CBEF 6 year Striped Bass & Menhaden Study


February 24, 2011 


An ongoing study by the Chesapeake Bay Ecological Foundation (CBEF) determined that low numbers of Atlantic menhaden have affected the growth, health and migration of Atlantic coast striped bass. Since 2004, the CBEF, with assistance from East Carolina University, has examined over 8,000 striped bass from the Chesapeake Bay and Atlantic Ocean along the Virginia and North Carolina coast. 

Diminishing striped bass numbers culminated in threatened species status in Maryland’s section of the Chesapeake Bay (upper Bay) in 1984 and a fishing moratorium in 1985. In 1990 the Atlantic States Marine Fisheries Commission (ASMFC), which is responsible for the management of menhaden and striped bass, partially reopened the fishery in state waters and in 1995 declared striped bass fully recovered. Within the upper Bay a harvest cap was imposed for the first time and the 14” minimum size was raised to 18” (4-5 years of age). This size limit protected more than 90% of the immature female striped bass which historically emigrated to coastal waters and became ocean residents before reaching 18”; only re-entering the Chesapeake Bay on spring spawning migrations after reaching maturity, at age 6 or older. Within ocean waters the minimum size was set at 28” to allow most females to spawn at least once before reaching legal harvest size. These actions resulted in a greatly expanded striped bass population, dramatically increasing predation on menhaden and bay anchovy. This high population of large striped bass, predominately females, has sustained the prey demand for all age classes of menhaden and adult bay anchovy at high levels since the mid 1990s. 

During the early 1990s, coincidental with burgeoning striped bass predation on menhaden, adult menhaden were severely overfished off New England concurrent with intensive fishing by the purse seine reduction fishery (large scale harvest of fish for processing into products such as fish oil and meal) on sub-adults (ages 1&2) and adult menhaden (ages 3+) in the Virginia section of the Chesapeake Bay (lower Bay) and in ocean areas from New Jersey to North Carolina. The Omega Protein Corporation currently owns and operates the only remaining menhaden reduction fishery. This fishery, the largest on the Atlantic coast, competes with striped bass, fish eating birds and many marine predators for declining numbers of age 1+ menhaden. The excessive harvest of adult menhaden from New England waters coincided with chronically low reproduction of menhaden and the onset of health problems in Atlantic coast striped bass - the Bay’s top predator. The age structure of menhaden has been unnaturally skewed toward younger fish and only a remnant population of fish older than age 4 exists even though menhaden can live for more than 10 years. 

Responding to concern about depletion of menhaden in the Chesapeake Bay, the ASMFC established (2006) an annual “Bay harvest cap” of 109,020 metric tons on menhaden reduction fishery landings from Virginia Bay waters. This measure has been ineffective in reducing harvest since reduction fishery landings in the lower Bay since 2006 have averaged approximately 30% below the harvest cap. 

Data collected by the CBEF study indicates that malnutrition in upper Bay striped bass 16” to 24” is a consequence of ecological depletion (insufficient numbers to provide adequate prey for dependent predators) of ages 0&1 menhaden (less than 10”) and bay anchovy, exacerbated by low numbers of other forage species. Studies of resident striped bass greater than 16” in Chesapeake Bay waters (year-round) and migratory striped bass greater than 28” in mid-Atlantic and Chesapeake Bay waters (late fall through spring) determined that in most years since 2005, menhaden constituted over 75% of their diet (by weight). Striped bass are the primary predator on menhaden from late fall through early spring. 

Chesapeake Bay tidal waters provide the principal spawning and nursery areas for striped bass. Historically, the Chesapeake Bay provided an ideal ecosystem for reproduction, survival and growth for high numbers of healthy striped bass. This natural productivity has deteriorated due to severe declines in populations of forage fish – primarily Atlantic menhaden, bay anchovy, river herring and spot. 

Most striped bass greater than 12” aggregate in the main stem of Maryland’s mid-Bay region from late spring through early fall. As a result of menhaden age distribution in the upper Bay during this time period, adequate numbers of menhaden over 10” (ages 2+) are available as prey for striped bass greater than 24”: Most age 0 menhaden (less than 6”) inhabit lower salinity regions of the Bay and tributaries; while most age 1 menhaden (less than 10”) have migrated out of the upper Bay by late spring and are unavailable to 16” to 24” striped bass inhabiting the Bay’s main stem. Consequently, striped bass over 24”, which can ingest menhaden over 10”, have significantly more body fat than striped bass 16” to 24” which prey on smaller, less available menhaden. During 2009 and 2010, approximately 500 million ages 0&1menhaden (less than 10”), approximately 43% of the total numbers landed, were harvested in the lower Bay and nearby coastal waters by the menhaden purse seine reduction fishery. These sub-adult menhaden are crucial to the diet of the Bay’s malnourished 16” to 24” striped bass, and should be protected according to the ecological objectives in ASMFC’s Interstate Fishery Management Plan for Atlantic Menhaden. 

Body fat accumulated by adult striped bass is used for gonadal development as post-spawning fat indices are near zero for both males and females. Sub-adult menhaden (less than 10”) and bay anchovy are also crucial for rebuilding body fat of post-spawned resident male striped bass (16” to 24”) during spring through early summer. This body fat helps sustain the health of these striped bass during late summer through early fall, when menhaden consumption is minimal and bottom dwelling prey dominates their diet. Large numbers of striped bass in this size range suffer from malnutrition during late summer through early fall. 

In the upper Bay, during years of low abundance of age 0 menhaden and other forage species, the average weight of striped bass caught during the fall can be less than 70% of their historical weight – a level symptomatic of starvation. This study determined that the weight of striped bass 14” to 18” caught in the Choptank River correlates with high and low year-classes of forage fish – primarily menhaden. Within the Chesapeake Bay, striped bass length-at-age and weight-at-length have decreased, a significant percentage of striped bass have mycobacterial infections and striped bass natural mortality rates have risen. 

This study revealed that large numbers of striped bass greater than 28”, predominately females, that historically migrated from summer habitat in New England waters during the fall to feeding grounds in coastal ocean waters off Virginia and North Carolina, now arrive in the upper Bay during late fall and remain through the spring spawning season - a previously undocumented event. This study also documented a significant increase in the population of immature female striped bass in the upper Bay during October through December of 2010. These females represented 25% of striped bass in the 18” to 24” range; two times higher than the 12% average in 2008 & 2009 and four times higher than the 6% average in 2006 & 2007. Immature females in this size range normally inhabit ocean waters and are protected by the 28” minimum size limit. However, within the Chesapeake Bay, immature female striped bass greater than 18” can be harvested by recreational and commercial fisheries. 

Diet analyses, body fat indices and the unprecedented shift in established feeding patterns by migratory striped bass indicate that menhaden and bay anchovy are ecologically depleted on their coastal feeding grounds. Consequently, migratory striped bass now inhabiting the upper Bay from fall through spring are competing with resident striped bass for menhaden of all sizes. 

CBEF studies of striped bass food habits determined that increasing the supply of menhaden less than 10” through harvest restrictions could mitigate nutritional stress on 16” to 24” striped bass. Also, closure of the Exclusive Economic Zone (three to 200 miles off the Atlantic coast) to menhaden harvest would increase prey availability for migratory striped bass in coastal waters and would enhance spawning stock survival. Optimistically, ASMFC decisions that address menhaden overfishing will eventually help resolve the fundamental problem - ecological depletion of Atlantic menhaden. 


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