– In past dietary studies kokanee Oncorhynchus nerka were prominent in the diet of Pend Oreille Lake's large piscivores: native bull trout Salvelinus confluentus, cutthroat trout O. clarki and northern pikeminnow Ptychocheilus oregonensis, and introduced lake trout S. namaycush and Kamloops rainbow trout O. mykiss gairdneri. However, kokanee have declined to 10–20% of their former abundance. We therefore initiated this study to understand current predation demands on kokanee and diet overlap among piscivores, using gut content samples and analysis of stable nitrogen (δ15N) and carbon (δ13C) isotopes from the lake's fish and invertebrate community. In gut content samples, kokanee were the main prey item of large [i.e., ≥400 mm total length (TL)] bull and lake trout; a conclusion that was affirmed by stable isotope analysis. Rainbow trout >500 mm TL consumed mostly kokanee, thus there was a high degree of diet overlap among large bull, lake and rainbow trout. Small (i.e., <400 mm TL) rainbow and cutthroat trout diets overlapped, and were composed mostly of littoral benthic invertebrates. However, gut content and stable isotope analysis did not accord for 400–500 mm TL rainbow trout, small lake trout, and large cutthroat trout. In these instances, a linear mixing model using stable isotope results predicted kokanee consumption for each species, but no kokanee were identified in rainbow or lake trout gut content. Gut content and stable isotope analysis of native northern pikeminnow indicated a diet of mostly littoral benthic invertebrates at smaller (100–150 mm TL) lengths, with kokanee becoming more prominent in the diet of individuals >300 mm TL. Percent of kokanee in the diet of northern pikeminnow has declined from a prior study; otherwise piscivore diets have apparently remained unchanged. In this study, judgments as to the feeding of some piscvores, based on gut content alone, would be tenuous because of small sample sizes, but stable isotope analysis provided an efficient means for confirming diets.
Seasonal temperature cues may play an important role in the parr–smolt transformation of Pacific salmon Oncorhynchus spp., but salmon reared in groundwater‐supplied hatcheries with seasonally stable water temperatures do not experience these cues. In this study, conducted over six release years, hatchery spring Chinook salmon O. tshawytscha that were initially reared in a groundwater‐supplied hatchery were transferred to acclimation ponds supplied with surface water either in November (November transfer [NT]) or January (January transfer [JT]) for rearing at ambient water temperatures prior to release into the Umatilla River, Oregon, in early March. After stream release, we monitored out‐migration travel times and survival to John Day Dam (JDD) by using passive integrated transponder tags in a subsample of each release group. Across all release groups, median travel time was slower for NT groups (51 d) than for JT groups (46 d); significant differences were observed in five of six release years. Average survival probabilities were 15% higher for NT groups than for JT groups, although this difference was not significant. We used recoveries of coded wire tags to estimate smolt‐to‐adult survival (SAS) and straying for NT and JT groups. Average SAS over the study was 27% higher for NT groups (0.76%) than for JT groups (0.60%), but straying was minimal for both types of release group. We found no significant linear relationship between SAS and travel time or survival to JDD. Adult returns of NT groups had a slightly older age distribution than those of JT groups. Our results suggest that for salmon juveniles reared in groundwater‐supplied hatcheries, providing an overwinter period of exposure to ambient water temperatures should optimize postrelease survival. We also discuss some risks and benefits to winter rearing in acclimation ponds for managers that are considering this approach.
We studied seasonal zooplankton community dynamics along spatial scales in Lake Pend Oreille, Idaho in 1997-1998 to determine if zooplankton community changes brought about by establishment of Mysis relicta in the 1970's persist today, and to relate zooplankton length and abundance fluctuations to zooplanktivory. Long-term trends in zooplankton species composition and relative abundance, as well as seasonal patterns of species succession are apparently unchanged since M. relicta establishment. Cladoceran zooplankters became abundant earlier in the spring and remained abundant later into autumn in 1998 than in 1997. Fluctuations in zooplankton lengths and abundance were more pronounced along horizontal gradients extending from littoral to pelagic zones than along vertical gradients extending downward from the surface, and these fluctuations were more pronounced in 1998 than in 1997. Zooplankton length variability along spatial scales occurred most often in late springlearly summer, and in autumn. The trends we observed are consistent with prior speculation that zooplanktivory by M. relicta structures the Lake Pend Oreille zooplankton community, although abiotic factors may also explain our results.
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