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salinity. Anderson (1985) reports that adult grass shrimp tolerate
salinities of 2 to 36 ppt and their larva exist in salinities from 3-31 ppt. As
a result of this broad euryhaline tolerance for salinity changes, and their
lack of commercial fishery value when compared with pink shrimp, grass
shrimp do not appear to be suitable for detailed consideration as a
potential indicator species for MFL related analyses in Biscayne Bay.
Because pink shrimp, however have more restricted salinity tolerances,
are more economically valuable and are a major food source for higher
trophic level organisms, are a potential indicator species. Pink shrimp
inhabit broad shallow continental shelf areas, shallow bays and estuaries.
In Florida, pink shrimp nursery areas are found in estuaries and nearshore
marine areas from Biscayne Bay, Florida Bay, the Ten Thousand Islands
area and up the west coast to approximately the Tampa Bay area (Bielsa,
et. al. 1983.).
The pink shrimp fishery is the most economically important of all the
fisheries in Florida (Bielsa et. al. 1983). Records of pink shrimp landings,
which are kept annually by the National Marine Fisheries Service ,
document that the millions of pounds of pink shrimp that are harvested
annually in Florida waters are a valuable financial commodity.
Pink shrimp are also an important component of the aquatic food web,
serving as food for a large variety of other organisms, including many
fishes that are important either recreationally or commercially.
Estuaries and other coastal seagrass communities are important nursery
grounds for pink shrimp during portions of their life cycle. Studies in
Florida Bay and/or Biscayne Bay have documented that post-larvae pink
shrimp settle in seagrass beds that are less than 1 m deep and that the
highest densities of juveniles were found in seagrass communities 1-2 m
deep on the western portion of Biscayne Bay (Diaz 2000).
Valued Ecosystem Components
Freshwater Flow and Ecological Relationships in Biscayne Bay 4-35
Although details of the life cycle of pink shrimp in South Florida are still
being learned, it is known that they emigrate from shallow, coastal
nursery areas to deeper offshore waters in the last juvenile or early adult
stage. They spawn throughout the year on the Tortugas shelf at water
depths of 15-48 m (Bielsa et. al. 1983), although the peak in spawning
activity generally is believed to coincide with maximum bottom-water
temperatures (Munro et. al. 1968). Pelagic larvae go through several
stages and are carried into the Florida Current from the Tortugas grounds
by westerly and southwesterly currents where they settle into estuaries.
Postlarvae become benthic and concentrate at locations where suitable
habitat exists. Pink shrimp then spend from 2-6 months in nursery areas,
where growth rate varies based on a variety of factors, including gender,
water quality conditions, including temperature and salinity, and
available food. They then emigrate back to offshore spawning areas.
Pink shrimp are omnivorous consumers, foraging primarily in seagrass
communities, primarily at night. Studies of feeding habits that revealed
that prey items include dinoflagellates, foraminiferans, nematodes,
polychaetes, ostracods, copepods, mysids, isopods, amphipods, caridean
shrimp and their eggs, molluscs, squid, annelids, other crustaceans, small
fishes and plant material (Bielsa 1983).
In turn, pink shrimp are prey for a variety of other organisms, including
fish, reptiles, aquatic mammals, including bottle-nose dolphins and birds.
Fishes that prey on them include inshore species (e.g., common snook,
spotted seatrout, and various snappers), reef species (e.g., groupers) and
pelagic species (e.g., king mackerel). Wading birds, such as herons, egrets
also forage for shrimp at locations where water depths are suitable.
(Bielsa et. al. 1983) provide an excellent summary of what is known about
the salinity preferences of pink shrimp, as follows:
“Pink shrimp exhibit different degrees of preference to salinity at
different stages of their life cycle. Hughes (1969) indicated that
tidal transport of postlarvae may be initiated by increases in
salinities of flood tides. Hildebrand (1955) reported that juveniles
exhibited a preference for salinities of 20 ppt or more. As they
grow, they move into deeper, saltier water, until finally they leave
the bays and enter the open sea (Williams 1955). Gunter et. al.
(1964) reported the greatest biomass of pink shrimp along the gulf
coast was distributed “around the South Florida islands, where the
salinities are oceanic.” Tabb et. al. (1962) found postlarvae at
salinities from 12 to 43 ppt, juveniles from 5 to 47 ppt and adults
Valued Ecosystem Components
Freshwater Flow and Ecological Relationships in Biscayne Bay 4-36
from 25 to 45 ppt in Florida Bay. Adult pink shrimp have been
found on the Tortugas grounds at salinities from 36.2 to 37.7 ppt
(Iversen and Idyll 1960).
Interactions between water temperature and salinity impose strict
environmental restraints on shrimp populations. At low
temperatures, all shrimp have difficultly adjusting to changes in
salinity; survival rates are higher at moderate to high salinities
under conditions of low water temperatures (Williams 1960).”
Maintaining desirable salinity regimes has been described as one of the
critical factors for maintaining seagrass communities and habitat for pink
shrimp. (Bielsa et. al. 1983) identify that “direct saltwater intrusion (or
diversion of natural freshwater discharge), which causes unfavorable
salinity regimes” is a leading cause of degradation of shrimp nursery
habitats”.
Browder et al. (1999) used two modeling approaches to explore the
potential basis for variation in recruitment of pink shrimp from its Florida
Bay nursery grounds to the Tortugas fishing grounds. The results indicate
a strong relationship between density of juvenile shrimp and sea-surface
temperatures. Three other variables (rainfall, water levels in Everglades
National Park and mean wind speed) were also correlated with juvenile
pink shrimp densities.
Although pink shrimp are vulnerable to a variety of diseases, none exerts
an appreciable impact on the commercial pink shrimp fishery (Bielsa et. al.
1983). For these various reasons, the pink shrimp appears to be an
excellent potential indicator species for maintaining desirable salinity
regimes in those portions of Biscayne Bay where the shrimp fishery is
currently thriving.
Crabs
There are three species of crabs (i.e., blue crab Callinectes sapidus, stone