Western Rock Lobster

The Leeuwin Current has been shown to have a major influence on western rock lobster catches. This impact is clearly seen on the number of rock lobster larvae, or puerulus, settling on the inshore coastal regions after their nine to 11 month planktonic journey in the open ocean. A Western Rock Lobster

The strength of the Leeuwin Current is affected by El Nino periods. However the strength of westerly winds during winter and spring, just before settlement, have also been shown to affect the abundance of puerulus settling along the coastal reefs and at the Abrolhos Islands.

It takes from three to four years for western rock lobsters to reach legal catch size from the puerulus stage, and there is a direct relationship between the level of puerulus settlement and future catches. This fishery is Australia's most valuable single species fishery with an average catch of over 10,500 tonnes and a value, in 1994-95 of about $300 million. The management of this fishery is assisted by predictions of catches up to 4 years ahead based on the relative abundance of puerulus settlement.

The Leeuwin Current starts to strengthen in March-April each year and appears to have a significant effect on the rock lobster larvae during this period. At this time the larvae have generally moved off the continental shelf into the open ocean after hatching during the late spring and summer (November-February) and before settling as puerulus the following September through to January.

The major impact of the south-flowing Leeuwin Current on the puerulus settlement at this time is felt in the southern most zone of the fishery (around Cape Mentelle) which receives very little settlement unless the Leeuwin Current strength is above average. The proximity of the Leeuwin Current to the coast off Cape Mentelle during the settlement period may also be important, with better settlement occurring when the current is close inshore than when it is offshore.

The latest El Nino event, with its weaker than normal Leeuwin Current, has caused particular concerns for fishermen in this region as no settlement has occurred over the past four years.

Exactly how the Leeuwin Current affects puerulus settlement is not yet clear. However the available information suggests two hypotheses.

Firstly, laboratory tests indicate that the warmer waters of the current could help the growth and survival of the larvae. Secondly, the current may help through larval retention by eddies and may assist in the transport of the late larval states and puerulus across the continental shelf into coastal reef nursery areas, especially in the southern areas like Cape Mentelle.

Scallops

Scallop catches throughout the world are acknowledged as being highly variable and the scallop fishery in Shark Bay is no exception. Scallop catches there have fluctuated enormously over the past 10 years and the Leeuwin Current appears to be strongly implicated.

The impact of the current apparently occurs during the spawning period, between April and July, with a weak current usually resulting in above average recruitment. A Western King Prawn Eva Boogard/Lochman Transparencies

This recruitment is measured by the abundance of scallops which are about six months old caught during a research trawl survey in November each year. It is this group of scallops which makes up the bulk of the fishery in the following season, and in years of weak Leeuwin Current the fishery usually has a high catch in the next year.

The years of good recruitment in 1982, 1987 and 1990 have all corresponded to El Nino periods which have been shown to be associated with years of weaker Leeuwin Currents.

However the very good recruitment in 1990 was responsible for record catches in both 1991 and 1992, with the 1992 catch of 20,000 tonnes live weight being five times greater than the next highest catch on record. This was because the recruitment in 1990 was so large that the fishery was only able to catch part of the available scallops and many of them had to be left until the following year.

The negative impact of the Leeuwin Current during the spawning period may be due to hydrological flushing of the larvae or due to the effect of the warmer waters on the spawning or fertilisation.

Western king prawn

The influence of the Leeuwin Current on the western king prawn fishery in Shark Bay occurs at the time of migration of the prawns to the fishing grounds from the nursery area. The impact during this part of the life phase is in contrast to its effect on other fisheries where the major influence of the current is during the larval life.

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