Effects of Menhaden Depletion on Atlantic Coast Striped Bass
The Atlantic menhaden decline along the East Coast of the United States has profound ecological significance, engendering concern and controversy regarding management of our coastal fisheries. Menhaden are a vital component of the coastal food web, important in the diet of many predators and crucial to the diet of large striped bass. Menhaden are a filter feeding fish in Atlantic coastal waters that consume large quantities of plankton and organic detritus. During the early l990’s, older adult menhaden (ages 3+), which spawn along the Atlantic coast during their annual migrations, were severely overfished in the Gulf of Maine concurrent with intensive fishing on sub-adult and adult menhaden (ages 1+) in the Chesapeake Bay and off the coast of Virginia and North Carolina. Exacerbating the menhaden decline was a dramatic increase in predation by unprecedented numbers of large striped bass (>18”), which depend on menhaden of all sizes for their survival in the Chesapeake Bay as well as along the Mid-Atlantic coast. Consequently, few menhaden older than age 4 are now present in the population even though life expectancy exceeds 10 years. Increased predation and harvest of older menhaden coincided with low recruitment of age-0 menhaden in all major nursery areas and the onset of health problems in Chesapeake Bay striped bass. These observations indicate that the remnant menhaden population can no longer fulfill its role as the primary prey species for striped bass because of ecological depletion (insufficient numbers to meet nutritional needs of dependant predators). The Atlantic States Marine Fisheries Commission (ASMFC), which is responsible for menhaden management in state and coastal waters, continues to state: “Menhaden are not overfished and overfishing is not occurring on a coast-wide basis”. However, ASMFC’S statement is based on overfishing targets calculated for their Fishery Management Plan (FMP) and questionable estimates of spawning stock biomass that do not account for increased menhaden mortality caused by striped bass predation.
Insufficient numbers of menhaden to support the nutritional needs of Chesapeake Bay striped bass was documented in the early 1990’s by Hartman and Brandt. In the late 1990’s, the Maryland Department of Natural Resources (MD-DNR) funded an expansive striped bass diet study by Anthony Overton which determined that cumulative prey demand by ages 4+ striped bass (>18”) exceeded supply, and striped bass had altered their diet to include more Bay anchovy and blue crab in order to survive. Overton’s research also documented striped bass health issues including poor nutrition, lesions and disease. Large numbers of striped bass have mycobacterial infections, length and weight-at-age has decreased and natural mortality rates have risen.
In 1984 striped bass were declared a “threatened species” in Maryland tidal waters after the overfished population reached an extremely low level. Maryland’s striped bass fishery was closed in 1985. In 1990 the ASMFC allowed the fishery to reopen coast-wide; within the Chesapeake Bay the striped bass historical minimum size of 12” total length (age 2) was raised to 18” (age 4) and the allowable harvest was substantially reduced. This greatly increased striped bass numbers and prey demand within Maryland’s portion of the Chesapeake Bay (upper Bay). Also during the 1990’s, increasing numbers of large migratory striped bass over 28” total length (ages 8+) preyed heavily on menhaden during the winter along the Virginia and North Carolina coast (historical winter feeding grounds for large striped bass). By the late 1990s the striped bass recovery had exceeded all expectations while menhaden landings declined. In 1997 and 1998 respectively, the Chesapeake Bay Ecological Foundation (CBEF) and MD-DNR informed ASMFC that the menhaden decline could be affecting the health of upper Bay striped bass. In response, ASMFC conducted its first Menhaden Peer Review in 1998 which led to a revised FMP in 2001. The main purpose of the new FMP was to maintain adequate menhaden numbers for dependent predators such as striped bass. This objective failed, in part because ASMFC’s menhaden management relies on a stock assessment model that assumes age-specific, fixed natural mortality rates that do not reflect current increases in striped bass predation. Since 2000 concern about low numbers of older adult menhaden has been substantiated by record low reduction landings of menhaden older than age 4, the most prolific component of the spawning stock. In 2006 ASMFC responded to concern about the depletion of menhaden in the Chesapeake Bay by establishing an annual “harvest cap” on the lower Bay menhaden reduction fishery (fish harvested for industrial purposes) and initiated a multi-million dollar research program to assess whether menhaden are locally depleted in the Chesapeake Bay. However, the ASMFC has been unable to specifically define localized depletion. The 109,020 metric ton harvest cap is considerably higher than recent reduction landings and has therefore been ineffective in conserving the menhaden stocks. Neither the “Bay cap” nor the revised FMP prevent ecological overfishing (when excessive harvest of a species threatens the health or survival of other species) because both have failed to protect and rebuild the menhaden spawning stock which inhabits the coastal ocean.
CBEF and MD-DNR have conducted cooperative striped bass studies since the early 1980s. In 2005, CBEF initiated a Predator/Prey Monitoring Program (PPMP) to determine the type of prey and age structure of Atlantic menhaden consumed by striped bass along the Atlantic coast and in the Chesapeake Bay. Partial funding for the PPMP was provided by the U.S. Fish & Wildlife Service, MD-DNR, CBEF and East Carolina University. Over 4,000 striped bass have been examined and analyses of PPMP and MD-DNR data demonstrate that striped bass are undernourished because their primary forage, Atlantic menhaden, has been depleted. Food habit studies of striped bass from the upper Bay show that age 0 menhaden less than 7” in total length are crucial to the diet of small striped bass (<18”) and that both age 0 and sub-adult menhaden are crucial to the diet of large resident striped bass (>18”) from fall through spring. Migratory striped bass over 28” in length (approximately 80% females) now prey on menhaden of all sizes while in the upper Bay.
Both sexes of young striped bass live and feed within the Chesapeake Bay system; however, by the time they reached 16″ most of the females have migrated to coastal waters. Approximately 85% of striped bass (16” to 18”) that remain in the upper Bay are males and are at the size where age 0 menhaden become their primary prey. From fall through spring, they feed heavily on age 0 menhaden and accumulate body fat. This body fat is used to develop their gonads prior to spawning. (The PPMP found that although resident striped bass 4 years and older prey heavily on menhaden from fall through spring, they become opportunistic predators during summer and early fall when upper Bay water temperatures are relatively high and feeding activity concentrates on benthic-type prey.) Since the mid 1990’s, consistently poor menhaden recruitment has resulted in the depletion of age 0 menhaden in the Chesapeake Bay. Consequently, many striped bass now enter the summer months lacking sufficient body fat to maintain their weight and health until intensive feeding resumes in late fall. The average weight of upper Bay age 4 striped bass caught in the Choptank River during the fall is now less than 70% of their historical weight – a level symptomatic of starvation. Weight-at-length of striped bass caught in the Choptank River increases and decreases with high and low recruitment levels of age-0 menhaden, demonstrating that striped bass in this size range are unable to maintain their weight when young menhaden are depleted. Diet analyses confirm that the number of age 0 menhaden in the stomachs of striped bass caught in the Choptank River increase when the MD-DNR juvenile menhaden indices are high and decrease when they are low.
The PPMP detected that large numbers of striped bass (mostly females >28”) that historically migrated south to the coastal waters off Virginia and North Carolina (winter feeding grounds for large striped bass), migrated to the upper Bay during 2006-07 and 2007-08 and remained over the winter. This was a previously undocumented event. These large migratory striped bass (>28”) accounted for a significant portion of upper Bay winter gill net landings. They preyed heavily on menhaden, primarily sub-adults, indicating menhaden were more available in the upper Bay than on their historical winter feeding grounds. This conclusion is supported by the condition of large migratory striped bass examined from the two areas; those from the upper Bay contained approximately twice the amount of body fat than those from the coastal ocean. CBEF’s stomach analyses on 98 of these large migratory striped bass caught in the upper Bay during the winter of 2006-07 found that 90 contained a total of 446 menhaden: age 0’s represented approximately 20% of the 90, sub-adults approximately 70%, and adults approximately 10%. The body fat index of these 98 striped bass averaged approximately 2 on a scale of (0 to 4), compared to an average body fat index of approximately 1 for 80 migratory striped bass caught during late winter in coastal waters near the mouth of the Bay. The change in historical migration patterns is another indicator that the depressed coastal stock of adult menhaden no longer provides sufficient prey for large migratory striped bass. The use of the upper Bay as winter feeding grounds for many large migratory striped bass (mostly females >28”) has resulted in competition with upper Bay resident striped bass (mostly males) for similar size menhaden. This additional competition for the depleted numbers of menhaden, coupled with depressed populations of bay anchovy and blue crab, could exacerbate growth and health problems currently affecting upper Bay resident striped bass.
After spawning large migratory striped bass resume feeding, primarily on age 1+ menhaden, while migrating out of the Chesapeake Bay to northern coastal waters. Large striped bass must compete with the reduction fishery for similar size menhaden, and because menhaden are overfished, older menhaden no longer meet the prey demand of large migratory striped bass. Weight of ages 8+ migratory striped bass (>28”) has been declining in recent years. In late fall they migrate south and many arrive on their winter feeding grounds in poor nutritional condition. Consequently, they must feed heavily over the winter in order to accumulate body fat essential for egg development and spring spawning success. Following the decline of older menhaden, these large migratory striped bass now prey more heavily on the depleted numbers of bay anchovy and age 0 menhaden which migrate to coastal ocean waters during the winter.
Chronically low recruitment of age 0 menhaden deprives striped bass in the upper Bay of sufficient prey to maintain their nutritional health. Following the decline of adult menhaden, coastal migratory striped bass suffer from poor nutrition and are more dependent on bay anchovy and younger menhaden as prey. Cumulative data from PPMP and MD-DNR studies since 2003 show menhaden are crucial to the diet of adult resident striped bass in the upper Bay and large striped bass (>28”) in ocean waters from fall through spring when menhaden constitute over 80% of their diet by weight. ASMFC’s single species management approach understates the importance of predator/prey relationships and their influence on fish health and productivity in the Chesapeake Bay and along the Atlantic coast. The remnant menhaden population should be protected until management agencies implement a comprehensive multi-species management plan based on nutritional needs of predators dependent on this keystone prey species. The decline of Atlantic menhaden has disrupted the food supply and threatened the health of striped bass in the Chesapeake Bay and along the Atlantic coast.
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