The goal of the project was to characterize the reproductive biology of Atlantic bluefin tuna (Thunnus thynnus) and to develop the biotechnology necessary to generate maturity schedules for bluefin without the need for the collection and processing of gonad tissue. Plasma, gonads, muscle tissue, otoliths, and length measurements were collected from wild bluefin on an annual basis in order to develop the means to nondestructively or noninvasively identify the sex and maturational status of individual fish. Sex-specific hormones and proteins present in muscle biopsy samples vary with sex and maturation in fish and therefore can serve as biochemical indicators for gender and maturational stage. The approach was to be validated by detailed histological examination of gonads to definitively identify the sex and state of maturation of individual animals. Levels of vitellogenin (VTG), the egg-yolk protein precursor, were measured as a marker for maturity, following development of an enzyme-linked immunosorbent assay (ELISA) for yellowfin tuna (Thunnus albacares) VTG. Once the assay was established and validated for detection of VTG in the blood plasma of yellowfin, it was adapted for use with muscle tissue.
Reproductive hormones were measured in bluefin by radiommunoassay (RIA). Levels of testosterone (T) and 111-ketotestosterone (11KT) were used as an index of male function; T and estradiol-17B (E2) levels were used as an index of female function. The RIAs were modified for use to detect the steroids in muscle biopsy samples. The reproductive cycle of bluefin is most likely similar to that of other teleosts. Peaks in VTG and reproductive steroids should be seen immediately prior to the spawning season in females, with peaks in T and 11KT occurring in males. These cycles are correlated with growth and maturation of the gonads. At this time, the biotechnology to complete this work is established. The techniques developed can be used to identify reproductively mature bluefin and perhaps other tunas as well. The only limitation encountered to date is that insufficient samples have been collected to complete final validation of the assays and construct maturity schedules. Extensive efforts to collect the necessary samples are now being coordinated through the National Marine Fisheries Service.