Freshwater crab abundances in headwater streams (Monteverde, Costa Rica)

We used capture-mark-recapture methods (CMR) to quantify Ptychophallus tumimanus (Pseudothelphusidae) freshwater crab populations across 20 headwater stream sites spanning a range of land uses, including forest reserves and an urban center in Monteverde, Costa Rica. 

We conducted the CMR survey from June to August 2018. At each site, two people collected crabs from riffles (fast-flowing, shallow areas) and runs. We sampled by conducting a visual search and turning over rocks, positioning a D-net downstream in areas with swift flow. We also searched for crabs under emergent rocks located on sand bars and stream banks. Pools were not sampled due to difficulty of catching crabs in deep areas with low visibility. We sampled crabs between 0800 and 1600 h. Surveys were time-limited to a minimum of 30 min, but sometimes extended to 45 min to account for crab handling time and to keep search time similar among sites (i.e., when large numbers of crabs were encountered, requiring increased handling time, we extended the survey duration to 45 min). The total sampled reach length varied among sites due to differences in numbers of crabs, habitat accessibility, and variations in stream width (longer reaches were sampled for narrower streams). We conducted the CMR surveys over three consecutive days at each site and sampled the same reach length each day. Over this short time interval, crab populations were assumed to be closed to immigration, emigration, recruitment, and mortality. 

All crabs were individually marked by using a drop of Loctite super glue to attach a piece of Rite in the Rain paper with a unique number to the middle of the carapace. A coat of quick-dry clear nail polish was applied on top of each glued number and allowed to dry prior to release. Crabs were released at random locations within the sampled reach. On the second and third sampling days at each site, we manually sampled crabs, recorded individuals with tags, marked newly caught individuals, and redistributed them throughout the sampled reach. 

We quantified a set of local environmental variables at each site: elevation, water temperature, conductivity, substrate type, wetted width, depth, stream velocity, pH, and % canopy cover. We estimated site-specific crab abundance and density using hierarchical Bayesian Markov chain Monte Carlo models and estimated the effects of potential covariates (elevation, water temperature, conductivity, % cobble, mean substrate size, channel width, depth, stream velocity, pH, % canopy cover) on crab abundances across sites.