In the marine waters off the southeastern United States of America (US), Northern red snapper, Lutjanus campechanus, is an iconic species. In US waters, this species is currently managed as an Atlantic stock and Gulf of Mexico stock, both of which are estimated to be overfished due to decades of heavy fishing pressure in the latter half of the twentieth century.
The status of red snapper has instigated rebuilding plans in both regions, as well as significant investment in monitoring and research programmes to facilitate stock recovery. One theme of red snapper research over the past two decades has been an examination of population structure that considered both genetic and non-genetic data. Collectively, the data indicate there are at least two stocks of red snapper, one in the Atlantic and one in the Gulf of Mexico, but also genetically distinct stocks in the Gulf of Mexico that had not been identified from these earlier efforts. However, several additional lines of evidence, such as regional catch rates, larval dispersal modelling, and adult movement data, have suggested that red snapper in the Gulf of Mexico may be composed of multiple distinct populations.
In the latest Editor's Choice article in ICES Journal of Marine Science, Portnoy et al. assessed red snapper genetic population structure. Throughout the species' range in the Atlantic and Gulf of Mexico US waters, the authors sampled using a dataset of 2,111 microhaplotype loci (a novel type of molecular marker), containing 4,124 single nucleotide polymorphisms, characterized in 391 adult red snapper. To account for metapopulation dynamics, the authors employed a landscape genetics approach that allowed for simultaneous evaluation of the effects of geographic position, larval-dispersal, and post-settlement movement on the partitioning of genomic variation.
Study results indicated the inclusion of dispersal and movement data improved the ability to explain patterns of variation better than geographic position alone, with adult movement explaining the largest amount of variation. The authors were also able to identify a previously undescribed genetic discontinuity along the West Florida Shelf in the eastern Gulf of Mexico, consistent with the idea that distinct eastern and western red snapper populations exist in the Gulf of Mexico.
For marine fish species that occupy discrete habitats, such as coral reefs or estuaries, standard genetic approaches that require a priori grouping may often be sufficient for detection and description of population structure. For shelf species, such as red snapper, that live across a variety of habitats and are seemingly continuously distributed, these types of approaches are less likely to succeed.
Spatially explicit approaches that incorporate information about larval dispersal or post-settlement movement into genomic assessments of stock structure, like the one used by the authors of the latest Editor's Choice article, are promising for these species. The study by Portnoy and co-authors also highlights the importance of considering how larval dispersal and adult movement patterns relate to gene flow, with the latter often overlooked in reef and other marine fishes with dispersive early life stages.
