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Stock identification and delineation are important in the management and conservation of marine resources. These were highlighted as priority research areas for Bali sardinella (Sardinella lemuru) which is among the most commercially important fishery resources in the Philippines. Previous studies have already assessed the stocks of S. lemuru between Northern Mindanao Region (NMR) and Northern Zamboanga Peninsula (NZP), yielding conflicting results. Phenotypic variation suggests distinct stocks between the two regions, while mitochondrial DNA did not detect evidence of genetic differentiation for this high gene flow species. This paper tested the hypothesis of regional structuring using genome-wide single nucleotide polymorphisms (SNPs) acquired through restriction site-associated DNA sequencing (RADseq). We examined patterns of population genomic structure using a full panel of 3,573 loci, which was then partitioned into a neutral panel of 3,348 loci and an outlier panel of 31 loci. Similar inferences were obtained from the full and neutral panels, which were contrary to the inferences from the outlier panel. While the full and neutral panels suggested a panmictic population (global FST ∼ 0, p > 0.05), the outlier panel revealed genetic differentiation between the two regions (global FST = 0.161, p = 0.001; FCT = 0.263, p < 0.05). This indicated that while gene flow is apparent, selective forces due to environmental heterogeneity between the two regions play a role in maintaining adaptive variation. Annotation of the outlier loci returned five genes that were mostly involved in organismal development. Meanwhile, three unannotated loci had allele frequencies that correlated with sea surface temperature. Overall, our results provided support for local adaptation despite high levels of gene flow in S. lemuru. Management therefore should not only focus on demographic parameters (e.g., stock size and catch volume), but also consider the preservation of adaptive variation.

DNA barcoding was done to identify the dominant species of sardines that comprised the landed catch of the sardine fishery in the Northern Mindanao Region (NMR), and juveniles or "lupoy" in the Northern Zamboanga Peninsula (NZP). Cytochrome oxidase I (COI) minibarcodes were sequenced for all samples, while 16S ribosomal subunit sequences were obtained from a subset of samples. In reference to a previously reported land catch survey, sequence data confirmed the identity of the most dominant sardines in NMR and lupoy from NZP as Sardinella lemuru. Meanwhile, those reported as S. gibbosa were identified as Herklotsichthys quadrimaculatus, while those reported as S. pacifica were identified as the recently described S. goni. Results highlight: 1) the limitation in identifying sardines during land catch surveys without further taxonomic verification, and 2) the utility of DNA barcoding as a tool for species identification. Moreover, genetic data revealed that S. goni occurs in the fisheries of NMR, thereby expanding its known habitat range. Since this study was limited only in using DNA barcodes for identification, a detailed morphological examination of the dominant sardines is recommended.