Preliminary results on crayfish populations in the freshwater
marshes/coastal mangrove ecotones of the C-111 basin, Everglades National
Park
M. C. Bruno, O. Beceiro, and S.A. Perry
South Florida Natural Resources Center, Everglades National Park, Homestead,
FL
Under present water management operations, the wetlands that fringe northeastern
Florida Bay fluctuate between freshwater and marine conditions within the annual
wet/dry cycle, experiencing higher salinities, longer periods of saline intrusion,
and shorter hydroperiods than would have occurred in a non-managed system.
Restoration could re-establish natural salinity gradients through the overland sheet
flow, restoring the freshwater/oligohaline coastal wetland.
The freshwater marshes/coastal mangrove ecotones in northeast Florida Bay are
important, since they are more directly influenced by discharge flow from Taylor
Slough and C-111 canals than the mangrove habitats. Due to lower salinity, the
freshwater faunal component is higher.
The crayfish Procambarus alleni Faxon is a major trophic component in
Everglades wetlands, and will be used as an indicator species for monitoring the
progress of restoration programs. The potential impact of restoration strategies on
population dynamics of this key trophic species has been investigated for ENP
marl prairies. Our research extends crayfish studies in the mangrove-marsh
ecotone, to the southwest of Acosta and Perry’s earlier study, to assess crayfish
responses to salinity gradients.
METHODS
We selected three north-south transects in the panhandle of the C-111 basin,
named from west to east 59, 60, and 61. Each transect included three stations,
named N (north), C (central), S (south), about 0.5 mi distant from each other,
representing different habitats: sawgrass for the northern stations, dwarf
mangroves for the central stations, and creeks in the mangrove fringe for the
southern stations. We set 6 minnow traps at each site, retrieved them after 24
hours, and measured water depth at each trap, water temperature, salinity,
conductivity, and oxygen percent saturation in the field.
Samples were collected after the sampling areas reflooded, on June 18-20, 2002,
July 25-26, 29-30, September 17-18, 19, and November 12-13, 2002. In
November two of the northern sites were dry (60N and 61N), and water levels
were so low at 59N and 61C that we were able to set only 5 and 3 traps
respectively.
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RESULTS
Salinity varied among sites and over time, from values of 0.2 p.p.m., as typically
measured in the surface water of the interior of ENP, to a maximum value of 4.1
p.p.m. The northern sites had an average salinity of 0.35 p.p.m., the central sites
0.53 p.p.m., and the southern sites 1.15 p.p.m., thus identifying a gradient that
corresponds to the shift in plant communities. Transect 59, which is closer to
Taylor Slough, had the widest variation in salinity, especially at the central and
southern sites. Transect 61 was less variable and had lower values, when
compared with the corresponding stations of the remaining two transects. At
transect 61 salinity varied very little at the northern and central site around
freshwater values and varied more, with higher values (average: 1.275 p.p.m), at
the southern site. Salinity was higher and more variable in June and November
(average 1.1 and 1.0 respectively) than in July and September (average 0.3 for
both months).
Water depth was less variable at transects 59 and 60. At transect 61 the central
station had depths lower than the northern site, and depth at the southern site was
much lower than the southern sites of the other two transects, and was comparable
to the values for the northern sites. Water depth decreased over time, and values
were more variable in June and November.
We standardized for different numbers of traps set at different months and
calculated the relative number of crayfish collected at each site (total number of
crayfish collected/ number of traps set). Relative numbers were higher in June
(40%), and lower in the remaining months (18, 22, and 200% for July, September,
and November respectively). Along the north-south gradient, the highest percent
of individuals was collected at the northern stations (42%), and along the eastwest one, at transect 61 (43%). When comparing the total number of crayfish
collected for all months, the southern transect differed (p=0.003) from central and
south transects, and transect 61 differed (p=0.003) from transects 59 and 60. The
southern sites differed because only at 61S high number of crayfish were
collected, and collection in November had high numbers of individuals only at
61S, and 0 individuals at 59S and 60S. Sites 59N, C, and S differed, probably
because high numbers of crayfish were collected at 59N and C, and low numbers
at 59S.
Large adult crayfish (>18 mm) predominated at the central transect at all months,
whereas the northern transect had mostly large adults in June and July, and the
southern transect had mostly large adults in July and September. For the east-west
transect, crayfish population size structure at transect 61 differed significantly
from that at transect 59 (p=0.0357) and at transect 60 (p=0.0068). At transects 59
and 60 the length of individuals generally decreased from north to south, whereas
at transect 61 the lengths were higher at the southern sites than at 61C and 61N.
At all transects the monthly average values varied from small adults emerging
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from burrows in June, larger individuals present in July and September, and
smaller individuals in November. Juveniles represented 45% of the total
individuals collected in June, 10% in July, 24% in September, and 48% in
November. They were mostly collected at transect 59, and in the north sites.
The linearized growth curves of length-weight data and linear regression on male
and female crayfish differed between sites. For males, the growth slopes were
significantly less steep at 61S, 60S and 60N than at the other sites, for females at
59N.
DISCUSSION
Variation in salinity and water depth characterize a north-south and an east-west
gradient of increased salinity and water depth. As a consequence, high numbers of
P. alleni were collected at transect 61, which was more similar to freshwater sites,
and mainly at 61S which is more similar in water levels and salinity to the
northern sites at transects 59 and 60.
The lower growth rates at 59N and 60N may be due to the adverse impact of
locally shortened hydroperiods and associated habitat quality, as already shown
for marl prairies wetlands of ENP.
Small adults of P. alleni dominate in non-optimal habitats, such as short
hydroperiod habitats. In this study they were mostly present at the southern sites,
where salinity values are higher and their variation is greater.
Overall, results from our research suggest:
 Crayfish can disperse at the beginning of the wet season to colonize new
habitats where resources are abundant. As a consequence, crayfish may
occupy the habitats on the basis of local salinity optimal values, which
were not reflected in major changes in vegetation composition during that
time frame.
 The wide monthly variations in salinity and water depth in the proximity
of Taylor Slough have an adverse impact on P. alleni growth and
densities. When water levels become too great, P. alleni can migrate to
shallower areas, to avoid fish predation, as observed for P. alleni
populations in deep water sloughs (Jordan et al., 1996).
M. Cristina Bruno
South Florida Natural resources Center, Everglades National Park,40001 State
Road 9336, Homestead, FL. Pho: 305-247-8110. E-mail:
Cristina_Bruno@contractor.nps.gov
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