Rusty Crayfish and Smallmouth Bass in the Susquehanna River: Who’s Eating Whom?
Two nonnative crayfish species, Rusty Crayfish (Faxonius rusticus) and the Allegheny Crayfish (Faxonius. obscurus), are currently found in the Susquehanna River. In 2015, we hand collected crayfish from among six sites spanning almost 115 river miles. Thirty stomachs were dissected from similarly sized individuals at each of the six sites; two of the sites had only Allegheny crayfish present, and the remainder had rusty crayfish. On average 49% of the stomachs contained food across all sampled sites, but at the individual sites this ranged from 27 to 77%. Aquatic macroinvertebrates comprised most of the observed prey, and in particular, mayflies. The 174 total individual prey items we recovered represented 29 different taxa. Macroinvertebrate prey in individual stomachs ranged from 5-17 taxa, indicating substantial breadth among prey groups. We found 10 genera of mayflies represented in the stomachs, but Leucrocuta was the dominant genus. Most surprisingly, however, was our discovery of many prey species within the stomachs entirely intact. These findings suggests that these crayfish were engulfing their prey whole. Both crayfish species also preyed upon snails in the river. However, Rusty Crayfish demonstrated higher mean predation rates on Physa acuta than did the Allegheny Crayfish. In the lab, Rusty Crayfish conspicuously favored smaller-bodied, softer-shelled snails like P. acuta and Leptoxis carinata, particularly in comparison to Pleurocera virginica, a snail species with a thicker, larger shell. Our findings suggest that this selective predation on snails by Rusty Crayfish could significantly alter gastropod communities in the river.
While we could not measure crayfish predation pressure directly on Smallmouth Bass nests due to high and turbid water during the spawning seasons, our proxy experimentation using trap arrays indicated that predation pressure by crayfish varied greatly between river locations. Twice as many crayfish were captured in high crayfish density sites versus at low density sites, suggesting that bass in these areas would work considerably more to defend their nests from marauding crayfish attempting to consume bass eggs. We designed, built, and field-tested a crayfish density device (CDD), and in conjunction with a sampling protocol that we also developed, obtained reliable and repeatable estimates of crayfish density. Crayfish density tended to increase in a downstream direction in the Susquehanna River, and in one individual CDD sample we collected 30 crayfish/m2. Interestingly, we found no statistically significant relationship between crayfish density in the river and crayfish relative abundance (estimated using baited crayfish traps).
Crayfish were a common prey for Smallmouth Bass in the Susquehanna River, and stable isotopes indicated diet variation between some of the sites. Crayfish dominated bass diet at all six sampling sites, but seemed to increase in importance moving downstream (into higher crayfish density areas). Our data suggest that crayfish were a larger dietary component at sites where Rusty Crayfish occurred, even when the density and relative abundances of rusties were low. This led us to hypothesize that perhaps there was a behavioral component of Rusty Crayfish that contributed to their consumption by bass (e.g., an antagonistic response to predators).
Finally, the condition of Smallmouth Bass in the river, estimated by their relative weight (Wr), varied considerably among the sampling sites. The average Wr at most of the sampling sites fell short of the recognized standard for Smallmouth Bass, and most of the individual fish at the sites were also substandard. Anecdotally, at one of the sites we made field notes that the fish there obviously looked “skinny.” However, a general longitudinal pattern evident in fish condition led us to investigate a relationship between crayfish abundance and bass condition. Our investigation revealed a significant positive relationship between Smallmouth Bass condition and crayfish relative abundance, such that areas with more crayfish seem to produce more robust bass. It seems that an invasive species of crayfish is enabling an introduced fish species in the Susquehanna River.