The Use of GAM Modeling Techniques to Evaluate the Effects of
Freshwater Flow Into Florida Bay- Part 1- Forage Fish Models
Darlene Johnson and Joan Browder
NOAA Fisheries, Miami, FL
An integrative database was assembled from three decades of
forage fish studies conducted in Florida Bay.
GAM models for 10
species of fish and pink shrimp were developed from this
database. Samples were linked to other information: tidal
amplitude, sea level, rainfall, freshwater discharge indices, wind,
salinity, and habitat type (physical habitat, seagrass density, and
seagrass type).
The 11-species community selected for statistical
models comprised 87% of the throw-trap samples by number for
1980’s and 1990’s.
The group comprised 86% of the 1980’s trawl
data, 66% -89% of the 1990’s trawl data, and 74% of the 1990’s
seine samples.
During the 1970's, the 11-species group comprised
64% of the fauna sampled in the western bay, 92% in the central
bay, and 39% in the eastern bay.
A second database was
prepared that combined ENP creel census data with environmental
variables.
Freshwater flow was a significant model variable for all species,
while month, seagrass density, seagrass type, and tidal amplitude
were significant variables for 10 of the 11 species, followed by
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depth (9 species), habitat and temperature (8 species), and salinity
and rainfall (7 species).
Gear and wind (6 species), and sea level
(5 species) were less important variables in predicting the
abundances of forage species in the Bay.
All the resident species (except goldspotted killifish) and the
pelagic species, bay anchovy (which spawns both offshore and
inshore), showed a positive relationship with freshwater flow into
the Bay.
In contrast, the three species that spawned outside the
Bay showed either a negative relationship with the previous
month’s freshwater flow that flattened out at higher flows (pinfish)
or a negative/positive relationship to lagged freshwater flow.
Three species (bay anchovy, code goby, and gulf pipefish) were
negatively correlated with salinity and two species (dwarf seahorse
and mojarras) declined under hypersaline conditions.
Pink shrimp
and gulf toadfish showed a parabolic relationship to salinity
peaking at 29 ppt and 32 ppt respectively.
goby) was positively correlated with salinity.
One species (code
Predicted abundance
of the two smallest species (rainwater killifish and goldspotted
killifish) was highest at extreme conditions (low and high salinity)
which suggests competitive exclusion or increased predation at
moderate salinities.
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The density of two resident species (rainwater killifish and clown
goby) was negatively related to rainfall, one resident species (code
goby) was positively related to rainfall, and another resident
species (goldspotted killifish) had a parabolic relationship with
rainfall.
Among the outside spawners, pinfish was positively
correlated with rainfall (previous month), and mojarras were
negatively correlated with lagged rainfall.
Two resident species (rainwater killifish and gulf pipefish) and the
three offshore spawning species (pinfish, mojarras, and pink
shrimp) were positively correlated with seagrass density.
Predicted densities of gulf toadfish, dwarf seahorse, and
goldspotted killifish were significantly higher at moderate seagrass
densities. Densities of the pelagic bay anchovy were also
significantly higher at moderate seagrass densities. Densities of the
two gobies were highest in sparse seagrass.
Increased densities
of five species (gulf toadfish, dwarf seahorse, clown goby, code
goby, and bay anchovy) may have resulted from decreases in
seagrass density due to seagrass die-off in the late 1980's.
The
models suggest that declines in seagrasses from moderate to
sparse would result in increases in the two goby species, but
declines in gulf toadfish and dwarf seahorse.
The models predict
that declines in dense seagrass would lead to declines in rainwater
killifish, mojarras, pinfish, pink shrimp and gulf pipefish.
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Seagrass type was important for all species but gulf toadfish, with
significant higher densities in Syringodium (pinfish, dwarf seahorse,
rainwater killifish), Halodule (pink shrimp, gulf pipefish, goldspotted
killifish), mixed seagrass with Thalassia dominant (code goby),
mixed seagrass with Syringodium dominant (mojarras, clown goby). The
bay anchovy showed the strongest correlation to areas with no seagrass.
Densities of the three offshore spawners--pink shrimp, pinfish, mojarras were
positively related to one or more physical parameters that may be indices of
transport mechanisms for recruitment: tidal amplitude, sea level, and wind
forcing. Densities of bay anchovy, a species that may spawn outside of the Bay as
well as within it, also were positively related to these factors. In contrast, those
species that spawned exclusively within the Bay and may be less dependent upon
recruitment mechanisms showed less correlation with these factors.
Johnson, Darlene, NOAA Fisheries, 75 Virginia Beach Drive, Miami, FL 33149
Phone: 305 361-4490, Fax 305 361-4478, darlene.johnson@noaa.gov, Question 5
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