Growth and maturation in two successive seasonal groups of the short-tinned squid, Illex coindetii from the Strait of Sicily (central Mediterranean)

Abstract

Age and growth rates of the Mediterranean short-finned squid Illex coindetii (Ommastrephidae) were studied using statoliths from 704 specimens collected during trawl surveys within the Strait of Sicily in April and October-November 1995. In both samples, almost the whole ontogenetic spectrum was present, from juveniles to spent adults. Most of the squid captured in April had hatched between October and December 1994 (autumn-winter hatched group, AW), whereas-most captured in October-November had hatched between May and July 1995 with a peak in June (spring-summer hatched group, SS). Age of the AW squid did not exceed 240 days (mainly 200-210 d), while that of the SS squid did not exceed 191 days (mainly 170-180 d). Growth in length and weight was best described by the logistic growth function. In both groups, daily growth rates (DGR) of males decreased at younger ages than in females, and sexual dimorphism in sizes (females larger than males) became evident in maturing and mature adults. The AW squids grew rather slowly during their juvenile and immature phases, which occurred in cool winter and spring seasons. They were characterized by a maximum DGR in mantle length (ML) and body weight (BW) at old ages, a prolonged period of maturation and old ages of mature specimens. AW squids attained large sizes and spawned at ages of 6-7 months in spring-summer. Their progeny (SS group) grew rapidly during the warm summer and autumn seasons and achieved their maximum DGR in ML and BW at younger ages than their AW predecessors. However, their younger age at maturation (5-6 months) reduced their somatic growth during the late ontogenetic stages, resulting in rather small sizes at maturity and early spawning. It is concluded that I. coindetii in the Strait of Sicily produce at least two alternating generations each year, which are adapted to live in different seasonal environmental conditions. (C) 2000 International Council for the Exploration of the Sea.