Striped bass heavily impacted by the collapse of bay anchovy and menhaden

Striped bass heavily impacted by the collapse of bay anchovy and menhaden
By Mid Atlantic Fisherman Staff

A study newly released by the Chesapeake Bay Ecological Foundation, Inc. shows an ecological depletion of Atlantic menhaden and bay anchovy is having a detrimental impact on Atlantic Coast striped bass. The CBEF, with assistance from East Carolina University, has been examining striped bass since 2004 through the Predator/Prey Monitoring Program (PPMP) to investigate biological characteristics, characterize diet composition and determine the age structure and size spectrum of Atlantic menhaden consumed by striped bass. Funding has been provided by the U.S Fish & Wildlife Service, Maryland Department of Natural Resources, ECU and CBEF.  Striped bass were obtained from Maryland, Virginia, and North Carolina sport fisheries and Maryland commercial fisheries.

According to the study, while a five-year moratorium and ensuing larger size limits and harvest caps have protected unprecedented numbers of striped bass since 1985, the enlarged striped bass population dramatically increased predation on menhaden and bay anchovy in Chesapeake Bay and Atlantic coastal waters.  Within the upper Chesapeake, the minimum size was raised to 18 inches, and a harvest cap was imposed for the first time.  In Atlantic waters, the minimum size was set at 28 inches.

According to the summary of the study, the rapid expansion in numbers of striped bass larger than 18 inches, which feed predominately on menhaden, has sustained the prey demand for menhaden at record high levels for more than a decade. “Chesapeake Bay resident striped bass are now nutritionally stressed by ecological depletion of their forage base ˆ particularly bay anchovy and age-0 menhaden,” the study says. “For example, studies show that year to year weight-at-length of Choptank River striped bass approximately 14 inches to 18 inches varies with annual recruitment of age-0 menhaden. “During years of low recruitment, the average weight of striped bass 14 inches to 18 inches caught during the fall can be less than 70 percent of their historical weight ˆ a level symptomatic of starvation.”

According to the study, PPMP research detected that large numbers of striped bass larger than 28 inches (80 percent females) that historically migrated from summer habitat in New England waters to winter feeding grounds off Virginia and North Carolina, have migrated into the upper Chesapeake each fall since 2006 and remained through the spring spawning season – a previously undocumented event. These striped bass account for a significant portion of upper Bay late fall and winter sport catches and commercial landings.
This unprecedented shift in established feeding patterns indicates that menhaden are more available as prey in the Chesapeake Bay than on their historical winter feeding grounds off the Atlantic Coast.

The Cooperative Winter Tagging Cruise found during the winters from 2006 to 2007 that captures of migratory striped bass in waters off northeastern North Carolina and southeastern Virginia (historical striped bass winter feeding grounds) averaged 531 per cruise compared to an average of 2,212 striped bass per cruise during the previous 19 winters. “Deviations from established feeding patterns since the winter of 2006 indicate that menhaden, essential prey for migratory striped bass larger than 28 inches, are ecologically depleted on their historical inshore winter feeding grounds within mid-Atlantic coastal waters,” the study says. “The depleted coastal stock of ages 3+ adult menhaden currently fails to provide sufficient prey for migratory striped bass larger than 28 inches.” Consequently, many migratory striped bass larger than 28 inches now enter the Chesapeake Bay during late fall and prey on the over-wintering population of predominately sub-adult menhaden.

This intensified competition for food could exacerbate growth and health problems affecting resident striped bass. Migratory striped bass supplement their diet with high value recreational / commercial species that include: blue crab, white perch, shad, herring, spot, Atlantic croaker, weakfish, flounder and American eel.  At the mouth of the Chesapeake Bay during late fall and early winter, large migratory striped bass prey heavily on emigrating adult eels, which are en route to Atlantic Ocean spawning grounds. When large migratory stripers arrive in Maryland, Virginia and North Carolina waters during late fall and winter, they have low levels of body fat.  As these fish begin feeding, primarily on menhaden in both Chesapeake Bay and mid-Atlantic coastal waters, internal fat deposits initially increase and then decrease as sperm and egg production accelerates prior to spawning.  After spawning, migratory striped bass larger than 28 inches (predominately females) resume feeding primarily on age 2+ menhaden, while migrating out of the Chesapeake Bay en route to northern coastal waters.  After spawning in April or May, most adult resident striped bass (predominately males) return to summer habitat in the main stem of the upper Bay.  Chesapeake striped  bass smaller than 24 inches feed primarily on benthic organisms, while those larger than 24 inches still prey on menhaden ˆ predominately ages 2+.

Migratory striped bass over-wintering in mid-Atlantic coastal waters prey heavily on the depleted bay anchovy spawning stock (ages 1). Research conducted by PPMP and the Southeast Area Monitoring and Assessment Program shows consumption of bay anchovy (by weight) by over-wintering migratory striped bass in Virginia and North Carolina coastal waters has dramatically declined since 2000.  This decline parallels poor recruitment of bay anchovy documented in Chesapeake Bay surveys, and the substantial increase in striped bass numbers following the change in minimum legal size from 14 inches to 18 inches.

“Ecological depletion of menhaden and bay anchovy in mid-Atlantic waters has disrupted the coastal biotic community and lowered the carrying capacity for striped bass greater than 28 inches,” the study says. The collapse of these primary forage species has undermined the ability of the Chesapeake Bay ecosystem to support a nutritionally healthy striped bass population.” The study says harvest restrictions on menhaden larger than 10 inches would make available additional ingestible-size prey for resident striped bass during late spring through early fall ˆ a period when upper Bay striped bass larger than 24 inches are nutritionally stressed.

Based on National Marine Fisheries Service data for 1999 to 2008, an Atlantic menhaden minimum size of 10 inches for the purse seine reduction fishery would have annually reduced the harvest by an average of approximately 175 million juvenile menhaden ages 0&1, which would represent 12 percent of the harvest by weight. Purse seine reduction landings during 2009 contained an estimated 356 million juvenile menhaden ages 0&1 — approximately 50 percent of the total number of menhaden harvested.  An increasing percentage of juveniles in annual landings could be symptomatic of a collapsing menhaden population.

The CBEF study also says the closure of the Exclusive Economic Zone (EEZ) to menhaden harvest would reduce spawning stock mortality and increase prey availability of adult menhaden for migratory striped bass larger than 28 inches in coastal ocean waters.

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