Abstract
An age prediction model based on individual morphometric characteristics (total length; weight) and otolith morphometric characteristics (diameter; weight) was investigated for juvenile sardine, Sardina pilchardus (Walbaum, 1792). Juveniles were collected from northern Portugal between May 2004 and January 2005. Daily growth rings were counted on the otoliths of 114 juveniles of 7-16 cm total length. The sample was divided into a training sample used to develop the age prediction model and a test sample used to evaluate the predictive ability of the model. The best model for predicting the logarithm of age was a linear regression with otolith diameter. The prediction of daily age was more accurate for younger ( < 200 days) juveniles. Overall, ages predicted from the model were unbiased in relation to ages determined from otolith microincrement counts. Moreover, predicted daily ages reproduced the overall shape of the observed age distribution and provided comparable growth estimates (0.041 cm day–1). The back-calculated birthdate period ranged from 29 September 2003 to 22 July 2004, with a peak in January 2004, which is consistent with the spawning season. The model presented here could be used as a method for increasing the volume of juvenile daily age data. Since growth and survival varies spatially and temporally, relationships between age and otolith/fish morphometry should not be extrapolated outside sampled periods, areas and fish size/age.
Highlights
Data on the birthdates of juvenile fish are important for investigating periods when environmental conditions are most favourable to recruit survival (Wilhelm et al 2005, Baumann et al 2008)
This study aimed to identify the best model for predicting the age, in days, of sardine juveniles from otolith and fish morphometric characteristics
An age prediction model was developed using samples of sardine juveniles collected off the northwest coast of Portugal during the 2004-2005 recruitment season and was applied to predict the age of individuals in another sample collected in the same period and area
Summary
Data on the birthdates of juvenile fish are important for investigating periods when environmental conditions are most favourable to recruit survival (Wilhelm et al 2005, Baumann et al 2008). Such knowledge is important in the case of small pelagic fish such as sardine (Sardina pilchardus) that have extended spawning seasons and extensive inter-annual fluctuations in recruitment (Stratoudakis et al 2007, ICES 2014). The idea of extending the technique to overcome the difficult task of determining age in juvenile fish has been put forward by Pawson (1990). The few published applications on juvenile fish age prediction from morphometric characteristics that we are aware of (Fey and Linkowski 2006, Megalofonou 2006) indicate that the idea has received limited attention in the past two decades
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