Abstract
Sequential hermaphroditic species, such as blue cod (Parapercis colias), are particularly vulnerable to selective harvesting as it can directly influence the sex ratio, timing of sex change, and consequentially recruitment success. We analyzed the population structure, at which males dominate the populations, and modeled fecundity of blue cod from the Marlborough Sounds in the north of New Zealand's South Island, and compared these results with previously collected data from a blue cod population in Fiordland on the southwest coast of the South Island. In addition to the geographical difference, there were differences in the time the two populations were sampled, and the fisheries management regime between regions. Within the Marlborough Sounds population, the length-frequency distribution was skewed toward high numbers of smaller individuals with an average length of 290 +/- 59 mm. In Fiordland, the average length of blue cod was 380 +/- 78 mm. Larger proportions of males within each length class were observed in the Marlborough Sounds, whereas females were the dominant sex in the Fiordland population. The estimated length at which males dominate the population was significantly smaller for the Marlborough Sounds population (182 mm) than for blue cod from the Fiordland population (419 mm). Geographical and temporal differences in sampling likely played important roles in the observed life history differences, particularly in the observed differences in growth. However, when compared to length and age data from national surveys, the life history traits of both sampled populations have remained relatively stable over the past 15 years, indicating that differences in population management and fishing pressure are the most likely explanation for the observed differences in length frequency and length-at-sex-change. For effective and sustainable management of species with sequential hermaphroditism, size-sensitive management, particularly protecting larger individuals, is required to avoid declines in length structure and recruitment limitations.