I doubt sharks & trout have the same burst strength. There are many fish out there that make trout seem like pikers when it comes to fighting ability & overall strength.

That being said I base my leader or line choice on several things but the size fish I anticipate is seldom the biggest factor since the reel drag or lack thereof and the rod can soften surges that far exceed the pound test rating of the line. When fishing for trout I let the fly size determine my tippet size.