Feminized cod on the high seas.

Male cod in the open ocean are producing vitellogenin, an egg-yolk protein ordinarily made only by females.

Vitellogenin "is a highly specific indicator of a fish's exposure to estrogens"--female sex hormones--as well as to pollutants that mimic them, notes Alexander P. Scott of the Centre for Environment, Fisheries and Aquaculture Science in Weymouth, England. Research has shown that the livers of male fish can make the quintessentially female proteins, but that typically occurs in males swimming near treated-sewage outflows (SN: 6/17/00, p. 388).

Scott's team wanted to know whether males that spend their entire lives at sea, far from sewage and other likely inputs of estrogen-mimicking chemicals, also show this feminization. They examined Atlantic cod from various regions around Britain and as far west as Iceland. Affected males showed up at all sites--sporting up to 160 parts per billion (ppb) vitellogenin in their blood. However, only males above a critical size, about 5 kilograms in weight and 80 centimeters in length, produced the protein.

Most such fish are 2 to 5 years old. By that age, they've switched to an adult menu offish and eels that live in or near bottom sediments, where many pollutants end up, says Scott. Prey may be passing estrogen-mimicking pollutants up the food chain, he suggests. Indeed, the researchers have preliminary data showing that among dabs (Limanda limanda), a bottom-dwelling flatfish on which cod dine, males inappropriately produce vitellogenin, although far less than male cod do.

Although the vitellogenin concentrations recorded in the cod signal a potential for reproductive harm, Scott says, the good news is that before male cod begin making this protein, "they should have had a chance to spawn at least once"--J.R.