Population Connectivity Of Small Yellow Croaker(Larmichthys Polyactis) In The Southern Yellow Sea Using Otolith Microchemistry

Small yellow croaker Larimichthys polyactis is an important commercial fish species that is endemic to the northwestern Pacific Ocean,including the Bohai,Yellow,and East China Sea,playing a pivotal role in offshore fishery of China,South Korea,North Korea,and Japan.For the past three decades,owing to intense fishing pressure,the current population of L.polyactis is characterized by individual miniaturization,decreased asymptotic length and weight,fast growth,and younger average age.However,the potential evolution of L.polyactis has reached a bottleneck.Given that the current status of L.polyactis needs urgent attention,recently many studies based on the analysis models for adjusting and optimizing the marine protected areas of germplasm resources of L.polyactis has reported.However,the effect of implementation of these studies is scant because the analysis models have not incorporated the essential characteristics of life history for L.polyactis.As a highly migratory species,the whole life migration of L.polyactis has been thoroughly studied.However,knowledge about its early life migration and site fidelity are scarce,and population divisions are inconsistent,limiting the ability of fishery scientists and administrators to evaluate the design and potential benefits of thorough conservation and resourcemanagement strategies.In the first study,otolith Sr/Ca was analyzed to investigate the early migratory patterns and discriminate the populations of L.polyactis in the southern Yellow Sea,including two spawning groups and one overwintering group.The variation in Sr/Ca ratios of ontogenetic growth zones,including the nucleus(N),larval(L),metamorphosis(M),juvenile(J),and edge(E)zones,was measured by electron probe microanalysis.The results show that(1)the variation in Sr/Ca ratios in early developmental growth zones was generally characterized by an evident downward trend from the N to J zone,which suggests that the early migratory pattern of L.polyactis might be from inshore to nearshore water.(2)Canonical discriminant analysis,based on the otolith Sr/Ca ratios of the N,L,M,and J zones,allowed the successful discrimination of the two populations,namely,the northern and southern Yellow Sea groups,whose differences were mainly reflected in the L and J zones.(3)Compared with previous studies,the traditional geographic boundaries(34°N)separating these two populations might be moving northward.The application of otolith Sr/Ca ratios based on ontogenetic stage could improve our understanding of the migration and population discrimination of L.polyactis from the Yellow Sea.Then,the second study used laser ablation inductively coupled plasma-mass spectrometry to analyze the elemental composition of otoliths from 60 adult small yellow croakers caught in the southern Yellow Sea,including two spawning groups with 1-and 2-year-old fish(S1 and S2,respectively)sampled close to China and one overwintering group of two-year-old fish(O2)sampled close to South Korea.(1)The ratios of elements(Li,Na,Sr,and Ba)to Ca in the otolith core zones were significantly higher(P < 0.05)than in those of the year one(Y1)and year two(Y2)annual rings,but there were no significant differences in the elemental ratios between the Y1 and Y2 zones.(2)Principal component analysis(PCA)of the elemental otolith signatures of the core,Y1,and Y2 zones in the three groups revealed two distinct clusters(cluster 1: S1-core,S2-core,and O2-core zones;cluster 2: S2–Y1,O2–Y1,S2–Y2,and O2–Y2 zones)and one zone(S1–Y1),suggesting spawning-site fidelity and natal-site fidelity uncertainty,especially considering the dispersal by current in prolonged period(50 h)from fertilized eggs to hatching and internal effect,such as yolk sac and maternal effect.(3)Furthermore,these results indicated that the S2 and O2 groups could represent the same population,suggesting a stable migratory route for L.polyactis in Chinese and South Korean waters,whereas the S1 group could represent another population.This suggests the possibility a mixed L.polyactis population in the southern Yellow Sea.Characterizing L.polyactis spawning-site fidelity is a crucial step toward linking spawningsite fidelity of this overexploited species with thorough conservation and management strategies.In the end,synthesizing the results of the studies of otolith Sr/Ca and elements fingerprints shows that(1)the range of spawning grounds of L.polyactis in southern Yellow Sea is from the Changjiang Estuary fishing grounds in the south to the Northern Haizhou Bay fishing grounds in the north.(2)L.polyactis is born in inshore waters,then forages and grows in nearshore waters.(3)There exists spawning-site fidelity in L.polyactis.The above results have important implications for adjusting and optimizing the marine protected areas of L.polyactis.