Physical and chemical factors affecting the upstream migration of amphidromous shrimp in the Luquillo Experimental Forest D.A. Kikkert, T.A. Crowl, and A.P Covich
MIGRATION Persistent, undistracted movement from one habitat to another at specific life stages (sensu Dingle 1996). Among aquatic biota the best studied migrations are those of diadromous fishes such as salmonids and eels.
DIADROMY MIGRATIONS BETWEEN MARINE AND FRESHWATER ENVIRONEMNTS AT PARTICULAR LIFE STAGES (MYERS 1949) ANADROMY SPEND MAJORITY OF LIVES IN THE SEA, MIGRATE INTO FRESHWATER TO BREED CATADROMY SPEND LIVES IN FRESHWATER, MIGRATE TO THE SEA TO BREED AMPHIDROMY
AMPHIDROMY • Migration between fresh water and sea for purpose • other than breeding • Migration usually occurs during the larval life stage • Most common on tropical oceanic islands where it • may be necessary for dispersal.
Freshwater Adults Adults Post Larvae Larvae Larvae Ocean
Luquillo Experimental Forest (LEF) • Humid, subtropical climate • Annual rainfall ranges from 1000 to 6000mm • Slightly seasonal with a dry season from • February-April • Nine streams originate within the LEF
Luquillo Experimental ForestSTREAMS • Headwater streams characterized by steep gradient (10-20%) • Primarily boulder and cobble lined with bedrock glides and waterfalls common • Discharge is highly variable with flows increasing rapidly during rainfall
SHRIMP • Headwater streams dominated by 2 species of atyid shrimp (Xiphocaris elongata and Atya spp.) and the freshwater prawn Machrobrachium spp. • Adult populations in headwater streams are well studies as part of an NSF long term monitoring program • Little is known about the upstream migration of post-larvae
UPSTREAM MIGRATION • Nocturnal migration • Hypothesized increase in migration during times of low predation risk • Critical for recruitment into adult populations • Increasing human development in the coastal plain • Road / stream intersections could act as barriers (Biocomplexity project)
OBJECTIVES • Quantify upstream migration of Xiphocaris elongata, Atya spp., and Macrobrachium spp. • Identify regular patterns in shrimp migration rates • Identify environmental factors that may affect upstream migration
METHODS FIELD STUDY ARTIFICIAL STREAMS
FIELD STUDY • Sampled shrimp migration over 5 months (N=46 nights) • Used multiple regression to determine if environmental variables explain variation in shrimp migration rate and timing
RESULTS WET SEASON SAMPLING (N=34) DRY SEASON SAMPLING (N=12) Atya spp. F=3.18 p=0.07 Xiphocaris elongata F=12.77 p=0.0005 Macrobrachium spp. F=24.86 p=0.0001
RESULTS REGRESSION Atya Spp.
RESULTS REGRESSION Xiphocaris elongata
RESULTS REGRESSION Xiphocaris elongata - WET SEASON ONLY
RESULTS REGRESSION Macrobrachium spp.
RESULTS REGRESSION Macrobrachium spp. - WET SEASON ONLY
RESULTS ARTIFICIAL STREAMS Atya spp. * * * * * DENOTES SIGNIFICANCE AT P=0.05
RESULTS ARTIFICIAL STREAMS Xiphocaris elongata * * * * DENOTES SIGNIFICANCE AT P=0.05
CONCLUSIONS • Atya spp. make up majority of shrimp migration • Xiphocaris elongata and Macrobrachium spp migrations seasonal • Macrobrachium make up a large component of the shrimp migration in August • Ayta spp effected by light levels and flow regime with increased migration following spates • Xiphocaris and Macrobrachium spp. migrate during periods of low • Migration rates of all species decrease during extended periods of low flow
CONCLUSIONS • Atya and Xiphocaris avoid channels with possible low water quality (turbidity and leaf litter) • Spates may increase water quality and shrimp migration • Chemical cues from fish predators may play a role in shrimp migration • Atya appear to be positively rheotactic, following the highest flow
Acknowledgments • Todd Crowl • Katie Hein • Alan Covich • Wyatt Cross • Ecology Center (Utah State University) • Ruth Kikkert (All the fieldwork)