Depletion in dissolved oxygen (DO) occurred in 2 semi-enclosed shallow bays in Korea in which Crassostrea gigas were suspended for aquaculture. DO depletion started at the bottom and expanded rapidly upwards for days until it was blocked by a temperature-driven density differential. This resulted in the formation of pelagic oxyclines at a depth of 2-4 m in one bay (Hypoxic Site 1, HS-1) and 2-5 m in the other (Hypoxic Site 2, HS-2). Water above the oxycline was normoxic (> 4 mg l ). The oxyclines were accompanied by significant changes in environmental variables and phytoplankton composition, but these changes were not fatal to the oysters. However, the oxyclines themselves caused oyster mortality: complete mortality below the oxyclines and depth-dependent mortality within the oxycline. Interestingly, mortality was observed in a significant number of oysters above the oxyclines compared with the reference site, and greater mortality was ob served in HS-1 than HS-2. These findings suggest the existence of toxic compounds that diffuse up from below the oxycline in shallow waters and exert effects that overshadow those of DO. The higher mortalities in the HS-1 normoxic layer supported this influence from the bottom layer. In a subsequent experiment, we observed additional mortalities among the surviving oysters in the upper normoxic waters after the bays had completely reoxygenated during fall turnover. These data provide useful observations of hypoxia in highly sheltered shallow waters and can be used to guide site selection for oyster longline aquaculture.
This study determined the level of fermented soybean meal (FSM) that could be substituted for fish meal in the diet for parrot fish Oplegnathus fasciatus. Fish meal (FM) was used the sole protein source in the control diet. FSM was substituted for 20, 40, 60, 80, and 100% of the fish meal in the experimental diets. The FSM resulted in increased crude protein, crude fat, and crude ash, but decreased crude fiber. The methionine and tryptophan contents in the FSM increased, while lysine and threonine decreased. The 18:2n-6 fatty acid content decreased from 55.30% to 28.67%. Fermentation increased 20:5n-3 (EPA) and 22:6n-3 (DHA) by 2.03% and 15.54%, respectively, although the differences were not significant. Based on growth performance, we concluded that FSM could replace up to 60% of FM for the maximum growth of juvenile parrot fish.
Most cold-adapted enzymes possess higher K and k values than those of their mesophilic counterparts to maximize the reaction rate. This characteristic is often ascribed to a high structural flexibility and improved dynamics in the active site. However, this may be less convincing to cold-adapted metabolic enzymes, which work at substrate concentrations near K. In this respect, cold adaptation of a shikimate kinase (SK) in the shikimate pathway from psychrophilic (CpSK) was characterized by comparing it with a mesophilic homolog (EcSK). The optimum temperatures for CpSK and EcSK activity were approximately 30°C and 40°C, respectively. The melting points were 33°C and 45°C for CpSK and EcSK, respectively. The ΔG (denaturation in the absence of denaturing agent) values were 3.94 and 5.74 kcal/mol for CpSK and EcSK, respectively. These results indicated that CpSK was a cold-adapted enzyme. However, contrary to typical kinetic data, CpSK had a lower K for its substrate shikimate than most mesophilic SKs, and the k was not increased. This observation suggested that CpSK may have evolved to exhibit increased substrate affinity at low intracellular concentrations of shikimate in the cold environment. Sequence analysis and homology modeling also showed that some important salt bridges were lost in CpSK, and higher Arg residues around critical Arg 140 seemed to increase flexibility for catalysis. Taken together, these data demonstrate that CpSK exhibits characteristics of cold adaptation with unusual kinetic parameters, which may provide important insights into the cold adaptation of metabolic enzymes.
The sea cucumber Apostichopus japonicus is a commercially valuable aquaculture species in Korea. Aquaculture species require specific nursery culture conditions to increase survival and growth rates. Sea cucumbers hibernate during the high temperatures of summer and during the low temperatures of winter, and suboptimal temperature conditions decrease sea cucumber growth and survival rates. The natural South Korean environment is very unfavorable for culturing sea cucumber; therefore, developing a recirculating aquaculture system (RAS) capable of breeding and growing sea cucumber year-round is necessary. The aim of this study was to investigate growth performance of juvenile sea cucumber in a RAS. Growth and survival rates of juvenile sea cucumber were high during our 24-week experiment. Sea cucumber survival rates were 87.8-93.3%, and specific growth rates were 0.4689-0.7846.
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