We have been asked this question a number of times on FirstStep.me and we have summarised it into the following:
1. Make sure any Gap Year placement agency that you enlist with on this form of adventure is reputable and accredited accordingly.

2. Check out the visa requirements for working in Alaska through an official United States Government website, State of Alaska.
3. Please check out the following articles on careers and the industry of fish packing in Alaska from About.com and How Stuff Works.  
Good luck and have an awesome Gap Year adventure!