No abstract

Laboratory experiments were conducted to test the hypothesis that temperature (2–8 °C) and recent feeding history (1–6 days of food deprivation) influence feeding motivation and attacks on baits by adult sablefish (Anoplo poma fimbria). Activity level after introduction of a bait-related olfactory cue increased with increasing water temperature and food deprivation. Fish at the lowest temperature (2 °C) attacked and consumed fewer squid baits (33–71%) than those at higher temperatures (100% at 8 °C). Baits were frequently inspected or handled multiple times before consumption in low temperatures, and time to attack (seconds to >40 min), time to consume, and time to handle baits (2–20 s) all decreased significantly with increasing temperature, with some temperature × feeding history interactions. The total ration consumed increased (<1–8% of body weight) with increasing water temperature. The observed variability in activity and feeding motivation in sablefish may have a large impact on the effective area of baited gear but is rarely considered in stock assessment models. In particular, population sizes based upon baited gear surveys may be underestimated in conditions where temperature is low, natural prey density is high, or where other environmental variables reduce feeding motivation.

Juvenile steelhead Oncorhynchus mykiss from a northern California Central Valley population were reared in a controlled laboratory experiment. Significantly different rates of growth were observed among fish reared under two ration treatments and three temperature treatments (8, 14 and 20°C). Wider circulus spacing and faster deposition was associated with faster growth. For the same growth rate, however, circulus spacing was two-fold wider and deposited 36% less frequently in the cold compared to the hot temperature treatment. In a multiple linear regression, median circulus spacing and water temperature accounted for 68% of the variation in observed O. mykiss growth. These results corroborate previous research on scale characteristics and growth, while providing novel evidence that highlights the importance of water temperature in these relationships. Thus, this study establishes the utility of using scale analysis as a relatively non-invasive method for inferring growth in salmonids.