SUMMARY We investigated the effects of the chorion on movement and growth in rainbow trout (Oncorhynchus mykiss) embryos. To test if the chorion restricts movement and growth before hatching, we manually removed the chorion 3–6 days before the natural time of hatching (dechorionated) and compared movement, growth and oxygen consumption in dechorionated embryos and in embryos whose chorions remained intact until the time of hatching(chorionated). Dechorionated embryos exhibited 36 times more movement before hatching compared with intact embryos. By 10 h post-hatch there was no difference in the number of movements between the two groups. At the time of hatching [30 days post-fertilization (d.p.f.)], dechorionated embryos had a significantly greater embryonic body dry mass compared with chorionated embryos, which persisted up to 45 d.p.f. At first feeding (50 d.p.f.) there was no significant difference in embryonic body dry mass between the two groups. Dechorionated embryos had a significantly greater embryonic body protein content after hatching (32, 33 d.p.f.) compared with chorionated embryos. Despite the differences in movement and growth, there were no significant differences in oxygen consumption between chorionated and dechorionated embryos. Furthermore, there was no correlation between the number of movements and oxygen consumption in rainbow trout embryos(chorionated, dechorionated, and hatched). Taken together, the data indicate that rainbow trout embryos have the capacity to be relatively active before hatching, but that the chorion restricts or inhibits movement. Moreover,precocious activity in pre-hatch embryos is correlated with accelerated growth and higher protein content, suggesting that the exercise training effect observed in adult salmonids is also present in early developmental stages.

The objective of our study was to estimate the overall (aerobic and anaerobic) energy expenditure involved in hatching in rainbow trout (Oncorhynchus mykiss) embryos. During hatching, there were significant increases (50% and 19%, respectively) in movement and oxygen consumption. There were no differences in tissue indicators of anaerobic metabolism (adenosine triphosphate (ATP), adenosine monophosphate (AMP) deaminase, ammonia) between embryos that hatched naturally (chorionated) compared with embryos in which the chorion had been manually removed and therefore did not hatch (dechorionated). Furthermore, phosphocreatine (PCr), ATP, and lactate were unaltered immediately after hatching compared with embryos at rest before hatch. The anaerobic capacity of just hatched larvae was further evaluated by forced exercise and hypoxic exposure. PCr and ATP were unaltered following a 2 min manual chase, whereas lactate concentration was significantly increased (30%). Hypoxia (5 mg O2·L–1, 5 days) resulted in a 2.7-fold elevation of tissue lactate concentration. Our results reveal that the increase in activity during hatch may be involved in escaping the egg capsule, but the metabolic cost of this increased movement is relatively small. Moreover, the anaerobic capacity of newly hatched rainbow trout is low relative to juvenile or adult fish.