Are there long-term temporal trends of size composition and the length- weight relationship? Results for chokka squid Loligo reynaudii during the peak spawning season off the south coast of South Africa

Temporal trends in the size composition (length frequency) and length-weight (L-W) relationship of chokka squid Loligo reynaudii on the south coast of South Africa were assessed over periods spanning 22 years: length frequencies from 1996 to 2017 (with 15 years represented); and L-W relationships over 9 years between 1994 and 2016. To allow for comparison, identical data selection and processing was adopted for all years considered (i.e. identical period of 60 days in spring-summer; the same depths and areas; chokka with empty stomachs; and squid of the same maturity stage). Although there were no significant long-term temporal trends in the mean lengths, there was a significant short-term drop in the mean lengths over the years 2014-2017 (especially in females), which could not be attributed with certainty to any cause. A tentative explanation is that this drop might be linked to the introduction of an additional closed season in these years. The estimated parameters of the L-W relationship also revealed no trend over the years considered. Investigation of the caecum colour, which indicates the state of starvation (white: 8 h on average after food ingestion; yellow: 6 to 7 h after food ingestion), showed significantly more starving males than starving females. Starvation of males on the spawning grounds might be associated with the spawning behaviour of chokka.