I'm not a teacher and not thoroughly familiar with the fishing industry, but if the application is using these words as they would be used for many other food source industries, the following may help:
1. a catering point is often the end retailer - a person/place who actually sells to the consumer
2. a labeler is usually a company who places labels on a packaged product; a relabeler may take an initially packaged and labeled product and put a brand label on it before end retailing.
3. a packer is the person/industry who takes the raw product (fish) and processes it and packages it for a retailer; a repacker may add or change some ingredients and repackage the product.
4. a salvage operator may refer to any salvage or cleanup process along the production line - from salvage operations on the fishing ships, to salvage of whatever of the catch is not taken to be processed, or salvage/cleanup anywhere on up the production line.
Note that the production process goes from #3 to #2 to #1 - with #4 anywhere along the line...
This said, the FDA itself may have very specific meanings for these words - which you may have to contact them to find out how exactly to respond on the application. Good luck.