Abstract
This thesis addresses several issues that need resolving before population age/length structure, growth rates, movement and reproductive biology of blue cod (Parapercis colias) can be properly understood. It provides validations of stock assessment methodologies that are perquisites to the analysis of population data to assist in the sustainable management of South
Island blue cod.
Chapters two and three deal with age and growth, initially by validating otoliths and comparing other aging methods. Length at age models were then compared among regions of the Marlborough Sounds. Female blue cod showed no detectable differences in growth, but males differed among areas within the extreme and outer Pelorus Sound strata, and between strata in both Pelorus and Queen Charlotte Sounds. Growth was fastest in the extreme outer strata of both Sounds. Results were then considered in terms of fishing pressure, catch-per unit- effort, sex ratios, female fecundity, and sex inversion to conclude that stock assessment of this species should be done at a smaller spatial scale than is currently used.
Chapters four and five described the movement patterns of blue cod in terms of current stock boundaries in Southland. From both laboratory and field trials, tagging methods were optimised to retention of 88% over two years and a 5.9% return rate. A total of 9368 blue cod were then tagged within Foveaux Strait (fishing statistical area 025) using a balanced stratified series of replicated sites (n=9) within three latitudinal and three longitudinal strata. Three different bottom habitat types were also identified and included opportunistically into the model.
After 20 months, 743 blue cod had been returned (7.9%). The largest distance moved was 156 km, however the median was only 800 meters, 60% moving less than 1 km. The habitats that blue cod were released into had no effect on distance moved. However, spatial location was an important but complex determinant of distance travelled. A strong trend toward countercurrent, north-west movements was evident and mixing rate calculations showed higher levels of emigration to the west and immigration from the east of area 025. A significant increase in the proportion of blue cod moving in the spring of both 1998 and 1999 further implied some seasonal migration associated with spawning. Mixing rate calculations showed moderate mixing with neighbouring areas (up to 14.7%). However, mixing was considerably higher between sub-areas within area 025 (up to 44.1 % ) suggesting that blue cod stocks are relatively stable at the fishing statistical return area scale.
Chapter six provided compelling evidence that blue cod are diandric protogynous hermaphrodites. Although males may be predominantly harvested, this is unlikely to cause fecundity problems, as sexual succession is relatively flexible. Observations of gonad developmental showed that Southland blue cod spawn from October through to January. Southland blue cod are not sexually dichromatic, however, macroscopic gonad observations are an acceptable sexing technique.
The final Chapter of this thesis discusses these findings and their implications for future blue cod fisheries management.