The marine heat waves (MHW) of 2010/11, 2011/12 and 2012/13 provide a benchmark with which to investigate changes in habitat composition and potential flow on effects to the fishery.
A 2018 independent review of the science used for stock assessment of the fishery recommended that studies should be undertaken to
a) investigate the impacts of the previous MHW on juvenile recruitment to the fishery and,
b) to better understand the role of habitat composition in recruitment
To understand the impact of habitat change on the fishery, either through warming events or changes in coastal processes, we need an increased understanding of the role of habitat on the survivorship and growth of puerulus, post-puerulus, juvenile and adult life stages of lobster.
In the 40 year time-series of puerulus settlement index a very strong relationship has persisted between puerulus abundance and commercial lobster catches 3 - 4 years larter (de lestang et al., 2010). An undersize catch rate index, based on historical (1985 - present) catch-rate records, evidenced the existence of a strong correlation between puerulus and undersize catch rate (de Lestang pers. com.). However, two-four years after the MHW of 2010/11, this relationship degraded, with the observed catch rate of lobsters being far less than expected at northern locations of the fishery. Although the relationship has now started to return to its historical form, the severity and longevity of its departure are cause for concern and highlight the marked impact future MHW or changes in coastal processes could have on the fishery.
Change in the extent and configurations of coastal habitats is already occurring in response to natural physical forcing (coastal processes) and accelerated by climate change related stressors. It is crucial to have a detailed knowledge of how habitat change affects survival and recruitment key life-stages of the western rock lobster, so these can be taken into consideration for management practices that ensure the sustainability of the fishery. Such information is currently lacking.
Final report
The West Coast Rock Lobster Managed Fishery (WCRLMF) is one of the most valuable and sustainable single-species fisheries in Australia. WCRLMF is managed, in part, using larval (puerulus) settlement indices obtained from artificial seagrass stations that are continuously monitored at eight locations throughout the shallow coastal habitats of the Western Bioregion of Western Australia. The settlement indices correlate to subsequent catch rates of the Western Rock Lobster (WRL) and are used to predict catch into the WCRLMF in typically 3-4 years times. Recently, the relationship of the settlement indices to catch have become less evident in some parts of the fishery, particularly after the marine heatwave that occurred in Western Australia in 2011. Since the heatwave reportedly impacted habitats, these ocean climate mitigated changes WRL recruitment habitats, such as seagrasses and macroalgae, are hypothesised to be the source of increased unexplained variation in the WRL population, but causal links remain unclear.
Modelling and tracking the changes in coastal habitat in space and time has become an important aspect of managing our environments more generally. This project set out to investigate if including measures of recruitment habitat into the WCRLMF stock assessment will improve management of the fishery.
