A male trout testicle containing a developing egg - an example of "intersex" conditions that can result from pollution.
Image credit: Oregon State University
The fish were sampled by trawling at depths between 700 m and 1400 m (approximately 2,300 ft to 0.9 miles deep) in the Bay of Biscay, west of France.
The study also found the first known case of a deep-water fish species with an "intersex" condition - a blend of male and female sex organs.
In the journal Marine Environmental Research, the researchers note that the samples came from an area of the Bay of Biscay with no apparent source of pollution, suggesting the findings could be indicative of general ocean conditions.
The study follows earlier research in national parks in the American West that found significant pollution and health effects in fish - including cases where male fish had been "feminized" to the point of producing eggs.
Changes in deep-sea fish suggest 'exposure to toxins and carcinogens'
Michael Kent, an international expert on fish diseases and a professor of microbiology in the College of Science at Oregon State University (OSU), worked on both the earlier study and the new one. He says:
"In areas ranging from pristine, high mountain lakes of the United States to ocean waters off the coasts of France and Spain, we've now found evidence of possible human-caused pollution that's bad enough to have pathological impacts on fish."
Prof. Kent says one might assume that levels of pollution and its biological effect would be less in deep ocean, but that is not what they seem to have found:
"The pathological changes we're seeing are clearly the type associated with exposure to toxins and carcinogens."
However, he and his colleagues warn that their findings should be regarded as preliminary; they do not prove that pollution is the cause of the changes in the fish.
Only follow-up chemical analyses can confirm whether man-made pollution is causing the biological changes in the fish. There could also be natural causes, they note.
The researchers found evidence of biological changes in black scabbardfish, orange roughy, greater forkbeard and other less well-known species.
Deep-water fish live longer and may accumulate more toxicants
It is rare for studies like this to look at the health of fish living on continental slopes. Usually, researchers are more interested in parasites in the fish, not whether the fishes themselves have liver damage, for example.
But interest in the health of fish in these deeper waters is growing, particularly as fisheries on the shallower continental shelf run out of fish.
The continental shelf slopes from the shallower coastal region toward the deeper ocean. The slope acts as a sink for heavy metals contaminants like mercury, lead and cadmium, as well as organic substances such as pesticides and PCBs.
Scientists who have found intersex fish in other regions suggest exposure to endocrine-disrupting chemicals that can mimic estrogens have caused their sex organs to mutate.
The fish that live in the deep-water, sloping regions usually live near the sea floor, grow slowly and mature later than other fish. Some can live to 100 years.
Partly because they live so long, the deep-water fish can accumulate a lot of toxicants in their bodies - some 10-17 times higher than fish in the shallow waters of the continental shelf. The researchers note this "may be a significant human health issue if those species are destined for human consumption."
On the other hand, many toxicants tend to gravitate to the fishes' livers and sex organs. So, perhaps - since it is generally the muscle tissue that people eat - the amount of toxic substances that people actually consume may be "generally not high enough for human health concern."
Man-made pollution may not be the only environmental change that could be causing fish to accumulate harmful substances. A study Medical News Today reported in October 2013 suggested that global warming may boost fishes' metabolism and accumulation of toxic metal.
Writing about their findings in the journal PLOS ONE, a team from Dartmouth College in Hanover, NH, says the rising ocean surface temperatures resulting from climate change may cause fish to accumulate more mercury, increasing health risks to seafood consumers.