ReproReproductive strategies of common dentex Dentex dentex: Management implications

Abstract

The common dentex Dentex dentex is an iconic endangered species in the Mediterranean, where it is a target species that is sought after for small-scale, recreational and spearfishing fisheries. The reproductive biology of D. dentex in the natural environment is poorly known; therefore, the reproductive strategy of the species was assessed through a combination of reproductive traits and growth characteristics (estimated from length-at-age data), the size/age of sexual maturity and the energetic dynamics. A total of 358 wild fish, ranging in total length (LT) from 19 to 84.7 cm, was sampled at Mallorca Island (Western Mediterranean) from March 1996 to June 1999. The sex ratio was skewed towards females (1.361); however, the length composition was not different between sexes (p = 0.551). Three young immature individuals (< 28 cm LT, 0.8% individuals) were rudimentary hermaphrodites, offering support for classification as a late gonochoristic species. The age composition, determined based on the sagittal otolith, ranged from 0 to 26 years (yr). The von Bertalanffy growth function did not differ between sexes (F = 2.58, p = 0.762): L∞ =76.581 cm, K = 0.127 yr-1, t0 = -2.335 in a combined function. The maturity ogives for size and age showed that females achieved 50% maturity at 34.922 cm LT and 3.3 yr, and males, at 33.812 cm LT and 2.5 yrs. The onset of annual ripening took place in December, whereas vitellogenesis occurred from February to April. The spawning peak was in April and May for both males and females. A generalised linear model (GLM) showed that female size did not significantly affect the spawning season, whilst the seasonal component affected both the onset and end of spawning (GLM, p < 0.005). The gonadosomatic index (IG), hepatosomatic index (IH) and relative condition index (KR) varied significantly with the reproductive season for females, and IG varied for males. The microscopic observation of the gonads showed that fecundity is likely determinate, with an asynchronous oocyte development before spawning and a clear ovarian bimodal organization after the onset of spawning. The oocyte density variance of 85% was explained by the oocyte diameter, which also offered support for the trait of determinate fecundity. Female weight explained 84% of the observed variance for fecundity. Liver storage seemed to be the primary source of energy for maturity, which suggests a combination of capital and income breeding. The results suggested a reproductive strategy of type A, with balanced trade-offs between survival, growth and reproduction. At present, no management measures are being directed to D. dentex; herein, we suggest a minimum landing size of 35 cm LT to ensure a higher proportion of larger fish to preserve the stock.