Influence of photoperiod and protocol length on metabolic rate traits in ballan wrasse Labrus bergylta.

In this study, ballan wrasse Labrus bergylta were subjected to either a conventional 1-day or an extended 5-day respirometry protocol. Additionally, in the 5-day protocol the fish were subjected to a 12 h light-dark cycle to assess the effects of photoperiods on metabolic rates (ṀO 2 ). Diurnal patterns in routine and resting ṀO 2 were not observed, suggesting that circadian rhythms in metabolism largely are driven by activity patterns rather than being of endogenous origin. Moreover, lack of a detectable circadian ṀO 2 may be an adaptation to lower costs of living in ballan wrasse. Protocol length influenced standard metabolic rates (SMR) where estimates decreased by 13% and 17% when using 48 h and 5 days, respectively, compared to 24 h. The maximum metabolic rate (MMR) and the derived absolute aerobic scope (MMR-SMR) were unaffected by protocol length. However, factorial scopes (MMR/SMR) were reduced from 8.5 to 6.4 in the 5-day protocol, showing that factorial scopes are more sensitive to how SMR are obtained. The critical oxygen tension (P crit ) was reduced from 15% PO 2 in the 1-day group to 11% PO 2 in the 5-day group. However, ṀO 2 in response to decreasing PO 2 was similar, which together with a similar oxygen extraction coefficient, α (ṀO 2 /PO 2 ), suggested that the higher P crit in the 1-day group was an artefact of overestimating SMR. Finally, α was 12% lower at MMR compared to at P crit , which either means that MMR was underestimated in proportion to this difference or that α is not constant in the entire PO 2 range. In summary, this study found that a conventional 1-day respirometry protocol may overestimate SMR and thereby alter the derived P crit and aerobic scope, while α is unaffected by protocol length. Moreover, alternating light conditions in the absence of other stressors did not influence ṀO 2 in ballan wrasse.