Back-calculation of previous fish size using individually tagged and marked Atlantic cod (Gadus morhua)

Abstract

The performance of five back-calculation (B-C) models was tested with individually tagged and multiple alizarin-marked Atlantic cod ( Gadus morhua ) reared at seasonally varying water temperature and food regimes over a 10-month period. The otoliths were transversely sectioned and the otolith growth between marks was measured along the dorsal, distal, and ventral radii and across the otolith width. Observed fish lengths at each marking time were compared with back-calculated lengths using linear and nonlinear scale and body proportional hypotheses (SPH and BPH) and biological intercept (BI) models. Measurements along the dorsal and distal radius with SPH and BI provided more accurate fish length estimates but with lower precision, whereas measurements of the ventral radius consistently produced greater overestimations by any model. Otolith width measurements produced moderately overestimated fish length estimates but with the highest precision and were significantly affected by temperature in all models. Certain combinations of model and otolith dimension were more sensitive to the length of the back-calculation time interval, whereas varying feeding regime induced temporary biases only. The selection of B-C model and otolith dimension thus depends not only on the environmental conditions that the fish experience, but also on the requirements of the application for which the B-C is used.