On the population ecology of the zebra seabream Diplodus cervinus cervinus (Lowe 1838) from the coasts of the Canarian archipelago, North West Africa

Abstract

Zebra seabream, Diplodus cervinus cervinus, caught off the Canary Islands is characterized by protogynous hermaphroditism. The male : female ratio is in favour of females (1 : 2.16). The reproductive season extends from spring to summer, with a peak in spawning activity in May–June. Males reach maturity at a larger total length, 327 mm (5 years old) than females 273 mm (4 years old). Recruitment occurs from late October to January in shallow waters of 0.5–8 m depth along the coastal line. The recruits are located over rocky substrates with an important algae vegetation forming schools lower than 2 m2. During the spawning season, schools of adults from 3 to 8 fish are observed. The schools are formed by one large individual and a few moderate size individuals. Mating takes place in small groups formed by one dominant male and a group of several females (polygamy). Moderate size individuals are often observed mixed in large schools (up to 30 individuals) of Diplodus sargus cadenati. Subadults form groups of a few fish (<5 individuals) or more commonly mixed groups (>15 individuals) with individuals of species of similar size. Otoliths age readings indicate that the population consists of 18 age groups, including a very high proportion of individuals between 2 and 4 years old. The von Bertalanffy growth parameters for the whole population are: L∞ = 603 mm, k = 0.149 year−1, and t0 = −0.22 year. The mean rates of total, natural and fishing mortality are 0.551, 0.215 and 0.336 year−1, respectively. The length at first capture is 183 mm. The exploitation rate indicates that the stock is overfished. The direct effects of fishing on the population result in changes in the abundance, with a reduction to 85% of the unexploited equilibrium level. The length at first capture by the commercial fishery is less than the length at maturity. With 58% of the total catch below this length there is a danger of recruitment overfishing.