Ocean Acidification

Genome-wide data revealed a mahimahi evolutionary history shaped by vicariance, ocean currents and local environments, needing models, functional genomics and selection tests for climate adaptation

This study examined the worldwide population structure of the cosmopolitan pelagic fish Coryphaena hippurus using a dataset of 8.7 million SNPs and complete mitochondrial genomes. The analyses reveal, for the first time, four genetically distinct populations corresponding to the major oceanic basins: the Atlantic, Pacific, Indian Ocean and Mediterranean Sea. Photo of Atlantic mahimahi by NOAA Fisheries.

The processes that generate and sustain marine biodiversity are still incompletely understood, especially in open-ocean systems where high dispersal potential is expected to prevent strong population differentiation. Understanding how evolutionary forces shape divergence or speciation in such environments requires detailed knowledge of a species’ life history and its interactions with key environmental variables.

In a study by Píndaro Díaz Jaimes and coworkers – at the Universidad Nacional Autónoma de México, the Tecnológico de Monterrey, and the Instituto Politécnico Nacional in Mexico; Universitat Pompeu Fabra in Spain, and the University of Palermo in Italy – the authors applied a genome-wide approach to examine the global population structure of the cosmopolitan pelagic dolphinfish or mahimahi (Coryphaena hippurus), an important commercial species. A comprehensive dataset comprising 8.7 million single-nucleotide polymorphisms together with complete mitochondrial genomes was generated during this research.

The analyses reveal, for the first time, four genetically distinct populations that correspond to the major oceanic basins: the Atlantic, Pacific, Indian Ocean and Mediterranean Sea. The Mediterranean population carries a unique genomic signature, most probably reflecting historical isolation and limited contemporary gene flow through the Strait of Gibraltar.

Despite this clear inter-basin structure, genome-wide data also retain extensive signals of historical connectivity. Results of various tests show significant excess allele sharing (meaning that two groups of individuals carry the same version or allele of a particular gene or DNA segment), particularly between the Indo-Pacific and Atlantic groups. In addition, phylogenetic network analyses (drawing evolutionary relationships that allows for mixing between groups, not just simple branching) show multiple migration events. These results underline the lasting influence of ancestral gene flow on present-day genetic patterns.

At finer geographic scales, modest but statistically significant structure was detected within both the Atlantic and Pacific basins. Seascape genomic analyses (how genetic differences in marine species are linked to the environment around them) further demonstrate that environmental gradients – including salinity, phosphate concentration, pH and others – are significantly associated with genomic variation, indicating that ecological factors play a role in shaping population differentiation across heterogeneous marine habitats.

Fig. 1: Graphical summary of the study. Adapted from the original.

Relevance of research findings to the industry

For fisheries managers and the fishing sectors, the discovery of four basin-scale genetic populations is important. Stocks that look continuous on a map may in fact be demographically independent. Management plans that treat all dolphinfish as a single global unit risk over-harvesting one basin while under-utilizing another.

Knowing that Mediterranean fish are particularly distinct and that Atlantic and Pacific populations also show internal structure gives managers a clearer basis for setting regional catch limits, seasonal closures or size regulations. The environmental associations also hint that climate-driven changes in salinity or nutrient levels could alter population boundaries in the future, which both industry and regulators should monitor.

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Perspectives

This research demonstrates that even highly dispersive oceanic species can maintain meaningful genetic structure. Future studies should combine these genomic results with ocean-circulation models to assess how currents move larvae and adults between basins. Functional genomic approaches and tests for loci under selection would help identify which genes allow each population to cope with local conditions and with rapid environmental change.

Expanding sample sizes, especially from under-represented regions of the Indian Ocean and southern Pacific, would refine the picture further. Overall, the four genetic groups identified in this study provide a solid starting point for designing management units that better match the true biology of the species.

Modeling ocean acidification and warming effects on Atlantic Sea scallop growth for adaptive management

Results of this study show that warming stress is stronger than previously recognized, especially in the southern Mid-Atlantic, while ocean acidification appears first in the north. Together these stressors shrink the optimal growth range of sea scallops, showing the value of the spatially coupled DEB model for adaptive fisheries management. Photo of a sea scallop (Placopecten magellanicus) – which has over 100 blue eyes along the edge of its mantle to sense light intensity – by Dann Blackwood, USGS (Public domain, via Wikimedia Commons).

Effective climate-ready fisheries management depends on robust predictions of how species will respond to environmental change across broad spatial scales. Bioenergetic approaches such as Dynamic Energy Budget (DEB) models link physiological processes to environmental conditions and can therefore forecast organismal growth under future climate scenarios.

In a recent study by Halle M. Berger and colleagues (University of Connecticut and NOAA Northeast Fisheries Science Center and Océanopolis in France) the scientists presented the first large-scale integration of a DEB model with downscaled regional oceanographic simulations. This coupling allows resolving spatiotemporal patterns and examine how climate stressors appear at biogeographic, economic and oceanographic scales that matter for management.

The researchers calibrated a DEB model for the Atlantic Sea scallop (Placopecten magellanicus) using output from a realistic oceanographic–biogeochemical model of the Northeast U.S. continental shelf. The model was used to project the combined effects of ocean acidification (OA) and warming on individual growth both historically and through the coming century. It successfully reproduced observed historical patterns in age at harvest size and maximum attainable size.

At mid-century (2035–2050), scallop growth was projected to increase across most of the region, with the exception of the southern Mid-Atlantic; OA effects remained confined to the deep Gulf of Maine. By the end of the century (2080–2095) under a high-emissions scenario, scallops were expected to grow more rapidly yet reach smaller maximum sizes. The results indicate that warming stress is more severe than previously recognized, especially in the southern Mid-Atlantic. Warming impacts appear first in the south, whereas OA stress precedes warming in the north.

Overall, together these stressors progressively shrink the geographic area that supports optimal growth, and the results of this study demonstrate the value of a spatially explicit, climate-forced DEB model as a practical tool for guiding adaptive fisheries management.

Fig. 5: Mean (± SD) dry flesh mass (top row) and shell height (bottom) of experimental juvenile scallops exposed to low (503 microatm – the microatmosphere is a unit of pressure equal to one-millionth of an atmosphere – black symbols), moderate (805 microatm, yellow symbols), or high (1168 microatm; red symbols) pCO2 (data from Pousse et al. 2023) used to calibrate the model. In ocean science (especially ocean acidification studies), a microatm is the standard unit used to express the partial pressure of carbon dioxide (pCO₂) in seawater. Typical surface ocean values today are around 400–450 microatm. Adapted from, and with additional information in the original publication.

Relevance of research findings to the industry

For the scallop fisheries industry, the most immediate value of the work is its ability to flag which fishing grounds are likely to become less productive and which may temporarily improve. Southern mid-Atlantic beds face the earliest risk of slower growth and smaller maximum size, while some northern areas may see short-term gains before acidification becomes limiting. Processors, vessel operators and managers can use these regional signals when planning long-term investments, lease decisions or rotational closures.

The finding that animals may grow faster yet end up smaller also has direct economic consequences: smaller scallops command lower prices and may require changes in gear or processing methods. Because the model is spatially explicit, it can be updated as new climate projections or survey data become available, giving the industry a living tool rather than a one-time snapshot.

Perspectives

Results of this study demonstrate that linking bioenergetic models to regional ocean forecasts is both feasible and useful for climate-ready management. Future work could refine the food-supply component of the model, incorporate density-dependent effects, and assess additional emission scenarios. Expanding the approach to other shellfish or finfish would allow multi-species comparisons and more integrated ecosystem planning.

Managers should consider that waiting until declines appear in the catch is no longer necessary. The spatial patterns of stress are already projected; adaptive measures – such as shifting effort northward, adjusting size limits, or protecting residual high-growth habitats – can be designed now. Overall, the model developed provides a practical bridge between climate science and the day-to-day decisions that will hopefully keep the Atlantic Sea scallop fishery viable through the coming decades.

Global Seafood Alliance, 3 August 2026. Full article.