4.1 Puerulus
4.1.2 Results
At each site, the surface water temperatures at the southern and northern ends of the collectors for that site are measured, and salinity is recorded. One collector at each site is also fitted with an archival data storage logger that records the water temperature every hour. It is replaced with a new logger every second month.
The environmental data and puerulus numbers are stored in an SQL-based database, which is used to construct monthly and annual trends of puerulus settlement (Figure 4.1–4 and 4.1–5).
Analysis
Puerulus settlement indices in each management zone (Figure 2.3–1) are based on one or more puerulus collection sites (Table 4.1–2). The settlement index for each collector site is the sum of each full moon period’s average number of puerulus sampled per collector, over the settlement season (May to the following April). These indices are standardized to having been sampled by tanikalon collectors by dividing puerulus averages from Boral-Kinnear collectors by 0.85.
When settlement index uses data from more than one puerulus location, a least-squares mean estimate (SAS 1987), standardised for location, is determined by GLM with location, season and month as factors.
If a location was not serviced for a month(s), usually due to bad weather, settlement is estimated from the proportion that month contributes on average. This proportion is identified for each location by using a GLM to model that locations monthly settlement overtime, in terms of the main effects of season and month. Using this model, seasonally standardized monthly effects were extracted and from these, the average contribution of each month to that locations annual puerulus index could be measured. If for a particular season, a group of “missing” months would normally contribute x%, then the sum of the “non-missing” collection of observed months was multiplied by a factor of 100/(1 - x).
At some locations, additional collectors have been added over the years for various reasons. To maintain the consistency over time of the indices produced from these sites use only the original collectors (see Table 4.1–2).
Table 4.1–2 Sites and numbers of collectors used to calculate puerulus settlement indices for the three management zones.
Puerulus collector sites Collectors used for indices
Port Gregory All (1–5)
Rat Island 1–4 (not 5)
Seven Mile Beach All (1–6)
Jurien Bay 1–5 (not 6)
Lancelin All (1–5)
Alkimos All (1–5)
Warnbro Sound All (1–5)
Fisheries Research Report [Western Australia] No. 217, 2012 63
Figure 4.1–2 Average puerulus settlement by month for sites in the northern (Dongara), southern (Lancelin) and offshore (Abrolhos Islands) zones of the WRL fishery.
Although the month in which settlement peaks differs slightly between years, there is a difference between the coastal sites where settlement generally occurs in September / October compared to the though the southern most site of Cape Mentelle and the offshore site at the Abrolhos Islands, which peak in ~December (Figure 4.1–3).
Figure 4.1–3 Most common month of peak puerulus settlement for puerulus collectors throughout the WRL fishery
Although catches of pueruli at the various sites throughout the fishery all exhibit similar inter- annual trends in puerulus settlement, the magnitude of settlement differs markedly between sites.
Puerulus collectors on the mid-west coast (e.g. Seven Mile Beach and Jurien Bay) generally record the highest numbers of pueruli. The two Perth metropolitan collector sites (Alkimos and Warnbro Sound) are lower than those of the mid west, though considerably higher than the most southerly collector site (Cape Mentelle). In contrast, the second most northerly collector site in Figure 4.1–4 (Port Gregory, light green) has a settlement rate that is intermediate between the high mid-west (Seven Mile Beach and Jurien Bay) and lower metropolitan sites (Alkimos and Warnbro Sound).
There is a strong correlation between years of high puerulus settlement between sites, due mainly to oceanic conditions affecting survival and settlement rates on a coast-wide scale.
Puerulus settlement is significantly affected by the water temperature in the area where the larvae occur in February–April (which is influenced by the strength of the Leeuwin Current) and rainfall at the southern sites (in October and November) (rainfall is used as a proxy for the frequency of westerly winds in these months) (Caputi et al. 1995b).
The Leeuwin Current in June to December has also been shown to affect the spatial distribution of puerulus settlement, with the mean latitude of puerulus settlement occurring further south in years of strong Leeuwin Current (Caputi et al. 2001 and Caputi 2008).
Figure 4.1–4 Puerulus numbers per collector for each season from 1968/69 at collector sites throughout the fishery.
Recent trends in puerulus settlement
The most recent above average settlement to occur in the fishery was recorded in the 2005/06.
Since this season the fishery has received four consecutive years of below average settlement.
Fisheries Research Report [Western Australia] No. 217, 2012 65 relationship (Figure 4.1–5). The unexplained nature of the most recent three settlements prompted the department to hold a Puerulus Risk Assessment workshop in April 2009 (http://
www.fish.wa.gov.au/docs/op/op071/fop71.pdf). The workshop developed a range of research projects designed to investigate the cause of these low settlement levels. Details of these projects with their preliminary results, which are helping to form the basis of our current understanding behind the low puerulus settlement, are outlined in section 7.1.
Water temperatures leading up to the 2010/11 puerulus settlement season (February/April 2010) were again been below average. The 2010/11 settlement was therefore not expected to be above average, but did see an increase in puerulus settlement rates at a number of sites along the coast, mainly at sites in the centre of the fishery, Jurien Bay and Dongara (Figure 4.1–4). The settlement, whilst below average, has now returned within the historical relationship between settlement and offshore water temperatures in February (Figure 4.1–5). These levels are still below the long-term average since water temperatures in were cooler than average.
Figure 4.1–5 Relationship with offshore seas surface water temperatures and puerulus settlement index for Dongara and Jurien for each season. The last four settlement seasons are in red text, with offshore water temperature for the 2011/12 puerulus season indicated by a blue arrow.
Water temperatures for January and February 2011 are well above average, and 1oC higher than previously recorded. This was a result of a strong Leeuwin Current associated with the La Nina conditions. The effect of these conditions on the next year’s (2011/12) puerulus settlement will be observed later in the year, starting in about August.