Population structure and relative abundance of blue cod (Parapercis colias) off Banks Peninsula and in Dusky Sound, New Zealand

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.