The Ecological Context of Aerobic Scope in Cost of Transport for Chum Salmon Migrating Upriver

Abstract Excerpt:
Ectotherms adjust their thermal performances to various thermal ranges by altering their metabolic rates (Angilletta 2009). Exploring the relationship between thermal niche and the performance of metabolic traits is a controversial topic in the thermal adaptation of ectotherms. Aerobic scope (AS), the difference between resting metabolic rate (RMR) and maximum (aerobic) metabolic rate (MMR), has been proposed as a functional metric for thermal performance (Pörtner and Farrell 2008). The thermal sensitivity of AS is described by the thermal performance curve, where AS increases with temperature up to a peak (which is defined as the optimum temperature of aerobic scope, T_optAS), and declines thereafter (Payne et al. 2016). Sub-optimal temperatures are known as pejus temperatures (T_pej). It has been reported that some salmonid species exhibit a variety of thermal profiles for AS curves, and the thermal performance curve of each species or population roughly corresponds to their temperature regime (Lee et al. 2003; Eliason et al. 2011; Clark et al. 2011; Eliason et al. 2013; Raby et al. 2016). Although the intra- or inter-specific adaptation of AS has been shown, however, the degree of correspondence between the thermal performance curves and temperature regimes varies in species or populations. For instance, the peaks of thermal performance curves in some populations do not always correspond with the mode of historically experienced temperatures (Clark et al. 2011; Eliason et al. 2013; Raby et al. 2016; Abe et al. 2019). While the studies of thermal performance curves of AS in Pacific salmonids indicated AS is an ecologically relevant trait, the ecological contexts of AS are not fully understood.