7.1.1 Background
Research projects have explored aspects of the rock lobster stock and recruitment relationship and the influences of environmental conditions. The results of the projects, amongst other things, will enable a review of the appropriateness of the 1980 level of egg production (breeding stock) as the threshold value for the coastal Zones (B and C) of the fishery and the historic level of catch that is currently used as the egg production (breeding stock) threshold value for Zone A.
The projects also provided information on the importance of particular breeding stock areas (e.g. Big Bank, northern and southern Zone A, deep water off Dongara), which could lead to specific breeding stock areas being given greater protection.
7.1.2 Project Objectives and Preliminary Findings
Identifying factors affecting the low western rock lobster puerulus settlement in recent years. (FRDC 2009/018)
Objectives
1. To use a larval advection model and the rock lobster population dynamics model to assess the effect of the spatial distribution of the breeding stock on the puerulus settlement.
2. To assess environmental factors (water temperature, current, wind, productivity, eddies) and breeding stock affecting puerulus settlement.
3. To examine climate change trends of key environmental parameters and their effect on the western rock lobster fishery.
4. Provide industry (WRLC) and Fisheries managers with an evaluation of relative impact of breeding stock and environmental effects on the puerulus settlement and its implications for management in the protection of the breeding stock.
Preliminary findings:
ENSO events, which are associated with weak Leeuwin Currents and cooler water temperatures during the larval phase, have long been known to be associated with low puerulus settlement.
Similarly, weak westerly winds (associated with fewer storms and lower rainfall) in late winter/
spring have also been known to be associated with lower puerulus settlement (Section 5.3).
The environmental conditions in the first year of low settlement (2006/07) had very cold water in February – April 2006 and hence a low settlement was expected. In 2008, for the first time, an above-average water temperature was associated with a very low settlement indicating that other factors are involved (Figure 4.1–5).
It is now clear that the Indian Ocean Dipole (IOD) also appears to have a significant influence on levels of puerulus settlement, with a positive index being related to poor settlement levels.
The effect of the IOD appears to be associated with the strength of the westerly winds in winter/
spring near the period that settlement is occurring.
The Indian Ocean Dipole remained positive for three consecutive years from 2006 – 2008, the first time three consecutive years have been recorded. Moreover, 2008 was the first time in 30 years that a positive IOD has occurred at the same time as a La Niña event.
Settlement remained relatively low in 2009/10 despite the IOD moving back to a neutral position in 2009 indicating again that other factors are involved.
Westerly wind patterns during the current settlement season (2010/11) have again been extremely unusual, with the westerly component being far weaker than average. The second lowest winter/spring rainfall in 2010 on record reflects this with the 2006 having the lowest rainfall on record.
Evaluating source-sink relationships of the Western Rock Lobster Fishery using oceanographic modeling. (FRDC 2008/087)
Objectives
1. To determine the relative contribution of larval production from different areas to the abundance and spatial distribution of puerulus settlement over 15 years using a larval advection model.
2. Provide industry (WRLC) and Fisheries managers with an evaluation of source-sink relationships and its implications for management in the protection of the breeding stock Preliminary findings:
Eggs released towards the very northern part of the fishery (e.g. Northern Abrolhos Islands
& Big Bank) appear to have a much higher chance, on average, of successfully recruiting as juveniles (Caputi et al. 2010, Feng et al. 2011). Possible annual variability in the relative success among regions will be examined in next phase of this project in 2010–2012.
Settlement success may also be greater for eggs released from deeper water areas (80–100 m) closer to the edge of the continental shelf.
The effect of temperature on the growth and survival of larvae was identified as an important component affecting settlement success. Higher success was associated with early larval release (November – December) compared with late releases (February) and the proportion of early release larvae was affected by the water temperature in May – October prior to spawning which includes the period when females are moulting back to setose and begin mating. When water temperatures are very cool (as occurred in May – June 2007) there is less early larval release. This could have affected the 2007/08 spawning period and in turn contributed to the low settlement in 2008/09.
This indicates that environmental conditions up to six months prior to actual spawning also need to be examined in addition to those present during the larval phase.
Overall, these preliminary results suggest that whilst the breeding stock in all regions needs to be maintained, the stock in the northern areas (despite being a relatively small percentage of the total), including the deep-water Big Bank stocks, may be particularly important. The relative importance of these northern stocks could be even more important during periods of unfavourable environmental conditions for the larval phase.
Evaluating the use of novel statistical techniques for determining harvest rates and efficiency increases in the Western Rock Lobster Fishery. (FRDC 2009/019)
Fisheries Research Report [Western Australia] No. 217, 2012 139 2. Evaluate whether additional sources of information are needed to produce more robust
estimates of harvest rate and efficiency increase.
3. Assess whether the estimates of harvest rate and fishing efficiency are reliable and could be used to assist in the management of the western rock lobster fishery.
4. Provide industry (WRLC) and fisheries managers with an evaluation of change-in-ratio and index removal techniques for determining harvest rates and efficiency creep.
Evaluation of population genetic structure in the western rock lobster (FRDC 2009/020) Objectives
1. Develop additional new microsatellite markers for western rock lobster.
2. Test whether the adult population of western rock lobster is genetically homogeneous throughout its range.
3. Test whether the spatial genetic structure in the next generation of recruits (pueruli) matches the spatial genetic structure found in adults. (If so, this suggests spatial structure is due to limited dispersal or local adaptation).
4. Estimate effective population size of the western rock lobster and test for severe bottlenecks in population size.
Assessing possible environmental causes behind the reduced colonization of puerulus collectors by a wide suite of species. (FRDC 2008/085)
Objectives
1. Begin monitoring the community composition of marine flora and fauna along the Western Australian coastline during this current poor settlement period.
2. Develop standard methodology for monitoring the spatial and temporal variability in the settlement of marine flora and fauna.
3. Determine what environmental parameters may be linked to the majority of variation in the floral and faunal communities colonizing puerulus collectors, focusing on those relating to puerulus settlement.
4. Identify indicator marine flora and fauna species for monitoring the influences of environmental change on Western Australian marine environment.
5. Detect any known or potential introduced marine pests within the Western Australian environment.
Biological Oceanography of the Western Rock Lobster
The Australian Marine National Facility RV Southern Surveyor was used in July 2010 to examine phyllosoma larval density at four locations from Shark Bay to Rottenest and the food web that supports phyllosoma growth at sea. The project was led by UWA in collaboration with other research groups including the Department of Fisheries. The data is still being analysed but they did find good quantities of lobster larvae associated with the eddy structures from the Leeuwin Current. A second cruise was undertaken September 2011 for some of these transects.
7.1.3 Finding from additional projects Patterns of ‘white’ lobsters migrations:
Reductions in the migration of ‘white’ lobsters north into Big Bank over a number of years, due to strong meridinal currents, high fishing pressure on the ‘whites’ and low puerulus settlement, is consistent with the lack of small lobsters and a relatively low catch rate of larger lobsters observed in Big Bank during a recent lobster survey (November 2009).
Fishing in the Big Bank area increased substantially during the 1990s. The recent catch trends indicate that the relative abundance in this region has declined to a far greater extent than has occurred at other locations throughout the fishery, probably as a result of this extended period of lower migration. While egg production has not historically been directly monitored in this area, these levels are likely to be reflected by the drops in catch rates.
Breeding Stock
The overall level of egg production for the 2009/10 spawning season was considerably higher than during the previous two seasons. Especially in Zone B the levels were at record highs and in Zone C it was close to the highest.
Separate estimates for the northern and southern parts of Zone B will be generated using the new model. Separate estimates of breeding stock in deepwater (80 – 100 m) compared to shallower depths (40 – 80 m) are also being examined. These may show different patterns.
The next breeding stock survey, including Big Bank and extending into deep water areas, is planned for October-November 2011.
7.1.4 Conclusions
Based on the information outlined above, the most likely scenario to explain the recent poor levels of puerulus settlement still involves a combination of events.
The long standing levels of fishing when combined with a run of environmental conditions unfavourable for the northwards migration of the whites, led to a reduction in lobsters and therefore egg production at the northern end of the fishery (e.g. northern Abrolhos Islands and Big Bank) which oceanographic modelling suggests may often be the most important for successful larval production. Combined with the environmental conditions during this period also being highly unfavourable for larval return/survival due to weaker westerly winds (including the increased frequency of positive Indian Ocean Dipole events) and cooler water temperatures (ENSO events, weaker Leeuwin Currents) may have resulted in the very low settlement levels recorded in recent years.
The continuation of low levels of settlement recorded so far this year (2010) despite a substantial lift in egg production levels for the relevant spawning season combined with the finding that phyllosoma levels this year appeared reasonably abundant and the very low levels of westerly winds (storms) experienced this year increases the likelihood that the major driver for the low settlements is environmentally driven. This is suggests that the transport of puerulus across the shelf is being adversely affected.
Fisheries Research Report [Western Australia] No. 217, 2012 141 animals that are or will in the future migrate) and by increasing protection of animals that do move into this area or are already resident there (e.g. through closed areas, reducing the maximum size of females) may be valuable but not sufficient to ensure a return to ‘normal’
settlement levels.
A second expert based workshop on low puerulus settlement was undertaken in May 2011.
This workshop formally examined the information that has been generated on this issue and re-examine the likelihoods for each of the proposed scenarios. The outcomes from this exercise included recommendations with respect to future research projects plus outlining any possible alternate direction or options for management.