Feeding Ecology Of Pawak Croaker (Pennahia Pawak) In The Beibu Gulf

The Beibu Gulf is rich in the fishery resources,however,the marine fish resources have declined obviously in recent years.The fish population has also undergone adaptive changes in population structure,feeding strategy and nutritional structure.Because of lack of relevant research of the Beibu Gulf at present,the samples of Pennahia pawak was the research object in this study,which were collected at the Jianghong and Beihai fishing ports in 2020.Stomach content analysis and carbon and nitrogen stable isotope technique were used to study population structure,prey composition,trophic level,trophic niche of P.pawak,and to analyze the changes of them.this study was an attempt to provide more comprehensive data support and scientific basis for the management of P.pawak resources in the Beibu Gulf,South China Sea.The main results of are as follows:1.Analysis of population structure:the standard length（SL）of P.pawak ranged from71 mm to 191mm,with the average SL of 136mm and the dominant SL of 110～169mm.The body mass（BM）of P.pawak ranged from 8.37 g to 182.55g,with the average BM of 62.40g and the dominant BM of 10～100g.The allometric parameter b of P.pawak in 2020 was 2.88.Combined with previous studies,it ranged from 2.84 to 3.01,showing negative allometric growth or isometric growth.Generally,its fatness decreased slowly with the increase of SL,and the fatness in spring and winter is greater than that in summer and autumn.Compared with 2008,the population decreased in the average SL,the average BM,the SL and BM range in 2020,indicating that the resources of the P.pawak have been probably in decline in the Beibu Gulf.2.Analysis of feeding habits:the food composition,trophic level and trophic niche of P.pawak were analyzed by using the method of the traditional stomach content analysis.The results showed that P.pawak fed on more than 60 prey species.The common prey items were Metapenaeopsis barbata,Alpheus.sp,Bregmaceros.sp,Apogonidae and Squillidae.Macrura were the most important prey groups to P.pawak,followed by Pisces.Compared with 2008,the number of prey species increased in 2020.The dominant prey items changed in large extent,and the proportion of Macrura increased further.With the growth and development of P.pawak,the proportion of Pisces was increasing in the diet,while the proportion of Macrura decreasing gradually.when its SL was about140～149mm,the proportion of Pisces exceeded that of Macrura.The locomotive and feeding organs of P.pawak gradually matured,which might be the main reason for the changes in feeding habits with increasing of body size.Pisces in the diet were less in all seasons,and their frequency percentage（F%）was the highest in summer,followed by autumn,while Macrura were more in all seasons,and their F%was the highest in spring,followed by winter.The reason was that small fish,as preys of P.pawak,were widely distributed in summer and autumn.In general,the feeding intensity increased with development of individuals,but Repletion index（RI）decreased gradually in length groups of 109-139mm,which may be related to the reproduction characteristics.RI was the highest in spring,followed by summer and autumn,and the lowest in winter.It is probably due to the numbers of prey resources and the temperature in the habitat.Based on the stomach contents,the trophic level of the P.pawak was 3.86.as indicators of the trophic niche,the means of the Shannon-Wiener index（H’）and Pielou index（J）were1.87 and 0.65,respectively.With the increase of SL,the trophic niche of the P.pawak showed basically a trend of gradually increasing.3.Analysis of trophic structure:The carbon isotopic ratio(δ¹³C)range of P.pawak was-18.08～-15.22‰and the extreme difference ofδ¹³C（CR）was 2.86‰.Theδ¹³C range of its prey was-21.24（Secutor ruconius,small pelagic species）～-16.27‰（Alpheus brevicristatus,benthic species）and the CR value was 4.87‰.The results suggested that P.pawak was likely to feed widely in various water depth,and its potential food sources were diverse.The average stable nitrogen isotopic ratio(δ¹⁵N)of P.pawak was 15.44‰.The averageδ¹⁵N of its prey was 12.26‰.The averageδ¹⁵N of P.pawak was 3.18‰higher than that of its prey.Based onδ¹⁵N,the average trophic level of P.pawak was 3.93,which was 0.94 higher than that of its prey.The average nutrient level based on nitrogen stable isotope ratio was 3.93,which was only 0.08 difference from that based on its prey items.With the development of individuals,the trophic level of the P.pawak was slightly increasing.Meanwhile,When the length groups were 70～99mm,the results showed that the nuclear niche separation of P.pawak was considerable.When the SL was higher than 99mm,the nuclear niches were similar.In terms of fishery management,the groups with similar ecological niches could be rationally exploited,but juvenile fish,the recruitment stock,is supposed to be reasonably protected at the same time in order to avoid damaging the nutritional structure of P.pawak.