of a pelagic stock, such a routine will provide a solid basis for assessing the rebuild of pilchard abundance and thereby pro-actively collect data which could subsequently assist our understanding of other ecosystem affects of the Herpesvirus mortality event. This project will compliment the annual DEPM surveys intended for SA pilchard stock. Collection of a continuous, albeit short-term, series of estimates of spawning biomass will permit the age structured simulation model being developed by Hall and Gaughan (in prep.) to be fitted to a likewise continuous series of these estimates. This should improve the fitting of the model, thereby making it more useful for predicting impacts of further mortality events involving S. sagax in southern Australia and facilitating the decision -making process in the ongoing management of pilchards in WA.
1. Document changes in the biomass of the four exploited adult assemblages of S. sagax in WA during the six year period following the 1998/99 mass mortality event.
2. Integrate results of biomass surveys into the age structured model for each assemblage of pilchards. Further develop the age-structured simulation models for S. sagax and investigate ongoing behaviour of these under conditions of different levels of information
3. Determine if there is a measurable change in biological parameters of pilchards important to ongoing development of the simulation models, in particular growth rate, in response to the decrease in stock size.
This project produced time series of estimates of spawning biomass for pilchards in four purse seine management zones in Western Australia, three on the south coast and one on the west coast. The pilchard stocks in Western Australia have recovered strongly since the 1998/99 mass mortality.
This project has shown that the fishery independent and fishery dependent methods applied to Western Australia’s pilchard fisheries cannot provide precise estimates of the size of the pilchard spawning biomass in each management region. Although the trends indicated by the age-structured simulation model are reliable and provide good evidence for a strong recovery of pilchard stocks, the magnitude of changes in stock size from year-to-year are not known with sufficient certainty to allow high rates of exploitation.
The demonstrated increases in pilchard spawning biomass has lead to optimism for the future of the purse seine industry in southern WA, which in turn has encouraged this industry to be proactive in ensuring that exploitation rates were not set at levels that were too high to impede the recovery of the stock or put undue pressure on the long term viability of the stocks. Tracking the progress of estimating of pilchard biomass in each region over the duration of this project was a crucial factor that assisted industry members to better appreciate the need to examine factors additional to the point estimate of biomass generated in any one year.
The project also determined that the growth rates of pilchards in WA can be highly variable, sometimes changing markedly from year to year while at other times exhibiting more gradual changes over several years. The tendency for a negative relationship between annual growth rates and stock size suggests density dependence, which in turn supports the contention that that pilchard stocks in this region are limited in size by the biological and oceanographic characteristics of the continental shelf waters of southern WA. The magnitude and variability of pilchard biomass observed during this project builds on the knowledge obtained during the 1990s, providing a strong basis against which the purse seine industry can expect future quota levels to be set.
Keywords: pilchard (sardine), recruitment, age-structured model, spawning biomass, daily egg production method, stock recovery, fisheries management.