| Huso dauricus is one of the existing two Huso in the world, it mainly distributed in Amur river and its tributaries (including Ussuri, Sungari, Zeya river, Bureya river, Shilka river, Argun river, Ob river, and Ingoda river), Khanka lake and Orel lake. There are two kinds of life history H. dauricus, they are fresh water type and estuary half migration type. Fresh water type completely settle in freshwater of lower and middle reach of Amur river and tributaries, estuary half migration type breed in rivers artery and migrate to the estuary for forage. In recent years, fishing survey show that H. dauricus fishing production almost focused on the lower reach and estuary of the river, fishing production in middle reach has nearly exhausted, China’s fishing mainly limited in the Fuyuan reach, and the annual output has been less than10t. In2010, H. dauricus was listed as critically endangered species by IUCN red list (CR). So far, Amur river main stream has not been built dam, the main reason that lead H. dauricus resources to exhaustion is the legal and illegal overfishing stimulated by the caviar prices, water pollution and lack of food in river channel.Due to its endangered status, the Chinese government has conducted large-scale population enhancement since2002, releasing over10million larvae and juveniles in Fuyuan county (lower-middle reach of the Amur River) from2002to2008. Cultured fish were also stocked at other places along the middle reach of the Amur River such as Heihe, Luobei and Tongjiang. The H. dauricus released in these places were7-10cm TL (Total Length). However, a set of standard on H. dauricus resource enhancement and releasing is still unavailable at present. The source of released fish, releasing location, the size of released fish and releasing scale are still haphazard and uncertain. In order to find out the source of the releasing fish, the developmental stage and its migration characteristics, and better guide future resource enhancement and releasing, this paper conducted a series of research on the habitat of H. dauricus spawning site, the morphological feature of H. dauricus on postembryonic developing, ontogenetic behavior and the effects of environmental factors on migration. The specific methods and the results obtained were as follows:1. According to the observation in situ, we know that the river reach near H. dauricus spawning ground has a low flow, and substrates on banks is mainly composed of sand and gravel, there are a few pebbles on banks, so, we can infer that river bed substrates is mainly composed of gravel and pebbles. The river is wide, and both side banks have lots of sandy shoal, the center of river also have sandy shoal, the mulch in river banks is mainly composed of river willow. By three places observations in situ, we measured the surface water flow near spawning ground ranged from0.15m·s-1to0.46m·s-1, and averaged0.31m·s-1. We also measured the maximal water depth near spawning ground was14m, the underwater illumination1m below the water surface decayed very much, in the depth of3-14m, underwater illumination almost stable in0.51x or so. Transparency is almost stable at0.3-0.4m.2. The morphological observation during postembryonic development of first filial generation of H. dauricus were studied under rearing conditions, whose parents were captured from the Amur river. New born prelarval [0-day age,(12.08±0.68) mm] was conducted till the early juvenile [60-day age,(129.21±7.69) mm]. According to morphological development, postembryonic development of kaluga was divided in to two different phases, the prelarval stage between hatching (0-day age) and first feeding (9-day age), and the postlarval stage between the initiation of external feeding and organ development completed, the body transparent features disappear, and fins folds anterior to the anus and oral teeth were completely absent (49-day age), then, fish develop into early juvenile[50-day age,(106.78±9.87) mm]. The morphological development and differentiation of prelarvae is obviously faster then postlarvae and juvenile, and all kinds of swimming, respiratory, sensorial and feeding organs during prelarval stage were rapidly developed of the synchronous coordination. Rapid development of those functional organs enabled prelarvae to forage and avoid predator, which contributed to its survival. The differentiation and improvement of bone plate, improvement of fins swimming organs, and occurrence and development of swim bladder were conducted during the postlarval stage, showing that the capability of this stage fish for foraging and avoiding predator was father improving.3. We conducted laboratory experiments with early life stages of H. dauricus, from the middle reach of the Amur River to quantify ontogenetic behavior and compare their behavior with similar laboratory data collected previously on young kaluga from the Amur River. Hatchling free embryos initiated an intense downstream migration that peaked on day1, and continued strongly to day3, and then, decreased strikingly during days4-6, and ceased on day7(8-day migration). Migrants preferred a bright habitat (illuminated and white bottom), open not cover habitat, and swam-up far above the bottom (daily median distance,3.5m). On days21-22, larvae initiated a second downstream migration of similar intensity (four passes per fish per5min), with a peak at34-36days. Juveniles continued a slow intensity migration (one pass per fish per5min) until day66, indicating a long-duration migration style by early life stages. When most free embryos developed into larvae on day9, early-larvae were non-migrant (like late-free embryos), preferring bright and open habitats, and swimming-up far above the bottom until about day35. All life stages (free embryo, larva, and early-juveniles (to day66when observations ceased) preferred a white colored bottom, indicating a strong adaptive need for this habitat feature by wild fish. Later-larvae and juveniles gradually lost their preference for illuminated habitat and also swam nearer the bottom, indicating a shift to a benthic life. Free embryos, larvae and early-juveniles also strongly preferred open habitat, suggesting a similarity in open habitat use by wild individuals of these three life stages. The innate migration pattern of free embryos and larvae was similar in general pattern, but different in some ways from young kaluga previously observed in similar experiments. This result suggests there are at least two breeding stocks with different early behavior in the river. Thus, great care must be taken when culturing fish for stocking during stock enhancement programs to insure the breeding stocks are not mixed, which would produce non-adapted early life stages.4. Four environmental factors (water velocity, substrate, rearing water velocity condition, and diel rhythm) were examined for their influence on migration of H. dauricus early-life stages (free embryos, age0-8day fish; larvae, age9-49day fish; and juveniles, age50-66day fish). Experiments were conducted in six artificial circular-endless-streams creating various combinations of environmental factors to test hypotheses. Velocity regime had a positive effect on migration intensity by free embryos, and further, high velocity delayed the stopping or resting period by late-free embryos and early-larvae. However, migratory free embryos and larvae in different velocity streams still showed they had the drive to migrate a similar distance. Moreover, results suggest a trigger velocity may be needed to initiate or to cease migration. Substrate type had no effect on migration by early free embryos, and had a different affect on larvae depending on age of larvae. Mid-larvae to juveniles reared in a still water tank migrated faster when placed in a high velocity stream tank, showing a compensatory migration, compared to same age fish reared in a high velocity stream tank. This compensation didn’t occur in fish reared in a tank with low velocity (mean,7.2cm·s-1), indicating a trigger velocity may exist between7.2to31.7cm·s-1for compensatory migration. Velocity regime had no effect on diel rhythm of free embryos and juveniles. A fast velocity regime may have indirectly retarded the occurrence of the diel migration rhythm by larvae by retarding fish growth and development. For population enhancement stocking of H. dauricus, the study indicates great care should be taken to insure populations are not mixed during culture. The emergence of compensation migration indicate that the rearing water velocity condition has no effect on the migration of H. dauricus, and the fish reared in still water is still suitable for population enhancement. It is the migrating larvae, not non-migrating or slow migrating juveniles, are stocked to maximize the downstream distribution of stocked fish and reduce competition for foraging resources, and larvae should be released in a river reach with a bottom velocity≥30cm s-1that contains sand-small pebble substrate.5. For the location and size of the fish released, we think it is better to distinguish two stock before specific population enhancement. If larvae are non-migratory, we can release the fish above Fuyuan county, such as Qideli, Zhaoxin and Luobei, even Jiayin and Xunke. The specific location is arranged in the sandy beach and sandy shoal of these above places for reducing feeding competition and enhancing the fishery resources in middle reaches. If larvae are migratory, we can release the fish near Fuyuan county, The specific location is arranged in river reach of Fuyuan county that bottom velocity is greater than30cm·s-1. According to our study results, migratory larvae will accelerate to migrate, so, fish is release earlier the more likely they are added to the wild fish team for migrating. But most of the time, we can’t distinguish larvae are non-migratory or migratory, so, for safety’s sake, we suggest releasing location is arranged in river reach of Fuyuan county that bottom velocity is greater than30cm·s-1, so that migratory larvae can migrate and find suitable feeding sites. Comprehensively consider of the predator, culturing cost and migration factors, we suggest5-8cm (23-36day age) are suitable to release, because this period larvae has well-developed bone plate, enhancing their anti-predator ability, and if this period larvae are mogratory stock, they are still show a migration characteristics. However, a tagging and releasing for effetiveness evalustion is needed to know the survival rate for releasing5-8cm larvae. |
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