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Government of Western Australia - Department of Fisheries

Puerulus settlement index

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 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.

Crucial to predicting catches is a monthly sampling program that looks at the abundance of late larval-stage lobsters (puerulus) settling on inshore reefs along the west coast mainly between August and January each year. This puerulus settlement index 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. Another important factor is the health of the rock lobster breeding stock.

Current settlement index

The puerulus stage of Panulirus cygnus settles naturally into areas with both reef platforms and attached algae during the dark phase of the moon (new moon period). Artificial collectors mimic the algal habitat and thus provide a mechanism that allows relative settlement rates of puerulus throughout the fishery to be measured.

Each monitoring location (indicated on the map below) 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 previous new moon period.

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

In cases when a location was not serviced for a given month(s), perhaps due to bad weather, 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, about one month old, probably after settling late in the previous month. These lobsters are added to the previous month’s 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 and combine the averages of the two settlements to get a total measure for that month. These may cause the figures reported on this website to change slightly (for months with two new moons).

About the figu​res

The graphs have recently been modified to provide more representative comparisons with both the long-term average and more recent average patterns of settlement.

Recent puerulus settlements have shown similar patterns to all settlements since 2007. This new pattern of settlement has a very poor August to October period relative to the pre-2007 period, with most settlement now occurring in December and January each year.

The relationship between offshore water temperatures and puerulus settlement also began to degrade in 2007, so we consider this the beginning of the recent puerulus settlement pattern. As this pattern has now occurred for nine consecutive years it seems fair to assume that it may represent a new long, relatively permanent, trend. In light of this, we believe it is more representative to show the current year’s settlement versus this new pattern, as well as the longer historical trend.

These figures show the pre-2006 average (old long-term average; black dotted line), the 2006-present average (red line) and current season’s puerulus settlement (blue line). On the monthly average figures the vertical dotted lines represent the start and end of each settlement season (May and April).

The new moon date prior to sampling determines which month's data is shown on the graph. For example, if the new moon occurred on 25 April 2009 and sampling occurred near on the full moon in May, it is recorded as the average settlement to April on the graph.

Please note that in some years we may have two new moon periods (new moons at beginning and end of the same month); we therefore combined the number of puerulus from each sample into one sample.

The environmental conditions that have historically influenced settlement include offshore water temperatures in February/March and winter/spring storms. However, in recent years the low puerulus settlement seems to also be related to the timing of adult spawning, with earlier spawning resulting in poor subsequent puerulus settlement. 

Historical data covering all areas

(Click the charts below to download larger versions.)


 

Current data - as of October 2017

Abrolhos - Rat Island

 

North - Dongara/P.Gregory (Seven Mile Beach)

Central - Jurien/Lancelin

South - Alkimos/Warnbro/Mentelle

(Click the charts above to download larger versions.)

Contact Mark Rossbach for further information.
T: (+61) 08 9203 0111
E: Mark.Rossbach@fish.wa.gov.au

Last modified: 16/10/2017 3:09 PM

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