Drifting invertebrates were collected hourly during 24-hour sampling periods at two stations in the Snake River . The greatest number of invertebrates was collected on 8 and 15 July 1966 at station 1 between 9 :00 p . m. and 12 midnight, then the numbers gradually decreased until the low daylight drift rate was reached at dawn . On 26 and 27 August 1966 at station 2 the diel periodicities of drifting invertebrates were different than at station 1 . Many species increased their drift rates slightly during the first hour of darkness but also exhibited a higher drift peak later in the night .Drift indices for 25 taxonomic groups of invertebrates were established from the ratio of standing crop, estimated from Surber samples, to numbers drifting . There was more apparent correlation between species life cycle stage and numbers drifting than between species abundance and numbers drifting .
We determined from fish collections made along the Atlantic and Gulf coasts that adult menhaden, genus Brevoortia, which school with juvenile menhaden in estuarine nursery areas are usually injured or are parasitized with the isopod Olencira praegustator, whereas adults in the ocean, bays, and sounds are undamaged. Olencira praegustator appears to be debilitory to menhaden, but the copepod Lernaeenicus radiatus does not, except when the fish are heavily infested. When many injured and parasitized adult menhaden are present in an estuary, they form schools independent of the juveniles. Large injured juveniles remain in the nursery areas longer than similar-sized healthy juveniles, and school with the remaining small juveniles.
Before the flow of water from Jackson Lake into the Snake River was stopped by closure of a dam, invertebrates were collected in five Surber samples from a downstream riffle. After the water had receded from the riffle, five 0.093‐m2 areas of the exposed streambed substrate were hand‐collected and the invertebrates removed.
Predrawdown samples contained 4,286 invertebrates weighing 6.3 g while postdrawn samples contained 15,490 weighing 13.6 g. Large differences between submerged and exposed samples resulted from passage of smaller insects through the fine mesh of the Surber sampler and from backwash created by the sampler.
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