The fish die from having stuck in place in the nets. They need to move to breath through the gills.
It's true in the warm summer waters some fish may die very surprisingly quick. Even in a matter of hours.
And then the spoilage starts fast too,
The cold waters may improve their lifespan to some extent, but I don't think it's that drastic.
From the local fishermen I've heard they check their wintertime nets twice a week, and still sometimes they find some fish dead. However, in the cold winter waters the spoilage doesn't go as fast as in the warm summer waters. And that could be tuned, with a day or two, but not with a week or so.
Something to consider, although we have a quite good all-year round average in use currently.
(For the realism sake the warm summer waters spoilage should be made faster too, but no-one is really suggesting that
)