Fisheries: Crayfish Effects on other Invertebrates and Basal Resources in the Presence and Absence of Trout
David A. Lieb - Graduate Research Assistant
Dr. Robert F. Carline - Adviser
Crayfish often account for a major portion of macroinvertebrate biomass and production in lotic systems. As omnivores, they consume items from multiple trophic levels and exert direct and indirect effects on basal resources (detritus, algae, macrophytes) and other macroinvertebrates. Crayfish are also an important food item for stream fishes including trout and bass. As such, crayfish behavior and habitat preferences often change in the presence of fish (presumably to reduce predation risk). Thus, fish may modify the effect of crayfish on basal resources and other macroinvertebrates. To date, few studies that address this possibility have been conducted. Additional studies conducted across a range of physical and biological conditions need to be completed before the role that crayfish/fish interactions play in the dynamics of freshwater food webs is fully understood.
1. Determine if crayfish affect basal resources (detritus and algae) and other invertebrates in riffle and pool areas of a limestone stream in central PA (Spruce Creek).
2. Determine if this effect is modified by the presence of wild brown trout (Salmo trutta).
A 54-day caging experiment with three treatments (cages with crayfish, cages without crayfish, and uncaged control areas) was conducted in Fall 2002 to determine if crayfish affect other invertebrates and basal resources in pool areas of Spruce Creek, a limestone stream located in central Pennsylvania. Samples were collected prior to adding crayfish to cages and 11, 33, and 54 days after crayfish addition. For all treatments/collection dates, invertebrate samples were collected from gravel, bricks, and leaf packs and algal samples were collected from bricks and gravel. Invertebrate brick samples collected on Day 33 have been processed and preliminary analyses have been completed. These analyses indicated that by Day 33 of the experiment there were 65% fewer invertebrates in the cages with crayfish than in the cages without crayfish resulting in highly significant (p=0.007) differences between treatments. Bricks from uncaged control areas generally supported the same invertebrate community (both in terms of density and community composition) as bricks from cages suggesting that caging artifacts were minor and probably did not affect the outcome of this experiment. Samples collected prior to adding crayfish to cages are currently being processed and will be analyzed to ensure that differences between treatments were due to differences in crayfish density and not some other unknown factor. Additional experiments will be conducted this fall to determine if trout presence modifies the effect of crayfish on other invertebrates and basal resources in pool areas of Spruce Creek.