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Metabolic thermal plasticity is central to the survival of fishes in a changing environment. The eurythermal three-spined stickleback Gasterosteus aculeatus displays thermal plasticity at the cellular level with an increase in the activity of key metabolic enzymes in response to cold acclimation. Nonetheless, it is unknown if these changes are sufficient to completely compensate for the depressive effects of cold temperature on whole organismal metabolic rate (ṀO2). The authors hypothesized that as a cold-tolerant, eurythermal fish, absolute aerobic scope (AAS), the difference between the maximum metabolic rate (MMR) and standard metabolic rate (SMR), would be maintained in G. aculeatus following acclimation to a range of temperatures that span its habitat temperatures. To test this hypothesis, G. aculeatus were acclimated to 5, 12 and 20(C for 20–32 weeks, and SMR, MMR and aerobic scope (AS) were quantified at each acclimation temperature. The maximal activity of citrate synthase (CS), a marker enzyme of aerobic metabolism, was also quantified in heart ventricles to determine if cardiac aerobic capacity is associated with AS at these temperatures. SMR increased with acclimation temperature and was significantly different among all three temperature groups. MMR was similar between animals at 5 and 12(C and between animals at 12 and 20(C but was 2.6-fold lower in fish at 5(C compared with those at 20(C, resulting in a lower AAS in fish at 5(C compared with those at 12 and 20(C. Correlated with a higher AAS in animals acclimated to 12 and 20(C was a larger relative ventricular mass and higher CS activity per 100 g body mass compared with animals at 5(C. Together, the results indicate that despite their eurythermal nature, AS is not maintained at low temperature but is associated with cardiac aerobic metabolic capacity. [ABSTRACT FROM AUTHOR]

Capelin Mallotus villosus is a coldwater, marine forage fish that responds quickly to environmental fluctuations; however, little is known about Capelin in Alaskan waters. The objective of the current study was to better understand the distribution and life history of spawning Capelin in northern Norton Sound, Alaska. Surveys were conducted from May through July 2018 to locate and estimate the size of nearshore Capelin aggregations prior to spawning, identify the location and timing of spawning events, characterize spawning habitat, and collect actively spawning fish to examine life history characteristics (e.g., body size, age, fecundity, etc.). Most (85.9%) of the nearshore aggregations were less than 12 m2 in surface area. Spawning Capelin were collected in Norton Sound between June 15 and June 21. At spawning locations, gravel and coarse sand accounted for over 70% of the proportional weight of sediment collected within a beach, and all sediment samples contained Capelin eggs. Spawning males were larger than spawning females in TL (mean ± SD = 148.8 ± 6.7 mm versus 137.0 ± 8.4 mm, respectively) and total weight (21.2 ± 2.9 g versus 13.7 ± 3.0 g), and both sexes were predominately age 3 (age range = 2–4 years). Absolute fecundity was 9,219 ± 4,529 eggs, and the gonadosomatic index was 1.09 ± 0.32% for males and 21.69 ± 8.21% for females. Nearshore aggregation sizes in Norton Sound were smaller than those reported for Newfoundland, but spawning behavior, timing, and water conditions were similar to observations from other Capelin spawning regions (e.g., Greenland), as were size, age, fecundity, and gonadosomatic index estimates. Although the results from the current study update baseline information on spawning Capelin in northern Norton Sound, continued research on their distribution and life history is needed to better understand ecosystem function in the North Pacific Ocean. [ABSTRACT FROM AUTHOR]