The behavioural thermal preference of the common triplefin (Forsterygion lapillum) tracks aerobic scope optima at the upper thermal limit of its distribution

Ocean temperatures are rising and fish are redistributing themselves poleward and into deeper waters to retain a favourable thermal environment (Dulvy et al., 2008; Perry et al., 2005). To investigate whether biogeographical shifts might occur through behavioural redistribution into optimal environments, we examined whether a common triplefin species (Forsterygion lapillum) would behaviourally select (i.e. track) a temperature that matches its physiological optimum under laboratory conditions. F. lapillum were acclimated to 15, 18 or 21°C for at least 4 weeks, after which various rates of oxygen consumption (MO2) were measured using automated respirometry and their behavioural thermal preferenda assessed using an electronic shuttle choice tank. Aerobic metabolic scope (resolved as the difference between maximal and maintenance MO2) did not differ across all thermal treatments (i.e. specimens acclimated to 15, 18 or 21°C) revealing that F. lapillum is a eurythermal species with a range of optimal physiological performance that closely matches the environmental conditions they are exposed to. A comparably wide range of behavioural preference would perhaps be expected but all three acclimation groups showed a surprisingly narrow behavioural preference range of 20–21°C. The results therefore suggest that, irrespective of acclimation, eurythermal species may have a tendency to select optimal temperatures at the upper limit of their thermal distribution range. The results are discussed in the context of the ecology and the expected response of F. lapillum to future thermal change. ► Potential tool for determining the distribution response of marine ectotherms to warming oceans. ► Identification of a eurythermal coastal New Zealand fish with a specific preferred temperature. ► Application of current respirometric and behavioural analysis techniques to a novel species.