Shoal occupancy estimation for 3 lotic crayfish species in the Tallapoosa River basin, Alabama

Abstract

The greatest diversity of crayfishes in the world is in the southeastern United States; however many species are at risk and lack of information on habitat requirements and the effects of habitat alteration hamper crayfish conservation efforts (Jones and Bergey 2007, Taylor et al. 2007). Two priority level 2 species (P2; ADCNR) of crayfish are endemic to the piedmont region of the Tallapoosa River Basin; Cambarus englishi, and closely related Cambarus halli, (Schuster et al. 2008). Additionally, widespread priority level 5 (P5) species, Procambarus spiculifer, have been documented in the region (Ratcliffe and DeVries 2004). Conservation of native fauna in large rivers is increasingly dependent on flow management therefore native fauna of the middle Tallapoosa are potentially strongly affected by flow management employed by Harris Dam (Irwin and Freeman 2002). Occupancy was estimated using methods outlined by Mackenzie et al. 2002 for crayfishes as part of adaptive management of the Tallapoosa River to gain understanding on how flow dynamics affect biota. Specific objectives were to determine variables affecting species specific detection probabilities and compare site level occupancy estimates between regulated and unregulated reaches. Additionally, catch data were examined for differences in size structure among sites. Lotic crayfishes were collected from shoals at 3 regulated and 2 unregulated reaches of the Tallapoosa River basin using pre-positioned area electrofishers (PAE). Detection probability and occupancy were modeled from presence- absence data as a function of a priori covariates and estimated in Program PRESENCE using the custom single-season single-species models. Model selection was based on the principle of parsimony and superfluous models were eliminated. Weighted model-averaged parameter estimates and unconditional sampling variances were calculated (Burnham and Anderson 2002). Multiple PAE’s (i.e. spatial replication; n= 5-20) were collected with habitat characters depth, velocity, percent vegetation, and substrate composition recorded and used to model detection. Site level occupancy covariates were based on the a priori hypotheses that occupancy was lower in regulated reaches due to negative impacts of hydropeaking on recruitment and /or occupancy varied along a linear downstream recovery gradient from Harris Dam and one a posteriori hypothesis that occupancy differed among the 5 reaches. Detection was low for all species in most years which affected precision of occupancy estimates. A few sites consistently had a high number of detections while others consistently had few. Variation in number of detections likely reflected changes in relative underlying populations of crayfishes potentially related to differences in habitat quality, food quality, number of available refuges, or predation risk. At least one individual of P. spiculifer, C. englishi, and C. halli were collected from almost every shoal at least once in the five year sampling period however occupancy estimates varied spatially and temporally. Modeling results suggested occupancy was similar in regulated and unregulated reaches of the basin in a ‘wet’ year while spatial differences were observed among reaches in all other years. Temporal differences were potentially related to basin hydrology. Data supports occupancy of P. spiculifer was close to one (Ψ ≈ 1) throughout the basin and occupancy of C. englishi was higher in the regulated reaches (Ψ ≈ 1) than unregulated reaches (Ψ ≈ 0.50 - 0.60) in most years. Extremely low detection due to (i.e., sparse data) resulted in model uncertainty making estimates for C. halli variable and difficult to interpret. Further investigation of distribution and habitat use for C. halli is warranted and C. halli may be more abundant in tributaries (Ratcliffe and DeVries 2004). Understanding habitat use of endemic species is important for recommending management actions directed towards conservation of crayfishes. Habitat covariates supported predicted biological responses, were sensitive to annual basin hydrology, and supported evidence of habitat partitioning among species. Vegetation was important for all species demonstrating a positive effect on detection. Depth influenced detection probabilities in ‘wet’ year and velocity influenced detection in a ‘drought’ year. Catch data also supported evidence of population level responses to drought including changes in size structure and potential density reductions and variation in recovery time among reaches. No evidence supported that the closely related Cambarus species competitively exclude one another; however, size differences were observed between species and C. halli may limit their use of shoals in the presence of C. englishi which may have resulted in consistently low detection of C. halli in our study. In addition, depth having a strong influence on detection of C. halli and the observed inverse relation to substrate size between the C. halli and C. englishi may be evidence of habitat partitioning among these closely related species.