Ef­forts are un­der­way to im­prove bill­fish man­age­ment


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.

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