ABSTRACT1. Little has been done to assess the potential impact of habitat modification by bottom fishing gear on the feeding habits of demersal fishes. An analysis is presented of the diet of blue cod in Foveaux Strait, southern New Zealand, based on the gut content of fish taken in winter 1999 from two sites where each site consisted of both undisturbed biogenic reefs and reefs modified by oyster dredging.2. Of the 420 guts collected, 13% were empty. The overall mean wet weight of gut content was 54 g. No significant habitat or site effects were detected for the proportion of empty guts or the amount of food consumed.3. A pattern was detected that blue cod on dredged habitats generally fed on more crustaceans than those on undistributed habitats. Blue cod from undisturbed habitat also displayed a more diverse diet than those taken from dredged habitat. These results suggest that long-term disturbance of seabed habitat by the oyster fishery in Foveaux Strait has caused changes to the diet of blue cod. The findings also suggest that actions should be taken to protect the biogenic reefs from further damage if the blue cod fishery and related resources are to be effectively managed.4. Changes in prey diversity with increasing fish size were also found, with prey diversity (Shannon-Wiener index) increasing from 0.83 to 1.35 over a range of fish size from 525 cm to larger than 34 cm. A total of 52 prey taxa were identified in the diet of blue cod. Crustaceans were the main component, followed by mollusca and polychaeta. Fish, echinodermata and 'other' were less important in the diet.

Epifaunal reefs in Foveaux Strait are oyster (Ostrea chilensis Philippi, 1845) habitat. One hundred and thirty years of oyster dredging has diminished the complexity and distribution of these reefs. Commercial densities of blue cod (Parapercis colias (Forster in Bloch and Schneider, 1801)) were discovered on epifaunal reef habitat in 1989 and became the focus of a major blue cod fishery. We document habitat changes that followed the closing of the oyster fishery in 1993 and interactions between the blue cod and oyster fisheries after the oyster fishery was reopened in 1996. Evidence from blue cod fishers and oyster surveys suggests that the benthic habitat of some oyster beds regenerated in the absence of dredging and that the relative density of blue cod, and then oysters, rebuilt to commercial levels. Benthic habitat was modified once more when oyster dredging restarted and the relative density of blue cod on oyster beds fell again. The observations suggest that rotational fishing of oysters could mitigate the effects of dredging on habitat and that marine protected areas could expedite habitat recovery. Increasing habitat complexity and blue cod density on a reef of oyster shells formed by an oyster fisher suggests that habitat enhancement might remedy effects of dredging. The questions raised by the observations could be answered by management experiments on the scale of the fisheries.

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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.

An experiment was conducted in early summer 1995 to determine the survival rate of sublegal‐size (<33 cm total length) blue cod Parapercis colias after being captured and returned to the sea by amateur fishers using two types of hooks (6/0 and 1/0). At the same time, commercial cod pots captured blue cod for use as a control group. As fish were caught, they were subjected to either good or poor handling techniques and then placed into holding pots built specifically for the experiment. These were lowered to the sea floor and sequentially monitored during daylight hours for 2 weeks. No mortality of blue cod occurred with the 6/0 hooks, but fish caught using 1/0 hooks suffered 25% mortality by the end of the experiment. No control fish died during the experiment, and the type of handling technique used had no detectable effect on blue cod survival. The fact that all mortality occurred within 26 h, combined with observations of fish behavior, suggests that mortality was induced by blood loss rather than disease. The location of the hook wound was related to hook size, small hooks lodging in the gut or gill usually proved fatal. These findings suggest that the mortality of released blue cod would be minimized if fishers used larger rather than small hooks. The management of blue cod by size limit regulation is discussed in relation to these findings.

In coastal areas, the identification of habitat types critical to fish life history strategies can provide useful information for ecosystem-based management. Recent studies show that species distribution modelling can be a cost effective tool for describing fish habitat. However, few modelling studies have examined ontogenetic habitat associations. This is critical, as fish species often have different habitat preferences depending on their life stage. In this study, we used boosted regression trees (BRT) to describe ontogenetic habitat associations in snapper Pagrus auratus across the inner Hauraki Gulf of New Zealand. The BRT models identified that juvenile snapper were most frequently associated with slow orbital velocities and slow tidal current speeds, as well as biogenic sedimentary structures (area under the receiver operating curve or AUC, a measure of model performance, 0.79). In contrast, larger snapper were associated with faster tidal currents and faster orbital velocities (AUC 0.78). Juvenile and adult snapper were spatially separated; juvenile snapper occurred in waters close to shore, whereas large snapper occurred mainly in the channels between the islands and the waters around the islands. The successful discrimination of adult and juvenile habitat associations suggests that a modelling approach such as this could be useful for ecosystem-based management.

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.