Population size structure, age and growth of Naucrates ductor in the small scale FADs fishery of Mallorca Island (western-Mediterranean)

Abstract

The pilotfish (Naucrates ductor) is mainly caught as a by-catch of the dolphinfish fishery using FADs deployed around Mallorca (western-Mediterranean) from the end of August to December. The age and growth parameters of this species were estimated by length frequency analysis and daily growth increments observed in the otoliths. To validate the daily formation of the ring observed in the sagittae otoliths, a marking otoliths experiment with SrCl2 was carried out with juvenile specimens kept in laboratory conditions. The growth parameters obtained from otolith interpretation were based on age at length of 99 specimens ranging between 15 and 31.2 cm FL. Length frequency analyses were based on the monthly length distribution obtained in 1990, 1991, 1995 and 1996. The results obtained from the two methods were similar and showed rapid growth during the first 6 months of life. The von Bertalanffy growth equation from otoliths was FL= 28.97 (1-e (-6.87(t - 0.055)). Positive allometric growth was observed in the length-weight relationship, with no differences existing between sexes: W (g)= 0.0147*FL3.040 (cm).