| Chinese sturgeon(Acipenser sinensis)is a large,migratory,and spawning fish species and a national first-class protected animal in China.Its populations are mainly distributed in the waters of the southeastern continental shelf and the main channel of the middle and lower reaches of the Yangtze River.Due to overfishing and environmental degradation,the natural population of Chinese sturgeon has been dramatically decreased.At present,the natural reproduction behavior of Chinese sturgeon has not been monitored for 6 consecutive years,and the species is at risk.Artificial freshwater conservation and breeding has become a key measure to ensure the survival of Chinese sturgeon species.The Chinese sturgeon(Acipenser sinensis)lives in the ocean for more than 90%of its life history.The lack of marine life history under long-term freshwater captivity may be an important reason for the smaller mature individuals,the lower proportion of male and female maturation,and the decline in reproductive capacity of the artificial conservation population.Marine conservation is the main means to supplement and improve the marine life history of Chinese sturgeon.However,traditional marine cage conservation has problems such as high cultivation density,low dissolved oxygen at the bottom,and poor permeability,resulting in poor conservation effects.Compared to traditional marine cage conservation,offshore fence aquaculture conservation has the characteristics of larger cultivation areas,stronger water permeability,bottomless nets,and strong resistance to wind and waves,which is a potential trial conservation mode.In this study,we investigated the environmental factors of the Bailong island marine ranch in Wenzhou,Zhejiang Province,and conducted a release experiment of Chinese sturgeon to understand its feeding ecology and behavioral adaptability.The main findings are as follows:1.The water quality and sediment quality of Bailong island marine ranch meet the national Class I quality standard.During the monitoring period in 2021,the annual wind speed in the study area ranged from 2.01 m/s to 25.22 m/s,with an average wind speed of 7.31 m/s.The annual water temperature varied from 12.28°C to 29.76°C,with an average temperature of 20.99°C.The flow rate ranged from 0.01 to 0.04 m/s,with an average flow rate of 0.03 m/s.The average tidal range of the year was 4.29m,with a range of 1.15m to 6.72m.During the low tide and slack tide(daily tidal range of 3.23m),the water depth ranged from 2.56 to 9.53m.In the study area,a total of 47 prey species belonging to 5 phyla and 8 classes were collected,including 38 benthic organisms and 9mid-lower layer organisms.The bait resources in the environment are the most abundant in autumn and the most deficient in winter,with an annual average biomass of 2.909±0.903 g/m~2.The study area is feasible for the release of Chinese sturgeon.2.The feeding preferences of Chinese sturgeon were assessed by examining their growth and the contents of their digestive tracts.The maximum carrying capacity of natural prey resources was also estimated.The results showed that all natural prey collected in the marine ranch were edible for Chinese sturgeon.According to the analysis of digestive tract contents,the released Chinese sturgeon could feed autonomously in the marine ranch,consuming a total of 16 prey species belonging to 4 phyla and 5 classes.The composition of their diet,ranked by the index of relative importance(IRI),was as follows:Malacostraca(mainly shrimp and crab)>Gastropoda>Pisces>Bivalvia>Polychaeta.The feeding preference,ranked by the feeding selection coefficient,was:Malacostraca>Gastropoda>Bivalvia>Polychaeta>Pisces.The estimated results showed that the maximum annual production of suitable prey resources for Chinese sturgeon in the 650-acre Bailong island marine ranch was approximately 818.389 kg.Relying solely on these natural prey supplies,69 Chinese sturgeon juveniles could grow and develop normally.3.The study used an ultrasonic telemetry system to track the released Chinese sturgeon.The precise positioning of the tagged fish was obtained by deploying an ultrasonic receiver array and using the three-point positioning method.The position information was processed using the Heat Map module in the Python programming language,obtaining the distribution heatmap of the experimental fish.The differences in habitat distribution between sexes,ages,diurnal rhythms,tidal rhythms,and typhoon avoidance behavior were analyzed.The results showed that,during the annual tracking period,the Chinese sturgeon was mainly concentrated in the northeastern area of the research region,with water depth range from 6.23 to 12.53 meters,and the overall habitat time in the dense area accounted for 29.06%.The distribution differences between male and female fish and the diurnal differences were not significant,possibly related to the small sample size.However,there were significant differences between adult and juvenile fish.The adult fish had a denser distribution,mainly in the overall distribution area,occupying 1.78%of the total research area,and accounting for 35.24%of the total habitat time,with an average water depth of 9.19 meters.Juvenile fish had more dispersed distribution areas,occupying 4.73%of the total area and accounting for 41.32%of the habitat time,with an average water depth of 7.22 meters.The experimental fish also formed tidal rhythms in the research area.During low tide,the distribution area was more concentrated,similar to the overall distribution,occupying 3.55%of the total area,with43.56%of the habitat time,and an average water depth of approximately 7.42 meters.During high tide,the distribution was more dispersed,with a larger dense area in the northwestern region of the research area,occupying 6.51%of the total area,accounting for 25.92%of the habitat time,and the average water depth was approximately 8.41meters.During typhoon periods,the experimental fish were highly concentrated in the northeastern area,occupying 1.78%of the total area,accounting for 40.37%of the habitat time,and with an average water depth of 8.27 meters.The results showed that Chinese sturgeon could adapt to the marine ranch environment and form tidal rhythms according to tidal phenomena while effectively avoiding typhoons.In conclusion,the environment of the Bailong island marine ranch is suitable for establishing a marine conservation base for Chinese sturgeon.The natural bait in the marine ranch meets the feeding preference of Chinese sturgeon and its resource productivity can continuously supply 69 Chinese sturgeons,which has the potential to become a marine conservation base for Chinese sturgeon;Artificial freshwater conservation of Chinese sturgeon in marine pastures has good behavioral adaptability,could generate a certain rhythm to the tidal cycle and effectively avoid typhoons.Offshore fenced conservation is a potential new marine conservation model for Chinese sturgeon,providing a foundation for the"land-sea-land"relay conservation of Chinese sturgeon. |
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