What is a "higher level" mammal?
A reference to the phylogenetic tree. I admit I might be biased towards certain animals, for instance if someone ate my dog, I'd probably do very bad things to them. While cows could be considered higher as well, you have to take into account that they are a "prey" species, and they have been bred to be stupid and productive in some way, like milking or more muscle mass. They also have less emotional cognitive abilities than us.
Which is maybe why a lot of people see things the way they do, the more like us they are, the less likely it's okay to eat them?
I dunno. A lack of what we associate with emotional processing in other animals does not equate to a lack of emotions in the animal. I'm not really convinced either way at this point, and the research on the topic seems pretty poor.
Perhaps your right on the research, but nothing suggest that they do have emotions as we know it. If it isn't the same neural associations as other animals, then wouldn't that make it something entirely different than emotion?
I wouldn't have a problem eating a salmon, but I couldn't in good conscious eat a cephalopod like a octopi, or cuttlefish.
People have to kill animals all the time for over-population, public safety reasons, but that's usually in response to human effects on the environment like the removal of natural predators, or things like introduced species. Should we let these animals live, resulting in the destruction of vegetation, indigenous species, disease carrying, etc...?
If you are to worry about conserving species, I'd say leave the fish for their natural predators like the whale, dolphin, and shark, which are all on the decline.
We've invariably put our hand into every metaphorical cookie jar in this world, and the consequences are still coming in. If they was ever a species that was over-populated, it would be ours...