Temporal stability in the otolith Sr:Ca ratio of the yellow croaker, Larimichthys polyactis (Actinopterygii, Perciformes, Sciaenidae), from the southern Yellow Sea

Otolith chemical signatures are sufficiently stable across time to allow for accurate stock classification. The classification of the southern Yellow Sea population for Larimichthys polyactis (Bleeker, 1877) and its connectivity with others from 1962 is controversial. The study aimed to study the inter-annual variation in otolith strontium:calcium (Sr:Ca) ratios of L. polyactis to determine whether otolith natural tags are representative over long periods and can then be used for population structure classification. Spawning L. polyactis individuals were captured by stow nets in the same site of the southern Yellow Sea coastal waters during April–May in 2003, 2012, and 2013. EPMA (electron probe microanalysis) was used to determine the Sr:Ca ratios of a total of 25 otolith samples. Mann–Whitney U-test was used to test the differences of otolith Sr:Ca ratios from the core to edge for each otolith. One-way ANOVA was performed to compare the mean otolith Sr:Ca values among 2003, 2012, and 2013. Otoliths from 2003, 2012, and 2013 showed similar patterns of Sr:Ca ratios and Sr:Ca ratios could be divided into higher and lower phases in the core and remaining regions, respectively. Inter-annual significant differences for each high or low Sr:Ca phase of otoliths were not observed over short- (between 2012 and 2013) or long-time (between 2003 and 2012, and between 2003 and 2013) scales. Univariate contrasts across the adjacent year and decade classes were statistically similar. The Sr:Ca ratio signatures in the otolith were relatively stable across years and can be used as a reliable natural tag for connectivity assessments and stock identification with little or no genetic differentiation among L. polyactis populations. The short- and long-term temporal stability of otolith Sr:Ca ratios also revealed, the existence of stable L. polyactis stocks in the southern Yellow Sea, consistent with a previous finding of capture survey.

Habitat segregation and migration in tropical anguillid eels, Anguilla bengalensis bengalensis and A. bicolor bicolor

Anguillid eels of the genus Anguilla, which have a unique catadromous life history, are widely distributed across many parts of the world. However, little research has been conducted on the behavioural mechanisms of habitat segregation between sympatric species in tropical anguillid eels. To understand the ecological and behavioural mechanisms involved in the life history and migration of tropical anguillid eels, strontium (Sr):calcium (Ca) ratios were examined in otoliths of A. bengalensis bengalensis (41 specimens) and A. bicolor bicolor (130 specimens) collected from ten rivers in northwestern Peninsular Malaysia. The otolith Sr:Ca ratios revealed different habitat use between the two species. The broad range of otolith Sr:Ca ratios and habitat shift found in A. bicolor bicolor suggested that its habitat utilization was opportunistic in environments of varying salinity. A. bicolor bicolor prefers to live in the midstream to downstream areas with tidal influences. A. bengalensis bengalensis, however, was found to only reside in freshwater environments throughout their continental growth. A. bengalensis bengalensis tends to live in upstream area with no tidal influence. Their habitat use, migratory history, and habitat distribution indicate that habitat segregation occurs between the two species, leading to the different habitat preferences in tropical river systems.

Facultative amphidromy and pelagic larval duration plasticity of Rhinogobius formosanus (Teleostei, Gobioidei)

Rhinogobius formosanus Oshima, 1919 has long been considered an amphidromous goby. However, a landlocked population recently found in the Jingualiao Creek upstream of the Feitsui Reservoir in Taipei suggests that R. formosanus may complete its life in the river. This study aims to verify the habitat use of the landlocked population of R. formosanus collected from the Feitsui Reservoir and an amphidromous population collected in Malian Creek using otolith Sr:Ca ratio analysis. The hypothesis that early life history varies between the landlocked and migratory gobies was also tested. Genetic analyses show that the Feitsui Reservoir and Malian Creek populations are not genetically different. Rhinogobius formosanus from Malian Creek showed high-to-low otolith Sr:Ca ratios suggesting that these specimens spent a planktonic larval stage in the sea followed by a freshwater life at later stages. In contrast, R. formosanus from the Feitsui Reservoir showed constant lower otolith Sr:Ca ratios, implying a landlocked life history of fish in the creek upstream of the reservoir. In addition, the analysis of growth increments showed a longer pelagic larval duration for the fish in the Malian Creek (58.8 days) than those in the Feitsui Reservoir (38.8). Variation of pelagic larval duration in two genetically homogenous populations implies acclimatization to the reservoir by the landlocked gobies. This study shows that R. formosanus, like some other congeners, is capable of adapting to a freshwater landlocked environment in its early developmental stage and supports the hypothesis that landlocked populations may have a shorter pelagic larval duration.