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ABSTRACT1. Little is known about the potential impact of habitat modification by bottom fishing gear on the growth of demersal fishes. An analysis is presented for the growth of blue cod in Foveaux Strait, southern New Zealand, based on otoliths of fish captured from two sites in Foveaux Strait in 1999.2. Each site contained two distinct areas of contrasting benthic habitat complexity, one area of relatively 'complex' recovering biogenic reef and another area of relatively 'simple' sand and gravel, both previously modified by oyster dredging.3. Data were fitted to von Bertalanffy growth models for each sex of blue cod from the four areas sampled. No significant difference in growth models was observed for either male or female blue cod compared between the two types of habitat complexity at the eastern site. However, growth differed significantly for both sexes of blue cod from the two habitat types at the western site. Pairwise t-tests further showed that growth differences only appeared biologically significant for the youngest blue cod sampled (3 years). These fish were, on average, 20% larger in complex biogenic reefs than in simple areas dredged by the oyster fishery.4. These results suggest that on-going disturbance and simplification of seabed habitat by the oyster fishery may impede the growth of juvenile blue cod. Areas of recovering biogenic reef may, therefore, provide important habitat for the recruitment and early development of blue cod in Foveaux Strait. Remedial actions may be required to protect some areas of recovering biogenic reef from further damage, and to allow dredged areas sufficient time to recover if the blue cod fishery and related resources are to be managed effectively.

Variability in the abundance of commercial catches of fish has been studied for centuries (Huxley, 1881), especially in highly productive and highly populated regions (Heincke, 1898;Newland, 1999).Globally, abundance of fishing stocks has displayed declines over the past several decades (Worm et al., 2009), with changes in commercial fish assemblages identified for important fishing grounds

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 overcompensate with size specific anthropogenic removal of large numbers of individuals that would otherwise inhibit the sex inversion process.