"Wisen" is antiquated. It's a remnant of Middle English (by way of Proto-Germanic) that is sometimes still heard in some dialects of the southern US and the Appalachian mountain region. The phrasal verb "wise up" is also used in lots of other places, but in most dialects the past form would be "wised up."

They "wisened up" means they got smarter. In this context, it means the fish have learned how to avoid getting speared.

In colloquial English people often manufacture words based on suffixes and patterns from other words. One common pattern is [adjective] + -en = to become more [adjective]. "To redden" means "to become redder." "To loosen" means "to become looser." "To weaken" means "to become weaker." "To ripen" means "to become riper," and so on. Based on this pattern, if someone who isn't bookish wanted to say "to become wiser," they might manufacture the word "wisen" by following that pattern. And they will be understood. Whether it's a "real word" or not, when a native English speaker here's "wisen," they will understand it to mean "to become wiser." I'll trust Jonathan Kimball on the word history. But variations like this are common. "To wise up" is an informal phrasal verb meaning "to stop being fooled by something." At first, the fish were caught by the hunter. Then they learned not to be. You could say "they wised up." It appears that this hunter, for whatever reason, uses a nonstandard form, "they wisened up."

To fasten something is to make it secure, or to attach it. A runner improves his speed with practice, but doesn't "fasten", in context.