The Use of Scales to Estimate the Prey Size of Nearctic River Otters (Lontra canadensis) and Other Piscivores

Abstract

Estimating the size of fish prey requires the use of relationships between the size of non-digestible fish remains (recovered as prey in scats or digestive tracts) and fish length. The applicability of scales for estimating the size of fish prey eaten by river otters (Lontra canadensis) or other piscivores was evaluated by conducting a linear regression analysis of scale size and fish length for 22 species and six multi-species groups from the Red River of the North tributaries of eastern North Dakota. Analyses included six scale measurements, and separate models were constructed for lateral line and non-lateral line scales. Positive relationships existed between scale size and fish length in most (42 of 44) single species models, with lateral line scales (r2 ranging 0.317–0.994) typically producing better relationships than non-lateral line scales (r2 ranging 0.136–0.959). Body-scale relationships also existed when including multiple species in a model (r2 ranging 0.471–0.984 and 0.305–0.953 for lateral line and non-lateral line scales, respectively). Scales, and particularly lateral line scales, are highly useful in estimating the size of fish prey, but there are limitations including: the lack of scales on some fish species, potential degradation in passage through the digestive system and the absence of lateral line scales from some prey remains.