Anguilla rostrata glass eel migration and recruitment in the estuary and Gulf of St Lawrence.

This study describes catches of Anguilla rostrata glass eels and associated oceanographic conditions in the St Lawrence Estuary and Gulf. Ichthyoplankton survey data suggest that they enter the Gulf primarily in May, migrate at the surface at night, and disperse broadly once they have passed Cabot Strait. They arrive in estuaries beginning at about mid-June and through the month of July. Migration extends west up to Québec City, in the freshwater zone of the St Lawrence Estuary, 1000 km west of Cabot Strait. Anguilla rostrata glass eels travel between Cabot Strait and receiving estuaries at a straight-line ground speed of c. 10-15 km day(-1). Catches of fish per unit effort in estuaries in the St Lawrence system are much lower than those reported for the Atlantic coast of Canada. Low abundance of A. rostrata glass eels in the St Lawrence system may be due to cold surface temperatures during the migration period which decrease swimming capacity, long distances from the spawning ground to Cabot Strait and from Cabot Strait to the destination waters (especially the St Lawrence River), complex circulation patterns, and hypoxic conditions in bottom waters of the Laurentian Channel and the St Lawrence Estuary.

Phenotypic flexibility of digestive system in Atlantic cod (Gadus morhua).

This study examined the restoration of the digestive capacity of Atlantic cod (Gadus morhua Linnaeus) following a long period of food deprivation. Fifty cod (48 cm, 1 kg) were food-deprived for 68 days and then fed in excess with capelin (Mallotus villosus Müller) on alternate days. Ten fish were sampled after 0, 2, 6, 14 and 28 days and the mass of the pyloric caeca, intestine and carcass determined. Two metabolic enzymes (cytochrome c oxidase and citrate synthase) were assayed in white muscle, pyloric caeca and intestine, and trypsin activity was measured in the pyloric caeca. A delay of 14 days was required before body mass started to increase markedly, whereas most of the increase in mass of both the pyloric caeca and intestine relative to fish length occurred earlier in the experiment. By day 14, the activities of trypsin and citrate synthase in the pyloric caeca as well as citrate synthase in the intestine had reached maxima. The growth of the digestive tissues and restoration of their metabolic capacities thus occur early upon refeeding and are likely required for recovery growth to take place. The phenotypic flexibility of the cod digestive system is therefore remarkable: increases in trypsin activity and size of pyloric caeca resulted in a combined 29-fold increase in digestive capacity of the fish during the refeeding period. Our study suggests that Atlantic cod are able to cope with marked fluctuations in food availability in their environment by making a rapid adjustment of their digestive capacity as soon as food availability increases.

Does condition of Atlantic cod (Gadus morhua) have a greater impact upon swimming performance at Ucrit or sprint speeds?

To compare the sensitivity of sprint and critical (Ucrit) swimming speeds to the condition of Atlantic cod (Gadus morhua) and to identify the best anatomic, behavioural and biochemical correlates of these types of swimming, we established two groups of cod that were fed or starved for 12 weeks. We evaluated sprint swimming and Ucrit performance as well as the speed at which repeated burst-coast movements began in the Ucrit test before measuring the metabolic capacities of red and white muscle sampled caudally, centrally and rostrally and the anatomic characteristics of the cod. White muscle lactate was measured directly after the Ucrit test. As expected, the twofold difference in Fulton's condition factor (0.5+/-0.04 for starved and 1.0+/-0.1 for fed cod) was accompanied by large differences in the anatomic and biochemical parameters measured. Despite the relative sparing of muscle aerobic capacity during starvation and despite the greater use of oxidative fibres during Ucrit compared with sprint swimming, these types of swimming differed by much the same extent between starved and fed cod. In the Ucrit tests, white muscle lactate levels and lactate accumulation per burst-coast movement were considerably higher in fed than starved cod, suggesting more intensive use of fast muscle fibres in cod in good condition. Multiple regression analysis indicated strong correlations between Ucrit, the speed at which regular burst-coasting began and the activity of pyruvate dehydrogenase (PDH) in red muscle (both caudal and central positions). PDH activity may limit the rate of oxidative ATP production by red muscle. The activity of cytochrome c oxidase in rostral white muscle was the strongest correlate of sprint swimming, suggesting that aerobic preparation of white muscle facilitates rapid contraction. The correlation between Ucrit and sprint swimming was weak, perhaps due to inter-individual differences in sensitivity during sprint tests.

Condition, prolonged swimming performance and muscle metabolic capacities of cod Gadus morhua.

This study evaluated the link between swimming endurance and condition of Atlantic cod Gadus morhua that had been fed or starved during the 16 weeks preceding the tests, and assessed whether muscle metabolic capacities explain such links. The condition factor [(somatic mass x fork length(-3))x100] of starved cod was 0.54+/-0.1 whereas that of fed cod was 0.81+/-0.1. In white and red muscle, we measured four glycolytic enzymes: phosphofructokinase (PFK), pyruvate kinase (PK), creatine kinase (CK) and lactate dehydrogenase (LDH), two mitochondrial enzymes: cytochrome c oxidase (CCO) and citrate synthase (CS), a biosynthetic enzyme, nucleoside diphosphate kinase (NDPK), glycogen and protein levels and water content. Muscle samples were taken at three positions along the length of the fish; starvation affected the metabolic capacities of white muscle more than those of red muscle. The levels of glycolytic enzymes and glycogen changed more in white than red muscle during starvation. Both in fed and starved cod, muscle metabolic capacities varied with position along the fish; starvation reduced this longitudinal variation more in white than red muscle. In white muscle of fed cod, the glycolytic enzyme levels increased from head to tail, while in starved cod this longitudinal variation disappeared. In red muscle mitochondrial enzyme levels were highest in the caudal sample, but fewer differences were found for glycolytic enzymes. Swimming endurance was markedly affected by fish condition, with starved fish swimming only 30% of the time (and distance) of fed fish. This endurance was closely linked with the number of burst-coast movements during the test and the activity of CCO and LDH in white muscle. The number of burst-coast movements was significantly linked with condition factor and PFK activity in caudal red muscle and gill arch mass. Our data indicated that cod use both glycolytic and oxidative capacities to support endurance swimming. Furthermore, swimming endurance is linked with the metabolic capacities of red and white muscle.

Once a fast cod, always a fast cod: maintenance of performance hierarchies despite changing food availability in cod (Gadus morhua).

To examine whether Atlantic cod maintain constant hierarchies of sprint speeds and muscle metabolic capacities under different feeding regimes, the physiological capacities of individual cod were followed through a starvation-feeding-starvation cycle. We examined sprint speeds and maximal enzyme activities in white-muscle biopsies at each period. We measured the glycolytic enzymes, phosphofructokinase (PFK) and lactate dehydrogenase (LDH), the mitochondrial enzyme, cytochrome C oxidase (CCO), and the biosynthetic enzyme, nucleotide diphosphate kinase (NDPK). Sprint speeds were measured in a laser diode/photocell-timed raceway. As expected, the feeding regime had a marked impact on the physiological capacities of cod, but the responses differed for sprint-swimming and muscle metabolic capacities. The different enzyme activities as well the condition index generally decreased during the first starvation, improved with feeding, and fell again during the second starvation. In contrast, sprint performance improved after feeding but did not fall with the second starvation. Although both the enzyme activities and the sprint speeds showed considerable interindividual variation, sprint speeds were not significantly correlated with the enzyme activities. The hierarchy of sprint performance of the cod was maintained, regardless of the preceding feeding regime, whereas those of muscle metabolic capacities were not.

Longitudinal and allometric variation in indicators of muscle metabolic capacities in atlantic cod (Gadus morrhua).

This study evaluated whether indicators of metabolic capacity of cod white muscle differ along the length of the body, whether this variation persists over a large range of body sizes, and whether the allometry of metabolic capacities is similar along the length of the body. We examined the maximal activities of two glycolytic enzymes, phosphofructokinase (PFK) and lactate dehydrogenase (LDH), a mitochondrial enzyme, cytochrome C oxidase (CCO), and the biosynthetic enzyme nucleotide diphosphate kinase (NDPK). All enzymes examined showed significant size dependence, which was generally apparent in all regions. The activity of glycolytic enzymes increased with size, whereas that of CCO and NDPK decreased with size. For PFK and LDH, the size dependence decreased caudally, whereas for CCO and NDPK it was strongest in the caudal sample. For each size range, the activities of PFK, LDH, and CCO were higher in the last third of the body than in the middle or just behind the head. In contrast, NDPK activity was higher just behind the head than at the middle or in the last third of the body, suggesting that nuclear proliferation is more rapid in this zone. The high capacity for adenosine triphosphate (ATP) generation in the caudal region suggests that increases in mass-specific ATP output are advantageous in this relatively thin section of the body.

Does the aerobic capacity of fish muscle change with growth rates?

To ascertain whether growth rate modifies the oxidative capacity of fish white muscle, we examined the effects of individual growth rate on the activities of four mitochondrial enzymes in white muscle of the fast growing Atlantic cod,Gadus morhua. Growth rates were individually monitored in cod held at three acclimation temperatures during experiments repeated in four seasons. The size dependence of citrate synthase (CS), cytochrome C oxidase (CCO) and ß-hydroxyacyl CoA dehydrogenase (HOAD) activities was established using wild cod ranging from 115 to 17,350 g. Given their negative allometry, CS and CCO activities in the experimental cod were corrected to those expected for a 1.2 kg animal. HOAD activities did not change with size. The specific activities of CCO and CS were positively correlated with growth rate. However, for both enzymes, season explained more of the variability than growth rate or temperature. Season was the only factor to significantly affect the activity of HOAD, while temperature and season interacted to determine glutamate dehydrogenase activity. CS activity was positively correlated with the initial condition of the cod, which differed among the seasons. The other enzymes did not show this relationship. The independent changes of these enzymes suggest that mitochondria undergo qualitative modifications with changes in growth rate, season and size. Although growth rate and the activities of CCO and CS are positively correlated, the activity of the mitochondrial enzymes is more affected by size, physical condition and season.