ICES Journal of Marine Science

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1 ICES Journal of Marine Science ICES Journal of Marine Science (2015), 72(Supplement 1), i49 i58. doi: /icesjms/fsu177 Contribution to the Supplement: Lobsters in a Changing Climate Original Articles What caused seven consecutive years of low puerulus settlement in the western rock lobster fishery of Western Australia? Simon de Lestang 1 *, Nick Caputi 1, Ming Feng 2, Ainslie Denham 1, James Penn 1, Dirk Slawinski 2, Alan Pearce 1,3, and Jason How 1 1 Western Australian Fisheries and Marine Research Laboratories, PO Box 20, North Beach, WA 6920, Australia 2 CSIRO Marine and Atmospheric Research, Private Bag No. 5, Wembley, WA 6913, Australia 3 Curtin University, GPO Box U1987, Perth, WA 6845, Australia *Corresponding author: tel: ; fax: ; simon.delestang@fish.wa.gov.au Present address: West Australian Fisheries and Marine Research Laboratories 39 Northside Drive, Hillarys Western Autsralia. de Lestang, S., Caputi, N., Feng, M., Denham, A., Penn, J., Slawinski, D., Pearce, A., and How, J. What caused sevenconsecutiveyears of low puerulus settlement in the western rock lobster fishery of Western Australia?. ICES Journal of Marine Science, 72: i49 i58. Received 4 August 2014; revised 17 September 2014; accepted 18 September 2014; advance access publication 19 October Puerulus settlement in the western rock lobster fishery has remained below average for seven consecutive years (2006/ /2013), with 2008/2009 being the lowest in over 40 years. Examination of the timing of the start of spawning using fishery-independent data since the mid- 2000s indicated that spawning has been occurring earlier. The low settlement appears related to higher water temperatures at the time of the onset of spawning (October) since the mid-2000s. Statistical analysis shows that the most (71%) of the variation in puerulus settlement was explained by the timing of spawning, storm activity during autumn/spring, and offshore water temperatures in February. Earlier spawning may cause a mismatch with other environmental factors such as peaks in ocean productivity and/or storms that assist the larvae return to the coast and offshore water temperatures that help the early stage larval growth. These variables produced a plausible hypothesis to explain the decline in puerulus settlement for these 7 years, including the recruitment failure of 2008/2009. They also predicted the substantial improvement in settlement for 2013/2014. Egg production levels did not to have a significant relationship with puerulus settlement levels after taking environmental variables into account. Further verification with additional years is required to see whether this relationship is maintained. Global climate change may influence these environmental factors: the timing of spawning is influenced by water temperature and there has been a reduced trend of autumn to spring storms off southwest Australia. Keywords: climate change, environmental effects, puerulus, rainfall, storms, timing of spawning, water temperature, western rock lobster. Introduction Puerulus settlement of the western rock lobster (Panulirus cygnus) fishery of Western Australia (WA; Figure 1) has remained well below average for seven consecutive years (2006/ / 2013), with 2008/2009 being the lowest in over 40 years over the whole area of the fishery (Caputi et al., 2010a). The puerulus settlement is important in the stock assessment and management of the fishery as it is a reliable predictor of recruitment to the fishery and catch 3 4 years later (Caputi et al., 1995a; de Lestang et al., 2009). Previous studies had shown that environment factors such as the strength of the Leeuwin Current and storms in late winter/spring typically affect the abundance and spatial distribution of puerulus settlement (Pearce and Phillips, 1988; Caputi et al., 2001; Caputi, 2008). A strong Leeuwin Current (associated with warm water temperatures and influenced by La Niña conditions in the Pacific) has historically been associated with above-average settlement. However, during the recent series of low recruitments, the low settlement has occurred despite a strong Leeuwin Current in 2008 and It was therefore important to identify other environmental and/or biological influences that may have contributed to the atypically low recruitment and whether any factors identified were # International Council for the Exploration of the Sea All rights reserved. For Permissions, please journals.permissions@oup.com

2 i50 S. de Lestang et al. Figure 1. Location of current (2013/2014) puerulus settlement sites along the coast and the different fishery zones (A, B, and C) operating in the fishery. exhibiting long-term trends, which could explain the new lower settlement levels. One of the possible causes of the decline in recruitment was whether the breeding stock had declined to levels low enough to cause recruitment overfishing. During the recent period of low settlement, the spawning stock was within historical levels at all sites that had been surveyed since the early 1990s which covered the main part of the fishery (de Lestang et al., 2012). However, there were concerns about the status of the stock in the northern part of the fishery (e.g. northern Abrolhos Islands and Big Bank), which did not represent a major area of the fishery and therefore had not been regularly surveyed. Significant effort reductions were introduced in 2008/2009 as a result of the poor puerulus settlement before these year classes recruited to the fishery. These reductions were designed to increase residual stock abundance that could be fished in future poor catch years and to maintain the spawning stock at safe levels, i.e. above threshold reference levels. The effort reductions resulted in the average catch declining from the longterm average of t in recent years (de Lestang et al., 2012). The lower west coast of WA has been identified as a hotspot for increasing water temperature (Pearce and Feng, 2007), particularly in the autumn winter period (Caputi et al., 2009). There has also been a significant decline in storm activity and rainfall in the region during winter (IOCI, 2012). These factors have been previously identified as affecting different aspects of the life history of the western rock lobster (Caputi and Brown, 1993; Caputi et al., 2001) and include the impact of water temperature and winter storms during the phyllosoma larval phase, Leeuwin Current effect on the spatial distributions of puerulus settlement, reduced migration of white lobsters to northern breeding stocks, and decreases in the size at maturity and migration (Melville-Smith and de Lestang, 2006; Caputi et al., 2010b). The western rock lobster has a relatively long spawning phase of 5 months and larval phase of 9 11 months. Changes in environmental conditions during any phase of the spawning and larval life could have contributed to a mismatch between environmental conditions and a later larval or puerulus stage. An oceanographic larval model has recently identified the timing of spawning as an important factor affecting puerulus settlement, with early egg releases providing the best fit between the model settlement and actual settlement (Caputi et al., 2014). Understanding the cause(s) of recruitment variability and identifying any potential long-term trends has important implications in the stock assessment and management of the fishery. That is, the management response would be significantly different if the major cause of the series of low recruitments was due to reduced egg production rather than variations in environmental factors outside of management control. A previous assessment of the stock recruitment environment relationship (SRR-E) for western rock lobster showed that environmental conditions explained most of the variation in puerulus settlement at the coastal sites with spawning stock not being significant (Caputi et al., 1995b); however, this relationship does not explain the variation in more recent years of puerulus settlement. This study examines whether the magnitude of the spawning stock and the timing of spawning are significant factors in the recent decline in puerulus settlement. It also examines previously identified key environmental factors that may also be influencing the puerulus settlement to determine an SRR-E. Methods Biological indices Puerulus settlement The puerulus settlement data are currently collected monthly from nine locations (Figure 1) throughout the rock lobster fishery by the Western Australian Department of Fisheries (DoF; de Lestang et al., 2012), with Dongara being the first location sampled in A standardized annual mean of fishery-wide puerulus settlement index was obtained from nine locations (Port Gregory, Horrocks, Abrolhos Is., Dongara, Jurien Bay, Lancelin, Alkimos, Warnbro Sound, and Cape Mentelle). The index represented an annual mean settlement rate per collector standardized for location and month of sampling using a generalized linear model (GLM) and the lsmeans package in R (R Development Core Team, 2014), and ranged between about 1 and 200. The timing of peak settlement was determined by fitting a normal distribution to the relationship between the date and the sum of all puerulus collected (locations pooled excluding the Abrolhos Islands) in each month of the settlement season (May April) and ranged between late September and early January. A 5-year centred moving average was fitted to the time-series to emphasize its long-term underlying trend. Egg production Annual egg production in the fishery was determined using data from the annual Independent Breeding Stock Survey (IBSS). This survey is conducted by the DoF at three locations every year (Lancelin, Dongara, and the Abrolhos Islands) and intermittently at four other locations (Fremantle, Jurien, Kalbarri, and Big Bank;

3 Low puerulus settlement in the western rock lobster fishery of Western Australia i51 de Lestang et al., 2012). The IBSS is conducted over a 10-d period during the last new moon before mid-november. This period is just before the annual peak of egg production, which occurs in November/December (Chubb, 1991). The egg production index is based on the catch rate of mature female lobsters and a fecundity size relationship (de Lestang et al., 2012) at each of the locations sampled. It represents a mean catch rate of eggs per pot lift standardized for location of capture, month of capture, water depth, ocean swell, and soak time of the pot using a GLM. The index ranged between about 12 and 15, and was estimated using the lsmeans package. For more information, see de Lestang et al. (2012). Timing of spawning The timing of the start of spawning of P. cygnus varies between years, most likely in relation to interannual variation in winter and spring water temperatures. The general pattern of reproduction is for mating to start around June and fertilization to begin in September before reaching its maximum with the most mature females being ovigerous (egg bearing) in November and December (Chubb, 1991; de Lestang et al., 2012). The numbers of ovigerous mature females then decline to only a few still being present by March. The timing of the onset spawning was examined using the reproductive state of mature females lobsters caught during the IBSS (October/November). Data from the Kalbarri and Big Bank regions were not used in constructing the index of onset of spawning as the timing of some of these surveys were too early for any significant interannual variation the proportion of spawning females to be detected (September). The annual estimate of the onset of spawning was a mean spawning stage standardized ( lsmeans ) for location of capture, year of sampling, the logarithm of timing (days since 1 September) of the survey, and the logarithm of carapace length using a GLM. Logarithmic transformations associated with the timing of the survey and carapace length were based on Tukey s Bulging rule (Tukey, 1977). Spawning stage was assigned according to the presence and developmental stage of eggs and the condition of a spermatophoric mass on mature females. Females were designated as being mature if they had mated (possess an external spermatophore) or had developing gonads. The stages assigned were: 0 ¼ mature female with no eggs and no used spermatophoric mass; 1 ¼ recently fertilized eggs; 2 ¼ mid-way through egg development; 3 ¼ eggs fully developed; 4 ¼ residual egg remnants from larval release; 5 ¼ a used spermatophoric mass but no eggs (i.e. the lobster had spawned). The resultant index ranged between just above zero when the most mature females had not yet extruded their eggs by mid-october (representing a late start to spawning) to above one when the most females had extruded their eggs by mid-october (representing an early start to spawning). Environmental indices Leeuwin Current and sea surface temperature The sea surface temperature (SST) off the lower west coast of WA s continental shelf is influenced by the strength of the Leeuwin Current and direct air sea fluxes in the region (Feng et al., 2008). The Fremantle sea level has also been used as an indicator of the strength of the Leeuwin Current and an indicator of the strength of the eddy structure associated with the current (Feng et al., 2009). The Leeuwin Current is influenced by El Niño Southern Oscillation (ENSO) events in the Pacific through the oceanic waveguide, and the Southern Oscillation Index is an indicator of the strength of the ENSO events. Levels of puerulus settlement have historically shown a strong relationship with offshore water temperatures in an area west of the fishery (25 288S, E) during the early larval stages (February), so this relationship has been updated with additional years (de Lestang et al., 2012). The temperature data used were the NOAA Optimum Interpolation SST V2 dataset provided by the NOAA website at noaa.gov/psd/data/gridded/data.noaa.oisst.v2.html (Reynolds et al., 2002). Winter storms Monthly rainfall data were collated from five coastal sites from Jurien (30.18S) in the north to Fremantle (32.08S) in the south ( shtml). Rainfall was used as a proxy for the effects of winter storms on the ocean environment (e.g. swell and waves action) and the westerly winds associated with storms crossing the coast, which is crucial to the onshore movement of late stage larvae (Feng et al., 2011b). Individual months of rainfall data were examined to determine what periods the storms having the biggest impact on puerulus settlement. Monthly rainfall data ranged from 0 to 342 mm. Bottom water temperature The effect of bottom water temperature on the timing of spawning was examined as it is the most likely cue for the initiation of spawning (Chittleborough, 1976). An empirical time-series of seabed water temperatures in the spawning grounds was not available; therefore, model-derived estimates of seabed water temperatures were collated by combining outputs from two of the Australian Bureau of Meteorology/CSIRO s data assimilating oceanographic models, BRAN and OceanMaps v1 for the periods prior and during the spawning period (Oke et al., 2008; Brassington et al., 2012). Outputs from the two models overlapped for a period of 14 months, i.e. BRAN spanned January 1993 April 2008 and OceanMaps October 2007 November This overlap period was used to develop a correction factor to scale the outputs from the OceanMaps v1 model to that from the BRAN model. Modelled water temperatures ranged from 17.4 to 20.38C. Statistical analysis Stock recruitment environment relationship Whether an SRR-E existed and in what form was examined by comparing annual variation in puerulus settlement, with a combination of indices, each previously identified as being capable of impacting on spawning or larval stages. A GLM was employed that utilized a log transformation for the puerulus and egg production indices to the skewed nature of their abundance distributions. The relationship between puerulus settlement and the factors was each examined individually and in combination. Whether the addition of factors contributed significantly to the overall model was assessed using an ANOVA. Multicoefficient models were simplified by removing components (main effects and/or interactions) if they contributed,1% to the model s overall R 2. Results Puerulus settlement The puerulus settlement time-series emphasizes the protracted period of very low settlement from 2006/2007 to 2012/2013, with the lowest level of settlement being recorded in 2008/2009 in all regions (Figure 2). Since this record low settlement, levels of settlement have increased, with the greatest increase occurring in the

4 i52 S. de Lestang et al. Figure 2. Standardized mean puerulus settlement time-series from 1968/1969 to 2012/2013 in four regional areas [Dongara/Port Gregory (a), Abrolhos (b), Jurien/Lancelin (c), and Alkimos/Warnbro/Mentelle (d)] and all regions combined (e). The horizontal dotted line represents each time-series median between 1984/1985 and 2005/2006 season. central and northern coastal regions (Figure 2a), where settlement levels have increased to well above their historical medians in 2013/2014 for the first time in 8 years. The weakest of these increases has been in the offshore Abrolhos region, where levels are still just below the median level (Figure 2b). The timing of peak settlement varied markedly between seasons ranging from late September during the 1982/1983 season to early January in the 2012/2013 season (Figure 3). A smoothing of the time-series displayed a progressive trend, with the timing of settlement changing from being delayed (December) during the late

5 Low puerulus settlement in the western rock lobster fishery of Western Australia i53 Figure 3. Mean timing of peak puerulus settlement (black line) each season from 1968/1969 until 2013/2014. A five-point centred smoothed time-series is shown in grey. 1960s/early 1970s, to variable between mid-october and mid- November in the 1980s then back to being late post 2010 (Figure 3). An important aspect of the recent decline in settlement has been the particularly low settlement during the early part of the season, August October, which has historically (before 2008) been a period of good settlement. Puerulus environment relationships The timing of spawning recorded during the IBSS (measured by egg stage development) increased during the mid-2000s, with this index reaching its maximum in 2007 due to the presence of large numbers of mature females with eroded (used) spermatophoric masses and no external eggs (Figure 4a). This state indicates that many females had started reproduction very early and spawned before the survey. This 2007 spawning season corresponds to the 2008/ 2009 puerulus settlement, which was the lowest on record. Apart from 1994, the most years when the timing of spawning was early Figure 4. (a) Standardized mean (and 95% C.L.) timing of spawning between 1992 and (b) The relationship between the timing of spawning and the logarithm of puerulus settlement, with the years representing the puerulus settlement season. (c) Standardized mean (and 95% C.L.) egg production from the IBSSs. (d) Relationship between mean egg production and the logarithm of puerulus settlement, with the dotted line representing the fitted relationships between the two factors and puerulus settlement.

6 i54 S. de Lestang et al. have occurred since 2004 (Figure 4a). This timing of spawning index displayed a significant (R 2 ¼ 0.23, d.f. ¼ 19, p ¼ 0.026) negative relationship with variation in the puerulus settlement during the subsequent settlement period (Figure 4b). Surveys conducted in years when the most mature females were yet to initiate spawning (as represented by a low index value of 0.2) generally coincided with years of above-average puerulus settlement. Egg production showed a general increase during the 1990s of 8%, followed by a similar decline down in the mid-2000s before increasing by 16% to record levels by 2012 (2013/2014 puerulus settlement year; Figure 4c). The most variation has been caused by major management changes introduced in 1993 and 2008, which reduced exploitation on the lobster stock, especially the spawning stock. The relationship between the magnitude of egg production and that of puerulus settlement in the following settlement season was very poor (R 2 ¼ 0.02, d.f. ¼ 18, p ¼ 0.60), with settlement remaining below average in 2011/2012 and 2012/2013 despite the record-high egg production at the time (Figure 4d). Since the timing of spawning showed a significant relationship with puerulus settlement, this index was then modelled in combination with environmental factors previously identified as being related to puerulus settlement (offshore SST in February and rainfall levels; Caputi et al., 2001) as well as the egg production index. The addition of rainfall data (as a main effect and interaction) into the model significantly improved its ability to fit the settlement data (Figure 5a). The months of rainfall data that most improved the model were May and October, i.e. at the start and end of the rainy season (Figure 5a). Since most months between these 2 months also increased the overall fit of the model (although not significantly), the entire period from May to October was combined to produce an annual index to represent the onshore storms fronts that the late larval stages and the puerulus stage would experience. This combined index significantly (Dd.f. ¼ 2, F ¼ 10.83, p, 0.001) increased the overall model fit, to a greater extent than any single month (Figure 5a). This mid-autumn to mid-spring period (May October) was then used to develop a long-term time-series of rainfall (Figure 5b). This series showed a relatively large interannual variation, ranging from a maximum average monthly rainfall of 181 mm month 21 in 1890 to a minimum of 47 mm month 21 in The years containing the 20 highest average monthly rainfalls all occurred before 1965, whereas 9 of the 20 lowest rainfalls have occurred since 2000 (Figure 5b). The 5-year smoothed average applied to the index remained relatively steady between 1880 until 1960 with a monthly average of 100 mm month 21. From 1970 through until the end of the century, this smoothed index dropped by 10% to 90 mm month 21 before declining progressively from the early 2000s until Figure 5. (a) R 2 values from a model between the timing of spawning/mean coastal rainfall in each month (and a grouping of months May October) and the puerulus settlement in that same year. The grey area represents the months chosen to develop a long-term rainfall index of May October. *, **, and *** identify in which models the addition of rainfall data significantly ( p, 0.05, 0.01, and 0.001, respectively) increased the models relationship. (b) Long-term time-series of coastal May October rainfall between 1880 and 2013 with a 5-year rolling average shown.

7 Low puerulus settlement in the western rock lobster fishery of Western Australia i55 the current year of 2013 to its record low of 70 mm month 21 (Figure 5b). SST during the early larval phase (February) has historically ( ) been a good indicator of strength of puerulus settlement that occurs later in the year, explaining 58% of the variation during this 26-year period. This correlation declined markedly with the addition of recent puerulus settlement data, explaining only 1% of the total variation when the years 2008/ / 2013 were added. When both breeding time and SST in February (as a main effect and interaction) were combined into the same model, that model s ability to fit the observed variation in puerulus settlement increased, but not significantly (DR 2 ¼ 0.135; Dd.f. ¼ 2, p ¼ 0.195) from a model with breeding time alone. When SST in February was added to the more complex model containing both timing of spawning and rainfall in May October, there was a 7% increase in model fit from this addition, although it was just not significant (DR 2 ¼ 0.07; Dd.f. ¼ 1, p ¼ 0.09). Since the addition of SST was marginally significant in its improvement to the overall model and SST alone displayed such a strong correlation with puerulus settlement before 2008, even when the dataset was extended back to 1968 (Caputi et al., 2001), we considered it valid to retain this factor in the optimal model. Egg production levels (as a main effect and interaction) were also re-examined in combination with breeding time (DR 2 ¼ 0.03; Dd.f. ¼ 1, p ¼ 0.39), breeding time and rainfall in May October (DR 2 ¼ 0.12; Dd.f. ¼ 4, p ¼ 0.20) and breeding time, rainfall in May October, and SST in February (DR 2 ¼ 0.06; Dd.f. ¼ 3, p ¼ 0.33). Egg production levels did not significantly improve the model description in any of these combinations. The optimum model (breeding time, rainfall, and SST) explained 71% of the variation in puerulus settlement, including two marked increases that occurred in 1995/1996 and 2000/2001, the historical minimum in 2008/2009, and the dramatic increase in 2013/2014 (Table 1 and Figure 6a). The model contained a two-way interaction term between the timing of spawning and SST and a three-way interaction between all three main effects. These two interaction terms indicated that both rainfall and SST were positive influences under most typical spawning stage conditions, but had little impact when the spawning stage was early. The relationship between SST and settlement was greater in concert with high rainfall and dampened under conditions of low rainfall (Figure 6b). Variation in timing of spawning The timing of spawning (log-transformed) displayed a significant (R 2 ¼ 0.30, d.f. ¼ 17, p, 0.015) positive relationship with bottom water temperatures throughout the breeding stock areas (Figure 7). Table 1. Summary of the optimal model used to describe the variation in (log) puerulus settlement. Independent variable coefficients (s.e.): Breeding time (30.395) SST (0.442) Btime:SST (1.364) Btime:Rain:SST 0.006* (0.001) Constant (9.640) Observations 21 R Residual s.e (d.f. ¼ 16) F-statistic (d.f. ¼ 4; 16) *p, The coolest water temperature in the time-series was predicted in 2001 which coincided with the latest onset of spawning, whereas the most warm years displayed average to early timings for the onset of spawning. All 7 years ( ) associated with a low puerulus settlement period, including the year when spawning started the earliest in 2007, coincided with above-average bottom water temperatures (Figure 7). Discussion and Conclusions Although levels of puerulus settlement have historically varied markedly between years, in the mid to late 2000s, the western rock lobster fishery received an extended period of below average levels of settlement including the lowest settlement on record in 2008/ This study showed that the timing of spawning (which is related to bottom ocean temperatures), coastal rainfall levels during late larval stages to peak settlement, and offshore water temperatures during the early larval phase collectively explain the most variation in the puerulus settlement. This relationship emphasizes the climate sensitivity of this species during its spawning and larval phase. The early spawning combined with the reduced storm activity (particularly during early winter) during the larval phase may have created a mismatch with the peak in food availability and/or larval transport mechanism back to the coast, as per the match mismatch hypothesis (Cushing, 1990). Under normal conditions, the enhanced ocean production in austral autumn/winter, the enhanced onshore flow that feeds the strengthened Leeuwin Current in austral winter, as well as winter storm activity support the onshore movement of the larvae. The strengthening Leeuwin Current in February April and winter/spring storms that have historically been identified as affecting the level of settlement for over 35 years (Caputi et al., 2001) may both affect larval food availability and transport, so that a mismatch may be occurring due to the early spawning. Schmalenbach and Franke (2010) identified that an increasing decoupling of the larval peak from optimal external conditions that affected food availability would result in a serious problem for the European lobsters in the warming North Sea. This hypothesis needs to be explored further for the western rock lobster using the oceanographic larval model and environmental data. Timing of spawning Demersal water temperatures throughout the spawning grounds of P. cygnus during the start of spawning (about October) were identified as a factor that may be influencing the timing of the onset of spawning. Increases in water temperature in October since the mid-2000s may have resulted in an earlier onset of spawning that has coincided with the decline in puerulus settlement since 2006/ Studies on historical water temperature trends in WA have identified the lower west coast of Australia as a hotspot for water temperature increases over the last years (Pearce and Feng, 2007). Increases were found to be particularly high in the austral autumn winter period with little or no increases evident in the spring summer period (Caputi et al., 2009). However, the bottom water temperature in the lobster spawning grounds during October obtained from oceanographic models shows a possible increasing trend since the early 1990s. This increase may reflect that in the strength of the Leeuwin Current since the early 1990s which has been described as a result of decadal climate variation in the Pacific (Feng et al., 2010), further enhancing the general increase in water temperature that has been occurring off the lower west coast of Australia (Pearce and Feng, 2007; Caputi et al., 2009). These two drivers of ocean temperature may have important

8 i56 S. de Lestang et al. Figure 6. (a) Observed (black) and estimated +1 s.d (grey) standardized puerulus settlement between 1993/1994 and 2013/2014 seasons. (b) The relationship between the timing of spawning (year t) and the subsequent level of puerulus settlement (year t + 1/t + 2) under good environmental conditions (high rainfall and SST in year t + 1) and poor environmental conditions. Seasons shown represent the puerulus settlement seasons in high (grey) and low (black) rainfall years, with high SST years identified by grey circles. Figure 7. Relationship between the modelled bottom water temperatures throughout the breeding stock areas of the fishery in October and the index of timing of spawning in that same year. long-term implications for the rock lobster fishery since the increases in water temperature under the decadal climate variation may be expected to revert back to previous levels under periods of weaker Leeuwin Current, and the future projections are for a longterm weakening of the current (Feng et al., 2012). However under the greenhouse gas-induced global warming, long-term increases of water temperature would not be expected to be reversed soon. Winter storms Storms (measured by rainfall and associated with westerly winds) in late winter/spring have historically been shown to be positively related to puerulus settlement (Caputi and Brown, 1993). They assessed the rainfall for the 22 years, 1969/ /1991, although the current study has examined the most recent 21 years, 1993/ /2014, as onset of spawning is only available for this period. The significance of winter storms in these two separate periods covering over 40 years confirms the importance of this factor on the settlement. Westerly wind patterns during the 2010/ 2011 settlement season have been unusual, with the westerly component being far weaker than average. This is reflected by the second lowest winter/spring rainfall on record in 2010, with 2006 having

9 Low puerulus settlement in the western rock lobster fishery of Western Australia i57 the lowest rainfall on record. It is likely that these conditions were adverse for the 2006/2007 and 2010/2011 puerulus settlement. Previous studies have identified rainfall during July to November as being a significant factor affecting puerulus settlement (Caputi and Brown, 1993; Caputi et al., 2001). However, in this study, the rainfall between May and October when combined with the breeding time index provided a better fit to the variation in puerulus settlement since the early 1990s including the recent years of low settlement. Rainfall represents an index of storm activity affecting the lower west coast of WA, which influences the water vapour transport and is generally associated with westerly winds that may help transport the larvae back to the coast. The rainfall time-series for May October shows a declining trend since the early 2000s with many of the lowest values occurring since this period. This decline is part of a long-term trend of declining rainfall in the southwest of WA that has intensified in the last 10 years and is expected to continue (IOCI, 2012). The IOCI study has demonstrated that storm development has reduced in the mid-latitudes but increased in the higher latitudes (50 708S) with the net result of fewer storms affecting southwest WA. Egg production The early management intervention in 2008, before the low puerulus settlement year-classes reached legal-size, has resulted in a record level of breeding stock in recent years ( ) as measured by the fishery-independent spawning stock survey, which is also supported by the model assessment of egg production. The effect of this level of egg production on the poor 2012/2013 and very high 2013/2014 puerulus settlements provides some insights on the effect of breeding stock on settlement. This relationship indicates that spawning stock is not a dominant factor, providing little contribution when modelled against puerulus settlement singularly or in combination with other factors. Relationship with decadal and longer term climate trends The Leeuwin Current has historically been identified as a key factor affecting the level of puerulus settlement, therefore understanding its long-term trend is important. The Leeuwin Current variability is essentially driven by the variations and changes in Pacific equatorial easterly winds: the Leeuwin Current has experienced a strengthening trend during the past two decades, which has almost reversed the weakening trend during 1960s to early 1990s (Feng et al., 2010, 2011a). Currently, most climate models project aweakening trend of the Pacific trade winds and a reduction in the Leeuwin Current strength (as well as the Indonesian Throughflow) in response to greenhouse gas forcing over the next century. Whereas the greenhouse gas forcing induced changes may be obvious in the long-term climate projection (such as 2100), natural decadal climate variations still need to be taken into account for assessment of short-term climate projection, e.g. 2030s (Feng et al., 2012). The strengthening of the Leeuwin Current in recent decades may have increased the southward warm water transport and bottom water temperature on the continental shelf off the west coast of Australia. In addition, the expansion of the tropics due to global warming may have pushed mid-latitude storm activity southward, reducing wave-induced onshore movement off the west coast in austral winter. Taking into account the status of the breeding stock and variations in the Leeuwin Current and the fact that the settlement has remained low for an extended period, a long-term change in the environmental conditions may be the main cause of the recent levels of poor puerulus settlement. The increasing bottom water temperature that may be influencing the start of spawning and the decline in winter storms are probably both influenced by long-term climate change trends that may continue to affect the settlement into the future. It will be interesting to see whether the very good settlement that has occurred in 2013/2014 heralds the return to an improved settlement period or whether it represents a short-term spike. These will need to be closely monitored. From a downscaling model simulation, SST may increase by 18C in the next 50 years; in the meantime the strength of the Leeuwin Current may reduce by 15% (Chamberlain et al. 2012; Sun et al., 2012). An examination of the eddy energetics in the downscaling model suggests that the eddies will be less energetic in the future climate, due to the reduction in the Leeuwin Current strength, which may also affect the ocean primary production in the region (R. Matear, pers. comm.; Matear et al., 2013). Climate model projections suggest that precipitation in southwestern Australia will continue to fall in the future due to further expansion of the tropics and poleward shift of the storm activity (IPCC, 2013). While the reduction in the Leeuwin Current may cancel some of the effects of the increasing ocean temperature, the reduction in the Leeuwin Current eddy energetics and storm activity off the coast may still cause the mismatch between the spawning/settlement and the physical environment. Continued monitoring of the physical environment, including shelf bottom temperature and settlement numbers, is crucial to understanding the sustainability of the fishery. Conclusions The onset of spawning combined with rainfall during May October and offshore SSTexplains a significant proportion (0.71) of the variation in puerulus settlement since the early 1990s and provides a plausible hypothesis to explain the decline in puerulus settlement in recent years. This relationship and that of the extended model that includes egg production and SST and their interactions need to be verified with additional years of data to see whether the relationships are maintained. The assessment of the possible contribution of a long-term environmental factor(s) to the low puerulus settlement enables the stock assessment model to factor the low settlement into its assessment, so that management and industry can take this into account in future planning. The fishery has been proactive in its adaptation to the low puerulus settlement by implementing early management actions to reduce fishing effort and catch before the low puerulus year classes recruited to the fishery. 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