Early life history of snapper (Chrysophrys auratus) in northern New Zealand

Abstract

Snapper (Chrysophrys auratus) is a commercially-important fish in New Zealand and Australia. Variability in interannual recruitment is high in this species, which affects subsequent adult abundances, and therefore, an understanding of the factors that affect recruitment is critical to the successful management of fished populations. Research presented in this thesis investigates some aspects of the early life history of C. auratus in four harbours (Kaipara, Manukau, Mahurangi and Huruhi) in northern New Zealand. Histological gonad analyses showed that recruitment to the Kaipara Harbour must originate from spawning activity outside the harbour. Adult fish captured within the harbour had underdeveloped gonads and reabsorbed their oocytes prior to spawning. Analyses of daily otolith increments showed that larvae spent 17–33 days in the plankton before settling in shallow waters of all four harbours. The successful spawning period that produced settled juveniles was only 29–109 days, despite a spawning season of ≤4 months, indicating that recruitment to some harbours may be limited by environmental conditions that affect larval survival or transport to settlement habitats. Daily settlement was significantly positively correlated with temperature, tidal range and on-shore winds (of the previous day) in some harbours, suggesting that these variables facilitate the on-shore transport of larvae. High temperatures are likely to be important for larval survival, with average water temperatures during the pelagic larval duration (PLD) >18 °C for 91% of fish captured. Recruitment may also be affected by growth rate and resource-allocation strategies. Growth rates varied significantly among sites, with fish from the Kaipara and Huruhi Harbour sites having the fastest growth. Faster-growing larvae spent less time in the plankton and continued to grow faster as juveniles, which is likely to decrease their mortality rate. Resource-allocation in 0+ year fish was found to change from maximising growth in summer to maximising lipid accumulation in mid-autumn. Lipid concentrations in fish during summer and early autumn were very low, making them very vulnerable to starvation mortality. Overall, these results provide us with a better understanding on the ecological processes that affect the recruitment of C. auratus, which can be used to more effectively manage populations of this important species.