Building a broodstock of the critically endangered sturgeon Acipenser sturio: problems and observations associated with the adaptation of wild-caught fish to hatchery conditions
This study deals with the establishment of a confined broodstock of the critically endangered sturgeon Acipenser sturio L., with special emphasis on two groups of wild captive fish. One is composed of 40 juveniles hatched in the wild in 1994 and caught in the Gironde estuary in 1995. The other is made up of older juveniles (n = 8) and adults (n = 6) caught at different times. The fish were fed frozen shrimps and reared in recirculated water systems. The main aims of the study were i) to determine optimal rearing conditions, ii) to improve our knowledge of the species (sex ratio, genetics), and iii) to determine how to manage adults in captivity to produce gametes. Short trawling, rapid transportation with water from the fish habitat and maintaining them in the hatchery in water of similar salinity reduced the initial weight loss which was measured at up to 30%. Growth of juvenile fish 1994-95 was similar in fresh and brackish water till mid 2001. Weight range increased with age. Analysis of 11-ketotestosterone (11 KT) levels shows that one third of these 1994-95-fish are probably males. Of the large juveniles kept in brackish water, some required several months to resume feeding but two 10-kg fish exhibited no weight loss, suggesting a great potential for adaptation. Some breeders did not recover their initial weight for several years, with growth best described as irregularly cyclic. The large fish (10 out of 12) exhibited better growth in a 2 m deep tank compared with a 1 m tank. Out of the 14 large fish, 6 died after one to three years holding for no known reason. The 8 remaining fish are sires, 5 of which matured in 2000 and 2001. Four of the five matured in the two consecutive years. Vernalisation (11°C in winter), natural daylight, one week in fresh water in late spring to mimic upstream migration, and hormonal stimulation with either carp pituitary homogenate or GnRH analogue provided high quality semen. The genetic variability of the Gironde population, assessed from the present experimental fish, is low and all specimens share the same mitochondrial haplotype. Characterisation of mitochondrial DNA fragments suggests that the juveniles born in the wild in 1994 were produced by the same dam and sire.