So not 100% airtight... still surely being sealed in like that, makes it so hard to breath they die due to lack of air? no?
Why does the UK have large amounts of salmonella infections compaired to other countries?
UK has purifying plants, but not enough for export
About breathing, fish and other seafood breathe oxygen from water, so how long they can resist outside of water depends just on how much water they can keep, external air, without waves and other turbulent water motion will be dissolved in water in minimal amount. Keeping cold blooded live seafood at the lowest temperature they can stand is probably the best that can be done to keep them alive longer.
About salmonella, dirty breeding farms, dirty processing and not removing quickly ill and dead animals are the most probable reasons.
When my mom studied for some time in England for her degree in foreign languages, she and her roommate, after days of eating milk, eggs, yoghurt and bananas to save money, decided to afford the "luxury" of a roast chicken, but they were so disgusted they had to give it to stray cats: it still had the final part of intestine, with shit inside.
EU is sometimes excessive on the opposite side, but just when it hampers the production of traditional and typical foods, that can be perfectly clean and safe, but with a wider range of bacteria, harmless if they don't go out of control, than those allowed by EU laws (France had to ask for exceptions for some typical cheeses and sausages, and they saved Italy too, as when this happened we had ministers so clueless on these matters that they said there was nothing they could do to save some products, like "lardo di Colonnata"), but it's perfectly right when it does its best to prevent thugs from preparing food in a dirty and unsafe manner just to save a few cents.