River otter (Lontra canadensis) populations in Illinois have rebounded considerably after >80 y of harvest protection and a successful re-introduction program. However, few studies of river otter survival ecology exist in the Midwestern U.S. We estimated survival rates and mortality causes for river otters in southern Illinois during 2014–2016. Thirty-four (16 F, 18 M) river otters were radio-marked and monitored for 8235 radio-days (x̄ days/river otter = 242.2 ± 20.6 [se throughout]). Two males died (one trapped, one unknown) during the period of radio-telemetry monitoring. Annual survival rates were 1.0 ± 0.00 (lower confidence bound = 0.83) and 0.85 ± 0.09 for females and males, respectively, and similar between sexes (P = 0.19). Pooled-sex breeding season survival was 0.96 ± 0.04. Trapping was the primary cause of mortality during our study, and three river otters were killed after radio-telemetry ended: two were harvested by recreational trappers and one by a vehicle collision. These primary mortality sources (i.e., trapping and vehicle collisions) for river otters in southern Illinois were similar to those reported elsewhere, but the high survival we observed is similar to published estimates for unexploited populations. Our study provides useful demographic information for river otter management in lightly harvested populations in the Midwest.
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Vol. 180 • No. 1