The dramatic, hidden, impact of marine heatwaves on Western Rock Lobster (Panulirus cygnus) survival has led to an expansion in data collection to better forecast the fishery’s stock levels.

By Brad Collis 

With support from the Western Australian Department of Primary Industries and Regional Development (DPIRD WA), Western Rock Lobster (WRL) fishers are recognised world leaders in fisheries management, so it’s not surprising they are stepping up on data collection to stay ahead of climate change impacts. 

This follows the recent discovery in the northern part of this Western Australian fishery that a high percentage of post-settlement puerulus – the first benthic stage in the lobster’s growth – have not been surviving to reach maturity.  

This has created a disturbing gap between puerulus data used for stock forecasting and actual lobster numbers. 

The finding led to an FRDC project (2019-159), funded through the FRDC/WRL Industry Partnership Agreement, to analyse this discrepancy and develop a more reliable basis for stock forecasts. As a result, a standardised, repeatable survey for shallow waters (less than 20 metres depth) where undersize lobsters grow before moving into the deeper fished areas, has been developed. 

Dr Simon de Lestang, Principal Investigator and Research Scientist with DPIRD WA, leads the FRDC project.  

He explains that accurate forecasting is crucial for stock assessment, so it was alarming when puerulus data was not mirrored by later juvenile lobster numbers. Puerulus levels are also used to make business decisions because they provide an indication of stocks, four years into the future. 

The discrepancy in puerulus data and undersize lobster numbers was first detected at Port Gregory in 2015, four years after the 2011 heatwave.  The puerulus count previously had been normal, but four years later there were no signs of lobsters.  

“It was the first time the relationship between puerulus and lobster numbers had broken down,” says Simon. 

“So, it was clear we needed to better understand what is happening in the shallower waters where the juvenile lobster live,” says Simon. 

Simon says the FRDC project implemented a process for annually surveying to the conditions within the shallow water habitats and how these habitats might be changing in response to increases in water temperature.  

POTBot cameras are installed into specially designed pots for surveying undersize lobster at 12 sites along 600 kilometres of coastline. The POTBot video footage uses a methodology developed in an earlier FRDC project.

Combined with sample pot catches, the footage provides fisheries managers and researchers with a much clearer understanding of the composition of these habitats and changes taking place.  

The project has been extended for two years as the Western Rock Lobster fishery takes over responsibility for conducting the new surveys.  

The surveys themselves are done in the five days either side of the new moon in March when there is minimal ocean swell. In 2024 this was during the second week of March.  

Since the research started, they are showing the difference in abundance of post-larval lobster (puerulus) settling in the shallow waters in the north and their numbers three years later (when they are just undersize lobsters) is now reducing. He has also indicated that some of the pre-heatwave algae communities are also returning to pre-heatwave levels. This supports the research hypothesis that the heatwave caused a near total food chain break.  

“The heatwave not only knocked off a lot of the algae and seagrasses post-puerulus use for habitat, but also destroyed juvenile lobsters’ food sources such as small gastropods (marine snails). 

“We’ve seen that it takes a long time for this food chain to come back to a level sufficient to support these juvenile lobsters. They live in the shallow waters for two to three years, just eating and growing, and if they don't have enough habitat and food, they're not going to survive.”  

The research into the impacts of the heatwave also found rock lobster populations further south off Fremantle increased for several years as the warmer water nourished habitats there. 

Simon believes the importance of the research and the new annual shallow-water surveys underpins world-wide data showing marine heatwaves are increasing in frequency. 

In response to marine heatwave forecasts, lobster and crab fisheries around Australia are already in the process of moving to a new stock assessment model. In WA, data from the juvenile lobster surveys will be incorporated into this. 

Simon believes the new surveys provide more timely data than the four-year puerulus projections and commercial catch statistics that were previously relied on. 

FRDC Research Portfolio Manager Adrianne Laird says the proactive approach taken by the Western Rock Lobster fishery demonstrates its commitment to ensuring the sustainability and resilience of the fishery, amid the challenges posed by a changing marine environment. 

“Industry members, often generational fishers with extensive on-water experience, possess invaluable firsthand knowledge of the ecosystem and can keenly observe changes as they occur.  

“This understanding of the marine environment enables them to identify emerging issues and contribute valuable insights to research initiatives,” says Adrianne. 

Related FRDC Project

2019-159: Developing an independent shallow-water survey for the Western Rock Lobster Fishery: tracking pre-recruitment abundance and habitat change