Large rockfish leave Chesapeake Bay to become ocean migrators; smaller fish remain

"Knowing the size at which they leave, we can do improved management that is tailored better to commercial and recreational fishing sectors those related to catch and size limits," said study author and Professor Dave Secor of the University of Maryland Center for Environmental Science. "It allows us to bring different parts of the fishery into an assessment model to evaluate stock health and test how effective regulations will be."

Chesapeake Bay striped bass, also known as rockfish, (Morone saxatilis) were implanted with two-inch acoustic transmitters and their coastal shelf migrations recorded over a four-year period by telemetry receivers throughout the Mid-Atlantic shelf waters and southern New England. Researchers found that only large striped bass from the Chesapeake Bay migrate to ocean waters when they reach 32 inches in length, and smaller fish remain resident to the Chesapeake Bay, regardless of sex.

"By our best estimates they are in the Chesapeake Bay for 9 years, and when they reach 32 inches they head north," said Secor.

All migrating striped bass spend summer and fall months in Massachusetts waters, where they contribute to important recreational fisheries. They undertake late fall migrations to southern waters, before returning to the Chesapeake Bay to spawn the next spring. They leave a few weeks after spawning to return to the ocean.

The small resident striped bass that remain in the Chesapeake Bay experience mortality rates of 70% per year, nearly twice as high as those that migrate to ocean waters. Mortality rates similar to these have been derived from recent conventional tagging studies in the Chesapeake Bay.

Striped bass with acoustic transmitters can be tracked as they move into and out of the Chesapeake Bay and seasonally migrate in coastal waters. Small, thumb-sized transmitters are surgically implanted in the fish. The transmitters then ping the individual fish's location to acoustic receivers set up at river and Chesapeake Bay mouth gateways and along the coast.

"Biotelemetry has allowed us to move beyond the question of whether Potomac River striped bass leave the Chesapeake Bay, to where do they go when they leave? All arrows point to Massachusetts," said Secor. "There is a remarkable connection between fisheries in Potomac and Massachusetts Bay and Cape Cod."

Ocean emigration by striped bass has challenged both anglers and fisheries managers. Quantifying the size at which fish leave the Chesapeake directly supports improved assessment models and tailored allocations between recreational and commercial fisheries.

"An improved understanding of differential migration allows fisheries managers to specify stock assessments according to different population sub-components, and tailor reference points and control rules between regions and fishing stakeholder groups," said Secor.

The study was supported by Atlantic States Marine Fisheries Program and NOAA Chesapeake Bay Office.

The paper "Differential migration in Chesapeake Bay striped bass" was published in PLOS ONE.