Residence of juvenile Chinook salmon, Oncorhynchus tshawytscha, in the Smith River Estuary, California
Wild juvenile chinook salmon appear to spend an extended period of time in the estuary of the Smith River, California. Based on recovery patterns of freeze-branded juveniles released from June through August, 2000, mean estuarine residence time ranged from 8.3-13.8 d during June and July, and increased to an estimated 38.3 d for fish marked and released in August. Based on computer simulations of the process of estuarine entrance and exit, and on evidence of estuarine growth of juvenile chinook salmon, it appears that these estimates of mean residence times were negatively biased. True mean residence times calculated in other studies may also be considerably longer than those estimated using existing published methods. Rowdy Creek Hatchery rears large numbers of chinook salmon and many are released as fingerlings into the Smith River estuary during May-June. In 1999, about 13% of Rowdy Creek fingerlings were marked with a left maxillary clip, whereas in 2000 about 22% of fingerlings were marked with an adipose fin clip and coded wire tag. Based on collection and examination of juvenile chinook salmon in the Smith River estuary, following release of these hatchery fish, it appears that the majority of chinook salmon juveniles are still produced by wild spawning fish rather than by Rowdy Creek Hatchery. Maximum estimated daily percentages of hatchery juveniles were 33% in 1999 and 31% during 2000. Within 30 days following releases in 1999 and 2000, the estimated percentages ofhatchery fish present in the estuary had dropped to 15% and 9%, respectively. Together, these estimates of estuarine residence time and percentages of hatchery fish suggest that the Smith River estuary may be an important habitat for wild juvenile chinook salmon.