Examples of ENSO-Society Interactions
High Mortality of Western Australian Rock Lobster Larvae During
El Niño
~~~
|
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).
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.~~~
Links
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.
|
