New Zealand Journal of Marine and Freshwater Research abstracts
Population structure and relative abundance of blue cod (Parapercis
colias) off Banks Peninsula and in Dusky Sound, New Zealand
Michael P. Beentjes
Glen D. Carbines
National Institute of Water and Atmospheric
Research Limited
P.O. Box 6414
Dunedin, New Zealand
email: m.beentjes@niwa.co.nz
Abstract Estimates of blue cod (Parapercis colias)
relative abundance and population structure were made from potting surveys
of Banks Peninsula and Dusky Sound, New Zealand in 2002. Five inshore and
two offshore strata around Banks Peninsula, and five strata between the inner
fiord and the open coast of Dusky Sound were surveyed. In Banks Peninsula
the overall mean catch rate (all fish) was 2.13 kg/pot per h (range 0.04–4.74)
and coefficient of variation (CV) was 10.8%. Blue cod from inshore strata
were significantly smaller than those from offshore strata, catch rates were
lower, and the sex ratio was skewed towards males (inshore 2.2:1, offshore
0.74:1). In Dusky Sound the overall mean catch rate was 2.69 kg/pot per h
(range 1.28–8.42), CV was 6.3%, highest catch rates were on the open coast,
and overall sex ratio was 0.78:1 (male:female). Blue cod in the 1940s from
throughout New Zealand were on average larger than blue cod in Banks Peninsula
and Dusky Sound during this survey. Low relative abundance and small size
of blue cod from the more accessible inshore areas, is consistent with fishing
pressure causing a reduction in size and probably abundance, particularly
in Banks Peninsula inshore strata. In Banks Peninsula, sex ratios in inshore
areas are skewed towards males, possibly a result of fishing pressure. The
results support other studies on protogynous fish species in which the removal
of the larger final sex fish (males), promotes sex inversion. The plasticity
of the sex inversion means that blue cod may be capable of restoring optimal
sex ratios in the natural state, but may over-compensate with size specific
anthropogenic removal of large numbers of individuals that would otherwise
inhibit the sex inversion process.
Keywords blue cod; relative abundance; catch rates;
potting survey; sex ratio; sex inversion; protogyny; fishing pressure
M04124; Online publication date 18 February 2005
Received 28 June 2004; accepted 18 October 2004
New Zealand Journal of Marine and Freshwater Research, 2005, Vol. 39:
77–90
0028-8330/05/3901-0077 © The Royal Society of New Zealand 2005
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