The North American river otter (Lontra canadensis) is a vitally important species, both biologically as an apex predator in riverine ecosystems, and economically as a furbearer species. River otters were once present throughout much of North America but were extirpated throughout much of the central United States by the 1970s due to excessive harvest, habitat loss, and other factors. Several reintroductions occurred throughout the 1980s and 1990s including an effort in Ohio between 1986-1993. The reintroduction effort in Ohio was considered successful enough that legal harvest was reimplemented in 2005. Since 2005 Ohio Division of Wildlife data has suggested that harvest is largely successful and sustainable, but little attention is currently being paid to population demographics and spatiotemporal variation of the populations in the state, especially as they relate to survival probability and harvest vulnerability.
Additionally, the diet of river otters in Ohio and how it varies with age group, sex, and region are not well understood. Several studies have examined river otter diet in other locations using scat analysis, and the diet of other species of otters have also been studied with stable isotope analysis. Previous research has indicated that river otters eat smaller fish in shallower waters or larger, less mobile, bottom-feeding fish, in addition to some invertebrates that help supplement their diet.
This study documented the current demographics of the river otter population in Ohio; modeled population trends, survival probability, harvest vulnerability, and recruitment trends for the river otter population in Ohio, determined the contribution of several prey items or prey item groups to river otter diet via stable isotope analysis, and determined how the contributions of these prey items or prey item groups varied with age group sex and location.
Population demographics, specifically the larger share of older individuals and older reproductive females, suggested that the population may be reaching a carrying capacity. The percentage of reproductive females of each age group did not demonstrate any clear trends relating to increasing or decreasing recruitment but did support the idea of a population reaching carrying capacity. The corpora lutea information indicated stable recruitment. The results of the SPR modeling were indicative of a population that is stable. The population abundance did not largely change over three years (λ ~ 0.99) and remained generally stable around a point. Harvest vulnerability was low (<5%) and juveniles were less vulnerable to harvest than yearlings and adults. Survival was high (>80%) for juveniles and was much lower (<60%) for yearlings and adults. Recruitment decreased each year and was notably lower (~57%) by the end of the study period than when it started.
Significant differences in the isotopic values of river otters sampled suggested regional differences (δ13C: F = 24.07, p = 5.46e-10, n = 183; δ15N: F = 31.28, p = 2.19e-12, n = 183) but no differences between age group and sex; additionally, the variation of isotopic signatures between individuals suggested that river otters tend to behave more as generalists than specialists. Results of a 95% mixing region analysis indicated that not all river otters were likely being accurately represented by the current mixing model, and that meaningful prey items or prey item groups are likely missing from the analysis. The mixing model results suggested that the Centrarchidae prey item group made up the largest portion of river otter diet (57.3%), followed by freshwater mussels (26.1%), the Cyprinidae prey item group (14.4%), crayfish (1.1%), and finally the Fish_1 prey item group, comprised of carp, catfish, darter, perch, and sucker species (1.1%). However, results of a 95% mixing region analysis indicated that not all river otters were likely being accurately represented by the current mixing model, and that meaningful prey items or prey item groups are likely missing from the analysis. The variation of prey item or prey item group contributions to river otter diet between individuals suggested that river otters seem to be more generalist predators. Lastly, the results of generalized linear mixed-effects models suggested that region was the biggest factor driving differences in river otter diet composition, not age group or sex. Even after otters were subset by region, age group and sex had no effect on river otter diet composition, with the exception of adult river otters in northeastern Ohio having a larger proportion of freshwater mussels in their diet compared to other age groups.
Within the river otter population modeling, the lack of a large sample, the lack of uniform spatial distribution, and the lack of continuous years of at-age harvest data are large limitations of this research and more data are needed before any meaningful changes in management practices can be recommended. Current, limited evidence suggests that river otter populations in Ohio may be stable at this time, suggesting current management policies are sufficient to maintain the conservation status of the otters in Ohio.
The results of the dietary analysis, despite several limitations, still provides useful insight that could offer beneficial perspectives for management. We recommend continued research pertaining to river otter diet in Ohio, we recommend the monitoring and maintenance of important prey items for river otters with a specific focus on regional variation, and we recommend further investigating potential human-wildlife conflict as a result of the large contribution of the Centrarchidae prey item group to river otter diet.