When Comparing Between Months, the Number of Crayfish Collected at Each Trap and Site (Wilcoxon

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.

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 east-west 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 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).