It's true that mercury contamination has been an increasingly large problem with fish these days, but what a lot of people don't realize is that generally, it's a problem with things higher up on the food chain. Tuna is a much larger fish, so canned tuna will have a much higher concentration of mercury than sardines, which are very small. However, I think the biggest problem is with things like swordfish and shark.
I then started thinking about food chains in general and I realized that, in general, stuff that's lower down on the food chain is healthier for humans to eat than stuff higher up, and smaller animals are healthier than large ones. Yet, I have never heard anyone discuss these sorts of issues. Are there exceptions? It certainly seems true for seafood but I wonder if it's true for land animals as well. It's interesting that almost all the major meat sources that humans eat are herbivores--that's probably because it's easier to cultivate them. But I think it goes deeper than this: many of the major problems arising from factory farming of meat can be traced back somewhere to feeding animals some sort of animal feed--bone meal fed to cattle causing the whole mad cow/BSE thing, fish meal being fed to poultry, etc.
I was wondering if anyone has explored these issues and has come up with any sort of "general themes" that relate to these...general principles, scientific studies, etc. It's obvious that heavy metals and certain organic pollutants (think DDT) become concentrated higher up on the food chain...but are there perhaps any other reasons it is better to eat things lower on the food chain? For example--think of the risks of consuming raw meat, vs. the risks of consuming raw veggies. Could higher-up animals perhaps be exposed to more parasites and other natural biological threats? Is there a reason that sharks have such effective immune systems--perhaps they need it? Are these patterns perhaps true for humans, but not necessarily for other omnivorous species? I don't know...just this little dialogue on sardines sparked all these interesting questions. I'm curious if anyone has any answers.