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High Mortality of Western Australian Rock Lobster Larvae During El Niño


David Griffin
CSIRO Marine Research
Hobart, Tasmania, Australia

Western rock lobster
Western Rock Lobster
(Photo: CSIRO Marine Research)


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Puerulus Larva
Puerulus larva
(Photo: WA Fisheries)
The western rock lobster, Panulirus cygnus, supports Australia's most valuable single-species fishery. The life cycle includes a long (~9 month) oceanic larval phase during which mortality is especially high during El Niño events. Hatching of eggs occurs in summer (mostly December-January) on the outer continental shelf. The larvae disperse to the open ocean then return to the coast from about July onwards as the final 'puerulus' larval stage. The number of pueruli settling has been monitored for decades and greater than five-fold variations of the annual settlement occurs, filtering through, three to four years later, to variations of +/- 2000t in the commercial catch (Caputi et al. 1995).

figure
Pearce and Phillips (1988) first noticed that coastal sea level (itself an indicator of El Niño-Southern Oscillation) was correlated with settlement rates of puerulus (see figure) but they could only speculate on the mechanism behind the correlation. Sealevel is high throughout the western equatorial Pacific during La Niña events and this signal propagates down the west coast of Australia, heralding a stronger-than-usual, southward flowing Leeuwin Current and high rates of puerulus settlement. Conversely, the Leeuwin Current is weak during El Niño events and puerulus settlement is low.
Griffin et al. (2001) tested the hypothesis that the advective and dispersive effect of the current velocity was the mechanism responsible for the correlation with sea level and El Niño. They computed year-long trajectories of many model larvae using ocean current velocity estimates for six actual years derived from Topex/Poseidon and ERS satellite altimeter data, and all that is known about the diurnal vertical migration behaviour of the larvae. The model succeeded in demonstrating that large numbers of larvae could return to the coast despite the strong southward current, due to the opposing effect of the wind. The model was also in general agreement with research observations of the distribution of larvae in the open ocean. However, it failed to produce large inter-annual fluctuations in the number of larvae being close enough to the coast at the appropriate age to swim shorewards as pueruli and settle, even though the effects of the waxing and waning current could be seen [animations available]. While it remains possible that the details of the advection process were not adequately modelled, this result suggests that it is one or more of the environmental parameters not included in the model, such as water temperature or prey abundance, that controls the variability of larval mortality through impacts on growth rate and survival.

There is not a strong need for short-term predictions of El Niño events for management of this species because the impact of El Niño is on the larval stage rather than on the adults (so the effects are delayed for 3 years). The value of knowing that settlement correlates with sea-level is that it helps decide whether adjustments need to be made to the rate of exploitation. For example, if a very low number of puerulus settle (as happened in 1998), one might have thought (at the end of 1998) that over-fishing had occurred in 1997. The managers of the fishery, however, knew this was not the case because (1) they expected low settlement following the El Niño of 1997-1998, and (2) independent breeding-stock surveys in 1997 showed the stock levels were on target (Caputi et al. 2002). What was significant about 1998 was that sea-level had quickly returned to normal by April and sea surface temperature was also normal, so these indicators did not have their usual skill (Caputi et al. 2001). Clearly, there is still plenty to learn about the relationship of El Niño with western rock lobster.~~~

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References

  • Caputi, N., Brown, R.S. and Chubb, C.F. (1995). Regional prediction of the western rock lobster, Panulirus cygnus, commercial catch in Western Australia. Crustaceana, 68, 245-256.
  • Caputi, N., C. Chubb and A. Pearce (2001). Environmental effects on recruitment of the western rock lobster, Panulirus cygnus . Marine and Freshwater Research, 52, 1167-74.
  • Caputi, N., C. Chubb, R. Melville-Smith, A. F. Pearce and D. A. Griffin (2002). Relationships between life history stages of the western rock lobster, Panulirus cygnus, in Western Australia. Fisheries Research (revision submitted Jul 2002).
  • Griffin, D.A., J. L. Wilkin, C. F. Chubb, A. F. Pearce and N. Caputi (2001). Ocean currents and the larval phase of Australian western rock lobster, Panulirus cygnus. Marine and Freshwater Research , 52, 1187-99.
  • Pearce, A.F., and Phillips, B.F.(1988). ENSO events, the Leeuwin current, and larval recruitment of the western rock lobster. Journal du Conseil International pour l’Exploration de la Mer, 45, 13-21.