Understanding billfish stock structure is fundamental to managing them sustainably. For example, biological or genetic evidence of multiple stocks in a given species might indicate that multiple reference points like maximum sustainable yield and different management approaches might be warranted. Unfortunately, actual stock status is poorly understood in many billfish species.
Nadya Mamoozadeh, a doctoral candidate at the Virginia Institute of Marine Science, is investigating the stock structure of striped and white marlin as part of her graduate research. The goal of her work is to reduce uncertainties currently associated with the management and assessment of billfish by providing information on genetic stock structure and connectivity among stocks, including the geographic location and number of stocks in an ocean basin, and the degree of genetic connectivity among stocks and between ocean basins.
Her specific research questions are: What is the genetic stock structure of white marlin in the Atlantic Ocean? What is the genetic stock structure of striped marlin in the Pacific and Indian oceans? And finally, what is the genetic relationship of striped marlin and white marlin?
To accomplish this, Mamoozadeh has relied on recreational anglers to provide her with genetic samples from around the world. In doing so, she sent participating anglers sampling kits that provided materials to preserve tissue samples, mostly consisting of fin clips, which were returned to her at the Virginia Institute of Marine Science. The DNA from each sample is extracted in the lab, and then it undergoes a process to characterize large numbers of molecular markers, which are the specific locations in an individual’s DNA that vary among individuals and are useful for evaluating the presence of genetic stock structure. She needed between 30 and 50 samples from each region in order to achieve a statistical inference.
Some of Mamoozadeh’s work on white marlin has recently passed peer review and has been published in the ICES Journal of Marine Science. She analyzed 24 molecular markers from 479 adult and 75 larval white marlin from six geographic locations. To cut to the chase, she didn’t observe any evidence to suggest that there is more than one genetic stock of white marlin in the Atlantic Ocean. However, she states that this is not necessarily a straightforward conclusion, as the lack of genetic structure could reflect several different scenarios that are important to consider.
The first is that no genetic stock structure exists, and this is consistent with a lack of biological stock structure. This would mean there is enough gene flow among white marlin in the Atlantic to prevent the accumulation of appreciable genetic and biological differences among groups of white marlin from different regions. The second would be that there is no genetic stock structure, but biological stocks do in fact exist. This would mean there is just enough gene flow among white marlin in the Atlantic to prevent the accumulation of an appreciable level of genetic differences, but not enough gene flow to prevent biological differences from developing that are specific to groups of white marlin from different regions.
A third conclusion would be that both genetic and biological stock structure is present, but the molecular markers characterized in her study were not powerful enough to detect it. In this case, using really large numbers of genetic markers is necessary to detect genetic stock structure, which is one of Mamoozadeh’s next research steps.
While her results support ICCAT’s management approach of a single Atlantic stock of white marlin, it is somewhat incongruent with rising catch rates of whites in the U.S. mid-Atlantic region that we discussed in the last issue, and the limited satellite tag data that shows cyclical movement confined to one region of the Atlantic. I’m particularly looking forward to Mamoozadeh’s next research question, which determines if morphologically similar white marlin represent a distinct species or rather an Atlantic population of striped marlin.