Western Rock Lobster Settlement Trends

Western Australia's Western Rock Lobster Fishery has received international acknowledgement as one of the best managed and most sustainable in the world.

The fishery involves about 390 commercial boats and 40,000 recreational fishers. Annual catches have been monitored since the late 1960s and vary between about 7,000 and 14,000 tonnes.

The fishery is one of a handful in Australia where a long-term scientific program has provided sufficient detailed information to predict catches up to four years in advance.

The scientific program also provides estimates of breeding stock levels, and a range of information on natural variations in the fishery as it responds to both fishing activity and environmental variations.

Rock lobsters spawn in spring and early summer and their larvae spend up to 11 months drifting in the ocean, before being swept by current systems to settle on inshore reefs.

A key to predicting catches is an annual sampling program that looks at the abundance of late larval-stage lobsters (puerulus) settling on inshore reefs along the west coast between August and January each year.

This “puerulus settlement index” has been running at one site since 1968 and has always shown a strong correlation with catches of lobsters three and four years later.

Environmental factors such as the strength of the Leeuwin Current, winter/spring westerly wind patterns and water temperatures play an important role in determining the survival and settlement of larval lobsters. However other environmental variables may also be affecting settlement.

Another important factor is the health of the rock lobster breeding stock.

The puerulus settlement during the 2008/09 West Coast Rock Lobster season was the second lowest in the 40 year time series of puerulus collections at Dongara and Jurien.  This poor settlement is expected to result in very poor rock lobster catch in the 2010/11 fishing season. 

The 2007/08 settlement has now been followed by an even lower level of settlement in 2008/09.  However, unlike the previous settlement season when environmental conditions were average for larval survival, conditions leading up to the 2008/09 settlement season were very favourable (strong Leeuwin Current) and were expected to lead to an improved puerulus settlement in 2008/09; although some environmental conditions observed during this period, such as westerly winds, were not favourable. Apart from these two years, the previous lowest puerulus settlement recorded was in 1969/70 resulted in a catch of 7,300 tonnes in the 1972/73 season.

It should be noted that the current overall breeding stock estimates are above the threshold levels and that the management settings implemented during the 2008/09 season have improved the status of the breeding stock.

In light of continued low puerulus settlement however, the Department of Fisheries has reviewed its methodology for estimating the breeding stock. In addition, the Department of Fisheries has also reviewed research and management priorities, in consultation with the respective Chairs of the Western Rock Lobster Council and the Rock Lobster Industry Advisory Committee (RLIAC). Key outcomes identified by this process have all been instigated.  These include:

1. The biological model has been further developed and now incorporates robust estimates of efficiency creep
2. funding from the Fisheries Research and Development Corporation has been received to (i) conduct oceanographic modelling with the CSIRO aimed at evaluating the effect of breeding stock in different regions and environmental factors on puerulus settlement; (ii) examine relationships between environmental factors and levels of settlement; (iii) investigate the settlement of other fauna and flora on the puerulus collectors; and (iv) examine the use of novel statistical techniques to investigate exploitation rates and efficiency creep.
3. The juvenile abundance sampling program using meshed pots has been expanded to evaluate settlement in shallow and deep water;
4. A research risk assessment workshop on factors affecting puerulus settlement has been conducted and the draft report has been sent to stakeholders for comment; and

The Department of Fisheries will continue to monitor puerulus settlement on a monthly basis, and is examining possible causes for the low settlement. It may be necessary to adjust the management settings for the fishery as a result of the low settlement. However, should this be the case, such changes would be introduced on the basis of available research advice and stakeholder consultation if time permits.

Current Puerulus Settlement

The puerulus stage of Panulirus cygnus settles naturally in seagrass beds or onto floating seaweed during the dark phase of the moon (new moon period). This behaviour is exploited by artificial collectors, which mimic this habitat and thus provide a mechanism that allows relative settlement rates of puerulus throughout the fishery to be measured.

Each monitoring location contains five or six collectors, each of which are sampled every full moon period (five days either side of the full moon) over the settlement season (May to the following April). This sample is considered representative of the settlement over the last new moon period.

For each site a monthly average is determined: total number of puerulus retrieved off each collector, averaged over all the collectors sampled. In April each year a seasonal average is also determined: sum of the monthly averages from May to April.

In cases when a location was not serviced for a given month(s), due to bad weather etc., an estimate of settlement is determined based on the historical proportion that month contributes to the overall catch.

Not all the lobsters settling on the collectors are early stage puerulus. Some are slightly larger, being about one month old, probably after settling late in the previous month. These lobsters are added to the previous months settlement tally. Also in most years a new moon will occur 13 times, resulting in one month having two new moons. In these months we record the puerulus settlement twice. In these cases we take an average of the two settlements to get just one measure for that month. These may cause the figures reported on this website to change slightly (for months with two new moons).

Note: In the figures below the grey areas with red boarders represent the long-term average of settlement at each location. The blue lines show the actual average monthly settlement at each site in each month since May 2005. Vertical dotted lines represent the start and end of each settlement season (May & April). The new moon date prior to sampling determines which months data is shown on the graph. eg the new moon occurred on the 25/04/09 and sampling occurred near on the full moon in May and recorded as the average settlement to April on the graph.

Click the chart above to view a larger version of the chart.

Click the chart above to view a larger version of the chart.

Click the chart above to view a larger version of the chart.

Click the chart above to view a larger version of the chart.

Click the chart above to view a larger version of the chart.

For further information please contact Mark Rossbach on 9203 0111.