This paper presents a comprehensive procedure for evaluating sedimentation impact on the streambed gravels used for spawning by salmonids. It establishes a relationship between embryo survival of incubating eggs and the geometric mean diameter of the substrate. It outlines simple procedures for estimating the overall composition of the gravel in spawning sites along stream reaches and provides the rationale for interpretation of these estimates in terms of overall embryo survival in a stream system. ESTIMATED EMBRYO SURVIVAL IN PERCENT i000 2.5 50 80 90 ,I--
SHIRAZI AND SEIM: SPAWNING HABITAT
Environmental toxicant exposure commonly vary in terms of duration and concentration. However, laboratory toxicity tests usually entail continuous exposures to constant concentrations. We compared survival, growth, and toxicant accumulation in early life-stage steelhead trout intermittently or continuously exposed to fenvalerate (FV) for 70 d after fertilization. Acute lethality was assessed in ancillary 96-h LC50 determinations with juvenile fish. Intermittent exposures were daily 4.5-h introductions of toxicant, and continuous exposures were to constant concentrations. All tests were conducted in a flow-through dilution apparatus, and mean concentrations for the entire exposure period were calculated for comparisons between regimens. The respective 96-h LC50 values for intermittently and continuously exposed were 88 and 172 ng/l. In the subchronic study, marked lethality (32%) and reduced terminal weight (50%) were found following exposures to cyclic FV concentrations that yielded an average of 80 ng/l (peak of 461). Continuous exposure to 80 ng FV/l did not affect these parameters. At mean FV concentrations above 20 ng/l, bioaccumulation was greater following intermittent than continuous exposure. Interaction of partitioning and elimination processes may partially explain differences in FV accumulation and subsequent toxicity.
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