Menhaden
are extremely abundant in nearshore coastal waters because
of their ability to directly utilize phytoplankton,
which is the basic food resource in aquatic systems.
Other species of marine fish are not equipped to filter
such small organisms from the water. Consequently, such
large populations of other species cannot be supported.
Because menhaden are so abundant in nearshore coastal
and estuarine waters, they are an important forage fish
for a variety of larger piscivorous fishes, birds, and
marine mammals. In ecological terms, menhaden occupy
a very important link in the coastal marine food chain,
transferring planktonic material into animal biomass.
As a result of this, menhaden influence the conversion
and exchange of energy and organic matter within the
coastal ecosystem throughout their range.
Because
menhaden only remove planktonic organisms larger than
13-16 microns (7 microns for juveniles) from the water,
the presence of large numbers of fish in a localized
area could alter the composition of plankton assemblages.
Studies have estimated that juvenile menhaden consumed
6-9% of the annual phytoplankton production in eight
estuaries on the east coast, and up to 100% of the daily
production in some instances.
A
large school of menhaden can also deplete oxygen supplies
and increase nutrient levels in the vicinity of the
school. Enrichment of coastal waters by large numbers
of menhaden can be expected to stimulate phytoplankton
production. One researcher measured ammonia concentrations
(from excretion) inside menhaden schools that were five
times higher than ambient levels 4.5 km away. At the
same time, chlorophyll values increased by a factor
of five over the same distance, indicating the grazing
effect of the fish on the phytoplankton standing crop.
Oxygen values were not significantly reduced by the
fish, but were much more variable inside the schools
than outside them.
Also,
in a study of energy and nitrogen budgets, food consumption
rates, energy expenditures, and growth efficiency were
examined. Results indicated that swimming speed, the
duration of the daily feeding period, and the concentration
of plankton in the water controlled the energy and nitrogen
budgets for this species.
Also
review:
 
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