Some aspects of the reproductive biology of the twaite shad, Alosa fallax (Lacépède, 1803), in the River Ulla (Northwest of the Iberian Peninsula)

Evaluation of fecundity is a key step to quantify the reproductive potential of individual fish and to understand the stock-recruitment relationship, increasing the knowledge of the state of a stock and improving the assessment of the spawning stock biomass. For the present work we investigated the fecundity and gonad development in anadromous female twaite shad, Alosa fallax (Lacépède, 1803), during the upstream spawning migration in the River Ulla. Thirty-four females were sampled with trammel nets between April and June 2011 in seating areas for this species, located at 24 km and 20 km from the Atlantic Ocean respectively. Fish were lengthed to the nearest millimetre, weighed to the nearest gram and aged by using scales. Factor condition (K) was estimated as K= (TW/TL^3)*100 where TW is fish total weight (g) and TL is fish total length (cm). The ovaries from each female were removed and weighted to the nearest 0.01 gram. Approximately one gram samples were taken from the anterior, central and posterior regions of each ovary and preserved in 70 % ethanol. The number of eggs in each subsample was counted and the fecundity computed from the mean. Individual fecundity (F) was computed and F was divided by the weight of the fish to estimate relative fecundity (FR). Also the gonadosomatic index (GSI) was calculated to assess changes in fish maturity and spawning stage during the upstream migration. Female age ranged from 4 to 7 years and values of TL, TW, K and GSI were [mean ± SE (range)]: 493 ± 7.3 mm (408-562 mm); 1,112 ± 53.7 g (592-1,626 g); 0.91 ± 0.02 (0.67-1.09) and 12.5 ± 0.47 (6.6-21.4) respectively. The value of fecundity (F) was [mean ± SE (range)]: 96,471 ± 6,493 oocytes (24,962 to 182,154 oocytes), while the relative fecundity (Fr) was [mean ± SE (range)]: 87,142 ± 3,592 oocytes (25,319 to 129,189 oocytes). A significant correlation (Spearman´s ρ test, p<0.05) between fecundity and all parameters has been determined. As observed in previous studies variables that best correlated with fecundity were the TW (correlation coefficient: 0.793), K (correlation coefficient: 0.640) and TL (correlation coefficient: 0.612).