Journal of the Royal Society of Western Australia, 92: 111-127, 2009 The ongoing influence of the Leeuwin Current on economically important fish and invertebrates off temperate Western Australia - has it changed? R C Lenanton, N Caputi, M Kangas & M Craine Department of Fisheries (Western Australia), Western Australian Fisheries & Marine Research Laboratories, PO Box 20, North Beach, WA 6920 Corresponding author: El [email protected] Manuscript received August 2008; accepted April 2009 Abstract Earlier reviews have identified that the strength of the Leeuwin Current is a key factor associated with changing abundance of a number of key invertebrate and scalefish species harvested by on-shelf commercial fisheries off the Western Australian coast. This review of these relationships has revealed that the addition of more recent data has strengthened the relationship for western rock lobster, tire only species whose larvae are primarily distributed in the area of the influence of the Leeuwin Current and its offshore eddies. For other species, particularly scallops and whitebait, while the addition of new data has weakened the LC relationships, other as yet unquantifiable factors now appear to be more important determinants of changing abundance. For a number of the scalefish species (e.g., south coast Australian salmon), discontinuation of juvenile monitoring and the lack of ongoing records of comparable abundance data, primarily as a consequence of changes in the distribution of fishing, relative to the distribution of the stocks, has precluded ongoing exploration of earlier relationships. However preliminary data for some hitherto unreported relationships for other coastal scalefish species suggest that some physical and biological variables that are likely to be influenced by both the Leeuwin and Capes Currents may also be important. To help unravel these relationships, the underlying mechanism of the influence of the current, particularly the role of the northward flowing, mid-shelf Capes Current and salinity of shelf waters, and factors controlling the availability of nutrients to on-shelf primary productivity need to be better understood. Keywords: Leeuwin Current, invertebrates abundance, scalefish abundance, time-series robustness Introduction commercial fisheries off the coast of South-Western Australia (SWA) (Figure 1). For most of these species The Leeuwin Current (LC) is a poleward flowing there are long-term data bases that either measure the eastern boundary current that carries warm, low salinity abundance of settlement life history stages, or the water southward along the shelf break off the Western abundance of adults. T hey include western rock lobster Australian (WA) coast (Cresswell & Golding 1980; Waite (WRL, Panulirus cygnus), Shark Bay scallops (Amusium et al. 2007). balloti), Shark Bay king prawns (Penaeus latisulcatus), Mesoscale interannual and seasonal variability in the Australian salmon (Arripis trultaceus), Australian herring LC strength (Ridgeway & Condie, 2004), measured as (Arripis georgianus), Albany pilchards (Sardinops sagax coastal sea level height, has been shown to be related to neopilchardus), and whitebait (Hyperlophus vittatus). All of the El Nino/Southern Oscillation (ENSO) cycle, with LC these relationships with the LC strength were "invoked" strength highly correlated with the Southern Oscillation from statistical correlations; they can be either positive Index (SOI); the current being weaker during ENSO (e.g. western rock lobster), or negative (e.g. Shark Bay events and stronger during La Nina events (Pearce & scallops). Few of the underlying mechanisms have been Phillips 1988, Caputi et al. 1996, Feng et al. 2003). While empirically verified, although the timing of the period the LC strength is also correlated with temperature and when the current affected the stock indicated that in most salinity of outer shelf waters, air-sea heat flux processes instances, the larval phase appeared to be the main life rather than advection associated with the LC appear to history stage influenced by the current. For example, the be the key factors influencing the temperature and contention by Fletcher et al. (1994) that "loss" of larvae salinity of shallower coastal waters inshore of the current due to the LC reduces recruitment of pilchards two years (Pearce et al. 2006). later was supported by Gaughan et al. (2001) who found that the larvae of south coast pilchards could be Earlier reviews (Lenanton et al. 1991; Caputi et al. 1996; transported up to 1000 km east, and in some cases well Pearce et al. 1998) have revealed that LC strength was into South Australian waters, primarily under the correlated with the abundance and/or catchability of a influence of the LC. Increases in availability and number of target species taken in a range of on-shelf resolution of oceanographic data (e.g. satellite derived steric height data to derive surface current patterns) have permitted advances in modeling of potential transport of © Royal Society of Western Australia 2009 the larval stages of WRL. However, there have been no 111 Journal of the Royal Society of Western Australia, 92(2), June 2009 26 °S - 27”S 28°S - 29 °S 30 °S 31°S - 32°S 33°S 34°S - 35°S 113°E 114°E 115°E 116°E 117°E 118°E 119“E Figure 1. Key locations along the lower west and western south coasts of Western Australia. other studies directed at determining the influence of the Gascoyne coast, during the austral summer. Meso- oceanographic variability on transport or survival of the scale features such as eddies and meanders are also larval stages of marine species. associated with the LCS. The way in which this system influences the water column behaviour and biological Ongoing research during the 30 years since the formal productivity off the WA coast has recently been reviewed recognition of the existence of the LC (Golding & Symonds in greater detail (Waite el al. 2007). In addition to the LCS, 1978; Cresswell & Golding 1980) has revealed that this global atmospheric changes are also contributing to the surface flowing current is one of four major currents that observed long-term temperature and salinity changes to comprise the Leeuwin Current System (LCS) (Batteen el al. continental shelf waters off Western Australia (Pearce & 2007). Beneath the southerly surface-flowing LC, the Feng 2007). However, as noted above, while such changes Leeuwin Undercurrent (LUC) flows northward in are acknowledged as factors that may further influence subsurface waters, while the wind generated Capes and the complex interrelationships between the LCS and Ningaloo Currents flow respectively northward on the fisheries production, indices of such changes are broad, shelf shoreward of the LC off the lower west coast, and 112 Lenanton et al: The ongoing influence of the Leeuwin Current making it inherently difficult to empirically demonstrate • whitebait (Hyperlophus vittatus) conclusive causal relationships; something few studies • tailor (Pomatomus saltatrix) * undertaken thus far have been able to achieve. Given this dearth of studies focused on determining causal • West Australian dhufish (Glaucosoma hebraicum) * relationships, the aim of this paper is to review the * species added since the last review. ongoing robustness of the hitherto established relationships between interannual and seasonal variability Oceanography and meteorology in LC strength and species abundance in the regions of Below are the sources of monthly values of the relevance to fisheries (i.e. fisheries production or physical variables that were used when exploring recruitment), and highlight any further examples where relationships with abundances indices of different the abundance of economically important species is being influenced by the LCS. species: • Southern Oscillation Index (SOI): http:/ This review updates the previous assessments from www.bom.gov.au/climate/current/soihtm.shtml); earlier reviews, either building on hypotheses identified earlier or invoking plausible conceptualizations of causal • Fremantle Sea Level (FSL): National Tidal Centre links between the LC and species abundance. In the in Adelaide; absence of information that can be used directly to • Sea surface temperatures (SST) on a 1° latitude/ ascribe mechanisms of influence, such hypotheses are longitude grid: Reynolds Global dataset (Reynolds framed around physical and biological effects linked to & Smith 1994); and variability in the LCS and which may affect different life history stages of the various species. Physical effects • Rainfall: the Bureau of Meteorology (http:// could be changes in temperature to levels sub-optimal for www.bom.gov.au/silo/products/Rain.shtml). growth, or transport of eggs or larvae (Muhling & Monthly at-sea measurements of surface and bottom Beckley 2007; Muhling et al. 2008a). Biological effects salinity and temperature have been made at four could include changes to nutrient regimes and broader locations, Fremantle, Lancelin, Jurien and Dongara, patterns of primary productivity leading to variability in across four depths, <10, 10-20, 20-30, and >30 fm during growth and survival. the WRL fishing season (15 November to 30 June) each financial year since 1971/72 (Caputi et al. 2009; Lenanton et al. 2009). Data sources Recruitment Indices The relationships between 10 species/fisheries and the The tailor recruitment index was expressed as a catch LCS are reviewed here, building on the seven covered in rate, calculated as the average number of 0+ and 1+ yr earlier reviews (Lenanton et al. 1991; Caputi et al. 1996). old tailor caught per standard angling hour over the Details of the biological data used here, such as method period February to April at a representative Perth of collection and any validations undertaken, are not Metropolitan site (Ayvazian et al. in prep.). dealt with here unless explicitly relevant. Rather, the pertinent data are mentioned as required to build the The dhufish recruitment index was derived from the conceptual understandings and hypotheses relevant to current age structure, back-calculated to estimate the progressing our knowledge of how the LCS affects abundance of the each age cohort during the year of its fisheries production off the WA coast. birth (St John unpublished). For each species or fishery the primary historical data The Australian salmon recruitment index is the consists of commercial catch and effort records, typically standardized catch rate of 0+ individuals settling in available since 1975. These compulsory catch and effort nursery areas off the lower west coast between data are aggregated monthly by fishing method for each September and December each year, and is derived from one degree by one-degree block (or to other spatial data collected during an ongoing coastal scalefish fishing zones for some fisheries), submitted to the recruitment index survey (Gaughan et al. 2006). Department of Fisheries, and stored in the Catch and Effort System (CAES) database. Results and Discussion The fisheries/species covered in this review are shown below. Further details of these fisheries can be found in Western Rock Lobster the Department of Fisheries, Western Australia annual State of the Fisheries reports available from While adults are confined to the shelf, larval stages www.fish.wa.gov.au. are distributed throughout oceanic waters adjacent to the continental shelf off the coast of SWA (Phillips 1981). The • western rock lobster (WRL, Panulirus cygnus) final larval stage (puerulus) actively swims from oceanic • Shark Bay scallops (Amusium balloti) waters across the shelf to settle in shallow coastal • Shark Bay king prawns (Penaeus latisulcatus) habitats. There are long-term databases recording indices of the annual abundance of settling puerulus, • Green or giant mud crabs (Scylla serrata) * environmental variables such as LC strength (April • Australian salmon (Arripis truttaceus) Fremantle sea level), satellite derived sea surface temperature (SST February-April), and westerly wind • Australian herring (Arripis georgianus) during peak settlement (October to November)(rainfall • Albany pilchards (Sardinops sagax neopilchardus) index of storm fronts). Correlations using data since the 113 Journal of the Royal Society of Western Australia, 92(2), June 2009 It seems that in years of strong LC, the survival and growth of WRL early stage larvae is enhanced, either via retention of larvae in anticyclonic warm core eddies, and/ or as a result of improved growth and survival as a consequence of warmer temperatures and/or higher productivity (chlorophyll A) during late autumn/winter. Potential sources of nutrients include upwelling associated with eddy formation, advection from the shallower nutricline further north, and seasonal cooling and storms which promotes convective mixing of the water column and shoaling of the nutricline (Koslow et nl. 2008). Understanding the causal mechanism of the effect of the LC is an area of ongoing research. Research has also demonstrated that strong LC in the second half of the year (June-December) which is near the period of peak settlement (August-December) has an advection effect as it distributes puerulus further south Year (Figure 4) (Caputi 2008). Thus annual puerulus settlement (abundance), LC Figure 2. Annual mean values of the Southern Oscillation Index, strength (Fremantle sea level), SST, productivity Fremantle mean sea level, and puerulus settlement at Seven- (chlorophyll A), and eddy kinetic energy are all Mile Beach Dongara Western Australia (Updated from Pearce positively correlated (Caputi et al. 2001; Feng et al. 2005), and Phillips 1986). with the more southerly distribution of higher puerulus settlement also positively correlated with LC strength (Caputi 2008). However the frequency of ENSO events late 1960s have revealed a strong positive relationship (weak LC) over the last 16 years (1991-2006) has been between puerulus settlement, mean Fremantle sea level greater than in the previous 20 years (1971-1990). If this and the SOI (Figure 2) (Pearce & Phillips 1988). Further, trend continues, is there likely to be a continuing trend in there is also a strong positive relationship between mean average to low puerulus settlement; or will the influence monthly SST (February to April) and puerulus of global climate change (increasing SST of the WA coast, settlement, with strongest settlement often occurring particularly the lower west coast) to some extent counter during years exhibiting high SST levels combined with the negative influence of weak LC? the strongest rainfall index (i.e. periods of strongest There has also been considerable effort expended westerly winds) (Figure 3). exploring the effect of long-term temperature increases Puerulus Jurien - Temperature (Feb - Apr) (Rain South Oct - Nov) 300 250 - (cid:9830) 96(68) (cid:9830) 84(106) 00(27) 200 - 95(110) </> 3 85(4^ t9(^9(89) (cid:9830) 05(68) C3L (cid:9830)04(5(cid:9830)7)o i (47) *88(63) 100 (cid:9632) 90(54^^29) (cid:9830) 83(55) 50 (cid:9632) (cid:9830) *86 (41(cid:9830)) 87(cid:9830)(5941) (8(cid:9830)6,) (cid:9830)9 7(47_,) .(cid:9830) p9a8r(5(cid:8482)8) *(cid:9830)93(38) 92(66> (cid:9830)qj, *02(49) 82<3i» 06(3?T93<3S) (cid:9830)07 08 20.50 21.00 21.50 22.00 22.50 23.00 Temperature (Feb - Apr) h ® 3'. Re*atlonsh|P between the annual puerulus settlement (mean number per collector) at Jurien, and sea surface temperature at wo levels of rainfall (red data points >, and blue data points < than 80 mm, an index of the strength of westerly winds) in October- 2onfibes' °ff WeSt. C°?St °f VYestern Australia (updated from Caputi et al. 1996). The predicted level of puerulus settlement for 2008 is presented. The multiple correlation is 0.82 (pcO.001) for 1982-2006 data. H 114 Lenanton et al.: The ongoing influence of the Leeuwin Current Correlation between Fremantle Sea Level and Mean Latitude of Puerulus Settlement Dongara 29 9495 03*04 86*87 02/03 29.5 97S8 84/85 91/9i 9091 05A36 M 9394»«75B8 85*86 * 92/J3 83*90* i .95/96 30 0001 Jurien _ci 98/99 96/97 <5 (51/02 g 30.5 • 99/00 31 Lancelin 60 65 70 75 80 85 90 Fremantle Sea Level (cm), June - December Figure 4. Relationship between the Leeuwin Current strength (measured by Fremantle sea level) over June to December, and the annual mean latitude of puemlus settlement at sites along the lower west coast of Western Australia (r=0.86, p<0.001 Caputi 2008). (Pearce & Feng 2007) on growth, size at migration, size at from the year before and possibly 2 years before, noting maturity, catchability, and timing of moulting (thus peak the life span is 2-3 years). For the Abrolhos Islands, a catch rates) of WRL (Caputi et al. in prep.). regular pre-season survey has also been undertaken in October since 1997, and as for Shark Bay, a catch Scallops prediction for the following season can be made using the survey abundance index derived from the pre-season Scallop recruitment is highly variable (Joll 1994; Joll & survey. Caputi 1995). In WA, this is seen by widely varied annual commercial scallop catches in the main scallop fisheries, The pattern of settlement in both Shark Bay and the which occur at Shark Bay, the Abrolhos Island (Mid-west Abrolhos Islands is variable between years and Trawl Fishery) and off the south coast (South Coast settlement patterns are patchier in the Abrolhos Islands Trawl fishery). The Shark Bay scallop fishery has compared to Shark Bay. The patchiness may reflect the historically been the most productive fishery in WA, with habitat and/or the water current circulation pattern and annual catches ranging between 600 to 22,000 tonnes eddies that occur at the Abrolhos Islands. However key (whole weight). However, in 2003 and 2005 the Mid-West areas where settlement may occur in any one year are Trawl fishery recorded higher annual landings for now reasonably well known for both fisheries. No fishery scallops and in 2000 the South Coast Trawl fishery independent surveys are conducted on the south coast. recorded the highest scallop catches in the state for that Normally in this region, one boat is elected to 'survey' year (Figure 5). potential settlement sites (derived using fishers knowledge) and if an encouraging "showing" of scallops Scallops are broadcast spawners (Kailola et al. 1993) is located, the other boats (only four licenses issued in with a larval duration of about two to three weeks (Rose the South Coast Trawl fishery) may elect to fish during et al. 1988). During this period larvae are susceptible to the season. being passively transported by tides and currents whilst in the water column. Larval survival is affected by food Leeuwin Current correlation with recruitment of availability and predator abundance, and the length of Amusium balloti the larval period (assuming survival is enhanced by reducing time in the plankton community) can also be Shark Bay influenced by water temperature. In the 1980s in Shark Bay, a good correlation was Annual scallop surveys, conducted between October observed between scallop recruitment strength and the and December, have been undertaken in Shark Bay since strength of the Leeuwin Current (Joll & Caputi 1995). 1983, and provide size class and abundance information The proxy used for the LC is the height of the Fremantle from over 90 trawl sites within the bay. These data are Sea Level (FSL) during May to August, lagged by 1 used to determine an index of recruitment strength month from the peak scallop spawning period of April to during that year (individuals derived from the current July to account for the latitudinal difference between years spawning) and provide the basis for predicting the Shark Bay and Fremantle. It was apparent that in years catch the following year. They also provide an index of of weaker LC flow, higher recruitment success was the size of the residual stock (older scallops remaining observed and vice versa. A more recent analysis of LC 115 Journal of the Royal Society of Western Australia, 92(2), June 2009 a) 35000 30000 e 25000 3 o £ 20000 (cid:9632)e I 15000 IQ c 10000 E o z 5000 0 c) Figure 5. Annual scallop landings (tonnes whole weight - bar histograms) and nominal hours of trawling (line graph) in a) Shark Bav b) Abrolhos Islands and c) South Coast trawl fisheries ' 116 Lenanton et air. The ongoing influence of the Leeuwin Current strength and scallop recruitment in Shark Bay using data There is some indication that a weak LC flow and/or collected between 1983 and 2006 indicates a much cooler water temperatures benefit recruitment but not weaker negative correlation (Figure 6) than the one sufficient in themselves to always result in good reported for the 1980s. Similarly, a relatively weak recruitment. Other factors (e.g. water current movements negative correlation was observed between scallop during larval phase) must also be strongly influencing recruitment and surface water temperature (Figure 7). recruitment. Figure 6. Correlation (log (Recruitment) = 9.6- 0.06FSL, R2 = 0.14) between recruitment index in the Shark Bay scallop fishery (northern ground) and Leeuwin Current strength (FSL) between May and August (1983-2006). Dashed line indicates low average FSL conditions where some recruitment was observed to be high. Note 2006 is highlighted as a year with higher recruitment even though FSL was average. Water temp (May - August) (oC) Figure 7. Correlation (log (Recruitment) = 15.8-0.46 SST, R2 = 0.08) between recruitment index in the Shark Bay scallop fishery and surface water temperature (°C) between May and August (1983-2006). The dashed line indicates years when higher recruitment has been observed when Reynolds water temperature in waters of Shark Bay have been less than 23.5°C on average between May and August (when larvae in water column). Also 2006 is highlighted, as high recruitment when LC flow was stronger than for 1987 or 1990. 117 Journal of the Royal Society of Western Australia, 92(2), June 2009 Sea level (September - November) (cm) Figure 8. Correlation (log (Index) = 2.02-0.0013FSL, R2 = 0.0022) between recruitment index in the Abrolhos Islands scallop fishery and Leeuwin Current strength (FSL) between September and November (1997-2006). The 2002 and 2004 are highlighted as years of high survey abundances during relatively low FSL resulting in high catches in 2003 and 2005. i i | | 19.5 20.0 20.5 21.0 21.5 Water temp (° C September - November) a q 9( C "'e atlon (log(Index) - 1.89 + 0.0010SST, R- = 1.156E-5) between recruitment index in the Abrolhos Islands scallop fishery and Surface Seawater Temperature (SST) between September and November (1997-2006). The 2002 and 2004 are highlighted as years of high survey abundances during relatively low FSL resulting in high catches in 2003 and 2005 1 118 Lenanton et al.: The ongoing influence of the Leeuwin Current Abrolhos Islands strong recruitment as years of high settlement have The high recruitment observed in Shark Bay in 1990 occurred under conditions of low LC flow and lower and 1992 (mainly in Denham Sound) resulted in four water temperatures (Figures 8 and 9). Another good years of good catches between 1991 and 1994, in recruitment occurred in 2007, and was associated with particular exceptional catches in 1992 (Joll 1994) (Figure below average FSL and water temperature. These 5a). In the Abrolhos Islands, the peak in catches assessments indicate that there are other significant occurred in 1993 and 1994 (Figure 5b) which would environmental variables that contribute to the success of have resulted from good recruitment in 1992 and 1993, recruitment besides the strength of the LC and water indicating a lag of a year or two between these and the temperature. high catches in Shark Bay. Standardised surveys were South Coast not undertaken in the Abrolhos Islands in the early 1990s so there is no comparative fishery-independent The high catch observed in the South Coast Trawl information for this region to indicate when a peak in fishery in 2000 (Figure 5c) was in a year of strong LC recruitment may have occurred, although based on flow (as was 1999 when recruitment is likely to have current information on lag between recruitment and occurred), which may have created more optimal catch, a good recruitment in 1992 and possibly 1993 is conditions (warmer than average South Coast water likely. Tire coincidence of high catches (with a lag) in temperatures) for scallop survival on the south coast. As these two fisheries in the early 1990s were associated the scallop larval duration is 2-3 weeks, it is unlikely with a period of extended ENSO events and subsequent that larvae could be transported from Shark Bay or the weak LC strength (see Figure 2) and this coincidence Abrolhos Islands to the south coast over that time period. has not been observed since. In the Abrolhos Islands, In fact, the highest catches recorded by fishers during high survey indices were recorded in 2002 and 2004 2000 were from Israelite Bay, the most eastern area of (Figure 8) (another period of ENSO events and weak suitable south coast scallop habitat. It is thus extremely LC) and resulted in very high catches in 2003 and 2005 unlikely that larvae originating from west coast respectively (Figure 5b). There was, however, no spawning populations could have been transported that carryover abundance in either 2004 or 2006 similar to far east. The high catches are more likely to be a those observed in Shark Bay in the early 1990s, even consequence of improved environmental conditions on though records show that scallops were left on the the south coast enhancing survival of larvae produced by grounds at the end of each fishing season. This may be the local populations in two successive years. due to differences in the timing of spawning resulting in the scallops at the Abrolhos Islands generally being Shark Bay Western King prawns six to twelve months older at the time of harvesting Earlier work during the 1980s and 1990s relating compared to those in Shark Bay. western king (Penaeus latisulcatus) catches and LC As was the case for Shark Bay, data from the Abrolhos strength showed a very good correlation (R2 = 0.6, Islands fishery revealed a relatively poor correlation Lenanton et al. 1991: R2 = 0.7; Caputi et al. 1996) with between the index of recruitment and both the LC higher catches related to stronger LC flows during the strength and water temperature (Figure 9). Flowever March to June period (Figure 10). This was considered to again these conditions appear to be pre-requisites for be due to strong LC causing an extension of the peak in Figure 10. Average Fremantle sea level height between March to June each year and annual western king (Penaeus latisulcatus) catches in the Shark Bay prawn fishery between 1982 and 2006. The years highlighted in open circles (from 89 onwards) are included in the dashed line relationship. 119 Journal of the Royal Society of Western Australia, 92(2), June 2009 the annual temperature cycle to match the main March to as Wilson Inlet just west of Albany (Figure 1) on the June fishing season at a time when king prawns are south coast (Bellchambers 2002; Gopurenko et al. 2003). recruiting onto fishing trawl grounds from the nursery These recruits survived and grew into mature and/or shallow inshore areas. Higher catches were individuals and during subsequent years, were caught thought to be related to improved catchability, growth by both commercial and recreational fishers. However and survival (Caputi et al. 1996). The updated they appeared not to have successfully bred in the lower relationship (Figure 10) reveals that this positive west and south coast locations. Thus although they correlation has persisted, but is now much weaker persisted in these more southerly locations for a few (overall R2 =0.37). Interestingly, while the relationship is years, they did not establish a permanent population. still positive for the years since 1989 (R2 = 0.42), there has The series of weaker LC since 2000 as a result of ENSO been an overall reduction in total prawn landings (Figure or neutral conditions may not have provided additional 10). The average landings have reduced from between recruitment from further north nor the right water 1400 and 1600 tonnes annually between 1982 and 1988, temperatures for successful breeding. However, mud to between 1000 and 1200 between 1989 and 2005. The crab populations have become established further south reasons for the lower landings may be related to different of their historical distribution (Shark Bay), perhaps made targeting and harvesting strategics since the early 1990s, possible by slight elevations in ambient water where fishers have tried to save fuel and reduce costs by temperature as a consequence of warming water focusing less effort on larger more valuable prawns as temperatures due to "climate change" (Pearce & Feng the price of small prawns decreased due to the 2007). One conclusive example is mud crabs having development of the prawn aquaculture industry. extended its southerly range to include the Murchison, and the Greenough Rivers (Figure 1). Mud crabs In Western Australia, there are numerous examples of Australian salmon the capture of tropical finfish and invertebrates well The western species of Australian salmon is south of their historical distribution range (Hutchins distributed from southwestern WA eastwards to the gulf 1991; Lenanton et al. 1991; Hutchins & Pearce 1994). The systems in South Australia (SA). The major part of the LC is the mechanism usually invoked as the vector breeding stock does not reside along the southwestern transporting pelagic larvae well south of the normal coast but undertakes an annual migration from the east range of distribution. Exceptionally strong LC during in autumn to spawn in this region, with adults then 1999 and 2000 distributed mud crab larvae as far south returning to the east. Progeny return to nursery areas in F gure 11. The relationship (Juv CR= - 0.2123 + 2.6414 x 10'2 catch (t)) between the annual index of the West Coast Australian Salmon December n°fC0 ApH M:W “Rh), and the annual index of juvenile (0+ yr class) recruitment (measured between September and “ of thf same year). Mean Fremantle sea level (February to May) is presented in parenthesis after each annualZanoint ote that recruitment data were not collected between 2002 and 2004. Recruitment forecast for 2008 is also presented (F). P 120