What could we learn from brown trout? We could learn to give up our dreams of continuously renewing our culture through immigration. Yep, blending populations works out badly for nearly everyone. Or at least for fish. We'll tell you how.
If you want to find out about fish cultivation, you should probably go to Erik Petersson and Torbjorn Jarvi. Researchers at Sweden's National Board of Fisheries, they've conducted many studies on fish behavior, and discovered everything from how being raised in a fish farm affects young fish to the fact that female brown trout fake "orgasm behavior" in order to collect sperm from multiple males and pick the best sperm with which to fertilize their eggs.
One study they've done is rather disturbingly dystopian. The Importance of Being Familiar studies how trout react to a stable, familiar population versus a dynamic, changing population. Turns out, familiar populations are pretty great. The researchers found that, "familiar fish in intact groups had more stable dominance ranks, higher food intake, and better utilization of food." Part of this was the fact that familiar fish were comfortable hanging out in close proximity to each other. Fish that knew each other weren't afraid to get close. Unfamiliar fish played it safe and stayed a certain distance away.
This isn't to say familiarity was good for everyone. For fish at the top of the hierarchy, a new group coming in presented a real problem. According to the research, "initiators of conflicts were more likely to win against familiar fish than against strangers." Essentially, aggressive fish in a familiar population knew exactly who they could take, whereas whenever aggressive fish in a new population started a fight, they faced the possibility that the new opponent could kick whatever, on a fish, counts as their "ass." So there is one societal situation in which large scale population change is good. When you've got one really mean bully at the top, pulling in a new group makes it more likely that some new person can overthrow the top guy and make the world better for everyone.
As a whole, however, "a picture emerges in which familiarity stabilizes the hierarchical structure of a group and governs behavioral modifications that will promote feeding and growth, in turn leading to higher fitness." Order is better, even if it comes at a high price.
It's worth noting that neither Jarvi nor Petersson ever remotely intended their study of fish behavior to apply to humans. This was strictly a look at how fish populations interact, and how a stable population of familiar individuals is the best way to grow big healthy fish. The fact that the study also promotes the ideology of every speech given by every dictator in a dystopian novel is just a coincidence.
As I imagine any reader can guess, there are important differences between blended fish populations and blended human ones. For one thing, brown trout don't turn on a small but familiar population because they look a little different, have different genitalia, or go to a different fish church. There are fish at the bottom of the hierarchy, but they aren't actively being crushed by the rest of the population. What's more, an immigrant population of fish isn't capable of bringing anything with them except unfamiliarity. They aren't able to produce new things like the telephone, the atomic bomb, or curry fries. Humans from new populations have more to offer an existing group of people. So while it might make sense to keep fish populations separate, humans don't work the same way.
[Source: The Importance of Being Familiar.]