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Settlement and growth of age 0+ cod were monitored using snorkel and self-contained underwater breathing apparatus (SCUBA) in four distinct habitat types (sand, seagrass, cobble, and rock reef) in St. Margaret's Bay, Nova Scotia. Newly settled cod were marked with acrylic dye, allowing repeated visual length estimates of individual fish. Settlement of cod did not differ between habitat types, but postsettlement survival and subsequent juvenile densities were higher in more structurally complex habitats. These differences appear to be due to increased shelter availability and decreased predator efficiency in structurally complex habitats. Growth rate was highest in seagrass beds, while the efficiency of cod predators was lowest and cod survival was highest on rocky reefs and cobble bottoms. Thus, trade-offs occur between energy gain and predation risk. In St. Margaret's Bay, the population structure of Atlantic cod may be less influenced by patterns of larval supply than by postsettlement processes such as habitat-specific growth and mortality.

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In anoxia, mitochondria change from being ATP producers to potentially powerful ATP consumers. This change occurs, because the mitochondrial F 1F0-ATPase begins to hydrolyze ATP to avoid the collapse of the proton motive force. Species that can survive prolonged periods of O2 lack must limit such ATP use; otherwise, this process would dominate glycolytic metabolism and threaten ATP delivery to essential ATP-consuming processes of the cell (e.g., ion-motive ATPases). There are two ways to limit ATP hydrolysis by the F1F0-ATPase, namely (i) reduction of the proton conductance of the mitochondrial inner membrane and (ii) inhibition of the enzyme. We assessed these two possibilities by using intact mitochondria isolated from the skeletal muscle of anoxia-tolerant frogs. Our results show that proton conductance is unaltered between normoxia and anoxia. However, ATP use by the F 1F0-ATPase is limited in anoxia by a profound inhibition of the enzyme. Even so, ATP use by the F1F0-ATPase might account for Ϸ9% of the ATP turnover in anoxic frog skeletal muscle.

The respiratory physiology, heart rates and metabolic rates of two captive juvenile male harbour porpoises (both 28 kg) were measured using a rapid-response respiratory gas analysis system in the laboratory. Breath-hold durations in the laboratory (12 +/- 0.3 s, mean +/- SEM) were shorter than field observations, although a few breath-holds of over 40 s were recorded. The mean percentage time spent submerged was 89 +/- 0.4%. Relative to similarly-sized terrestrial mammals, the respiratory frequency was low (4.9 +/- 0.19 breaths.min-1) but with high tidal volumes (1.1 +/- 0.011), enabling a comparatively high minute rate of gas exchange. Oxygen consumption under these experimental conditions (247 +/- 13.8 ml O2.min-1) was 1.9-fold higher than predicted by standard scaling relations. These data together with an estimate of the total oxygen stores predicted an aerobic dive limit of 5.4 min. The peak end-tidal O2 values were related to the length of the previous breath-hold, demonstrating the increased oxygen uptake from the lung for the longer dives. Blood oxygen capacity was 23.5 +/- 1.0 ml.100 ml-1, and the oxygen affinity was high, enabling rapid oxygen loading during ventilation.

Abstract. Genetic isolation by distance (IBD) has rarely been described in marine species with high potential for dispersal at both the larval and adult life‐history stages. Here, we report significant relationships between inferred levels of gene flow and geographic distance in the Atlantic cod, Gadus morhua, at 10 nuclear restriction‐fragment‐length‐polymorphism (RFLP) loci at small regional scales in the western north Atlantic region (< 1600 km) that mirror those previously detected over its entire geographic range (up to 7300 km). Highly significant allele frequency differences were observed among eight northwestern Atlantic populations, although the mean FST for all 10 loci was only 0.014. Despite this weak population structuring, the distance separating populations explained between 54% and 62% of the variation in gene flow depending on whether nine or 10 loci were used to estimate Nm. Across the species' entire geographic range, highly significant differences were observed among six regional populations at nine of the 10 loci (mean FST= 0.068) and seven loci exhibited significant negative relationships between gene flow and distance. At this large geographic scale, natural selection acting in the vicinity of one RFLP locus (GM798) had a significant effect on the correlation between gene flow and distance, and eliminating it from the analysis caused the coefficient of determination to increase from 17% to 62%. The role of vicariance was assessed by sequentially removing populations from the analysis and was found to play a minor role in contributing to the relationship between gene flow and distance at either geographic scale. The correlation between gene flow and distance detected in G. morhua at small and large spatial scales suggests that dispersal distances and effective population sizes are much smaller than predicted for the species and that the recent age of populations, rather than extensive gene flow, may be responsible for its weak population structure. Our results suggest that interpreting limited genetic differences among populations as reflecting high levels of ongoing gene flow should be made with caution.