Trout-perch: Is it a trout? Or a perch? or Neither? by Don Orth

The Trout-perch Percopsis omiscomaycus (Walbaum) is a widely distributed species in North America, yet they present a number of puzzles.  Percopsis means “perch like” (Greek, perke=perch and opsis=appearance) and omiscomaycus is an Algonkian name that includes the root word “trout.” This enigmatic fish is certainly not a trout – though it has an adipose fin and a scaleless head --  and certainly not a perch – though its coloration and spines are somewhat similar.   It has traits that are characteristic of each group.  For example, the dorsal fin has 2 weak spines and pelvic and anal fins each have one weak spine. And the scales are ctenoid with a single row of teeth on the edge.  But, there is that puzzling adipose fin.   The head is large and the premaxillary bones are nonprotractile, unlike the Yellow Perch Perca flavescens.  Trout-perch are ray-finned fish in the Class Actinopterygii (ACK-tih-NOP-tuh-RIJ-ee-eye).  They are a surviving remnant of mostly marine fish that marked the transition between soft-rayed and spiny-rayed fishes. They belong to a superorder called Paracanthopterygii (literally "like" spiny rays) and Trout-perch are the only fish in that group with an adipose fin.   

Trout-perch Percopsis omiscomaycus (Walbaum) Illustration by Ellen Edmonson  Source

Phylogeny of trout perch is just plain confusing because they possess a primitive trait, the presence of adipose fin, but many other derived traits.   This is likely an example of character reversal, where the adipose fin (the ancestral trait) reappears during the evolution of the lineage.  The genus Percopsis is  taxonomically very distinctive. There are two species, the Sand Roller Percopsis transmontana,  ranges from western Idaho in the lower Snake River westward through the lower Columbia and Willamette drainages.     Recently Thomas Near and colleagues estimated, after analyses of DNA sequences from 232 fish species, that divergence of trout-perches from marine relatives, such as deep-sea beardfishes, Polymixiidae, and cods, Gadiformes, began over 100 million years ago.  The closest extant relatives include the cavefishes, Amblyopsidae, and Pirate Perch, Aphredoderidae in the order Percopsiformes.  Although these relatives have external characteristics that are very different from Trout-perch, they are classified together due to internal similarities. All members of the Percopsiformes are freshwater fishes, but the group appears to have a marine origin.

The distribution ranges widely from north to south in deep lakes as well as large streams. Trout-perch are currently found from the Missouri River drainage east to the Connecticut River, south to the Potomac, and from the Ohio River north throughout the Red, Saskatchewan, Mackenzie, and Yukon Rivers to the north.  Following the last glacial age, the Trout-perch probably moved from the lower Mississippi valley into newly thawed water bodies as glaciers retreated northward. Despite this broad distribution and locally high abundance, there is seemingly little known about this small-bodied benthic fish. 

Range of the Trout-perch (Becker 2001)

What do they eat?  Trout-perch consume a variety of benthic animals, depending on their habitat.  The are nocturnal feedings and move inshore to feed.  Most common diet items include Chironomidae larvae, Hexagenia nymphs, amphipods, Cladocera, and  Copepoda.  Trout-perch are seldom captured in daytime shoreline seining even in waters where it is abundant.  They appear to move inshore at night to feed.  In the Great Lakes, they overlap with Ruff Gymnocephalus cernua and Yellow Perch Perca flavescens, juvenile Walleye Sander vitreus, and Round Goby Neogobius melanostomus.   Trout-perch diet preferences overlap with Yellow Perch and juvenile Walleye.   In Lake Erie, “trout-perch ingested benthic prey items in proportion to their availability in the environment” and Chironomidae, Hexagenia, Nematoda, Hirudinea, Amphipoda, and Trichoptera composed over 90% of benthic macroinvertebrates in the diet by number.  They appear to avoid competition with other fishes by using cooler areas of the lake. 

Near shore abundance trends in Lake Erie source (Kocovsky et al. 2014)

In Lake Michigan, native species (Emerald Shiner Notropis atherinoides, Lake Herring Coregonus artedi, and Deepwater Cisco Coregonus johannae) declined when Alewife Alosa pseudoharengus and Rainbow Smelt Osmerus mordax invaded; however, Trout-perch were seemingly not affected by the Alewife increase.  The Trout-perch is the most abundant fish in trawl samples from western Lake Erie (Kocovsky et al. 2014).  Through the multiple invasions of White Perch Morone americana, Quagga and Zebra mussels Dreissena spp., spiny waterflea Bythotrephes longimanus and Round Goby in Lake Erie, the  Trout-perch remains an abundant benthic invertivore.  

Growth curves (total length, mm, plotted against year of life) House and Wells (1973)

Trout-perch never grow very large (maximum length is only 13 cm).  Therefore, they are vulnerable to predation by many piscivores.   Trout-perch are eaten by many fishes, including the northern pike, burbot, yellow perch, walleye, freshwater drum.  However, they are not always a preferred prey and not always common in diets of Walleye or Lake Trout even where they overlap.  Why?   No one is sure.  They provide equivalent energy to soft-rayed fishes, yet walleye positively select soft-rayed fishes as prey (Knight et al. 1984; Hall and Rudstam 1999).   As a trophic link between benthic invertebrates and large lentic fish the abundant Trout-perch remain an enigmatic weak interactor.     

The Trout-perch will remain an enigma for some time.  However, students of Ichthyology will always appreciate this easy-to-identify small fish with a big head and an adipose fin.    It’s the only Percopsis in most regions and the genus, family and order names all have the same root.   They appear in many standardized fish trawl sampling programs and, therefore, a long-term record of abundance continues to accumulate.  

References
Becker, G.C. 2001.  Fishes of Wisconsin. University of Wisconsin Press, 1052 pp. 

Blouzdis, C.E., L.N. Ivan, S.A. Pathoven, C.R. Roswell, C.J. Foley, and T.O. Höök.  2013.  A trophic bottleneck?: The ecological role of trout-perch Percopsis omiscomaycus in Saginaw Bay, Lake Huron. Journal of Applied Ichthyology 29:416-424.

Hall, S.R., and L.G.Rudstam. 1999. Habitat use and recruitment:  comparison of long-term recruitment patterns among fish species in a shallow eutrophic lake, Oneida Lake, NY, U.S.A. Hydrobiologia 408/409: 101–113.

House, R., and L. Wells. 1973.  Age, growth, spawning season, and fecundity of the trout-perch (Percopsis omiscomaycus) in southeastern Lake Michigan.  Journal of the Fisheries Research Board of Canada 30:1221-1225.

Knight, R.L., Margraf, F.J., Carline, R.F., 1984. Piscivory by walleyes and yellow perch in western Lake Erie. Transactions of the American Fisheries Society 113, 677–693.

Kocovsky, P.M., A.T. Stoneman, and R.T. Kraus.  2014. Ecology and population status of Trout-perch (Percopsis omiscomaycus)in western Lake Erie.  Journal of Great Lakes Research 40:208-214.

Near, T.J., and eight coauthors. 2012.  Resolution of ray-finned fish phylogeny and timing of diversification.  Proceedings of the National Academy of Sciences of the USA 109(34):13698-13703.