Can ordinary single-day egg collection increase the effective population size in broodstock management programs? Breeder-offspring assignment in black sea bream (Acanthopagrus schlegelii) through two-hourly intervals

摘要

The management of the broodstock used for stock enhancement purposes is essential to preserve the genetic resources of the natural populations. Black sea bream (Acanthopagrus schlegelii) has been extensively released in Japan; nevertheless, the little emphasis paid on the genetic conservation, resulted in important drifts due to the small number of parental fish reared in the hatcheries. The present study shows how simple changes in the broodstock management procedure may greatly contribute to increase the effective number of breeders (N b ) and reduce the rate of inbreeding. In this regard, the collection of eggs at several timings over a single night allowed the identification of those contributors spawning for a short period and with little overall contribution. It enabled identifying those alleles presented in lower frequencies, an essential issue on conservation genetics. The second improvement involved the use of a reasonable large number of individuals as breeders. In this study, the broodstock (BR) comprised 143 specimens, a feasible number to be reared at any hatchery that produces juveniles for stocking in Japan. The BR-offspring assignment confirmed the contribution of an extremely high number of breeders (140), corrected to 136 after considering the differences in sex proportions. The number of offspring comprising each family was similar (up to 3). Nevertheless, large disparities in the contribution were observed among breeders. As a result of the large number of offspring produced by few parental fish, Nb was reduced to 37. Although lower than desirable, the fact that most of breeders took part in the mating process, confirmed the usefulness of this procedure. An alternative method to reduce and simplify the number of egg batches required was investigated. The three groups of eggs sampled in the middle of the sampling period and representing the period the releases were most intensive, were combined (nocturnal group), showing similar results. In this case 136 out of the initial 143 breeders took part in the mating process, and the final N b was 88. These results confirm that by either of these simple means, most of the genetic diversity would be inherited by the offspring and help to preserving the genetic resources of the natural populations subject to stocking. This experience, although promising, should also be applied to other species to confirm its effectiveness. In addition, it is necessary to confirm the viability and performance of the offspring produced in this way in relation to those collected over the whole spawning season.