Effects of Iron Gate Dam discharge and other factors on the survival and migration of juvenile coho salmon in the lower Klamath River, northern California, 2006-09

Abstract

Current management of the Klamath River includes prescribed minimum discharges intended partly to increase survival of juvenile coho salmon during their seaward migration in the spring. To determine if fish survival was related to river discharge, we estimated apparent survival and migration rates of yearling coho salmon in the Klamath River downstream of Iron Gate Dam. The primary goals were to determine if discharge at Iron Gate Dam affected coho salmon survival and if results from hatchery fish could be used as a surrogate for the limited supply of wild fish. Fish from hatchery and wild origins that had been surgically implanted with radio transmitters were released into the Klamath River at river kilometer 309 slightly downstream of Iron Gate Dam. Tagged fish were used to estimate apparent survival between, and passage rates at, a series of detection sites as far downstream as river kilometer 33. Conclusions were based primarily on data from hatchery fish, because wild fish were only available in 2 of the 4 years of study. Based on an information-theoretic approach, apparent survival of hatchery and wild fish was similar, despite differences in passage rates and timing, and was lowest in the 54 kilometer (km) reach between release and the Scott River. Models representing the hypothesis that a short-term tagging-or handling-related mortality occurred following release were moderately supported by data from wild fish and weakly supported by data from hatchery fish. Estimates of apparent survival of hatchery fish through the 276 km study area ranged from 0.412 (standard error [SE] 0.048) to 0.648 (SE 0.070), depending on the year, and represented an average of 0.790 per 100 km traveled. Estimates of apparent survival of wild fish through the study area were 0.645 (SE 0.058) in 2006 and 0.630 (SE 0.059) in 2009 and were nearly identical to the results from hatchery fish released on the same dates. The data and models examined supported positive effects of water temperature, river discharge, and fish weight as factors affecting apparent survival in the Klamath River upstream of the confluence with the Shasta River, but few of the variables examined were supported as factors affecting survival farther downstream. The effect of water temperature on apparent survival upstream of the Shasta River was greater than Iron Gate Dam discharge, which was greater than fish weight.

Benefits of Prescribed Flows for Salmon Smolt Survival Enhancement Vary Longitudinally in a Highly Managed River System

“…While some studies indicate that flow increases can have positive effects on smolt emigration survival, those effects can be context‐dependent and difficult to quantify independent of other influential abiotic and biotic factors, such as water temperature (Connor et al ., ; Petrosky and Schaller, ; Beeman et al ., ; Haeseker et al ., ), migration timing and fish size (Zabel and Williams, ), migration distance (Anderson et al ., ; Welch et al ., ) and predator densities (Beamesderfer et al ., ; Anderson et al ., ; Krueger et al ., ; Cavallo et al ., ). Despite these complexities, quantifying flow–survival relationships is necessary to optimize flow management regimes for anadromous salmonid production, and there is a need for field experiments that successfully discern whether migrating fish benefit from flow augmentation.…”