Author(s): Steven P. Cramer
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Abstract: Migrating juvenile chinook were sampled during 1993-1996 in the Stanislaus River, a tributary of the San Joaquin River, California, to determine the influence of pulses in flow on the propensity of juvenile chinook to migrate. Water storage reservoirs provide substantial control of flows in the river, and natural resource agencies have recommended at least 30 d of sustained high flows during spring to enhance survival of out-migrating chinook. The pattern of daily outmigrant abundance before, during and after artificial pulsed-flow events suggested that sharp increases in flow stimulated juvenile migration, the stimulus lasted only a few days, and not all of the population responded. The greatest increase in movement of chinook smolts during each year, 1993-1995, was associated with the first artificial pulse in flow during April. The number of outmigrants remained elevated for no more than 5 d following a jump in flow, regardless of the flow level. Some juvenile chinook of all sizes responded to pulses in flow. An increase in flow from 9.1 to 16.4 m~3/s in 1995, had the same result on juvenile chinook that larger magnitude pulses had in 1993 (11.3 m~3/s to 39.7 m~3/s) and 1994 (9.9 m~3/s to 34 m~3/s). In the absence of any sharp changes in flow in 1996, the outmigration of chinook smolts still peaked between mid April and mid May, although the period of high catches was protracted over 2-3 weeks rather than the 2-3 d associated with pulses in flow.
Year: 1997