Age, growth, and reproduction of the rock goby, Gobius paganellus L., in the Isle of Man

Summary

Age, growth and reproduction of the intertidal teleost, Gobius paganellus, has been investigated in the Isle of Man from March 1959 to September 1960. Samples were obtained by poisoning shore pools with a rotenone emulsion.

Age was determined by examination of otoliths where a translucent annual ring is laid down in spring. Although the potential life span is at least 10 years, immature fishes of the first two age-groups constitute most of the population. An onshore breeding migration is postulated. An empirical survival curve indicates a high mortality rate during the first two years of life. Instances of predation on G. paganellus are given.

Growth in length is logarithmic, with marked decrease in growth rate from about the age of sexual maturity. During the year, growth occurred from June to October. Maximum standard lengths recorded were 92*5 mm (feThe overall sex ratio of females to males was 1:1-326; this predominance of males persisted through the age-groups. The reproductive organs are briefly described, and a classification of gonad maturation is proposed. The breeding season lasted from mid-April to mid-June. Sexual maturity was reached at the end of the second or third year of life. Smallest ripening individuals found were 48-5 mm (female) and 505 (male). Fecundity increased with body size. Eggs were deposited on stones in shore pools and guarded by the male. Post-larvae occurred in pools from the beginning of July, and the first demersal individuals from August. Size-frequency distribution of oocytes in the ovary indicates that at least two broods may be produced in a single breeding season.

In maximum size, longevity, and age at maturity, G. paganellus differs from other European gobies so far studied. It is suggested that a similar life history is exhibited by other British species of Gobius. The short breeding season of G. paganellus in the Isle of Man is held to be a result of proximity to the northern limit of distribution of this species. Examples of this phenomenon in other teleosts are provided. The significance of pre-ovulatory corpora lutea in the ovary of G. paganellus is briefly discussed.