Latest data show decline in menhaden, despite spawning potential

 


For menhaden, it's same old story: The good news keeps getting better, and the bad news keeps getting worse.

Atlantic menhaden reproduction for the past several years has been poor. At the same time, the overall spawning potential has been unusually high.

Now, a recently completed analysis of the 1997 data shows a continuation of those contrary trends. "It's a concern, but it's not a panic situation," said Doug Vaughan, a National Marine Fisheries Service scientist who does a menhaden population
assessment each year.

The health of the population is important for the Bay because menhaden are one of the most important ecological and economic species.  They support the Bay's largest fishery, measured by weight, and are one of the most important food sources for predators such as striped bass, weakfish and bluefish. Concern has been raised that the huge number of striped bass in the Bay - which is near an all-time high - is running out of food, primarily menhaden.

Vaughan's latest assessment found that the spawning stock biomass - the estimated total weight of mature females, which is an indicator of the spawning potential for the stock - last year hit its highest level since 1962.

NMFS estimated the spawning stock biomass at 89,000 metric tons - far above the 17,000-metric-ton target which is considered a warning signal. It is similar to the spawning stock biomass of 1958, which produced the largest number of young - 15.1 billion - on record.

"In 1997, the stock - which is the adult portion - is healthy," Vaughan said. "But there are dark clouds looming."

The dark cloud is the continued failure of spawned larvae to "recruit" - or survive until age 1 - into the population.

For the second year in a row, the number of recruits failed to reach the target level of 2 billion. Vaughan estimated there were 1.4 billion 1-year-old menhaden last year. That tied 1996 for being the worst recruitment year since 1971.

Also in 1997, the proportion of adults caught in the fishery exceeded 25 percent, tripping another of the six indicators of the stock's health. Last year, 29.9 percent of the menhaden were age 3 or older, a figure that basically reflects the low number of young fish entering the population.

Combined low recruitment and increased spawning stock biomass means that although there are fewer fish in the population, the fish are getting bigger.

"There are less fish out there in each of the age classes, so the prey that is available to them is relatively more abundant, so they get fatter," said Joe Desfosse, fishery management plan coordinator with the Atlantic States Marine Fisheries Commission, a compact of East Coast states that manages migratory species. "There is more food for each one of them."

But without new fish entering the population to replace older fish that die or are caught, the spawning stock biomass ultimately has no place to go but down. Vaughan said that drop should start showing up in next year's figures.

It's unclear what is causing the continued recruitment failure, Vaughan said, although he said environmental conditions are most likely the problem. Because of the large spawning stock biomass, there should be a high recruitment potential if the right conditions exist.

Menhaden spawn all year on the continental shelf, peaking from fall to early spring. The larvae remain at sea for about a month before entering coastal bays and rivers where they remain until they are about a year old, at which point they begin migrating along the coast, often returning to bays to graze.

Vaughan presented his assessment in April to ASMFC's Atlantic Menhaden Advisory Committee, which recommended that no management action be taken this year.

Mainly, Vaughan said, that is because fishing pressure will decline substantially this year without action. The company with the largest menhaden fishing fleet is moving seven of its 20 boats from the Chesapeake to the Gulf of Mexico, an action that is expected to reduce menhaden fishing pressure in the Bay and along the Atlantic Coast by about 30 percent.

"What I would have recommended, if that hadn't occurred, would have been some reduction in [fishing] effort," Vaughan said. The committee's recommendation will be reviewed by an ASMFC management board, which will make a final decision in June.  Because of the concern that has been expressed about the menhaden stock, both in the Bay and elsewhere along the coast, it is possible that the board could amend the management plan that guides decisions about the menhaden catch, or call for an independent, outside review of the stock assessment.

Bill Goldsborough, senior scientist with the Chesapeake Bay Foundation, wrote to the ASMFC, urging that the management plan be revised so it considers whether there are enough menhaden to serve as food for other fish along the coast. 

His concern stems from anecdotal evidence that menhaden abundance in the Bay has been declining and that this could be linked to reports of striped bass that appear to be undernourished.

"While on a coastwide basis the menhaden stock may seem to be stable or even healthy according to the current plan, the Chesapeake situation suggests that localized and perhaps short-term depletion of menhaden does occur with adverse consequences for other fisheries," Goldsborough wrote.  He said the plan should consider whether enough menhaden are available to serve as "forage" for other species, and consider adjusting harvest levels accordingly.

Normally, about 40 percent of the coastwide population recruits in the Chesapeake Bay, but some evidence suggests that the Bay recruitment has been lower along the coast recently.

Besides being an important source of food for other fish, menhaden are thought to help improve water quality because they consume large amounts of algae. A single adult menhaden can filter nearly 4 gallons of water a minute.