Male dab, cod, tuna and swordfish caught many miles from land contain elevated concentrations of female egg yolk protein (vitellogenin; VTG)
The size of the fish is very important in all studied marine species. The larger the male fish, the higher the level of VTG in its blood.
Age per se is not an important determinant of VTG blood of male dab or cod.
Since the size of an individual fish is primarily determined by the amount of food that it has eaten, a likely explanation for these results is that, as they grow, marine fish gradually accumulate stable man-made pollutants that are widespread in the marine environment (e.g. DDT and PCB). Some such persistent organic pollutants have already been shown to be weak oestrogens.
The chemicals that have been most implicated in fresh water species (female hormones discharged from sewage treatment works) are highly unlikely to be implicated in cod, dab, tuna or swordfish (due to the relative instability of these hormones and the fact that some sites where affected fish are caught are many miles from land).
Data from dab and swordfish are not consistent with the presence of egg yolk protein in large marine fish being a natural phenomenon.
In terms of the implications of these findings for the health of the fish or other animals that eat them, it is impossible to do anything more at the moment than speculate. To make any decisions (or to know if there are any legislative actions that could be taken), it is essential to know the identity of the causative compounds.
f the causative compounds are, as the data appear to show, cumulative compounds, then they or their metabolites are likely to still be present in (and thus extractable from) the flesh, liver or bile of large cod.
We recommend that the next key step in this research is to determine the nature of the causative compound(s). On the basis that they and/or their metabolites accumulate in the fish, we suggest collecting flesh, liver and bile from large and small male cod and then extracting, purifying and identifying those compounds that (either before or after deconjugation) are able to activate the oestrogen receptor (using in vitro YES or ER-CALUX assays).
We have already looked into the problem of collecting cod tissues for this research and have dismissed Research Vessel cruises. The Cefas Fish Discard team have indicated that it is feasible to send someone to sea on a commercial boat that is specifically targeting cod. The costs would also be very reasonable (in comparison to commissioning RV time).
The tissues and bile samples will be extracted and assayed using YES and/or ER-CALUX assays, both before and after being treated with glucuronidase and sulphatase enzymes in order to remove conjugating groups that might be masking any activity. These groups are added to many natural and unnatural compounds by the livers of animals in order to make them more soluble and facilitate their excretion.
If our hypothesis is correct, extracts from the large fish should be more oestrogenic and contain compounds that are not present, or at considerably lower concentrations in the smaller fish. These are the compounds that we will then target for purification and identification.