baskets to the surface. After retrieval, the contents of the basket and the collection net are emptied into a bucket, and the associated organisms are removed by scrubbing the substrate with a fingernail brush. This material is then concentrated on a 425-Ám mesh sieve and composited with other basket samplers prior to field processing. Baited traps or baited artificial substrate samplers are appropriate for QMH sampling in deep, rocky rivers but not for RTH or DTH sampling. Baited traps can be particularly effective for collecting crayfish under conditions where other methods are ineffective.
Stovepipe samplers (fig. 3E) may also work in this type of habitat in water less than 0.75 m deep. For this technique, approximately 1 m of large-diameter 30-cm polyvinyl chloride (PVC) pipe with a beveled bottom edge is used. This sampler is driven int o the substrate deeply enough to produce a good seal around the bottom of the sampler. Substrate is removed from the sampler by hand, if physically possible, and composited. More typically, the substrate is too deep to reach by hand, so organisms are dislodged from the substrate using long-handled brushes and scrapers attached to poles. A hand- or battery-operated diaphragm pump equipped with suitable hoses and a PVC pipe is used to pump the water and suspended invertebrates into a 425-Ám mesh net. In dividual samples are composited to obtain a representation of the habitat type. The disadvantages of this type of sampler are that it is (1) often difficult to handle the sampler in very fast-flowing water, (2) usually impossible to assess the degree to which the substrate has been cleaned of invertebrates, and (3) often difficult to estimate substrate characteristics. However, despite these shortcomings, this type of sampler and its modifications provide one of the few means to collect samples on rock outcrops in water 50-75 cm deep (Voshell and others, 1992).
Grab samplers (fig. 3F, -G, -H, and -J) are appropriate for use in shallow, fine-grained riffles or pools. A pole-mounted Ekman grab is particularly useful in wadeable streams with sand or silt substrates, whereas a Ponar grab is a better choice for fine-gravel substrates. All screening on the grab should have mesh openings of 425 Ám or smaller. The appropriate screening mesh can be accomplished by gluing or sewing smaller mesh fabric over the existing larger mesh panels of grabs such as the Ponar. Grab samplers are lowered carefully to the stream bottom and released to avoid disturbing the sediments prior to contact and to aid in establishing a uniformity of substrate penetration. Additional weights can be added to grabs to achieve better and more uniform substrate penetration. Recovered grabs are carefully checked to make sure that sample material was not lost because of rocks, sticks, or other debris catching in the jaws of the grab. Each of the samples to be composited is taken within the sam e instream habitat type but at sufficient distances apart to avoid interference among samples. Individual samples can be composited in a suitable container prior to field processing or processed and then composited.
Grab samplers, such as Ponar (fig. 3G), Petersen (fig. 3H), Van Veen (fig. 3J), or Shipek (fig. 3I) grabs, can be used from boats to obtain samples from deep rivers with fine-grained substrates. A hand or power winch is recommended for sampling in deep waters or using weighted grab samplers. All screening on the grab sampler should have mesh openings of