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Geology topics

J.L. Nielsen

Publications and source records attributed to J.L. Nielsen.

22 records · Page 2Linked to original sources
USGS1999

The evolutionary history of steelhead ( Oncorhynchus mykiss ) along the US Pacific Coast: Developing a conservation strategy using genetic diversity

Changes in genetic variation across a species range may indicate patterns of population structure resulting from past ecological and demographic events that are otherwise difficult to infer and thus provide insight into evolutionary development. Genetic data is used, drawn from 11 microsatellite loci amplified from anadromous steelhead ( Oncorhynchus mykiss ) sampled throughout its range in the eastern Pacific Ocean, to explore population structure at the southern edge in California. Steelhead populations in this region represent less than 10% of their reported historic abundance and survive in very small populations found in fragmented habitats. Genetic data derived from three independent molecular systems (allozymes, mtDNA, and microsatellites) have shown that the southernmost populations are characterized by a relatively high genetic diversity. Two hypothetical models supporting genetic population substructure such as observed were considered: (1) range expansion with founder-flush effects and subsequent population decline; (2) a second Pleistocene radiation from the Gulf of California. Using genetic and climatic data, a second Pleistocene refugium contributing to a southern ecotone seems more feasible. These data support strong conservation measures based on genetic diversity be developed to ensure the survival of this uniquely diverse gene pool.

ICES Journal of Marine Science↗
USGS1988

Channel hydraulics, habitat use, and body form of juvenile coho salmon, steelhead, and cutthroat trout in streams

Habitat use by juvenile coho salmon Oncorhynchus kisutch , steelhead Salmo gairdneri , and the coastal subspecies of cutthroat trout Salmo clarki clarki in small streams in western Washington was influenced by hydraulic characteristics of different types of channel units. Coho salmon preferred pools with average velocities less than 20 cm/s; very few fish were found in riffles with high current velocities. Steelhead occurred in riffles and also utilized deep pools with relatively high velocities along the center of the channel. Cutthroat trout were intermediate between coho salmon and steelhead in their use of swiftly flowing habitats. Variation in body shape and fin size among the three species generally fit the predicted morphologies that would be favored in different locations within the channel. Coho salmon possessed a deep, laterally compressed body with large median and paired fins. These features are believed to facilitate rapid turns and quick but transient burst swimming. Steelhead possessed a more cylindrical body shape with short median fins and relatively large paired fins, attributes that appear well adapted to holding a position in swift water. The cutthroat troutˈs lack of morphological adaptation to either fast or slow water may help to explain why this species is dominated by coho salmon and steelhead in areas of sympatry.

Transactions of the American Fisheries Society↗
USGS1988

Summer production of coho salmon stocked in Mount St. Helens streams 3-6 years after the 1980 eruption

We monitored habitat use and summer production of stocked underyearling coho salmon Oncorhynchus kisutch from 1983 to 1986 in three streams affected by the 1980 eruption of Mount St. Helens, Washington. Two streams were in the blast area and one was on a volcanic mudflow terrace, Midsummer water temperatures frequently exceeded presumed stressful thresholds and occasionally surpassed the incipient lethal limit. Temperatures at the study sites (up to 29.5°C) may have been the highest ever recorded in small streams in western Washington. In addition, there was relatively little submerged cover and limited pool habitat. Despite the severe conditions created by the eruption, production rates of stocked coho salmon at all sites ranged from 15.1 to 143.8 mg/m 2 ·d (2.3–21.6 g/m 2 over an average 150‐d summer period) and were equal to or greater than those measured in other streams of comparable size in the region. Coho salmon production in the streams was more strongly influenced by population biomass and density than by average individual growth rate. Production was also influenced by timing and average weight at stocking; larger fish stocked later in the summer had higher survival than smaller fish stocked earlier. Apparent summer mortality (true mortality plus emigration) may have been influenced by the presence of other salmonids. Coho salmon density at the end of summer was consistently lowest in the mudflow stream, the only site to have a large population of steelhead (anadromous rainbow trout Oncorhynchus mykiss , formerly Salmo gairdneri ). We suspected that an abundance of both terrestrial and aquatic food was partly responsible for the high summer production of stocked coho salmon in what was an otherwise hostile environment.

Washington↗