Invasive Red Swamp Crayfish

I spent the day working with Stephanie, and we are in the process of obtaining permits to construct a nontidal wetland abutting the fresh tidal Chicamuxen Creek.  Part of our wetland hydrology will tie-into a sweetbay magnolia sand and gravel bench seep.  As we walked through the fresh tidal buffer, a rarely used roadway, mowed field and seep swamp, we were quite surprised at the number of Red Swamp Crayfish (Procambarus clarkii) that we saw.  They were everywhere, probably in excess of sixty 5-inch, adult individuals.


The Red Swamp Crayfish is native to the United States, but originally confined to the lower Mississippi River and Gulf Coast, primarily Louisiana.  These are the famous edible crayfish of the Louisiana Crawfish Boil.  Although native, they are not endemic to Maryland, and are considered an introduced invasive.


Fourteen crayfish species are known to occur in Maryland.  Five are introduced, non-native species.  Of the five introduced species, the red swamp crayfish are particularly invasive and can alter species composition, food webs and habitat in aquatic ecosystems.  These crayfish have only been in Maryland for the last decade or so, and primarily came from the release of live crayfish used as bait for fishing.


The most obvious impact of invasive crayfish in Maryland has been the concomitant declines in native crayfish species.  The Red Swamp and a few others such as the Rusty and Virile Crayfish tend to quickly colonize a new area and become abundant.


Invasive crayfish are known to adversely affect stream insects, mussels, snails, amphibians, reptiles, fish and sport fisheries, and alter community structure and function of aquatic ecosystems.


Once an invasive crayfish is introduced into an aquatic system, it is impossible to eradicate it without harm to native crayfish and other non-target species.  Since eradication is impossible without harming other species, preventing introductions is the one effective way to halt the spread of invasive crayfish.


The Red Swamp Crayfish now occupies Charles and Prince George’s County on the western shore and parts of Dorchester, Worcester and Queen Anne Counties on the eastern shore.


The Red Swamp’s can be found in creeks, swamps, ditches, rivers and ponds with muddy or sandy substrate, and can be found among woody debris, aquatic vegetation and detritus.

Based on today’s observations, these crayfish have great vision.  They tracked our movements, and every time we got close, they went into a defensive posture, with claws-up and ready to fight.  If I was a River Otter, I’d be all over this site for some serious snacking!


I laid down in the road to take this photo of a walker.  Swamp Red’s can live up to 5-years old and tolerate the slightly saline brackish water of the Chicamuxen and Potomac River.  They will eat largemouth bass eggs, and largemouth bass will eat Red Swamp’s!  Note the characteristic carapace, claws and back covered in rough-textured tubercles (bumps).  Everywhere we looked, in the lawns, roads, terrestrial side slopes, wetlands and waters, we saw Red Swamp’s, with more than not orienting toward going to the fresh tidal river water of the Chicamuxen.  What a sight!  Local migration?


The arrow arum and pickerelweed in the river were dying back and signs of early autumn were apparent.  The Virginia Creeper, Parthenocissus quinquefolia, was bright red, as it draped over swamp edge trees.


Winterberry, Ilex verticillata, grew along with sweetbay and highbush blueberry along the banks.  With some leaves turning and already fallen, the striking berries of the winterberry were beginning to show.


The Possumhaw, Viburnum nudum is the most obligate (OBL) wetland Viburnum, and typically grows well into the wetter portions of a swamp.  We saw some plants with red, white and blue berries!


The only flowering plant along with aster that was in bloom on our site today was the Wild Ageratum (aka Blue Mistflower), Eupatorium coelestinum.


Smooth Alder (Alnus serrulata) was the dominant wetland shrub on the site.


Portions of the steep, sandy side slopes had patches of Trailing Arbutus (Epigaea repens).


Trailing Arbutus is uncommon, and has a beautiful early spring flower, white, with sometimes pink tones and quite fragrant.  Great day in the field!