Modeling the size of snapper (Pagrus auratus) using temperature-modified growth curves

Abstract

Age-length keys from RV Kaharoa trawl surveys in the years 1984-1990 and 1992-1994 were used to estimate mean lengths-at-age of snapper (Pagrus auratus) in the Hauraki Gulf of New Zealand. A von Bertalanffy growth curve fitted to these lengths-at-age exhibited poor fit, and when the fit was restricted to only cohorts since 1968 (those for which sea surface temperature data were available) the estimated maximum length parameter dropped 5 cm. A power curve provided a better fit without an increase in the number of parameters and was not sensitive to the restriction to younger cohorts. When water temperature was added to the power curve model the residual plots indicated unexplained year effects and temporal correlation. These features were included using a mixed-effects model for repeated-measures data, and temperature was found to be statistically significant (p < 0.0001). This conclusion was corroborated via a simple randomization test. The model predicts growth over a year to change by the equivalent of 8 weeks if the average annual water temperature changes by 1°C. This is sufficient to predict a 4-year-old snapper from the 1986 cohort as having the equivalent of 6 months more growth (at the growth rate corresponding to average water temperature) than a 4-year-old fish from the 1990 cohort.