Evaluation of biochemical methods for the non-destructive identification of sex in upstream migrating salmon and sea trout

Female-specific markers of reproductive activity (plasma 17β-estradiol, E2; vitellogenin, VTG; alkali-labile phosphoprotein phosphorous, ALP) were measured over 12 months in a captive population of brown trout. During the early months of the reproductive season (February – May) and using the concentration of plasma E2 or plasma ALP as a marker for females the proportion of fish in which sex was misidentified was high (15% - 50%). The misidentification rate was considerably lower (1 – 8%) using plasma VTG. Preliminary evaluation of a commercial immunochromatographic VTG 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 by ELISA. These preliminary conclusions were verified by sampling upstream-migrating sea trout and salmon trapped over a 6 month period. Plasma E2 levels did not satisfactorily discriminate between male and female sea trout and salmon. However, plasma VTG levels in both species were bimodally distributed and it was assumed that this divergence corresponded to male (plasma VTG levels <10 μg ml-1) and female (plasma VTG levels >800 μg ml-1) fish. Plasma ALP provided a more accurate indication of sex in the wild salmon and sea trout than was suggested by the pilot study on captive brown trout. The commercial immunochromatographic VTG test system provided results that were wholly consistent with the data obtained from the trapped fish by direct measurement of plasma VTG.