Can production in the southern rock lobster fishery be improved? Linking juvenile growth, survival and density dependence to sustainable yield
Across all southern states there is a strategic need for research to improve assessment of the stocks, improve advice on management alternatives, and provide forecasts with reduced uncertainty of future stock size and,by implication, potential catches (see attached figure 1). This proposal addresses this need. In all states current management objectives include rebuilding of the stocks. Current legal sized biomass is principally based on recent recruits to the fishery. As stocks rebuild, the biomass will contain a greater proportion of lobsters that had recruited prior to the last season. As these lobsters will have increased in size since recruiting, the average weight of the legal sized biomass will have increased. Thus bigger stocks will certainly provide greater yields from each recruit and therefore lead to greater productivity. As the number of eggs that a female lobster broods is a cubic relationship to her size, a small increase in size will lead to a much larger increase in the number of eggs produced. Thus large gains may also be possible through the feedback from greater egg production from the rebuilding legal sized biomass. The potential for more eggs to lead to higher recruitment to the fishery will be strongly dependent on the rates of growth and mortality of the pre-recruit stages. Therefore there is a need to acquire this information to enable managers to take egg production into account when setting management measures. Tasmania is about to start a puerulus harvesting program as part of an attempt to establish a rock lobster aquaculture industry in the state. Part of this activity is a return of a proportion of the animals to the wild after a period in the laboratory, estimated to be equal or in excess of those that would have survived over a similar period in the wild. This aims to ensure neutrality of puerulus harvest. The return is currently based on "guesstimates" of mortality. The results of this study will therefore have a significant application in this related work. The high value of rock lobster fisheries in southern Australia means that even small increases in the catch may have substantial benefits. A 5% improvement might result in a $10 million increase in landed value with flow on benefits to southeastern Australian rural coastal communities. Concern has been voiced that increases in production may be offset by lower prices due to supply outstripping demand. However, price increases during the past decade suggest that demand is growing more rapidly than supply. It therefore seems reasonable to conclude that higher production (without increased effort) in southern Australia will have a positive impact on the economy.
1. To determine growth and mortality rates of juvenile (<80mm) lobsters throughout the range of the commercial fisheries.
2. To assess if increased juvenile density will translate to increased fishery production.
3. To evaluate techniques and obtain preliminary estimates of growth and mortality rates of puerulus / post-puerulus.
Principal Investigator: David Hobday, Stewart Frusher and Adrian Linnane