First of all I would say that it would be the members of the crew who would be doing this rather than the boat and it would be more usual to talk about hauling the nets up rather than the catch itself - ils étaient occupés à remonter les filets - is what I would suggest.
it's for a multiple choice question answer stating
"fishing boats hauling catch"
so we are going with one of the two following translations
"Les bateaux de pêche menant leurs activités"
"Les bateaux de pêche qui repère leurs captures."
Would say "leurs prise" but that is apparently more for one or two fishermen gathering a few trout, as opposed to a fishing boat hauling a whole net full of catch (as per Transport Canada's opinion on the matter).