The Birds and the Bees (and the Fish) of Paleolimnology: Did Sockeye Salmon Historically Spawn in Lake Roberta, WA?

Historically the Sanpoil River produced spawning runs of Chinook salmon (Oncorhynchus tshawytscha) and Steelhead trout (O. mykiss). Lake Roberta is connected to the Sanpoil River and oral history suggests the lake may have supported anadromous Sockeye salmon (O. nerka), which was last reported in the 1940’s around the completion of Grand Coulee Dam. Post-spawning mortality of anadromous salmon provides large pulses of marine-derived nutrients to aquatic and terrestrial ecosystems in the vicinity of spawning sites.  These unique marine isotopic ratios are often transferred to freshwater algae and archived in sediments. To investigate the possible historic presence of anadromous salmon in Lake Roberta, we compared carbon, nitrogen and sulfur isotope compositions for both pre-1940 and post-1940 sediments to those collected from control lakes with no history of anadromy. If Sockeye historically spawned in Lake Roberta we would expect pre-1940 sediment organic matter to reflect a marine signature indicative of the large pulses of nutrients from decomposing post-spawning salmon carcasses. Based on our isotope results, there is no conclusive evidence to support historical anecdotal accounts of anadromy in Lake Roberta.