Principal Investigator: Adrian Linnane
Key Words: Southern Rock Lobster, Northern Zone Rock Lobster Fishery, temporal management, spatial management, crustacean fishery, lobster biomass, exploitation rates, catch composition.
Summary: The Northern Zone Rock Lobster Fishery (NZRLF) of South Australia covers approximately 207,000 km2. The fishery has been managed as a single spatial unit using both input and output controls which includes a total allowable commercial catch (TACC).A closed season extends from 1 June to 31 October each year. The majority of the annual TACC is taken within inshore grounds (<60 m depth) in the eastern region of the fishery. This trend is largely driven by market demands for lobsters of a particular size and colour, specific to shallow-water areas. The confined nature of fishing behaviour under the TACC system has led to concerns within the industry of localised spatial depletion and the subsequent need for an adaptive management response. In addition to this spatial confinment, fishing in the NZRLF is temporally restricted by the current fishing season, with the majority of the catch taken from November to March each year.
In order to inform possible changes to management options, this project was developed in response to the following needs; (i) information on the catch composition in the NZRLF during the current the closed season, from June to October, inclusive; and (ii) spatial biomass estimates within the NZRLF to ensure that harvest strategies set annual TACCs based on sustainable and economically optimal levels of exploitation rate. Further to the identified needs, the objectives of this project were to; (i) provide a detailed breakdown of catch composition in each of four specific regions in the NZRLF from dedicated surveys undertaken from June to October inclusive; and (ii) generate spatial estimates of rock lobster biomass in the fishery using historical catch and effort data.
In 2014, a winter fishing survey was successfully completed with 104 out of a possible 120 days sampled (87%) and 3.6 t of lobsters landed. The overall survey catch rate was 0.49 kg/potlift, which is approximately 50% below that of the normal fishing season. This could be expected given that; (i) overall lobster catchability decreases during the winter period; and (ii) only males were retained during the survey. The highest catch rate was observed in August (0.68 kg/potlift) within the western part of the fishery. These estimates were driven by mean weight which was consistently higher across all survey months in western areas. High mean weights were further confirmed by length frequency data which showed that larger lobsters above 150 mm carapace length were more prevalent in the west compared to all other areas. These results are likely to reflect the known spatial variation in lobster growth across South Australia.
Based on consultation with industry stakeholders, estimates of biomass were generated for three spatial sub-regions of the NZRLF; West Coast, Deep Water and Inner Region using the South Australian lobster fishery model. Two-thirds of the biomass was estimated to lie in the Inner Region, with 28% and 6% respectively in the West Coast and Deep Water. However, the Inner Region comprised 93% of the average yearly catch since 2009, with 5% and 1.5% of landings coming from the West Coast and Deep Water. Accordingly, exploitation rates were much lower in the West Coast and Deep Water with only 3% and 4% of the lobster stock biomass in those sub-regions harvested annually since 2009, compared with 32% in the Inner Region.
Growth rates of lobsters varied spatially by sub-region. Deep Water lobsters grow much more slowly than Inner Region or West Coast counterparts, implying a lower stock productivity below 80m depth. The mean weight of lobsters during the current 7-month fishing season was very similar in the Inner and Deep Water regions, but was much higher in West Coast waters, consistent with the larger lobsters harvested in this area during the winter fishing survey.
The outcomes of this project have been directly applied to the management of the NZRLF resource. Spatial management of the TACC in this fishery has now been recommended by the South Australian Rock Lobster Fishery Management Advisory Committee (RLFMAC). The boundary line separating the Inner and Outer Regions has been accepted, with the Outer Region combining West Coast and Deep Water areas. An amended NZRLF harvest strategy has been proposed based on Inner and Outer regions, with a new schedule of TACCs underpinned by spatial biomass and exploitation rate estimates generated from this project. TACCs were computed to target 21% and 10% exploitation rates in the Inner and Outer Regions respectively.
At the request of the RLFMAC, to further investigate the option of a 12-month fishing season, in 2015, a second commercial fishing survey was undertaken based on a maximum of 175 fishing days (35 days for each of five months June to October inclusive).
It is envisaged that optimal economic benefit of these revised NZRLF management arrangements will accrue from the combination of winter fishing and spatial management. The expanded yearly quota made possible by spatial extension to previously under-exploited grounds is enhanced by the higher price for lobster seafood received by fishers in the winter months of traditionally low rock lobster supply and substantially higher export prices.