Ecological Depletion Of Atlantic Menhaden Effects On Atlantic Coast Striped Bass.
The Chesapeake Bay Ecological Foundation (CBEF) and the Maryland Department of Natural Resources (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 malnutrition observed in striped bass results from ecological depletion (insufficient numbers to meet nutritional needs of dependant predators) of Atlantic menhaden, their primary forage. Food habit studies of striped bass from Maryland’s portion of the Chesapeake (upper Bay) show that age-0 menhaden less than 6” in total length are crucial to the diet of small striped bass (<18”) during the summer, fall and winter. Both age-0 and sub-adult menhaden (ages 1&2) 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) prey on all age classes of menhaden while in ocean waters and the upper Bay from late fall through spring.
Both sexes of young striped bass live and feed within the Chesapeake Bay system; however, prior to reaching age-4 (about 16”) most of the females migrate to coastal waters. More than 85% of striped bass (16” to 18”) that remain in the upper Bay are males and are at the size when age-0 menhaden become their primary prey. From fall through spring, just prior to reaching age-4, these 3 year olds feed heavily on age-0 menhaden and accumulate body fat. This fat is used for gonad development. (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 feeding activity is low and upper Bay water temperatures are relatively high).
Since the mid 1990’s, consistently poor recruitment has contributed to the ecological 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 on menhaden 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 age-4 striped bass are unable to maintain their weight when young menhaden are ecologically depleted. Diet analyses confirm that the number of age-0 menhaden in the stomachs of striped bass caught in the Choptank River increases when the MD-DNR Choptank juvenile menhaden index is high and decreases when it is 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 – 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 from late fall through spring, primarily on sub-adults, indicating menhaden may now be more available in the upper Bay than on their historical winter feeding grounds along the coast. This conclusion is supported by the condition of large migratory striped bass examined from the two areas; those from the upper Bay contained about 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-0s were present in approximately 20% of the 90, sub-adults in 70%, and adults in 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 one of several indicators that the depressed coastal stock of older menhaden is ecologically depleted and no longer provides sufficient prey for large migratory striped bass. (Few menhaden older than age-4 are now present in the population even though life expectancy exceeds 10 years). The use of the upper Bay as a winter feeding ground 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. The additional competition for the declining numbers of menhaden, in conjunction with depressed populations of bay anchovy and blue crab, could exacerbate growth and health problems currently affecting upper Bay resident striped bass. The menhaden purse seine fishery and large striped bass compete for the declining numbers of older menhaden – depleting menhaden spawning stock and the prey supply for large striped bass. (During 2006 & 2007 menhaden purse seine landings in the Chesapeake Bay declined sharply to approximately 60% of the previous 20 year average).
After spawning in the spring, large migratory striped bass resume feeding, primarily on age-1+ menhaden to rebuild body fat, while migrating out of the Chesapeake Bay to northern coastal waters. Adult females use most of their body fat for egg production – leaving little or no body fat reserves for assimilation during the summer months of reduced feeding activity in New England coastal waters. In late fall they migrate south and arrive on their winter feeding grounds off Virginia and North Carolina in poor nutritional condition. (Weight-at-length of adult female migratory striped bass has been declining in recent years). They feed heavily from fall through early spring, primarily on menhaden, and accumulate body fat essential for weight maintenance and egg development. Following the decline of older menhaden, those migratory striped bass wintering in coastal ocean waters from late fall through early spring now prey heavily on bay anchovy and younger menhaden. Those migratory striped bass that enter the upper Bay prey heavily on menhaden from late fall through spring.
The ecological depletion of Atlantic menhaden has resulted in nutritionally stressed striped bass. Both resident and migratory striped bass now consume increased numbers of alternate prey that have high recreational and commercial value such as blue crab, spot, white perch and weakfish. Cumulative data from PPMP and MD-DNR studies since 2003 show menhaden are crucial to the diet of large striped bass (>18”) in the upper Bay and (>28”) in ocean waters from fall through spring when menhaden constitute over 80% of their diet by weight.
Direct questions or comments to James Price: firstname.lastname@example.org