The size distribution of the sediments in the bed of a stream channel is a significant characteristic of fish habitat. Spawning gravels are sampled during habitat assessments and monitoring programs. Within a given stream, effective, representative sampling requires a clear understanding of the spatial variation of spawning gravel characteristics. This study documents and explains the spatial variation of spawning gravels in two streams on the Queen Charlotte Islands. In many respects these are representative of the small, coastal watersheds of British Columbia.
An oversized McNeil sampler was used to collect representative subsurface samples at 29 spawning sites. Surface material was characterized at 35 sites using Wolman counts, and an expedient photographic technique provided limited surface information at 116 sites. Detailed sediment sampling, magnetic tracer stones, and scour chains were used to study the impact of log jams on grain size.
Strong hillslope-channel interactions and an abundance of large organic debris preclude the development of simple grain-size trends. Two fisheries indices of gravel texture, the Fredle Index and percent finer than 3.36 mm (64.0 mm truncation), do not differ significantly between spawning sites. In contrast, surface median size and percent finer than 0.85 mm (64.0 mm truncation) differ significantly and stochastically between sites.
A sampling strategy, which aims to establish the general fisheries value of the bed material in a given stream, is then proposed. Number of samples, site selection, sample size, and mode of collection are specified in view of practical field considerations. This is regarded as an improvement on existing sampling methodologies. An issue of pressing concern, and one which hinders the development of effective field sampling, is the poor understanding of the relationships among fish survival, gravel quality indices, and sample truncation limits.
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Updated October 15, 2009