Stable isotopes reveal variation in consumption of Pacific salmon by brown bears, despite ready access in small streams

Hair samples (n = 2026) were collected during the months of July and August of 2012 through 2015, from six streams that flow into Lake Aleknagik (southcentral Alaska as part of the Wood River sytem): Happy, Hansen, and Eagle creeks on the northeastern side, and Yako, Whitefish, and Bear creeks on the southwestern side (Quinn et al. 2014; Wirsing et al. 2018). We designate these two trios of streams as separate foraging neighborhoods because bears often foraged on two or three of the streams on one side of the lake but almost never crossed the lake within or between years (Wirsing et al. 2018, and additional unpublished data). Annual estimates of sockeye salmon abundance in each stream, based on multiple counts each year (Quinn et al. 2017), indicated much higher densities on the northeastern side compared to the southwestern side (Table 1). To collect the hair samples, two unbaited barbed wires (average: 8 m long) with a barb every 12 cm were strung across each stream, attached to trees on either side using fencing staples. The wires, about 50 – 55 cm above the streambed at mid-channel in different reaches of each stream, were carefully checked for samples every second day to minimize sample degradation that results from prolonged exposure (Dumond et al. 2015). Hair samples were removed from the wire with tweezers, placed in dry coin envelopes in the field, and then stored with desiccant in the field. At the end of the season, samples were sent to the Laboratory for Evolutionary, Ecological, and Conservation genetics at the University of Idaho for DNA analysis to verify the species (i.e., brown rather than black bear), identify the individual, and determine the sex of each. Genetic material was extracted from hair samples using the DNeasy Blood and Tissue Kit (Qiagen, Inc.). Bear genotypes were generated for each sample using 10 nuclear DNA microsatellite loci (Paetkau and Strobeck 1994, Paetkau et al. 1998, DeBarba et al. 2010) and one sex identification marker (Ennis and Gallagher 1994). The observed and expected heterozygosities for these 10 loci are 0.71 and 0.69. Each sample was amplified two to four times to ensure accuracy. Consensus genotypes were derived following the rule that each allele must be observed twice at each locus and must contain data at eight or more loci to be included in the matching analysis. Two genotypes were considered a match in the program Genalex (Peakall and Smouse 2006) if they were identical or included a one allele mismatch at two or fewer loci that could be due to allelic dropout. Under this protocol, the probability of a match for unrelated individuals across 10 loci was 0.0000000011, and the probability of a match between siblings across 10 loci was 0.00025 (Waits et al. 2001). The probabilities of a match for unrelated individuals and siblings at eight loci ranged from 0.000000011 to 0.00000021 and 0.00072 to 0.0017, respectively. For all single captures the reliability of their genotype was estimated using the program Reliotype (Miller et al. 2002) and retained in the dataset if the reliability score was ≥ 90%. For the years used in this analysis, the amplification success rate was 63%.