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Standard metabolic rate (SMR), maximum metabolic rate (MMR), absolute aerobic scope (AAS), and critical oxygen tension (Pcrit) were determined for the Gulf killifish,Fundulus grandis, an ecologically dominant estuarine fish, acclimated to lowered salinity, elevated temperature, and lowered oxygen concentration. Acclimation to low salinity resulted in a small, but significant, elevation of Pcrit; acclimation to elevated temperature increased SMR, MMR, AAS, and Pcrit; acclimation to low oxygen led to a small increase in SMR, but substantial decreases in MMR, AAS, and Pcrit. Variation in these metabolic traits among individuals was consistent and repeatable when measured during multiple control exposures over seven months. Trait repeatability was unaffected by acclimation condition suggesting that repeatability of these traits is not context dependent. There were significant phenotypic correlations between specific metabolic traits: SMR was positively correlated with MMR and Pcrit; MMR was positively correlated with AAS; and AAS was negatively correlated with Pcrit. In general, within-individual variation contributed more than among-individual variation to these phenotypic correlations. The effects of acclimation on these traits demonstrate that aerobic metabolism is plastic and influenced by the conditions experienced by these fish in the dynamic habitats in which they occur; however, the repeatability of these traits and the correlations among them suggest that these traits change in ways that maintains the rank order of performance among individuals across a range of environmental variation.SUMMARY STATEMENTAerobic metabolism of an ecologically dominant estuarine fish is influenced by acclimation to environmental changes without altering trait repeatability. Furthermore, specific metabolic traits are phenotypically correlated.

For most fishes, there is an oxygen level, the critical oxygen tension (Pcrit), below which oxygen consumption (MO2) becomes dependent upon ambient oxygen partial pressure (PO2). We compare multiple curve-fitting approaches to estimatePcritof the Gulf killifish,Fundulus grandis, during closed and intermittent-flow respirometry. The traditional approach fitting two line segments ofMO2versusPO2produced high and variable estimates ofPcrit. Nonlinear regression using hyperbolic or Weibull functions resulted in either variablePcritestimates or, in some cases, failed to converge upon meaningful solutions.Pcritdetermined as thePO2whenMO2equals standard metabolic rate (SMR) based upon a linear relationship ofMO2andPO2at lowPO2were consistent across fish and experimental trials. Therefore, we recommend that Pcrit specifically refer to thePO2below which SMR cannot be maintained. Its determination, therefore, requires accurate measurement of SMR.

The effects of passive integrated transponder (PIT) tagging on cortisol release, standard metabolic rate (SMR) and daily specific growth rate (GS ) were evaluated in the Gulf killifish, Fundulus grandis, a small estuarine fish native to the Gulf of Mexico. Cortisol release by individual fish was measured non-invasively prior to PIT tagging, immediately after tagging and once per week for 1 month following tagging. Within the first 2 h of tagging, cortisol release rates were significantly elevated compared with values measured prior to tagging and significantly higher than that of fish handled identically except not implanted with PIT tags. By 1 week after PIT tagging, cortisol release rates returned to control levels. SMR, determined by intermittent-flow respirometry and GS , defined as per cent change in body mass per day, were measured prior to PIT tagging and weekly for 1 month after tagging. Neither SMR nor GS was significantly different in tagged v. untagged fish for the duration of the study. One month after tagging, haematocrit, plasma cortisol, blood glucose and blood lactate did not differ between tagged and untagged individuals. Therefore, after a transient stress response that subsides within 1 week, PIT tagging had no significant effects on these physiological variables in F. grandis, validating its use as a method of marking this and other small fishes.