Anthropogenic warming of the ocean and atmosphere, concurrent with ocean acidification and deoxygenation, has made it even more pressing to quantify the link between environmental stressors and marine organism population dynamics. In marine environments, low food availability, low feeding rates, and/or increased metabolic costs can cause energetic limitation. Energetic limitation affects some functional traits, such as growth rates of body tissue and reproductive output. Other functional traits that are linked with short-term survival are often prioritized in conditions of energy limitation, despite their energetic cost. Mussels are ecosystem engineers in rocky shore ecosystems, and they produce byssal threads to attach to hard substrate and aquaculture line. Previous studies of mytilid mussel bioenergetics suggest tissue and shell growth are energetically-constrained, while production of byssal threads presents a fitness trade-off and could potentially be a fixed or ‘constitutive’ response regardless of energetic state. In this study, we conduct a field test with two congener mussel species, Mytilus trossulus and Mytilus galloprovincialis to determine whether an index of energetic availability, scope for growth (SFG), correlates with growth and byssal thread production, and the extent to which other potential stressors (hypoxia, low pH, low salinity and high temperature) modulate this response. We find a positive correlation between SFG and growth (both tissue and shell) but not the number of byssal threads produced. We also find low pH or low DO, two co-varying physiological stressors, negatively affect tissue growth of both species, but only marginally affect byssal thread production. We also observed mortality in the late summer/early autumn that coincides with the period of greater hypoxia and low pH. Overall, this work suggests that some functional traits, such as shell and tissue growth, are energetically-constrained while other functional traits, such as mussel byssal thread production, may be best described as a fitness trade-off.