Handling a Slippery Baitfish
A novel approach to determining the future of menhaden
by Rona Kobell
If you want to know how many fish of a particular species you can safely take out of the sea, it’s useful to know how many of those fish are in the sea. How many die annually of natural causes? How many migrate? Where and when do they go?
Those were questions facing Emily Liljestrand when she was a Maryland Sea Grant fellow and a master’s student at the Chesapeake Biological Laboratory at the University of Maryland Center for Environmental Science (UMCES) a couple of years ago. Now working on her doctorate at Michigan State University, Liljestrand has answered some of those questions with regard to one key species: Atlantic menhaden (Brevoortia tyrannus). In collaboration with Michael Wilberg, her UMCES advisor, and Amy M. Schueller, research biologist at the NOAA Fisheries Service, Liljestrand has published two papers on mendaden population models and mortality in Fisheries Research.
Atlantic menhaden, an oily baitfish not consumed directly by humans but a food staple for striped bass, are processed into dietary supplements, fertilizer, and animal feed. Watermen can make a living catching menhaden, which come from the Chesapeake Bay and the Atlantic Ocean, but the bulk are harvested by Omega Protein Company in Reedville, Virginia.
In the late 1960s, researchers from the NOAA Beaufort Laboratory in North Carolina injected menhaden with individualized magnetic tags — usually placed under the dorsal fin — that recorded the current length and age of the fish. These tagged menhaden were released along the Atlantic coast. Processing boats later caught the fish and took them back to the plants, where workers extracted the tags along with the recorded information. The fish were subsequently ground and converted into various products. Over the decades, some of the information recorded by the tags was lost, and it was mostly forgotten — that is, until Liljestrand and a team rediscovered the information and digitized it at the Chesapeake Biological Laboratory. Equipped with new statistical techniques and computer models, they wanted to see if they could make any determinations about the menhaden population with the recovered data.
The range of Atlantic menhaden extends from Nova Scotia to North Carolina. Scientists had assumed that in the spring, most of them moved north — and in the winter, most headed south toward the Chesapeake Bay. Liljestrand’s modeling, however, showed that only about half of the population moved south. Many overwintered in the northern part of the range, north of the Chesapeake Bay. The data analysis also indicated that natural mortality was 2.3 times greater than previous estimates. That conclusion challenged assumptions that much of the mortality was the result of fishing pressure; instead, it was influenced by temperature and predation and factors not connected to fishing.
During the 1960s, 18 companies took massive amounts of menhaden. Today, only one company, Omega, is still operating — but natural mortality rates basically have stayed the same. Knowing where menhaden migrate and at what rate they die can help inform management decisions. The Atlantic States Marine Fisheries Commission regulates the stock, except in Virginia; the only state that still has a reduction fishery, it regulates menhaden through the state legislature.
Recreational anglers long have contended that Omega takes too many menhaden and is harming the striped bass population, while scientists have said that menhaden are not overfished. Liljestrand said her team’s findings could “open the possibility to fishing at certain areas and at certain times” that, historically, were believed to be unproductive. North Carolina’s fall fishery, according to the study, is one such area that could support higher catches.