THE HOLOCENE HISTORY OF SALMON IN THE COLUMBIA BASIN

Archaeological fish bone records from the Columbia River system provide a ~13,000 yr history of fishes in the basin. The sample includes over 38,000 bones from 37 dated archaeological components along the main stem and tributaries of the Columbia and Snake Rivers. Salmonids (mainly Oncorhynchus) represent over 2/3 of the specimens, the rest being sturgeon (Acipenser sp.), various Cypriniformes (minnows, suckers), and other freshwater fish. Temporal and spatial variation in salmon:nonsalmon bone ratios suggest ways fishes adjusted to post-glacial stream conditions and changing Holocene climates. The earliest salmonid records are from the Snake River (13,450-11,130 cal yr BP), while earliest salmonids on the Upper Columbia River date ~4000 years later (8810-7140 cal yrs BP). These ages suggest the Snake River system provided spawning habitat much earlier than the Upper Columbia. This is to be expected, since the Snake River was ice-free throughout the Pleistocene and the Upper Columbia was blocked by continental ice until ~15,000 cal BP, and then would have carried a significant silt load as continental ice melted and streams reworked fine glacial sediments. Huge numbers of salmon remains from 8000-9000 cal yr BP deposits at The Dalles, OR indicate salmon populations were well established in some parts of the river system by that time. Salmon abundance declined sharply in the Mid-Holocene (~7500-4000 cal yr BP), reflecting deteriorated stream conditions (higher temperatures, increased silt loads, lower stream flows), which would have affected adult and egg mortality, and limited spawning and rearing habitat. Increased salmon abundance during the Neoglacial (~4000-2500 cal yr BP) likely reflects improved conditions in the freshwater system (cooler temperatures, lower silt load) that led to increased survival of developing young. Higher flows at this time would have lowered the gradient of barriers (waterfalls) allowing returning adults consistent access to large sections of the basin that had been unavailable during the mid-Holocene. Archaeological bone records help establish baseline conditions for fish populations prior to major habitat changes associated with Euro-American activities and suggest how fish may respond to future habitat alterations associated with both natural and anthropogenic factors.