Seasonal growth dynamics and maximum potential growth rates of Australasian snapper (Chrysophrys auratus) and yellow-eyed mullet (Aldrichetta forsteri)

Abstract

Seasonality can have a major influence on growth dynamics of temperate teleosts that alternates between sustaining the maximum growth performance and confining it to its minimum. Yet the effects may not always be easily discernible, and species use different strategies to deal with annual oscillation in environmental variables. In this comparative study carried out in New Zealand snapper (Chrysophrys auratus) and yellow-eyed mullet (Aldrichetta forsteri) were exposed to seasonal conditions for 12 months under an ad libitum feeding regime to investigate whether an unrestricted diet can produce a clear seasonal response in their growth profile and, assuming that food is limiting in the wild, what would be their maximum potential growth rates. Data showed a strong seasonal effect on growth parameters and energy partitioning in snapper, whereas there were no seasonal dependent responses in mullet. Snapper showed exponential growth during the warm period and the opposite was evident in winter when growth ceased and become even negative. Mullet grew steadily all year around with only a slight deflection from the linear growth trajectory in winter. Snapper’s maximum growth rates corresponded with the highest ambient temperature in the peak of summer. Therefore, this study was not able to determine the maximum potential growth rates for snapper as it would possibly increase with further temperature rise. In contrast, mullet appeared to grow fastest at ∼17/18 °C. In terms of fish condition – snapper partitioned resources to maximise winter survival by switching from investment into growth to energy stores on transition between autumn and winter, whereas yellow-eyed mullet focused on maximising accumulation of visceral fat to reach optimal levels of ∼15 % body mass, which was independent of environmental temperature. Results from this study suggest that wild length-based growth rates in mullet may indeed be feed limited, whereas this cannot be suggested for wild snapper.