CHESAPEAKE BAY ECOLOGICAL FOUNDATION, INC.
ECOLOGICAL DEPLETION OF ATLANTIC MENHADEN & BAY ANCHOVY
EFFECTS ON ATLANTIC COAST STRIPED BASS
First Year-Round Ecological Study Of Large Chesapeake Bay Striped Bass
The Chesapeake Bay Ecological Foundation (CBEF), with assistance from East Carolina University (ECU), 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 (MD-DNR), ECU and CBEF. Striped bass were obtained from Maryland, Virginia, and North Carolina sport fisheries and Maryland commercial fisheries.
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 fishery reopened coast-wide under harvest restrictions approved by the Atlantic States Marine Fisheries Commission (ASMFC). Within the upper Bay the 14" (total length) minimum size was raised to 18" and a harvest cap imposed for the first time. Within ocean waters the minimum size was set at 28". These actions protected unprecedented numbers of striped bass, dramatically increasing predation on menhaden and bay anchovy in Chesapeake Bay and Atlantic coastal waters. The rapid expansion in numbers of striped bass greater than 18" (>18"), which feed predominately on menhaden, has sustained the prey demand for menhaden at record high levels for over a decade. (Prey population dynamics is an intrinsic element of scientific fisheries management which is frequently overlooked during the formulation of fishing regulations.)
During the early 1990's older adult menhaden were severely overfished off New England concurrent with intensive fishing on sub-adult (ages 0-2) and adult menhaden (ages 3+) in the Chesapeake Bay and off the coast of Virginia and North Carolina. The resulting decline in older menhaden coincided with low recruitment of menhaden (age 0) in all major nursery areas and the initiation of health problems in Chesapeake Bay striped bass. Depletion of the menhaden spawning stock is substantiated by record low purse seine reduction fishery (large scale harvest of fish for processing into products such as fish oil and meal) landings of menhaden older than age 4, the most prolific component of the spawning stock. The age structure of the Atlantic menhaden population 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 (Classic overfishing). The Virginia-based menhaden purse seine reduction fishery, which operates in Virginia's section of the Chesapeake Bay (lower Bay) and in ocean areas from North Carolina to New Jersey, competes with striped bass for declining numbers of ages 1+ menhaden. Responding to mounting concern about the declining menhaden population in the Chesapeake Bay, the ASMFC established (2006) an annual "Bay harvest cap" of 109,020 metric tons on reduction fishery landings from Virginia Bay waters. Since 2006, reduction fishery landings in the lower Bay have averaged approximately 30% below the harvest cap.
Insufficient numbers of menhaden to meet the prey demand of upper Bay striped bass was first documented in the early 1990s by Hartman and Brandt. In the late 1990s the MD-DNR funded an expansive upper Bay striped bass diet study by Anthony Overton which determined that cumulative prey demand by striped bass >18" exceeded supply, and striped bass had altered their diet to include more bay anchovy and blue crab. Overton's research also documented striped bass health issues including malnutrition, lesions and disease. Within the Chesapeake Bay, striped bass length-at-age and weight-at-length has decreased, a significant percentage of striped bass have mycobacterial infections and striped bass natural mortality rates have risen. Recently published research (Jacobs et al. 2009) established a causal relationship between poor nutrition and a serious and potentially lethal striped bass disease (mycobacteriosis) prevalent in Chesapeake Bay striped bass since the late 1990s.
Since 2004 over 7,000 striped bass have been examined through the PPMP and accumulated data demonstrates that malnutrition observed in upper Bay striped bass less than 24" (<24") is a consequence of ecological depletion (insufficient numbers to provide adequate prey for dependent predators) of bay anchovy and ages 0&1 menhaden. Diet analyses of upper Bay striped bass show that bay anchovy are a crucial prey species for most striped bass <16". Age 0 menhaden 3 1/2" to 7" (total length) and bay anchovy are crucial to the diet of striped bass approximately 12" to 16". Ages 0&1 menhaden approximately 3 1/2" to 10" are crucial to the diet of striped bass approximately 16" to 24". Ages 0-3 menhaden approximately 31/2" to 12" are consumed by striped bass 24" to 28". All age classes of menhaden are consumed by striped bass >28". During spring and fall, soft crabs and small hard crabs < approximately 3" are a significant dietary component for striped bass >12". In mid-Atlantic ocean waters all age classes of menhaden and adult bay anchovy (ages 1+) are consumed by migratory striped bass >28". PPMP studies of resident striped bass >16" in Chesapeake Bay waters (year-round) and migratory striped bass >28" in mid-Atlantic and Chesapeake Bay waters (late fall through spring) determined that menhaden constitute over 75% of their diet (by weight). Striped bass are the only significant menhaden predator in the Chesapeake Bay and along the mid-Atlantic coast from late fall through early spring.
Chesapeake Bay tidal waters provide the principal spawning and nursery areas for Atlantic coast striped bass. Historically, Chesapeake Bay provided an ideal ecosystem for reproduction, survival and growth for high numbers of healthy striped bass. This natural productivity has deteriorated over the past two decades due to sever declines in populations of small prey - primarily bay anchovy and age-0 Atlantic menhaden. Although most male striped bass remain within the Chesapeake Bay system, most females migrate to ocean waters by age 3 (approx. 16"). PPMP data shows that about 90% of upper Bay resident striped bass 16" to 24" are males and approximately 10% are females. All striped bass over 50" are females, while males rarely exceed 40".
Most striped bass >12" (age 2+) aggregate in the main stem of Maryland's mid-Bay region from late spring through early fall. In the Bay's main stem during this period, prey consumption by striped bass 16" to 24" decreases significantly, with benthic organisms tending to replace the fall/winter diet of fish - primarily menhaden. During summer through early fall, the period when most annual linear growth occurs, individual weights of most striped bass decline below that of the previous winter.
In the Bay's main stem from spring to early fall, the prey supply for striped bass 16' to 24", which feed on menhaden <10", is severely limited compared to striped bass >24" that can prey on menhaden >10". These findings are substantiated by relative body fat levels recorded from 2007 through 2009 which averaged approximately three times higher in striped bass >24" than in striped bass <24". (From summer through fall, millions of age 1 menhaden (<10") are caught annually in the lower Bay and nearby coastal waters by the menhaden purse seine reduction fishery).
As Chesapeake Bay water temperatures decline in the fall, striped bass movements accelerate and striped bass predation intensifies. Some striped bass begin leaving summer residency in the Bay's main stem and enter Bay tributaries to intercept bay anchovy and age 0 menhaden that are leaving summer nursery areas on a down-Bay migration to the ocean. Many striped bass follow and prey on these bay anchovy and age-0 menhaden before returning to their natal rivers prior to the spring spawning season. Significant numbers of age 0 menhaden do not join the fall migration, but remain in the upper Bay over the winter along with a population of older menhaden. Sexually mature striped bass inhabiting the Bay during late fall and winter feed heavily, primarily on menhaden (all age classes), and spot and croaker juveniles which are moving up the bay from ocean spawning areas. The vigorous resumption of predation on menhaden restores physical robustness degraded by reduced feeding from early summer through early fall and accrues fat reserves for impending spawning demands. Striped bass acclimate readily to cold water as evidenced by voracious feeding at water temperatures <45 degrees F. during the winter months. Based on these findings, (excepting movements related to spawning) late fall and winter migrations and localized movements within Chesapeake Bay and mid-Atlantic coastal waters are a response to distribution patterns of prey - particularly menhaden - rather than water temperatures.
From 2007 through 2009 body fat indices were recorded for 1,842 resident striped bass caught in the main stem of the upper Bay from June 21 to Oct. 15 and in the Choptank River from May 2 to Oct. 15. The average index value for striped bass 12" to 24" was significantly lower than for striped bass >24". These findings indicate that populations of bay anchovy and ingestible size menhaden (<10") in the upper Bay from late spring through early fall are too low to provide adequate prey for striped bass 12" to 24". In contrast, age 2+ menhaden (>10") inhabit Chesapeake Bay throughout the year and are available as prey for resident striped bass >24". Also, the decimated populations of historically abundant prey such as juvenile river herring have reduced prey availability for striped bass <24" during summer through fall.
Following a resurgence in numbers during the fishing moratorium, Chesapeake Bay resident striped bass are now nutritionally stressed by ecological depletion of their forage base - particularly bay anchovy and age-0 menhaden. For example, studies show that year to year weight-at-length of Choptank River striped bass approximately 14" to 18" varies with annual recruitment of age-0 menhaden: During years of low recruitment the average weight of striped bass 14" to 18" caught during the fall can be less than 70% of their historical weight - a level symptomatic of starvation. The striped bass recovery which occurred during the moratorium is now threatened by the ensuing imbalance between prey and predator populations. This imbalance was exacerbated by the increase in resident striped bass following the change in minimum size from 14" to 18".
PPMP research detected that large numbers of striped bass >28" (80% 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 Bay 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 indicate that menhaden are more available as prey in the Chesapeake Bay than on their historical winter feeding grounds. Recent changes in winter distribution of migratory striped bass are supported by data compiled by the Cooperative Winter Tagging Cruise during 22 consecutive years of fish sampling: During the past four winters (beginning 2006 - 07) 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 >28", are ecologically depleted on their historical inshore winter feeding grounds within mid-Atlantic coastal waters. The depleted coastal stock of ages 3+ adult menhaden currently fails to provide sufficient prey for migratory striped bass >28". Consequently, many migratory striped bass >28" 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.
Following their summer residency in northern coastal waters, migratory striped bass >28" arriving in Maryland, Virginia and North Carolina waters during late fall and winter 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. Large female striped bass over approximately 40" tend to accumulate body fat earlier than small females. Resident male striped bass enter the spawning areas prior to the arrival of females. The first females to arrive on the spawning grounds are generally the largest striped bass. They often spawn and leave before the smaller females arrive. Body fat is utilized for gonadal development as post-spawning fat indices are near zero for both male and female striped bass. (Body fat regained during the intensive feeding interval between spawning and early summer is assimilated during mid-summer through early fall). After spawning, migratory striped bass >28" (predominately females) resume feeding primarily on age 2+ menhaden, while migrating out of the Chesapeake Bay en route to their summer habitat in 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. In the main stem, striped bass 18" to 24" feed primarily on benthic organisms while those >24" still prey on menhaden - predominately ages 2+.
Migratory striped bass over-wintering in mid-Atlantic coastal waters prey heavily on the depleted bay anchovy spawning stock (ages 1+). (During late fall adult bay anchovies migrate to ocean waters from the Chesapeake Bay.) 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" to 18". PPMP diet studies of striped bass since 2004 corroborate the depletion of bay anchovy in Chesapeake Bay and mid-Atlantic coastal waters.
Ecological depletion of ages 0&1 menhaden and bay anchovy in the upper Chesapeake Bay has lowered the carrying capacity for striped bass <24". In Chesapeake Bay striped bass, bacterial disease and lesions are prevalent, natural mortality has risen, and weight-at-length and length-at-age (growth rate) have declined. 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 >28". PPMP studies of striped bass in Chesapeake Bay and mid-Atlantic waters determined that menhaden are crucial to maintenance of healthy striped bass populations. Within the Chesapeake Bay, from late fall through spring, migratory and resident striped bass now compete for depleted numbers of menhaden. Malnutrition in upper Bay resident striped bass 12" to 24" indicates insufficient prey: primarily bay anchovy and menhaden <10" (ages 0&1). The collapse of these primary forage species has undermined the ability of the Chesapeake Bay ecosystem to support a nutritionally healthy striped bass population.
Harvest restrictions on menhaden <10" would make available additional ingestible-size prey for resident striped bass during late spring through early fall - a period when upper Bay striped bass <24" are nutritionally stressed. Based on National Marine Fisheries Service data for the ten year period (1999-2008), an Atlantic menhaden minimum size of 10" (total length) 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 weigh approximately 21,000 metric tons (12% of the harvest by weight). Purse seine reduction landings during 2009 contained an estimated 356 million juvenile menhaden ages 0&1 - approximately 50% of the total number of menhaden harvested. An increasing percentage of juveniles in annual landings could be symptomatic of a collapsing menhaden population. A menhaden minimum size of 10" would benefit predators nutritionally dependent on this keystone prey species by augmenting the severely depleted prey base in Chesapeake Bay and mid-Atlantic coastal waters. Also, closure of the Exclusive Economic Zone to menhaden harvest would reduce spawning stock mortality and increase prey availability of adult menhaden for migratory striped bass >28" in coastal ocean waters.
Direct questions or comments to James Price: firstname.lastname@example.org