Abundance estimates for a remnant population of extirpated Lake Erie cisco Coregonus artedi (albus) in Crystal Lake, Pennsylvania

The discovery of a remnant Lake Erie cisco ( Coregonus artedi “ albus ”) population in Crystal Lake, Pennsylvania provides a unique opportunity to evaluate the status of a fish that was extirpated, with the possibility that the unique Lake Erie form was extinct. Historically, Lake Erie cisco supported the largest freshwater fishery in the world, with annual harvests exceeding 22 million kilograms — nearly equivalent to the contemporary commercial harvest of all fish species in all of the Great Lakes, combined. The loss of the ecologically and economically important albus cisco has been a major impediment to restoring native coregonine diversity and food-web function in Lake Erie. To assess this remnant population, we integrated nighttime split-beam hydroacoustic surveys with daytime remotely operated vehicle (ROV) videography during stratification to estimate abundance and characterize habitat use by cisco. Hydroacoustic detections were restricted to an oxythermal habitat band suitable for cold-water fishes and filtered to include target strengths consistent with age-2 + cisco and lake trout, while ROV footage provided species-level confirmation and informed apportionment of acoustic targets. Cold-water fishes were confined to the metalimnion and occurred at low but measurable densities. Based on combined acoustic and visual information, mean age-2 + cisco abundance was estimated at N = 815. Biomass estimates were similar to estimates from other self-sustaining inland lake populations. The presence of multiple year classes and older fish indicated high survival and ongoing recruitment. These results provided the first quantitative estimates of the only known population of Lake Erie “albus” cisco and suggests that Crystal Lake supports a viable source population for restoration efforts.