Age/size effects on juvenile green sturgeon, Acipenser medirostris, oxygen consumption, growth, and osmoregulation in saline environments

The green sturgeon, Acipenser medirostris, is an anadromous species that migrates from freshwater (FW) to seawater (SW) relatively early in its life history, although the ages and sizes of juveniles at SW entry are not known. Developmental constraints of osmoregulatory organs may either prohibit (i.e., due to salinity tolerance limits) or minimize (i.e., due to substantial osmoregulatory or ionoregulatory energetic costs) SW entry in small fish. Interestingly, larger green sturgeon are often encountered in brackish water (BW) estuaries, perhaps due to an energetic advantage in occupying these near-isosmotic environments. To test hypotheses concerning fish-size effects on the energetic costs of occupying habitats of different salinities, we measured oxygen consumption rates in green sturgeon representing three age groups (100, 170, and 533 days post hatch; dph), which were acclimated for 5 weeks to one of three salinities (FW, <3‰; BW, 10‰; or SW, 33‰). Also, after 7 weeks, final wet masses were compared and blood and muscle tissue samples were taken to assess osmoregulatory abilities. There were no differences in body-mass-adjusted oxygen consumption rates between any salinities or ages, indicating that the energetic costs were not prohibitively high to occupy any of these salinities. The only mortalities occurred in the 100 dph SW group, where 23% of the fish died, from apparent starvation. Final wet masses were comparable between FW and BW for each age group and with the 533 dph SW group, but were lower in SW groups at 100 and 170 dph. Similarly, osmoregulatory abilities, in terms of plasma osmolality, Na⁺, K⁺, lactate, and protein concentrations, and muscle water content, were comparable in FW and BW groups at all ages, and with the SW group at 533 dph. These results indicated an age/body size effect in hyperosmotic adaptability, and that juvenile green sturgeon may be found in FW or BW at any age, but only have the ability to enter SW by 1.5 years (75 cm, 1.5 kg) of age.