Spatial and temporal availability of Pacific salmon mediate compensatory growth in coastrange sculpin and sex-age specific spatial habitat use in brown bears

Abstract

Pacific salmon bring marine nutrients into freshwater ecosystems and provide food for countless species. Two major consumers of salmon along the Pacific coast are Coastrange Sculpin Cottus aleuticus and brown bears Ursus arctos. I examined the effects of salmon temporal availability on the growth rate of sculpin and the spatial use of habitat by brown bears. In a study on Coastrange Sculpin I found that treatment groups denied food, in the form of salmon eggs, for two or three weeks exhibited compensatory growth following resumption of feeding in the laboratory and field respectively. The compensatory growth response was mediated by stream of origin and temperature, controlled in the laboratory. Sculpin groups from different streams of origin had different overall growth rates in the laboratory despite being housed under identical conditions. At the end of the food-deprivation period, sculpin housed at 20°C exhibited a 5% greater weight loss compared to sculpin housed at 10°C. High temperatures (20°C) prevented compensatory growth in one treatment group and lengthened the catch up period for other treatment groups compared to those housed at moderate temperatures (10°C). In a study on brown bear spatial and temporal use of habitat in Berners Bay, Alaska, I found that brown bears use salmon when they are available and reproductive status of adult female bears affects use of this resource. Brown bears used space within 250 meters of salmon spawning reaches 50% more when salmon were present (69%) than when salmon were not present (19%) at spawning reaches. However, sex, age, and reproductive status all affected the use of space near spawning reaches. When salmon were available, adult females without cubs used space similarly to dominant adult males, while females with cubs used space similarly to subdominant juveniles. Adult females without cubs used space within 100 meters of spawning reaches 13% more than females accompanied by cubs. Salmon runs in Southeast Alaska are exhibiting reduced duration and earlier arrival of migration timing, reducing their temporal availability to consumers. Additionally, climate change is projected to increase temperatures throughout Southeast Alaska. Warmer temperatures resulting from climate change may improve digestion and growth for sculpin in colder streams, but could negatively affect the ability of sculpin to exhibit compensatory growth. Reduced salmon availability and increased human activity might lead to changes in behavior and dominance among brown bears feeding on salmon.