Earl M. Dawley

A Surface Trawl to Detect Migrating Juvenile Salmonids Tagged with Passive Integrated Transponder Tags

Abstract We developed a surface pair-trawl system to detect juvenile Pacific salmon Oncorhynchus spp. marked with passive integrated transponder (PIT) tags as they migrate through the upper Columbia River estuary. The trawl was fitted with a detection antenna in its cod end and was deployed by two vessels. Fish entering the trawl body exit after passing by the detection antenna. Detection data were recorded by electronic components housed in a small boat tethered to the trawl. The system was modified extensively after its first deployment in 1995 and by 2001 was performing reliably under a variety of weather conditions and river flows. From 1995 to 2001 the device detected 29,699 PIT-tagged juvenile salmon. During extended daily sampling periods, we detected nearly 2% of all PIT tags previously detected at Bonneville Dam, which is located 159 rkm upstream from the study area.

Modeling the Effects of Supersaturated Dissolved Gas on Resident Aquatic Biota in the Main-Stem Snake and Columbia Rivers

Abstract Dissolved-gas levels in the Columbia and Snake rivers during the spring freshet often exceed 110% of saturation, the maximum level permitted by the U.S. Environmental Protection Agency. The highest levels of supersaturation result from high springtime river flows and turbine outages, conditions over which there is little control and that cause high volumes of water passing over the dams and through spillways instead of through turbines. During the spring freshets of 1994–1997, we surveyed nonsalmonid fishes and invertebrates for signs of gas bubble disease (GBD) and conducted holding experiments in three river reaches where gas saturation commonly exceeds 120%. We developed a dissolved-gas exposure index for nonsalmonid fishes sampled at specific times and locations; mean daily total dissolved-gas saturation (TDGS) was ranked and then summed over a 7-d period. We analyzed observations of 39,924 nonsalmonid fishes in an iterative process, which led to development of a mathematical equivalence mode...

Feasibility of Siphons for Degassing Water

Abstract Siphons of 5.1-, 10.2-, and 15.2-cm inside diameter, tested at several vacuum heads and flow rates, lowered the nitrogen content of supersaturated water by up to 20% of saturation (to 103% or less). Turbulence was a primary factor in lowering gas content of water passed through a siphon.