Publication Abstract

Non-destructive biochemical sex determination in adult salmonids.

Non-destructive biochemical sex determination in adult salmonids.

T.G. Pottinger, T.R. Carrick, K.T.G. Pulman and A.P. Scott

Efficient management of salmon and sea trout stocks, which focuses on optimising recruitment, operates through the setting of targets for critical phases of the recruitment process. A key target is that set for spawning. This requires that the spawning biomass can be estimated each year with confidence. A major impediment to accurate estimates of spawning biomass is lack of river-specific information regarding the sex ratios of returning adult fish.

Two approaches are possible to the problem of identifying the sex of an adult salmon or sea trout in which no definitive secondary sexual characteristics are evident. The first is to utilize a probe for a sex specific marker at the level of the genotype. The second approach is to exploit sex-specific phenotypic alterations associated with reproductive development. Sex-specific genetic markers for Atlantic salmon and sea trout are currently unavailable and therefore the second option was evaluated.

A preliminary 12 month study of a captive population of brown trout was carried out during which plasma 17ß-estradiol (E2), vitellogenin (VTG), and alkali- labile protein bound phosphorous (ALP; a surrogate measure of VTG) were measured at intervals in male and female fish. The results of this study revealed that the sex of individual fish was identified with greatest confidence by plasma VTG levels. The risk of misidentifying the sex of the fish was unacceptably high (15% - 50%) during the early months of the reproductive season (February – May) using plasma E2 or plasma ALP but acceptably low using VTG (1 – 8%). A preliminary evaluation of the "Best Checker" immunochromatographic vitellogenin test system as a screen for the presence or absence of VTG in plasma from brown trout provided results that were consistent with those obtained from direct measurement of plasma VTG levels. It was not found possible to detect VTG in the epidermal mucus of mature female brown trout by either direct assay or by the assay of ALP. Collection of blood samples for analysis is therefore unavoidable.

A 6 month study of upstream-migrating sea trout and salmon on the River Lune confirmed and expanded upon the findings of the preliminary experiments. The results of this field study confirmed VTG as the most reliable marker of sex but, because differences in the levels of indicators in the blood of male and female salmon and sea trout were more pronounced than was evident in the preliminary study on captive brown trout, also suggested that plasma ALP may be a more useful indicator of sex than was initially concluded.

It is therefore recommended that one of the biochemical methods identified as providing a high confidence in the determining of sex be adopted, until such time as a genetic marker of sex is available for Salmo trutta and S. salar.

The combination of moderate cost and added informational content make the direct analysis of VTG the most effective option. The "Best Checker" method was reliable but is expensive in comparison with the two alternatives. It is not recommended that this method be adopted, unless considerable savings can be negotiated by bulk purchase. Analysis of ALP levels did not provide particularly effective sex discrimination during the preliminary study. However, when applied to samples collected from feral fish this method performed quite well. Because of the comparatively low cost of this method in comparison to both the VTG and "Best Checker" methods it is recommended that its usefulness be further evaluated, in tandem with the implementation of the VTG assay, or possibly the "Best Checker" test system.

Reference:

T.G. Pottinger, T.R. Carrick, K.T.G. Pulman and A.P. Scott. 2003. Non-destructive biochemical sex determination in adult salmonids. Non-destructive Environment Agency Research and Development Technical Report W2-061/TR, 54 pp. Environment Agency, Bristol, UK.

T.G. Pottinger, T.R. Carrick, K.G.T. Pulman and A.P. Scott

Non-destructive Environment Agency Research and Development Technical Report W2-061/TR, 54 pp. Environment Agency, Bristol, UK.