We are familiar with seasonality for fruits, vegetables even milk. But Seasons for fish and crustaceans, it is not that obvious. Why can it be considered that a particular fish, shellfish or crustaceans has variations of qualities because of the natural phenomenon of the seasons.
Fishes live out a life ruled by a dual type cycle. During the first phase they grow and mature, accumulating energy reserves and reaching their peak of culinary quality. The second phase is a period in which they expend those reserves to migrate and create masses of eggs or sperm for the next generation. Unlike most land animal, which store their reserves in layers of fat tissues, fishes use the proteins of their muscle mass as their energy pack.
During migrations and spawning, they accumulate protein-digesting enzymes in their muscle and literally transform their own flesh into the next generation.
This makes their meat meager and spent, resulting in spongy,mushy dishes.
Crustaceans (crabs, shrimps, lobsters) quality is also, subject to fluctuation due to season cycles. One typical aspect of crustaceans is their shell. To grow they must periodically cast off the old cuticle and create a larger new one. It is called molting.
During this process the animal constructs a new, flexible cuticle under the old one from its body's protein and energy reserves. Then, it squeezes its shrunken body through weakened joints in the old shell, then pumps up itself with water (from 50-100% of its original weight) to stretch the new cuticle to its maximum volume. Finally, the shell hardens and gradually replaces its body water with muscle and other tissues.
Molting means that crustacean flesh quality is highly variable. An actively growing animal has dense, abundant muscle while an animal preparing to molt or newly molted have a poor muscle mass and is mainly filled with water.