A NEW METHOD FOR MAXIMIZING THE USE OF SPERM CELLS FROM GONADS - A MULTI-SPECIES APPROACH

During sexual maturation, the gonads (testes) of an Atlantic salmon male will increase in size as sperm production commences. At the start of the spawning season, the gonads can comprise 2-10% of the total body weight, and in one male contain enough sperm to fertilize

> 1 million eggs. This is an enormous reproductive potential that normally goes unrealized. In aquaculture, each salmon male broodstock may only be stripped a maximum of 3-4 times during a typical spawning season, in reality utilizing just 10-30% of the sperm available. This means a large surplus of sperm is "wasted" each season. In addition, reliance on stripping means a large surplus of males must be held "on hand" throughout the season to ensure sperm is available in case stripping fails to obtain sufficient sperm.

Due to the fecundity of most fish species, and families traditionally being considered the smallest breeding unit, maximum utilization of each male has not been given much attention by most egg producers. However, with the implementation of modern genotyping technology in breeding as well as in production of eggs tailored to each customer's needs, the value of each male has increased significantly. Some males may be considered more attractive due to their genetic properties, and there is more emphasis on utilizing the genetic contribution of these males to the full.

By extracting sperm directly from the gonads, the full reproductive potential of each male can be realized. Our trials have included development of a special extraction buffer (AquaBoost® SpermCoat) for gonadal sperm. The gonad is homogenized in this buffer and the sperm extract stored for 24 hours before fertilization. Used in this way, our tests have shown that the estimated potential of 1 million eggs per Atlantic salmon male may even be a conservative estimate. Data from other salmonids (Arctic charr and rainbow trout) show that there is 3- to 10-fold increase in sperm extraction efficiency by using gonad extraction. For marine species such as lumpsucker and wrasse that have a low sperm output after stripping, gonad extraction is clearly the superior method of sperm collection.

In combination with cryopreservation, gonad extraction of sperm at the beginning of the season can provide sperm to be used throughout the spawning season, providing flexibility for each egg producer. Trials this season will collect data on how early in the season gonads can be harvested and produce viable sperm with acceptable fertilization rates.