Time series analysis of daily growth in Dicentrarchuslabrax L. otoliths

Abstract

Variations in the thickness of daily growth increments in the otoliths ( n = 40) of sea bass ( Dicentrarchus labrax L.) hatched and reared in the laboratory and ranging in age from 2 to 116 days were studied. The complete increment series and three partial series corresponding to distinct stages of development were also analysed by means of time series. The relationships between otolith growth and water temperature fluctuations during the period studied were analysed using transfer functions. In the experimental conditions the major uncontrolled external variable affecting growth was water temperature, observed daily at noon. Time series analysis of the daily increments was carried out by estimated auto-correlation and partial autocorrelation functions. An IMA model was identified, estimated, and verified. The bivariate time series process — water temperature and otolith growth increments — was analyzed by means of a linear transfer function model. The examination of the cross-correlation function showed dependence between both series. The influence of water temperature on otolith growth was adequately described by an undelayed third-order transfer function model, with a steady state of −0.47μm·day −1·°C −1. An updating forecast of otolith growth was made using the transfer function model. As otolith size and body length were closely related, the predicted values of otolith growth could be used for body growth estimations. This paper presents evidence suggesting that otolith growth in sea bass is a conservative process, with growth on any given day depending upon growth and the disturbances occurring on the day of deposition, ( t, and the day before, t − 1. Water temperature exerted an effect on the growth the same day, and thereafter the effect decreased exponentially every 3 days. Multidaily growth patterns were not present in sea bass otoliths. This lack of periodicity is probably related to the lack of periodic cycles in water temperature.