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Bioenergetics approach to fish reproductive potential: case of Trisopterus luscus (Teleostei) on the Galician Shelf (NW Iberian Peninsula)
NW Iberian Peninsula
|Abstract:||The fish species Trisopterus luscus (Linnaeus 1758), commonly known in English as pouting, is a member of the cod family (Gadidae) and it is of major commercial importance to the artisanal fleet of a number of European countries, primarily France, Portugal and Spain. Since the 1960s and 1970s (with average landings of around 3000 t per annum) a slow but steady decrease in catches has been observed on the Galician shelf, down to landings below 1000 t per annum in recent years. This situation is aggravated by the fact that the information available about this species is generally scarce, especially regarding our knowledge of its reproductive biology.
Knowledge of the reproductive biology of a fish species is essential for effective fishery management. There is increasing awareness that the traditional indicators of stock viability are inadequate because the capacity of a population to produce viable eggs and larvae each year is extremely important for stock viability and recovery. Improved estimates of stock reproductive potential should thus lead to improving the current models used in fisheries management.
Histological study of gonads of T. luscus was consistent with asynchronous ovarian development. This study shows that pouting on the Galician shelf presents a protracted spawning season, from January to May, with a peak of spawning activity in February, a month when female pouting release a batch almost every day. Length-at-maturation, 150.6 mm, was lower than previously estimated in other studies, and it seems to be in concordance with the established minimum legal size, 200 mm. The development of the pelagic eggs and hatched larvae has been described for the first time ever in this study. This has also been the first natural spawning experience in captivity for this species.|
One of the most important findings in this study is that pouting exhibits determinate follicle recruitment. However, the possibility of an indeterminate period of follicle recruitment in response to surplus energy is also likely to occur. Maternal effects in potential and batch fecundity are clearly manifested through positive allometric coefficient of fecundity-female size relationships. Energy reserves in pouting females are mainly associated to the lipids stored in the liver. Water content appeared to be a 3 4 useful index in order to obtain very accurate predictions of energy density, particularly for gonads and the liver. Condition indices, both the common ratio index and the residual index, are suitable to analyze nutritional status of fish, since they reflect changes in energy content and proximate composition in the different tissues studied. The seasonality of energy reserves was clearly detected and is closely related to the sexual cycle, indicating that pouting females display a period of energy storage, mainly through lipid deposition in the liver. Therefore, pouting should be considered a capital rather than an income breeder, as the fuel for reproduction comes essentially from stored energy. Reproduction in T. luscus is financed from stored energetic capital, but energetic provision through concurrent feeding during reproduction may also contribute to egg production, i.e. pouting exhibits a mixture of determinate/indeterminate fecundity and capital/income breeding strategies. Individual female size influences reproductive biology at different levels, as discussed previously in this chapter: timing and length of spawning and fecundity (potential and batch). Therefore, it is not surprising that the simulation model has verified that a population biased towards larger fish can produce a considerably higher TEP than a population with the same SSB but biased towards smaller females. These huge differences clearly support the inadequacy of the assumption of direct proportionality between egg production and Spawning Stock Biomass. Additionally, and as shown in our study, larger fish not only contribute disproportionally to SRP in terms of quantity but also of quality. Consequently, effective and sustainable fishery management requires considering several aspects of fish reproductive strategies to better preserve a stock’s reproductive potential.
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