Age determination of Illex coindetii from the Strait of Sicily by statolith increment analysis

Abstract

A total of 649 short-finned squid Illex coindetti caught in the Strait of Sicily (Central Mediterranean) was examined to investigate statolith microstructure and estimate the number of growth increments. Two different methods of analysis were employed to count increments, the conventional method by eye and an automatic image-analysis system (IAS). Age estimates (based on the assumption of daily deposition of increments) obtained with the two methods were compared. Maximum ages estimated by eye were 230 and 240 days for males and females respectively. Average IAS estimates were significantly higher (2 days more for males and 5 days more for females) than the conventional ones. Size-at-age relationships were computed from both sets of age estimates using three size indicators: mantle length, body mass and an index calculated as the ratio between the logarithmic values of body mass and mantle length. Size-at-age relationships were similar for both methods of counting increments. Best fits were obtained using the quadratic, power and yon Bertalanffy models applied to mantle length, body mass and the new index respectively.