| Aspects of the seawater (SW) adaptation of juvenile green sturgeon (Acipenser medirostris), an anadromous species along the Pacific Coast of North America, were examined. In their natal rivers, temperatures have changed from historic patterns. To determine the effect of elevated and cycling temperature regimes on post yolk-sac larva to early juveniles, growth rates were measured. Significant differences were found between final wet weights, total lengths (TL), and specific growth rates (24°C & cycling 19--24°C > 19°C), indicating that these temperatures do not adversely affect juveniles (∼0.1--10 g) with abundant food and oxygen. Because some salmonids show decreased swimming performance ( Ucrit) during their parr-smolt transformation, we tested the hypothesis that juvenile green sturgeon decrease U crit during their analogous ontogenetic stage. U crit increased up until the size that SW tolerance was achieved, and then either decreased or increased depending on the season. Therefore, ontogenetic and endogenous factors appear to influence swimming abilities. A salinity tolerance study was conducted to determine the earliest age/size at which SW survival was possible for short-term (72-h) and long-term (28-d) exposure. Juvenile green sturgeon salinity tolerance increased with increasing size and age, with 100% survival in 72-h exposed fish occurring by 134 days post-hatch (dph; 86 g, 266 mm TL), and 100% survival in 28-d exposed fish by 7 months in two size groups (139 g, 336 mm TL; 342 g, 434 mm TL). To determine the metabolic costs and osmoregulatory mechanisms involved in SW adaptation, sturgeon (100, 170, and 533 dph) were acclimated to salinities of <3‰, 10‰, and 33‰. There were no differences in body-mass-adjusted oxygen consumption rates, and osmo- and ionoregulatory abilities were comparable among <3‰ and 10‰ groups, and with the 533 dph 33‰ group. Na+, K+-ATPase activities increased in pyloric caeca at 33‰, presumably due to drinking and SW absorption. Gill chloride cell size increased between 100 and 170 dph, and abundance relative to wet mass decreased with increasing age/size. These results indicated an age/body size effect in hyperosmotic adaptability, and that juvenile green sturgeon most likely enter SW by 1.5 kg (75 cm TL). |
