EVIDENCE OF IMPAIRED REPRODUCTION BY WESTERN MOSQUITOFISH INHABITING SELENIFEROUS AGRICULTURAL DRAINWATER

Western mosquitofish Gambusia affinis from the San Luis Drain (SLD), a major conveyance system for seleniferous agricultural subsurface (til e) drainage water in the San Joaquin Valley, California, experienced l ower reproductive success than did mosquitofish from a nearby referenc e area (Pond 26 at the Volta Wildlife Area, VOLTA) that had no history of receiving tile drainage. At birth, mean survival of SLD fry was 70 % in 1984 and 77% in 1985, whereas mean survival of VOLTA fry was grea ter than 99% in 1984 and 97% in 1985. Measurements of total length (TL ) of offspring born to SLD females in 1985 showed that stillborn fry ( mean TL, 7.5 mm) were significantly shorter than live fry (mean TL, 9. 4 mm). In addition, most stillborn fry were at earlier stages of devel opment (contained prominent yolk sacs or occasionally resembled eyed e mbryos) than most live fry (yolk sacs were fully resorbed). In compari son, stillborn and live fry born to VOLTA females did not differ signi ficantly in TL (mean far stillborn fry, 7.1 mm; mean for live fry, 7.5 mm), and nearly all fry had resorbed their yolk sacs. Mosquitofish fr om the SLD contained over 100 mu g/g (whole-body concentration in dry weight) of selenium, whereas those from VOLTA averaged 1.5 mu g/g. Alt hough experiments are still needed to establish cause and effect, the high incidence of abnormal births of mosquitofish from the SLD is seem ingly a response to selenium toxicity.