Fisheries don’t collapse all at once. Instead, they telegraph their distress with signals that may be subtle at the start but grow more insistent as stocks decline.

Often, at least in the case of migratory stocks, the first sign of trouble might be a contraction of the fish’s range. On the East Coast of the United States, most migratory inshore species have centers of seasonal abundance. There is a core summer range, where the fish feed and are most abundant after their northward migration, and there is typically a core winter range, where they escape the worst of the cold.

Striped bass provides a good example.

LINK (via Marine Fish Conservation Network)