Stocking density at early developmental stages affects growth and sex ratio in the European eel (Anguilla anguilla)

To investigate the effect of stocking density on growth and sex ratio in European eel, four constant density conditions were tested during the transition from the glass to the elver stage for 90 days (Period 1). The test conditions combined the weight of fish per unit surface or volume (surface density or volume density) resulting in four experimental groups: low surface density (0.5 kg/m(2)) and low volume density (5 kg/m(3)) (group S(0.5)V(5)); low surface density (0.5 kg/m(2)) and high volume density (10 kg/m(3)) (group S(0.5)V(10)); high surface density (2 kg/m(2)) and low volume density (5 kg/m(3)) (group S(2)V(5)); and high surface density (2 kg/m(2)) and high volume density (10 Kg/m(3)) (group S(2)V(10)). Subsequently, fish from the S(0.5)V(5), S(2)V(5), and S(2)V(10) groups were transferred to low density conditions (0.1-0.4 kg/m(2) or 0.1-0.3 kg/m(3)) for another 21 months (630 days; Period 2). After Period 1, fish maintained at high surface density, regardless of the volume density, showed higher standard growth rates (SGRs) and RNA/DNA ratio in muscle than those cultured at low surface density. The percentage of mortality was similar in three of the groups (34.2%-41.8%), but not in the S(2)V(10) group (83.3%). At the end of Period 2, most fish (about 95%) exhibited fully differentiated gonads, but different sex ratios were observed in each group. Thus, the S(2)V(5) group showed a higher proportion of females (36.1%) and a lower proportion of males (56.8%) than the S(0.5)V(5) group (11.4% and 72.5%, respectively), while all survivor fish from the S(2)V(10) group developed into females. The gonadosomatic index and SGR were higher in females than in males. These results suggest that glass eels maintained at high surface density during the first months of growth tend to develop into females. The data also indicate that growth and sex ratio are linked processes during eel development, with growth seeming to be sex dependant rather than being influenced by the density conditions in which glass eels are maintained.