| Popis: |
Dissolved oxygen (DO) level is a critical factor for the survival of fishes and sustainable management of an aquatic ecosystem. This study aims to investigate the lethal thresholds of three major fish species in the Love River, Kaohsiung, S. W. Taiwan, in an attempt to provide ecological points of view for the further improvement of the river-water quality, as well as for the restoration of its fish community. Laboratory tolerance experiments of fish fries, including mullet (Liza macrolepis), milkfish (Chanos chanos) and tilapia (Oreochromis mossambicus), exposed to low levels of dissolved oxygen were conducted in the period of January 2002 to April 2003. At water temperature of 23¢J, median lethal times (LT50) and 48 hours median lethal concentrations (48h LC50) at the salinity of 10‰, 20‰ and 30‰ were determined, respectively. We also carried out a lethal DO experiment to mullet fry in a sudden drop of salinity from 30‰ to 20‰, which is always the case when the sewage gate of the Love River interception system is opened to avoid flooding after heavy rainfall. Under the condition of 20‰ salinity, the tolerance of low dissolved oxygen for the three species of fish fries were significantly different. Among them, milkfish was found to be the most sensitive species, followed by mullet, whereas tilapia was the most tolerant species. The 48h LC50s of milkfish, mullet and tilapia were 15.7, 8.6 and 7.8% air saturation, respectively, which were 1.16, 0.63 and 0.57 mg/l at 23¢J. Similarly, the LT50s also showed species-specific differences. At DO level of 0.52 mg/l, the LT50s were 77.9 minutes for milkfish, 245.7 minutes for mullet and over 1000 minutes for tilapia. The changing of salinity affecting the tolerance of low dissolved oxygen in fish fries was species depending. The DO lethal concentrations of mullet fry increased with the decrease of salinity, but not the case for milkfish fries. The 48h LC50 of mullet fries in 30‰ saline water was 11.5% air saturation (0.8 mg/l), which was higher than those in 10‰ and 20‰ saline water. However, no significant difference in 48h LC50 was found between milkfish kept in 10‰ and 20‰ saline water. The 48h LC50 for the former was 14.9% air saturation while the latter was 15.7%. Both are equivalent to 1.16 mg/l at water temperature of 23¢J. A rapid change of salinity reduced the low Do tolerance of fish fry. When mullet fry were introduced to 20‰ saline water from acclimated salinity of 30‰, its 48h LC50 raised significantly (13.9% air saturation; 1.02 mg/l). Under the DO concentration of 0.52 mg/l, the LT50 also dropped to 7.6 minute. It means that the opening of sewage interception gate largely challenges the survival fish fry. Based on the results of current study, the DO level of Love River is suggested to be maintained beyond 2.19 mg/l (28 % air saturation, 48h LC50 of milkfish at 10‰ salinity and 23¢J water temperature), in order to achieve a sustainable environment for the present fish community. In case of that DO occasionally fall to the above level, it is strongly recommended that a manual aeration system should be operated within 100 minutes. |
