The role of nutritional state in the relationship between standard metabolic rate and locomotor activity in juvenile white sturgeon (<i>Acipenser transmontanus</i>), with implications for anthropogenically altered food webs

White sturgeon (<i>Acipenser transmontanus</i>) are in decline globally, and populations in the Sacramento-San Joaquin River Basin are particularly vulnerable due to habitat impacts, variable recruitment and altered food availability, all of which are exacerbated by climate change. The minimal metabolic expenditure required to maintain homeostasis, termed standard metabolic rate (SMR), is thought to have broad ecological relevance because it correlates with other important measures of metabolic demand and a range of fitness-related behavioural traits. SMR is variable among individuals and this variation may also underlie variation in behaviour. Additionally, SMR has been shown to be phenotypically flexible in the presence of changing food availability. The objective of this study was to assess how nutritional status may affect the relationship between SMR and locomotor activity in juvenile white sturgeon. We reared white sturgeon at 15°C under an optimal feed rate (OFR, 5.3% bodyweight/day) and low feed rate (LFR, 2.6% bodyweight/day) for 6 weeks, measuring SMR and locomotor activity at the 3- and 6-week timepoints. OFR fish were significantly larger than LFR fish at both timepoints, but mass-specific SMR was not significantly different across treatment or time. We found that only fish under the greatest nutritional stress (6 weeks at LFR) showed a significant relationship between SMR and locomotor activity. This is evidence that observable correlations between physiological and behavioural traits may only become apparent under the influence of environmental stressors. As changing climate is projected to impact food web dynamics and food availability, understanding how nutritional state affects physiological and behavioural traits may help to predict how animals respond to future shifts.