Things that eat fish, taste of fish. For example muttonbird in New Zealand is a seabird that tastes as strong as anchovy. The chickens are possibly being fed on fish processing waste, or ocean-caught “inedible” species like krill https://www.google.com/search?q=alibaba+krill+chicken+feed
It's also likely that if it's from a big processor (e.g. Tyson) that it has been washed in bleach to kill bacteria. This may make for safer food but it certainly leaves an aftertaste.