SynopsisRoutine respiratory metabolic rates of mosquitofish (-0.2g live weight) were determined at 10, 15,20,25, 30, and 35°C at normoxic, hypoxic (= 40 torr P@), and extreme hypoxic (= 25 torr P@) conditions. Rates generally increased with increases in temperature (overall Q,, = 2.11 at normoxia). Significant depressions (P<0.05) in rates were measured at 30 and 35°C at extreme hypoxic conditions. Males exhibited higher mortality at extreme hypoxia than females, especially at 35" C. Metabolic rates were significantly elevated at hyperoxic conditions (= 300 torr P@) at 25 and 30" C, but not at 35°C. Resting routine rates of less-confined fish were determined at 20,25, and 30" C at normoxia and were significantly lower than the routine rates at the same temperatures. Behavioral experiments showed that aquatic surface respiration is initiated by mosquitofish at -20-65 torr Po, and is obligatory below 20 torr at 20" C. Overall, respiratory metabolic rates provide a quantitative metabolic basis from which predation rates of mosquitofish in various environments can be estimated.

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SynopsisSuspension-feeding fishes use gill structures for both respiration (lamellae) and food capture (rakers) . During hypoxic exposure in eutrophic lakes or poorly circulated sloughs, many fishes, including Sacramento blackfish, Orthodon microlepidotus, increase their gill water flows, in part by increasing ventilatory stroke volumes . Stroke volume increases could compromise particle sieving efficiency by spreading interdigitated gill rakers from adjacent gill arches, although blackfish capture food particles by raker-guided water flows to a sticky buccal root. Using van Dam-type respirometers, blackfish respiratory variables and feeding efficiency (Artemia nauplii) were measured under normoxia (> 130 torr P0 2) and hypoxia (60 torr P0 2). Compared with non-feeding, normoxic conditions, gill ventilation volume, frequency, stroke volume, and gape all increased, while 0 2 uptake efficiency decreased, during hypoxia and during feeding . 02 consumption increased during feeding treatments, and % uptake of nauplii showed no difference between normoxic and hypoxic groups. Thus, blackfish display respiratory adaptations, including increased ventilatory stroke volumes, to survive in hypoxic environments such as Clear Lake, California . Importantly, they have also evolved a particle capture mechanism that allows efficient suspension-feeding under both normoxic and hypoxic conditions.