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 fishsize 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 bodymass-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.

To investigate the timing of the initial entry of green sturgeon Acipenser medirostris into seawater, we examined the ratios of strontium and barium to calcium in pectoral fin rays via laser ablationinductively coupled plasma-mass spectrometry. In a reference group of six hatchery fish that was reared 1-2 years in freshwater and 1-3 years in seawater, we found a significant increase in the Sr:Ca ratio, a decrease in the Ba:Ca ratio, and an increase in the Sr:Ba ratio in calcified growth zones during the transition from freshwater to seawater. In 10 wild adults captured in the Klamath River, California, combined evaluation of the Sr:Ca and Ba:Ca ratios provided a clearer understanding of fish movements than the Sr:Ba ratio, probably because wild fish have a more complex life history. The Ba:Ca ratio dropped significantly between growth zones 1 and 2 (ages 0.5-1.5), indicating a transition into saline waters, such as the Klamath River estuary. The Sr:Ca ratio increased slightly in the same location but attained the values found in reference fish held in seawater between growth zones 3 and 4 (ages 2.5-3.5). The Sr:Ca, Ba:Ca, and Sr:Ba ratios in growth zones 4-6 (ages 4.5-6.5) of wild fish were similar to those found in reference fish held in seawater and corroborated the results of previous physiological studies as well as the limited field data. These results demonstrate the usefulness of trace element analyses of green sturgeon pectoral fin rays and confirm an early age of entry into seawater for this species.

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An increased understanding of the juvenile life history stage of the lake sturgeon, Acipenser fulvescens, has been recognized as a key requirement for improving conservation efforts for this once abundant species. The objectives of the current study were to develop an effective methodology for capturing juvenile lake sturgeon in a large riverine environment; to describe the physical habitat characteristics (depth, water velocity and substrate) associated with the areas where juvenile lake sturgeon were captured; and to determine basic population parameters (length, body mass and condition factor) for juvenile lake sturgeon captured in those areas. Gillnets (mesh sizes 25-108 mm) were used to sample the Winnipeg River, a large river in the northern extent of the species range, from 12 June to 6 November 2006. A total of 2154 juvenile (<530 mm FL) lake sturgeon were captured, which represented over 74% of the total fish catch. Moderate (51-108 mm) and small (25 mm) mesh gill nets were found to be efficient for sampling juvenile lake sturgeon, with the former capturing a wider range of sizes and the latter capturing fewer fish but resulting in decreased mortality. Juvenile lake sturgeon were captured at high densities in discrete areas characterized by water depths >13.7 m, detectable water velocities >0.20 m s )1 , and various substrate types.

Because some salmonids show decreased swimming performance (Ucrit) during their parr-smolt transformation, we tested the hypothesis that juvenile anadromous green sturgeon (Acipenser medirostris) decrease Ucrit during their analogous ontogenetic stage. Juvenile green sturgeon (mean age: 98 days) that had not reached seawater tolerance had a positive relationship of Ucrit (cm·s–1) with total length (TL), whereas seawater-tolerant sturgeon (mean age: 150 days) had a negative relationship of Ucrit with TL. This Ucrit decrease was presumably seasonal because a similar-sized group of fish tested later (mean age: 288 days) showed a linear increase in Ucrit with TL. Smaller sturgeon displayed both morphological (larger relative pectoral fin surface areas) and behavioral (rostrum wedging and pectoral fin holding) attributes for station holding in a riverine environment. In a second experiment to test the effects of increased river temperature near the predicted time of downstream migration, Ucrit and heat-shock protein concentrations increased at 24 °C compared with at 19 °C (mean age: 155 days). Thus, although there was evidence for cellular stress, the swimming ability of the fish was not compromised. Therefore, water velocity, temperature, and time of year should be considered in life-stage-specific efforts to manage or protect green sturgeon.

We measured the ontogeny of salinity tolerance and the preparatory hypo-osmoregulatory physiological changes for seawater entry in green sturgeon (Acipenser medirostris), an anadromous species occurring along the Pacific Coast of North America. Salinity tolerance was measured every 2 weeks starting in 40-day post-hatch (dph) juveniles and was repeated until 100% survival at 34‰ was achieved. Fish were subjected to step increases in salinity (5‰ 12 h(-1)) that culminated in a 72-h exposure to a target salinity, and treatment groups (0, 15, 20, 25, 30, 34‰; and abrupt exposure to 34‰) were adjusted as fish developed. After 100% survival was achieved (134 dph), a second experiment tested two sizes of fish for 28-day seawater (33‰) tolerance, and gill and gastrointestinal tract tissues were sampled. Their salinity tolerance increased and plasma osmolality decreased with increasing size and age, and electron microscopy revealed three types of mitochondria-rich cells: one in fresh water and two in seawater. In addition, fish held on a natural photoperiod in fresh water at 19°C showed peaks in cortisol, thyroid hormones and gill and pyloric ceca Na(+), K(+)-ATPase activities at body sizes associated with seawater tolerance. Therefore, salinity tolerance in green sturgeon increases during ontogeny (e.g., as these juveniles may move down estuaries to the ocean) with increases in body size. Also, physiological and morphological changes associated with seawater readiness increased in freshwater-reared juveniles and peaked at their seawater-tolerant ages and body sizes. Their seawater-ready body size also matched that described for swimming performance decreases, presumably associated with downstream movements. Therefore, juvenile green sturgeon develop structures and physiological changes appropriate for seawater entry while growing in fresh water, indicating that hypo-osmoregulatory changes may proceed by multiple routes in sturgeons.

Relatively little is known about salinity acclimation in the primitive groups of fishes. To test whether physiological preparative changes occur and to investigate the mechanisms of salinity acclimation, anadromous green sturgeon, Acipenser medirostris (Chondrostei) of three different ages (100, 170, and 533 dph) were acclimated for 7 weeks to three different salinities (\3, 10, and 33 ppt). Gill, kidney, pyloric caeca, and spiral intestine tissues were assayed for Na ? , K ? -ATPase activity; and gills were analyzed for mitochondria-rich cell (MRC) size, abundance, localization and Na ? , K ? -ATPase content. Kidneys were analyzed for Na ? , K ? -ATPase localization and the gastrointestinal tract (GIT) was assessed for changes in ion and base content. Na ? , K ? -ATPase activities increased in the gills and decreased in the kidneys with increasing salinity. Gill MRCs increased in size and decreased in relative abundance with fish size/age. Gill MRC Na ? , K ? -ATPase content (e.g., ion-pumping capacity) was proportional to MRC size, indicating greater abilities to regulate ions with size/age. Developmental/ontogenetic changes were seen in the rapid increases in gill MRC size and lamellar length between 100 and 170 dph. Na ? , K ? -ATPase activities increased fourfold in the pyloric caeca in 33 ppt, presumably due to increased salt and water absorption as indicated by GIT fluids, solids, and ion concentrations. In contrast to teleosts, a greater proportion of base (HCO 3 -and 2CO 3 2-) was found in intestinal precipitates than fluids. Green sturgeon osmo-and ionoregulate with similar mechanisms to more-derived teleosts, indicating the importance of these mechanisms during the evolution of fishes, although salinity acclimation may be more dependent on body size.

An examination was made of whether social interactions can have a beneficial effect through the attenuation of the stress response in a social species. In the first experiment, one larger (mean +/-s.e. 194.0 +/- 12.5 g) and seven smaller (32.0 +/- 2.6 g) juvenile lake sturgeon Acipenser fulvescens were placed in tanks to determine whether a classic dominance effect would be established based on body size (n = 6). Large fish did not establish a territory or aggressively interact with smaller fish, as there were no significant differences in nearest-neighbour distances and an absence of aggressive behaviour (biting, chasing and pushing). In the second experiment, it was hypothesized that the presence of conspecifics would have a beneficial effect through an attenuation of the stress response. Fish in groups or isolation were stressed by a brief aerial exposure (30 s), and blood plasma was measured at regular time intervals (0, 20, 40, 60, 120 and 240 min) following the stressor via an implanted cannula (n = 9-11). The presence of conspecifics did not affect the peak cortisol response, however, the overall cortisol response was shorter in duration compared to fish in isolation. Furthermore, secondary stress variables (plasma ions and glucose) showed differences between fish in groups and isolation. The results of these experiments suggest that social interaction plays an important and beneficial role in regulating the stress response in cohesive social species such as A. fulvescens.