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TROPHIC INTERACTIONS – MENHADEN AND STRIPED BASS

PREDATOR/PREY MONITORING PROGRAM

CHESAPEAKE BAY ECOLOGICAL FOUNDATION, INC.

EAST CAROLINA UNIVERSITY

September 10, 2007

Menhaden are a vital component of the coastal food web and are crucial in the diet of many predators, including striped bass. Menhaden are also an abundant filter feeding fish in Atlantic coastal waters, consuming plankton and organic detritus. Within the Chesapeake Bay the filtering capacity of a healthy menhaden population could remove a significant amount of phytoplankton, thereby improving water quality and clarity. The age structure of the Atlantic menhaden stock has been truncated and very few fish older than age 6 exist in the stock even though menhaden can live for more than 10 years.

When Maryland’s striped bass fishery reopened in 1990 after a five year closure, the Atlantic States Marine Fisheries Commission (ASMFC) approved raising the historic 12” (age-2) minimum size to 18” (age-4). The protection of striped bass (<18”) greatly increased striped bass numbers and prey demand within Maryland’s portion of the Chesapeake Bay (upper Bay). By the late 1990s the striped bass recovery exceeded all expectations. Also, increasing numbers of large migratory striped bass over 28” (ages-8+) preyed heavily on older menhaden during winter along the Virginia and North Carolina coast (historic striped bass winter feeding grounds).

During the early 1990s older adult (ages-3+) menhaden were severely overfished off New England concurrent with intensive fishing on sub-adult (ages-1&2) menhaden in the Chesapeake Bay and off the coast of Virginia and North Carolina. The resulting decline in older menhaden coincided with low recruitment of age-0 menhaden in all major nursery areas and the initiation of health problems in Chesapeake Bay striped bass. Since 2000, concern about low numbers of adult menhaden has been substantiated by record low purse seine reduction landings of menhaden older than age-4, the most prolific component of the spawning stock. Menhaden reduction landings in Virginia’s portion of the Chesapeake Bay (lower Bay) totaled 145 million pounds in 2006, the lowest harvest in 25 years, and 100 million pounds below ASMFC’s 2006 “Bay cap”.

An insufficient number of menhaden to support the nutritional needs of the upper Bay’s striped bass was first documented in the early 1990s by Hartman and Brandt. In the late 1990s the Maryland Department of Natural Resources (MD-DNR) funded an expansive upper Bay 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 nutrition, lesions and disease.

In 2004, the Chesapeake Bay Ecological Foundation Inc. initiated a Predator/Prey Monitoring Program (PPMP) with Dr. Anthony Overton at East Carolina University to determine the age structure and biomass of Atlantic menhaden consumed by striped bass along the Virginia and North Carolina coast and in the Chesapeake Bay. Over 3,000 striped bass have been examined since the PPMP began and the results further demonstrate that large upper Bay age-4 striped bass (>18”) are undernourished because their primary prey, Atlantic menhaden, has been depleted. Age-0 menhaden are crucial to the diet of small striped bass (<18”) during the summer, fall and winter. Both age-0 and sub-adult menhaden are crucial to the diet of resident striped bass (>18”) from fall through spring. Migratory striped bass (>28”) prey on menhaden of all sizes while in the upper Bay from late fall through spring. Within the Chesapeake Bay system both sexes are protected until they reach 18” (age-4); however, at age-3 (about 16”) most females migrate to the ocean. More than 85% of striped bass (16” to 18”) that remain in the upper Bay are males that must compete for the limited number of age-0 menhaden which are now 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 internal body fat. This fat is assimilated during the following summer and early fall when feeding activity by age-4+ striped bass in the upper Bay is greatly reduced. (The PPMP found that although resident striped bass 4 years and older prey heavily on menhaden from fall through spring, feeding activity declines during summer and early fall when upper Bay water temperatures are relatively high.) Since the mid 1990s consistently low Atlantic menhaden recruitment has resulted in the depletion of age-0 menhaden and the onset of health problems in Chesapeake Bay striped bass. Consequently, age-4 striped bass now enter the summer months lacking sufficient body fat to maintain their weight and health until intensive feeding resumes in late fall. Upper Bay age-4 striped bass now weigh about 30% less than their historic weight by early fall – a level symptomatic of starvation. Length-at-age has decreased, disease is a concern because large numbers of striped bass have mycobacterial infections and striped bass natural mortality rates have risen in Chesapeake Bay.

The average weight of ages 8+ migratory striped bass (>28”) has been declining in recent years and current research shows that most large female striped bass now use all of their body fat for egg production - leaving no reserves for assimilation during the summer months of reduced feeding activity in northern coastal waters. In late fall they migrate as far south as North Carolina and arrive on their winter feeding grounds in poor nutritional condition (low body fat). Over the winter they feed intensively on menhaden to rebuild fat reserves essential for egg development and spawning success the following spring. Following the decline of older menhaden, large migratory striped bass have preyed more heavily on the small age-0 menhaden that migrate south along the coast during winter. The PPMP detected that the winter feeding grounds of migratory striped bass has shifted northward since the winter of 2004-05 and in 2006-07 included the upper Chesapeake Bay. The depressed stock of adult menhaden may no longer be supplying sufficient prey for the migratory striped bass population. This may explain why large numbers of migratory striped bass abandoned their traditional winter feeding grounds off Virginia and North Carolina where older menhaden congregate and migrated to the upper Chesapeake Bay to feed on the more numerous younger menhaden. This event during the winter of 2006-07 resulted in migratory striped bass (>28”) competing with upper Bay resident striped bass for similar size menhaden. Additional competition for the limited menhaden supply could exacerbate growth and health problems already affecting upper Bay resident striped bass.

During the winter of 2006-07 large numbers of migratory striped bass, which typically over-winter off Virginia and North Carolina, over-wintered in the upper Bay – 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 historic winter feeding grounds. This conclusion is supported by the relative physical 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. Stomach analyses on 98 of these migratory striped bass caught in the upper Bay during the winter found that 90 (92%) contained a total of 446 menhaden (4.5 per striped bass examined): 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 ocean waters near the mouth of the Bay. These fish contained a total of 220 menhaden (2.7 per striped bass examined). After spawning 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 purse seine fishery for similar size menhaden, and because menhaden are overfished, older menhaden no longer meet the prey demand of large migratory striped bass. Large striped bass become opportunistic predators during the summer when feeding activity declines; however, cumulative data compiled since 2003 from MD-DNR and PPMP studies determined that large striped bass in the upper Bay and in ocean waters are menhaden dependent during the crucial fall through spring feeding period when menhaden constitute over 80% of their diet by weight.

As numbers of age-0 menhaden declined in the upper Bay, striped bass supplemented their diet with bay anchovy. Currently both crucial prey species are depleted. Chronically low recruitment of age-0 menhaden now deprives most upper Bay (age-4) striped bass (>18”) of sufficient prey to maintain their weight and health. Following the decline of adult menhaden in the ocean, migratory striped bass have suffered from poor nutrition and have become more dependent on younger menhaden as prey. The coastal biotic community has been disrupted and the striped bass fishery is being threatened by the decline of Atlantic menhaden.

REFERENCES:

Ahrenholz, D.W., Nelson, W.R. & Epperly, S.P. (1987) Population and fishery characteristics of Atlantic menhaden (Brevoortia tyrranus). Fish. Bull. 85:569-600.

Gottlieb, Sara (1998) Nutrient removal by age-0 Atlantic Menhaden (brevoortia tyrranus) in Chesapeake Bay and implications for seasonal management of the fishery. Ecological Modelling 112: 111-130.

Griffin, J.C. (2001) Dietary Habits of an Historical Striped Bass (Morone saxatilis) Population in Chesapeake Bay. Masters Thesis, University of Maryland Eastern Shore, 135

Hartman, K.J. & Brandt, S.B. (1995b) Predatory demand & impact of striped bass, bluefish, weakfish in the Chesapeake Bay: applications of bioenergetics models. Canadian Jour nal of Fisheries Sciences 51, 1667-1687

Hartman, K.J. & Margraf, F.J. (2003) US Atlantic coast striped bass: issues with a recovered population Fisheries Management and Ecology, 10, 3009-312.

Overton, A.S. (2003) Striped Bass Predatory Prey Interactions in the Chesapeake Bay & Along the Atlantic Coast.

Pollock, Kenneth H. (2007) Tag Return Models Allowing for Harvest and Catch and Release: Evidence of Environmental & Management Impacts on Striped Bass Fishing and Natural MortalityRates. North American Journal of Fisheries Management 27:000-000, 2007

Uphoff, Jr.,J.H. (2003) Predatory-prey analysis of striped bass and Atlantic menhaden in upper Chesapeake Bay. Fisheries Management and Ecology. 10, 313-322.

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Is this because there are fewer other prey species to eat? Did the Rock come back too much? Should the limits be lower to cull?

Hmmm, no wonder why Coop caught 'em all last winter...

It's kind'a like fewer Sharks make more Skates/Rays.

But this is opposite, more Rocks, less bait.

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yeah but its more total Stripers, less big ones and many are emaciated and diseased.

Hell Sam caught a 45" fish that was around 30lbs, that's not right! Critter Gitter caught that 49" in the fall that should have been close to 50lbs...he said it was thin. They should not be thin in the fall.

In the bay I think it all comes back to Omega and the bunks.

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In the bay I think it all comes back to Omega and the bunks.

Without a doubt.

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