Age, size and vertebral calcification in the basking shark, Cetorhinus maximus (Gunnerus)

ABSTRACT The inshore shoals of basking sharks that appear in spring off the western seabord of Europe have been variously claimed to be part of a seasonal south to north migration, or local, perhaps nuptial, aggregations. The decline in the catches of the Achill Island Fishery in the last decade suggests that the size of the shoals has diminished there. Size/frequency analysis of available records shows a midsummer peak of specimens with a mean length of 3.09 m and a mid-winter peak at 3.52 m; the difference is significant and is ascribed to growth. From this value, coupled with the recorded maximum and minimum sizes, a hypothetical asymptotic growth-curve is calculated. Comparison of this age/length relationship with the observed relationship between length and the number of calcareous "rings" in the vertebral centra suggests that at birth there are seven rings in a precaudal vertebra and that two rings are added annually thereafter. This suggests a possible gestation period of 3.5 years. This in turn, leads to the expectation that, since pregnant (or gravid) females are almost or quite unknown, they must live a hidden, perhaps demersal, life for three consecutive years after their first mating. The size/frequency analysis shows that only three specimens, none of them females, have been recorded between 5.8 and 7.4 metres in length, which on the tentative growth curve, corresponds with fish of average size in their sixth, seventh and eighth years. The fishery catches, though probably insignificant in relation to the total population of the north-eastern Atlantic, would have a devastating effect on a local population of a fish with such a low reproductive rate. Thompson (1856) states that the western coasts of Ireland are much frequented by the sunfish (basking shark) which appears in March or April and stays until November. The Parliamentary Gazetteer of Ireland (Anon., 1846) refers to their sporadic appearance off the north-west coast but says