Variability in age-at-recruitment and early growth in English Channel Sepia officinalis described with statolith analysis

Abstract

In the English Channel, the French cuttlefish landings of the smallest commercial category (body weight < 100 g) indicate that recruitment begins in autumn. Samples of recruits were collected in the Port-en-Bessin (Normandy) fish market once every fortnight from October to December 2000 to determine age-at-recruitment and juvenile growth variability. In addition, pre-recruit samples (mantle length: 53–90 mm) were collected in the Bay of Seine from August. Age was determined by statolith analysis: daily growth increments on the statolith lateral dome were observed under a light microscope. Increments were counted automatically, and manually when necessary, using the image analysis software TNPC. Readable statoliths were obtained in 236 cuttlefish, which revealed changes in age-at-recruitment during the study period. The cohort 2000 is recruited at an age ranging between 2 and 5.5 months (the bulk of recruits are 60–120 d old) and an increasing trend in average age is observed in autumn. The duration of the hatching period was derived from back-calculated hatching dates. A main hatching peak was observed between June and August. Juvenile growth variability was analysed at the individual level and between batches of hatchlings. Growth rates suggest that early hatched individuals grow faster than animals hatched later in the summer.