El atún blanco [ Thunnus alalunga (Bonnaterre, 1788) ] en el Mediterráneo occidental: biología de la reproducción y métodos aplicados al estudio de la fecundidad

Abstract

Albacore Thunnus alalunga (Bonnaterre 1788) is an important commercial tuna species distributed in the three major oceans and in the Mediterranean Sea. The Mediterranean albacore stock was first assessed in 2011. Taking into account the incomplete fishing statistics, and the lack of knowledge on the life cycle and biological population parameters, the stock was classified as data-poor. This thesis presents new information about the reproductive biology of albacore in the western Mediterranean Sea, which is one of the most important spawning grounds of tuna species in the Mediterranean Sea. The size frequency, sex ratio, gonad reproductive stages, spawning season, minimum length at maturity and gonadosomatic indexes were examined. The sex ratio was female biased in fish <70 cm fork length (FL) and male biased in those >75 cm FL. Histological analysis of the ovaries and the monthly variation of the gonadosomatic index for both sexes showed that spawning occurred from June to August. The minimum length at maturity for albacore was 56 cm FL.

The present PhD thesis also addresses methodological issues related to fecundity estimates that are of interest to fish reproductive researchers. The new Oocyte Packing Density theory was used in order to examine any differences in both the number of stage-specific oocytes per gram and the relative batch fecundity between the most commonly used embedding media in fish reproductive studies. The results showed that the mean oocyte diameters were smaller in paraffin than in resin with differences ranging between 0.5% for previtellogenic oocytes and 24.3% for hydrated oocytes. In spite of the higher degree of oocyte shrinkage in paraffin, the Oocyte Packing Density formula could equally well be used for estimating the number of migratory nucleus (MG) oocytes in paraffin as well as in resin histological sections. However, the mean relative batch fecundity estimated from hydrated oocytes was significantly lower than when using MG oocytes, for both embedding media. In addition, batch fecundity estimates of albacore resulting from the application of different methods were compared. The relationships between batch and relative fecundity estimates and the associated biological metrics (length, body weight and ovary weight) were also investigated. Batch fecundity estimates ranged from 0.42 to 2.16 million oocytes, with a mean relative batch fecundity of 136 oocytes per gram of body weight.