Measuring nighttime spawning behavior of chum salmon using a Dual-frequency Identification Sonar (DIDSON)

K.F. Tiffan and D.W. Rondorf

United States Geological Survey, Western Fisheries Research Center, Cook, Washington, USA

The striking body coloration and morphology that Pacific salmon display during spawning coupled with elaborate courtship behaviors suggest that visual cues are important during their reproductive period. To date, virtually all existing information on chum salmon (Oncorhynchus keta) spawning behavior has been derived from studies conducted during the daytime, and has contributed to the assumption that salmon do not spawn at night. We tested this assumption using a new technology - a dual-frequency identification sonar (DIDSON) - to describe and measure nighttime spawning behavior of wild chum salmon in the Columbia River. The DIDSON produces detailed, video-like images using sound, which enabled us to collect behavioral information at night in complete darkness. The display of DIDSON images enabled fish movements and behaviors to be spatially quantified. We collected continuous observational data on 14 pairs of chum salmon in a natural spawning channel during the daytime and nighttime. Spawners of both genders were observed chasing intruders during nighttime and daytime as nests were constructed. Regardless of diel period, females were engaged in digging to both construct nests and cover eggs, and courting males exhibited the pre-spawning behavior of tail crossing. We observed a total of 13 spawning events, of which nine occurred at night and four occurred during the day. The behaviors we observed at night suggest the assumption that chum salmon do not spawn at night is false. Once chum salmon begin nest construction, visual cues are apparently not required for courtship, nest defense, and spawning. We speculate that non-visual cues (e.g. tactile and auditory) enable chum salmon to carry out most spawning behaviors at night. Our findings have implications for how nighttime flows from hydroelectric dams on the Columbia River are managed for power production and protection of imperiled salmon stocks.

Paper presented at Measuring Behavior 2005 , 5th International Conference on Methods and Techniques in Behavioral Research, 30 August - 2 September 2005, Wageningen, The Netherlands.

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