| River otters (Lontra canadensis) in Yellowstone Lake, Yellowstone National Park, depend on native cutthroat trout (Oncorhynchus clarki bouvieri) as prey, especially during summer when the fish migrate from the lake into its tributary streams to spawn. As part of their social behavior, otters deposit excreta at latrine (scent-marking) sites along these spawning streams and fertilize riparian plants with aquatically-derived nutrients such as nitrogen (N). Over the last two decades, however, nonnative lake trout trout (Salvelinus namaycush) and whirling disease (caused by the parasite Myxobolus cerebralis), have contributed to a dramatic decline in the lake's cutthroat trout population. Declines in cutthroat trout could reduce the abundance and distribution of otters, and sever the nutrient link they provide. This dissertation describes aspects of the biology of river otters in Yellowstone Lake in the context of these changes to the lake's food web.;This dissertation is presented in four journal-formatted chapters. Chapter 1 compares the hematology of high elevation Yellowstone otters with a sea level population. Comparisons of oxygen dissociation curves revealed no significant difference in hemoglobin-oxygen binding affinity between the two populations. Instead, high elevation otters had greater hemoglobin concentrations than their sea level conspecifics. Yellowstone otters displayed higher levels of the vasodilator nitric oxide, and half the concentration of the serum protein albumin, possibly to compensate for increased blood viscosity. Theoretical aerobic dive limits were similar between high elevation and sea level otters due to the lower availability of oxygen at altitude.;Chapter 2 presents the first estimates of population size and survival for river otters in Yellowstone National Park. Although the cutthroat trout population continued to decline during the study, there was no significant change in otter population size. However, overall densities (1 otter/9.91 km of shoreline) were lower than reported for most river otter populations, and evidence of a recent genetic bottleneck suggested the population may have already declined by the onset of the study. Annual survival for otters was 0.67 and correlated with cutthroat trout abundance in some models. Otters primarily responded to reductions in cutthroat trout via behavioral shifts in their spatial distribution and prey consumption. Activity at latrine sites decreased with declines in cutthroat trout on several tributary streams. Cutthroat trout remained the dominant prey item in otter scats throughout the study, but showed a declining trend from a frequency of occurrence of 73% in 2003 to 54% in 2008.;Chapter 3 documents the effects of otter-derived N fertilization on trees at latrine sites. Stable N isotope (15N) values were higher in needles of Engelmann spruce (Picea engelmannii) and lodgepole pine (Pinus contorta) on otter latrines than at reference sites. However, tree growth was greater on reference sites. No relationship existed between cutthroat trout abundance and tree ring 15N, precluding the ability to detect whether the trout-otter-excreta link has changed with the cutthroat trout population.;Chapter 4 is a note on seasonal breeding in Yellowstone otters. Although river otters in temperate regions typically breed in March and April, Yellowstone otters showed evidence of late seasonal breeding in June. This observation suggests otters in Yellowstone Lake have a relatively long breeding season, or their breeding schedule is delayed so that energetically-demanding lactation coincides with spawning runs of Yellowstone cutthroat trout. (Abstract shortened by UMI.). |
