Physiological Benefits of Being Small in a Changing World: Responses of Coho Salmon (Oncorhynchus kisutch) to an Acute Thermal Challenge and a Simulated Capture Event

Abstract

Evidence is building to suggest that both chronic and acute warm temperature exposure, as well as other anthropogenic perturbations, may select for small adult fish within a species. To shed light on this phenomenon, we investigated physiological and anatomical attributes associated with size-specific responses to an acute thermal challenge and a fisheries capture simulation (exercise+air exposure) in maturing male coho salmon (Oncorhynchus kisutch). Full-size females were included for a sex-specific comparison. A size-specific response in haematology to an acute thermal challenge (from 7 to 20°C at 3°C h−1) was apparent only for plasma potassium, whereby full-size males exhibited a significant increase in comparison with smaller males (‘jacks’). Full-size females exhibited an elevated blood stress response in comparison with full-size males. Metabolic recovery following exhaustive exercise at 7°C was size-specific, with jacks regaining resting levels of metabolism at 9.3±0.5 h post-exercise in comparison with 12.3±0.4 h for full-size fish of both sexes. Excess post-exercise oxygen consumption scaled with body mass in male fish with an exponent of b = 1.20±0.08. Jacks appeared to regain osmoregulatory homeostasis faster than full-size males, and they had higher ventilation rates at 1 h post-exercise. Peak metabolic rate during post-exercise recovery scaled with body mass with an exponent of b∼1, suggesting that the slower metabolic recovery in large fish was not due to limitations in diffusive or convective oxygen transport, but that large fish simply accumulated a greater ‘oxygen debt’ that took longer to pay back at the size-independent peak metabolic rate of ∼6 mg min−1 kg−1. Post-exercise recovery of plasma testosterone was faster in jacks compared with full-size males, suggesting less impairment of the maturation trajectory of smaller fish. Supporting previous studies, these findings suggest that environmental change and non-lethal fisheries interactions have the potential to select for small individuals within fish populations over time.

Aerobic scope measurements of fishes in an era of climate change: respirometry, relevance and recommendations

“…However, as noise and drift are low in modern optode systems, there is little purpose in letting kg −1 obtained after 16h) due to spontaneous activity of the fish. Raw data taken from Clark et al (Clark et al, 2012).…”

“…Once this knowledge has been obtained, experimental approaches and techniques can be refined to ensure the most accurate estimates of standard M O2 and the most efficient use of time. Notably, some species settle in respirometers faster than others, and smaller fish often recover baseline metabolism faster than larger fish following a stressful event (Clark et al, 2012), hence these factors should be considered when determining the most appropriate respirometry techniques and protocols. Additionally, it is possible that measured values of M O2,min can decrease with subsequent, independent periods in the respirometer, as the fish may become progressively more comfortable with the handling and measurement procedures.…”

“…1min) to the treatment to help ensure that the fish is maximally exhausted and most likely to elicit M O2,max during the recovery period (e.g. Clark et al, 2012). While a short period of air exposure is common in standard fisheries practices for most species, some animal ethics committees may object to air-exposing fish for scientific purposes and therefore this should be considered before deciding on appropriate protocols.…”

Exploring key issues of aerobic scope interpretation in ectotherms: absolute versus factorial

“…However, while interpretation of aerobic scope defined as AAS can help us understand how an animal can function within its environment, it does not account for the fact that different individuals may require a greater proportional investment of oxygen to perform a given physiological task, perhaps most obviously due to size-dependent differences in the relative costs of activity, whereby smaller animals typically (cf. Clark et al 2012) have a higher relative metabolic rate (Schmidt-Nielsen 1972). Analysis of FAS, on the other hand, evaluates the capacity of the animal's oxygen transport system relative to its own baseline rate of oxygen uptake.…”

“…An example of when the choice of FAS or AAS may affect interpretation of results is when aerobic scope is compared across a range of body sizes and life stages (Bishop 1999;Gillooly and Allen 2007;Glazier 2009;Weibel and Hoppeler 2005). Relatively few studies have quantified changes in both SMR and MMR over orders of magnitude of body sizes for a single species (Brett 1965;Clark et al 2012;Killen et al 2007). In each of these three studies, the authors presented changes in aerobic scope with mass in terms of FAS, or presented AAS on a log scale.…”