By: Bryan Watts
7/2/2021

Bunker or Atlantic menhaden (Brevoortia tyrannus) have supported the largest commercial fishery by weight along the Atlantic Coast for more than a century.  The menhaden is a large schooling fish that is distributed in nearshore waters from Nova Scotia south to Florida.  Menhaden are filter feeders that depend on plankton and spend portions of their life cycle within estuaries including the Chesapeake Bay.  Although menhaden are increasingly replacing other fish within the bait industry, 75% of the landings come from the purse seine fleet for the “reduction” industry where fish are processed into fish meal and oil.  These products are used as poultry, hog and aquaculture feeds, fertilizer, pet food and dietary supplements.  During its peak in the 1950s, the reduction industry was harvesting 700,000 metric tons of menhaden annually and supported 20 processing plants from Maine through Florida.  Today only a single plant located in Reedville, Virginia remains in operation.

In addition to their economic value, menhaden play a significant role in coastal ecosystems representing forage fish for birds, marine mammals and predatory fish.  Many economically important fish such as striped bass, bluefish and bluefin tuna, as well as marine mammals including dolphins and humpback whales, depend on menhaden during specific seasons.  Many bird species such as brown pelican, bald eagle and great blue heron depend on menhaden during the breeding season while others such as common loon, northern gannet and double-crested cormorants depend on them during migration or winter.  Of all the bird species that depend on menhaden, osprey seem to stand alone in terms of how tightly their recovery along the outer coast has been linked to menhaden.  Throughout the early phase of recovery during the 1970s and 1980s, menhaden were believed to be a keystone prey species over a large geographic area.

Because menhaden stocks are shared by multiple states along the Atlantic Coast, by federal law their management falls to the Atlantic States Marine Fisheries Commission (ASMFC).  The ASMFC’s approach to menhaden management has evolved over the decades from no quotas and independent management by states to coordinated coast-wide management.  Since 1980, the ASMFC has recognized the importance of menhaden as a forage fish for predator populations and the need to consider these consumers in management decisions.  However, policy actions that protect consumer populations from overharvest by the reduction industry have not risen to the occasion.  It was not until the recovering striped bass (an important recreational fish) began to show signs of food stress attributed to low juvenile menhaden abundance in the 1990s that the ASMFC began to shift from single-species to multi-species models to set reference points that inform harvest limits.  Striped bass, with lesions and poor condition, represented a line in the sand and highlighted the conflict between the fishing public and the reduction industry in terms of the role of menhaden.  Not surprisingly, the multi-species models and the ecosystem-based models that followed have focused on the impact of menhaden harvest on predatory fish populations to establish reference points.  The current state-of-the-art model for menhaden harvest regulation utilizes striped bass as an umbrella species for the broader consumer community under the assumption that as long as striped bass are performing to threshold levels, all other consumers should be fine.

Osprey are fish obligates that have a near global distribution.  Due to this dependency, osprey represent one of the best biological indicators for many fish stocks throughout the world.  Low fish availability and related low brood provisioning rates in osprey lead to clear behavioral and demographic responses.  Pairs that maintain high provisioning rates feed young equally and siblings do not form dominance hierarchies.  Broods experiencing low provisioning rates form linear dominance hierarchies and females feed dominant young to satiation.  The consequence of continued low provisioning is brood reduction where subordinate young die sequentially until brood size matches available food.  Because under low food conditions dominance hierarchies form early, most young with food stress are lost in the first weeks of development just before the most rapid phase of mass growth.  Extreme food stress leads to breeding failure.  The graded response to food stress makes osprey pairs sensitive indicators of fish availability. 

Several osprey populations from New England south to North Carolina have collapsed in recent decades due to food stress, likely related to declines in menhaden stocks.  Mobjack Bay within the lower Chesapeake Bay has been a focus of osprey research since 1970 when Mitchell Byrd first conducted surveys there.  Over the past 50 years, several William & Mary graduate students have worked with osprey within this study area including Bob Kennedy, Gary Seek, Chris Stinson, Tim Kincaid, Peter McLean and Andy Glass.  The collective work of these students provides insight into the response of osprey to changing menhaden abundance.  During the mid-1970s when menhaden stocks were high, there was little evidence of food stress in osprey broods.  By the mid-1980s, several signs of food stress were present within the population, including an increase in hunting time by adult males, lower provisioning rates, the occurrence of sibling aggression and subsequent brood reduction.  By the mid-2000s, the amount of menhaden in the diet had dropped by 40%, brood reduction was common and reproductive rates had dropped to precarious levels.

One of the central tenets of the ecosystem-based model for setting menhaden harvest limits is that menhaden stocks should be maintained at levels that will sustain stable populations of the consumer species that depend on them.  Population stability means that adults are capable of producing enough young to offset adult mortality, which in osprey represents about 1 young/pair/year.  With respect to setting menhaden harvest limits, the question translates to what stock levels are required to allow osprey to reliably reach threshold production.

Michael Academia is a new William & Mary graduate student working with osprey-menhaden interactions within Mobjack Bay, with a particular focus on harvest policy and the possible role of osprey in setting ecosystem-based harvest limits.  Michael has a passion for osprey but has a background in fisheries science from Humboldt State.  Fieldwork in Mobjack was initiated during the spring of 2021 and is examining osprey reproductive rates, diet, brood provisioning rates, the growth rate of young and time budgets of males.  Osprey exhibit a gender-based division of labor, with the males responsible for providing fish during the critical period of young development.  In some ways, they are the key to linking menhaden stocks to population stability.  In addition to observational work, Michael is performing a menhaden-addition study to evaluate how additional menhaden may mitigate food stress.