Effects of temperature and ration on the otolith-to-somatic size relationship in juvenile Chinook salmon (Oncorhynchus tshawytscha): a test of the direct proportionality assumption

Fish otoliths are commonly used to estimate somatic growth rate, but this depends on the assumption that the otolith and body grow in direct proportion. Environmental conditions contribute to variability in somatic growth and can result in deviations from direct proportionality in the otolith-to-somatic size relationship. In the present study we examined the otolith-to-body size relationship for juvenile Chinook salmon (Oncorhynchus tshawytscha) subjected to simulated seasonal (summer, autumn and winter) water temperatures and feeding rations. The otolith-to-somatic size relationship became uncoupled during summer between fish subjected to the cool (15°C) and hot (21°C) water temperatures. A food ration effect was also observed during the summer, such that fish fed an unlimited ration had smaller otoliths than equivalently sized fish fed a limited ration. The effects of water temperature and ration disappeared by the end of autumn, indicating that a seasonal compensatory response occurred in the otolith-to-somatic size relationship after the extreme temperatures and food limitations were alleviated. In winter, this relationship became uncoupled again, but only between fish that were fed throughout the winter and fish that were starved during the 3-month experimental period. The effects of water temperature and rations on the otolith-to-somatic size relationship of juvenile Chinook salmon could have implications for accurately estimating somatic growth from otolith growth in natural populations and should be incorporated into back-calculation techniques.

Impact of differential energy allocation in Atlantic salmon (Salmo salar) precocious males on otolith–somatic size proportionality: a longitudinal approach

We studied juvenile Atlantic salmon (Salmo salar) males that become precociously mature or not at age-1+ to test the hypothesis that differential energy allocation affects the relationship between otolith size and fish size and to validate the use of a back-calculation method to estimate size over 30 weeks. We used a longitudinal approach by repeatedly measuring marked fish and obtaining corresponding otolith radius measurements. Differential energy allocation of mature males did not affect the proportionality ratio between otolith and somatic size. Short-term otolith growth varied with short-term somatic growth, but only weakly with temperature. Some correlation coefficients of the covariation of otolith growth estimated over a longer time interval with somatic growth were significantly greater than the short-term estimate. For mature and immature males, back-calculated lengths accurately estimated the observed individual length on practically all occasions. These results indicate that back-calculation can be used to estimate size for Atlantic salmon with different energy allocation patterns. Variable strength of coupling of otolith and somatic growth depending on time interval suggests that these processes are completed on different time scales.