N. J. Demetras, D. D. Huff, C. J. Michel, J. M. Smith, G. R. Cutter, S. A. Hayes and S. T. Lindley

Recent acoustic tagging of juvenile Chinook salmon (Oncorhynchus tshawytscha) in the southern portion of California's Sacramento San Joaquin Delta has revealed extremely low survival rates (<1%), possibly due to predation by piscivorous fishes. We evaluated predation as a cause of low survival by designing and testing freely floating GPS-enabled predation.-event recorders (PERs) baited with juvenile Chinook salmon. We estimated predation rates and identified predation locations within. a 1-kilometer reach of the Lower San Joaquin River. We modeled the relationship between time to predation and environmental variables with a Cox proportional hazards analysis that accounts for censored data. Our results indicated that an increase of 1 m/s in water velocity elevated the minute-by-minute hazard of predation by a factor of 9.6. Similarly, each increase in median depth decreased the predation hazard by a factor of 0.5. The mean relative predation rate in the study area was 15.3% over 9 sampling events between March and May 2014. Waterproof video cameras attached to a subset (48 of 216) of PERs successfully identified predator species 25% of the time. Our GPS-enabled PERs proved to be an inexpensive and reliable tool, which quantified predation, identified predation locations, and provided complementary information for acoustic telemetry and predator diet studies.

National Marine Fisheries Service (NMFS)