Study On The Fish Larvae And Juveniles And Food Organism In Xiangshan Bay

Fish foraging ecology analysis is fundamental for studying the structure and function of marine ecosystem,which provides an elemental assement for conducting fishery manage.Fishes in their early life are susceptible to predation,competition,disease and environmental changes.Among them,foraging is the main factor affecting the survival and growth of juveniles.Foraging can have significant consequences for the condition,growth,survival and recruitment of fishes.The distribution of larvae and juvenile fishes and zooplankton in the Xiangshan Bay was investigated during April-June in 2015,especially the composition,distribution and the feeding ecology of the larvae and juvenile Japanese Spanish mackerel(Scomberomorus niphonius)in the Xiangshan Bay.The relationship between the feeding properties of larvae and juvenile fishes and their food organism' biomass was also studied.Fourteen sampling stations were established in the present study,zooplankton were weekly collected by a plankton net ?(0.37 m diameter,0.3 mm mesh size)from April 3 to June 10 in 2015.Larvae and juvenile fishes were collected by frame trawls(1 m × 2 m,1.0 mm mesh size).The water temperature and salinity at each sampling site were also measured at each sampling time.In laboratory,zooplankton,larvae and juvenile fishes were classified,identified and counted to analyze the species composition and distribution.Margalef species richness index(D),Shannon-Wiener diversity index(H')and Pielou evenness index(J)were used to analyze the diversity of zooplankton and larvae and juvenile fishes.Person correlation analysis was used to examine the relationship between the density of zooplankton,the density of fishes and the environmental factors respectively.The feeding ecology of Japanese Spanish mackerel was analyzed based on the stomach contents.Both the relationships between gape-size and body length of Japanese Spanish mackerel larval fish,and the relationship between its density and its food organism density were analyzed by Person correlation analysis to study the relationship between Japanese Spanish mackerel and its food organism.The results indicated that,first,A total of 36 species of zooplankton(including 1 species of Pelagic larvae)were identified in XiangShan Bay.All the species were belong to 7 major classess.Copepods were the main classes,besides there were Ctenophora,Siphonophora,Amphipoda,Cumacea,Chaetognatha,Tunicata.Paracalanus crassirostris,Paracalanus aculeatus,Acartia clausi,Acartia pacifica,Centropages mcmurrichi,Corycaeus japonicus were the 6 dominant species.The ecological type of zooplankton in Xiangshan Bay were mainly composed of Nearshore low-salinity species.During the investigation,the density of zooplankton increased with the increase of water temperature.The lowest mean density of zooplankton was recorded in first cruise(39.4 ind/m3),while the highest mean density(755.2ind/m3)occurred in nineth cruise.Results of the Person Correlation between the density of zooplankton and same-time environmental factors showed that the zooplankton density had a distinct correlation with water temperature and salinity.Second,180 254 larvae and juvenile fishes in total were collected representing 25 families and 45 species.The dominant fish species were Konosirus punctatus(25.8%),Chaeturichthys stigmatias(23.9%),Chelon haematocheilus(19.2%),Acanthogobius ommaturus(9.9%),Amoya pflaumi(8.5%),Parachaeturichthys polynema(4.5%),Acanthopagrus schlegelii(2.5%),Glossogobius giuris(1.2%).The density of larvae and juvenile fishes varied significantly among cruises.The highest mean density of larvae and juvenile fishes was recorded in third cruise(587.9 ind/100m3),while the lowest mean density(8.2ind/100m3)occurred in eighth cruise.However,the number of species increased gradually from the first cruise to the last cruise.Fish larvae in preflexion stage accounted for 63.6% of the total catch.The flexion stage of larvae made a 26.4% contribution to the total catch.Results of the Spearman Correlation between the density of fish and same-time environmental factors showed that the fish larvae density had a distinct correlation with water temperature and zooplankton density,but the salinity had no remarkable relationship with the fish larvae density.As zooplankton is the vital food source for most larvae and juvenile fishes species,variations in zooplankton distribution and quantity can directly or indirectly affected larvae and juvenile fishes.Third,total number of Japanese Spanish mackerel was 1 108 in the present study,of which body length ranged from 4.0 to 41.1 mm.Gut content analysis was carried out on 587 intact larvae and juveniles,of which 256 had not food in their guts.In total 11 food organism species were found in guts,and fish larvae were the dominant food organism(98.6%).The main food organism of S.niphonius were various at different development stage.In yolk-sac larval stage,the main food organism were Konosirus punctatu and Goby.In pre-flexion larval,the main food organism changed to K.punctatu,Chelon haematocheilus and Goby,while K.punctatu and C.haematocheilus were the main food organism in other development stages.The results suggested that S.niphonius was poscivorous and fed mainly on fish larval from the first feeding stage.Except yolk-sac larval,the feeding incidences were high for other development stages: 62.0,81.3,80.0,and 86.8% in pre-flexion,flexion,post-flexion larval and juveniles,respectively.The gape-size of S.niphonius was large compared with other fish,which was significantly correlation with body length.And the ration of gape-size to body length ranged from 16.4% to 25.1%.The size of food increased as S.niphonius individual growth and development,which was significantly correlation with body length and gape-size.There was no significantly different between food organism size in yolk-sac and pre-flexion larval,but significantly different among in other development stages.The average food organism length ranged from 2.6 mm in yolk-sac larval to 11.2 mm in juveniles.Feeding of larval and juveniles depends on a spatial and temporal match with food organism.There were three peeks at the density of main food organism in Xiangshan Bay during sampled.The variation of S.niphonius distribution was significantly synchronized with the density of food organism.The feeding ecology of the larvae and juvenile of Japanese Spanish mackerel and high food availability may indicated the reason that S.niphonius spawned in the nearshore water.