The first few lines (what shows up on a search or what shows up first on the video before the "SHOW MORE" link) of the description are most important, so I wouldn't put any links there. I'd put links further down.
Keep in mind that YouTube wants people staying on YouTube, so links that take people away from YouTube will end the "watch session", which will hurt your analytics. However, that's only if they click on the links (which you will probably find that most people don't click on links in the description -- generally, most people won't pay attention to your description unless you mention it in the video, so the description is really more for YouTube's purposes, since YouTube cannot "see" your video...they can only process the information you give in the title, tags, description, and any submitted subtitles). I think one thing you can do is post links to relevant content and then describe those links. The links themselves won't have much of an SEO impact, but by describing them, you can reinforce your keywords.
For example, if you had a video about one of your fish, you might say, "To learn more about [insert fish name here to reinforce that keyword], check out the wikipedia page on them. [insert fish here] is known for [insert relevant facts summarized from the wikipedia page.]"