Sardines are "kovush" with non-kosher species?
It seems the issue was found by sardines, most recently from Morocco by the Aidah.
They usually do 3 inspections of removing the non sardine species allong the way till they rid the whole batch of sardines from the non kosher species.
In the meantime, till that's done it's "kuvush" for more than 24 hours with non kosher species. There is no shishim. We aren't aware of any that don't have this issue at this time.
When fish are removed from their enviroment and put into a chilled brine solution they are dead, hence kovush is a factor.
Rav Landau of Bnai Brak wouldn't give a Hashgocha on tuna fish because he said that there are a quantity of non kosher e.g. dolphins in the catch and there is an issue of "kuvush" in the brine (salt) water in the hold of the boat.
Others have not found it to be the case by their productions of Tuna e.g. the Aidah, Rav Rubin, etc.