Characteristics of sperm of captive seabass in relation to its fertilization potential

Seabass Dicentrarchus labrax sperm concentration was high (up to 60 x 10 de grees spz ml(-1)) but decreased significantly at the end of the reproductiv e season (mid-March) in monthly sampled fish. The spermiation period may be shortened by frequent stripping. Sperm can be prediluted up to 1 : 128 in non-activating medium without loss of initial motility and motility duratio n. Immediately after activation by transfer to sea water, all the spermatoz oa were motile for 10 s and then the number of motile cells decreased progr essively but sharply to zero, so that the duration of sperm motility was ve ry short (40 s). As a consequence, the fertility of seabass sperm decreased exponentially after 10 s following sperm activation and was zero by 1 min. The sperm requirements for optimal fertilization were c. 66 000 spermatozo a per egg. Scaling-up of the experimental insemination procedure yielded be tter fertilization rates while conserving the individual differences due to the breeder pairs. (C) 1999 The Fisheries Society of the British Isles.