There are several types of fish that are in season in the UK during May, including:
Sea Bass: This popular fish is in season from May to October, and can be found in both the sea and estuaries.
Mackerel: This oily fish is at its best from May to September and is often found in large shoals off the coast of the UK.
Dover Sole: This flatfish is in season all year round, but is at its best in the summer months from May to August.
Salmon: This fish is in season from April to September, and is often found in rivers and estuaries.
Sea Trout: This fish is in season from April to September and is often caught in estuaries or rivers.
Brown Crab: This popular shellfish is in season from April to November and is often found in coastal waters.
It's worth noting that the specific fishing seasons may vary depending on the region and weather conditions, so it's always a good idea to check with local fishermen or fishmongers to find out what is currently in season and available in your area.