Study On The Osmotic Physiology And Salinity Preference Of Chinese Sturgeon During Seawater Adaptation

Chinese sturgeon, Acipenser sinensis Gray, a class I endangered species by the Chinese government, is an anadromous species that presently only remains in the Yangtze (=Changjiang) River, the East China Sea and Yellow Sea. Adult Chinese sturgeon, which are a maximum of 400 cm total length (TL) and 452 kg body weight (BW), are one of the largest fish to enter fresh water. Juveniles hatched in the Yangtze River migrate to the sea and return to spawn at the age of 8-10 years. Chinese sturgeon juveniles concentrate at the river estuary during the period of May-September, and almost all of the young fish found in the estuary are younger than 1 year old. As a result of overfishing and construction of Gezhouba Dam in 1981 at Yichang, Hubei Province, 1766 km from the river estuary, blocked the spawning migration of Chinese sturgeon to the Yibin spawning reach, populations of Chinese sturgeon have greatly declined in abundance. Some successful spawning occurs in the short reach below the dam (Gezhouba spawning site) as verified by the capture of early-life stages in 1982 and during 1996-1999. By the beginning of this century, research on Chinese sturgeon has focused on the fields of morphology, reproductive biology and, especially, reproductive ecology and stock assessment of the spawning population below the Gezhouba Dam. To date, little information has been gathered concerning the osmoregulatory physiology and salinity preference of Chinese sturgeon. The author studied the development of osmoregulatory mechanisms and salinity preference behavior of 1-year-old juvenile Chinese sturgeon during seawater adaptation. Conclusions from the studies are as follows:1. Changes of serum osmolality, serum ion concentrations and Na⁺, K⁺-ATPase activities in juvenile Chinese sturgeon during seawater adaptationThe osmoregulation capabilities of 7-month-old juvenile Chinese sturgeon (128.8±15 g) transferred directly from freshwater (0%o, 46 mOsmol kg^-1) to brackish water (10‰, 273 mOsmol kg^-1) were studied over a 20-day period. Changes in serum osmolarity, chloride (Cl^-), sodium (Na⁺), potassium (K⁺) and calcium (Ca²⁺) ion concentrations, as well as gill, spiral valve, rectal and renal Na⁺, K⁺-ATPase activities were measured at 3, 12, 24, 72, 216 and 480 h after transfer to BW. The serum osmolarity and ion concentrations (Na⁺, Cl^- and Ca²⁺) increased immediately after the transference to BW, reaching maximum at 24 h and returned to a new steady state at 216 h, while the FW control group maintained basal levels which showed lower (p<0.05) than the BW group. Serum potassium ion concentration lagged behind the chloride, sodium, potassium and calcium ion concentrations after exposure to BW. K⁺ ion concentration of BW group maintained steady state in the first 12 h after transfer, but began to increase at 24 h, reaching maximum at 216 h after transference and returned to the levels of FW control at 480 h. Gill Na⁺, K⁺-ATPase activity of BW group exhibited an abrupt decrease in the first 3 h after transfer, but began to increase at 3 h, reaching a peak value at 24 h, and returned a new steady state at 216 h. The differences between gill Na⁺, K⁺-ATPase activity of BW and FW fish were significant (p< 0.05) after 12 h. In contrast, Na⁺, K⁺-ATPase activity of the spiral valve showed transient increase after transference from FW to BW, and then decreased rapidly at 3 h, reaching the lowest at 24 h after transference. At 216h after exposure to BW, Na⁺, K⁺-ATPase activities of the spiral valve increased slowly to the levels of FW control. Rectal Na⁺, K⁺-ATPase activity of BW group maintained steady state in the first 3 h after transfer, but began to decrease at 3 h, and returned slowly to the levels of FW control at 216 h. Renal Na⁺, K⁺-ATPase activity decreased rapidly after exposure to BW, and the differences between renal Na⁺, K⁺-ATPase activity of BW and FW fish were significant (p< 0.05).2. Changes in serum hormone levels in juvenile Chinese sturgeon during seawater adaptationAfter exposure to BW, the fish bated rapidly its secretion of prolactin (PRL), and accelerated to secrete cortisol and thyroxine. The serum PRL level decreased immediately after the transference to BW, and reached a new steady low state at 12 h which was significantly lower than in the FW control fish (p<0.05). The serum cortisol level of BW group exhibited an abrupt increase after transfer, reaching a peak value at 3 h and returned rapidly to the steady level at 24 h. The new steady level of cortisol in BW group was higher than the level of the FW control group, and did not show significant differences. After transference from FW to BW, Serum total thyroxine (TT₄) level showed transient increase, reaching maximum at 3 h and returned slowly to the level of the FW control group at 216 h. Serum total triiodothyronine (TT₃) increased immediately after the transference to BW, reaching a peak value at 3 h and returned to a new steady state which was significantly higher than in the FW control fish (p<0.05). Serum free thyroxine (FT₄) and free triiodothyrinine (FT₃) increased immediately in the first 3 h after the transference to BW, and returned rapidly to the level of the FW control group at 24 h.3. Structural changes of gills chloride cell of juvenile Chinese sturgeon acclimated to various salinitiesModifications in the chloride cells of gill epithelia of juvenile Chinese sturgeon in FW and BW were examined by light and transmission electron microscopy. In freshwater, a few chloride cells were present on the base of lamellae and in the interlamellar region of the filament with undeveloped tubular network and vesicle-tubular as freshwater-type chloride cells. While in brackish water, fish showed a marked increase in the number and size of chloride cells. On the lamellae of these fish, chloride cells were generally centralizing to the base of lamellae. Ultrastructural modifications included: presence of a more compact tubular network, a greater development vesicle-tubular and an enlarged apical crypt bearing some short microvilli asα-subtypes (seawater-type) chloride cells. The gill chloride cell, with increasing in the number and modifying structure, participate in extruding excess Na⁺, Cl^- and adjusting osmolality of body fluid in the hyperosmotic medium.4. The activities of digestive enzymes of juvenile Chinese sturgeon during seawater adaptationThe protease, amylase and lipase activities of the alimentary canal in 7-month-old juvenile Chinese sturgeon were increased in the few hours after exposure to BW, and then decreased rapidly. The minimum activities of protease and amylase occurred at 12h after exposure to BW. The protease and amylase activities were increased continually after 48h after exposure to BW, and were the same as activities of freshwater control group at 216h. The minimum activity of lipase occurred at 48h after exposure to BW. The lipase activity was increased continually after 72h, and was the same as lipase activity of freshwater control group at 216h after exposure to BW. The activity of lipase was affected strongly by salt. Environment salinity didn't affect secrete of digestive enzymes in liver. The activities of protease and amylase were from high to low: pylorus vesica > duodenum > ileum > stomach > liver. The lipase activity was from high to low: ileum > duodenum > stomach > liver > pylorus vesica.5. Design a device for the study of salinity preference in Chinese sturgeonWhen two kinds of different salinity water met, there showed vertical stratification phenomenon and appeared a stable interface, forming the light water in the upper layer and the heavy water in the lower layer. According to the law of salinity diffusion, a six-chambered device for the study of salinity preference in Chinese sturgeon was designed. The device, including six salinity chambers, was circular, 1-m-high, 6-m-diameter, and the center area was the transition region. The structure was even. Salinities in the six chambers could steady 10 days without disturbing, while those salinities only steadied one day in the state of continuing bubbling or fish turbulence. To test salinity preference, fish were placed in the testing device from six chambers or the center area. We could perceive salinity preference, or behavioral selection of a particular concentration of dissolved salts, according to the difference of the frequency or duration in six salinity chambers.6. Salinity preference behavior of juvenile Chinese sturgeonJuvenile Chinese sturgeon which was strictly confined to freshwater showed preference for fresh water. While, wild juvenile Chinese sturgeon which concentrated at the river estuary without completing conversion from hyper-osmoregulatory to hypo-osmoregulatory chose salinity 5%o. In contrast, juvenile Chinese sturgeon which had been acclimated in 10‰BW "preferred" salinity 5‰. In the present study, condition salinity appeared to have affected the salinity preference of juvenile Chinesesturgeon. It is the plasticity of salinity preference to drive juvenile Chinese sturgeoninto the sea.