. The Canadian field-naturalist. 1988 Johnston and Cheverie: Drift of Eggs and Larvae 511 TEMPERATURE / \ '^•.•.•«. 8LU EBACK JULY 22-23. Figure 5. Diel abundance of Blueback Herring larvae in the drift. Water temperatures recorded at the sampling site are shown for that day. Seasonal drift of blueback larvae and eggs peaked in the first two weeks of July when mean water temperatures were between 14-15°C (Figure 7). Adult blueback were first observed at the head of tide at Bonshaw on 8 June and upstream migrations did not occur to any extent until 21 June when water temperature was between 13-
. The Canadian field-naturalist. 1988 Johnston and Cheverie: Drift of Eggs and Larvae 511 TEMPERATURE / \ '^•.•.•«. 8LU EBACK JULY 22-23. Figure 5. Diel abundance of Blueback Herring larvae in the drift. Water temperatures recorded at the sampling site are shown for that day. Seasonal drift of blueback larvae and eggs peaked in the first two weeks of July when mean water temperatures were between 14-15°C (Figure 7). Adult blueback were first observed at the head of tide at Bonshaw on 8 June and upstream migrations did not occur to any extent until 21 June when water temperature was between 13-15°C. By August eggs and larvae were no longer in the drift. Discussion The downstream displacement or drift of fish eggs may occur as a consequence of (1) spawning activity, (2) loss of adhesiveness of the egg membrane, (3) increased velocity of stream flow, (4) attachment of eggs to floating vegetation or (5) dislodgement of eggs by man or animal activities in the river. The present study suggests that spawning activity and stream discharge influence the level of drift the most. Based on the abundance of stage 1 eggs in the drift, most spawning of smelt and blueback occurs between dark and 1 This agrees with McKenzie's (1964) report that 80% of smelts spawn in the Miramichi river at night, but does not agree with Loesch and Lund's (1977) report that blueback in the Connecticut River spawn in the late afternoon. The absence of stage 2 smelt eggs and small numbers of stage 3 eggs in the drift suggests that the chorionic membrane is very adhesive at this stage and holds the egg firmly to the substratum after fertilization. During stage 4 the strength of this adhesiveness appears to be reduced for a much larger number of eggs were released to the drift. Higher water temperature or enzymes associated with hatching may be responsible for this loss of membrane adhesiveness. During periods of increased stream flow there was approximately double the number of stage 4 sme
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