Predicting the occurrence of Atlantic bluefin tuna (Thunnus thynnus) larvae in the northern Gulf of Mexico: building a classification model from archival data

ABSTRACTAlthough blueﬁn tuna are found throughout theAtlantic Ocean, spawning in the western Atlantic hasbeen recorded predominantly in the Gulf of Mexico(GOM) in spring. Larval blueﬁn tuna abundancesfrom the northern GOM are formulated into an indexused to tune the adult stock assessment, and the var-iability of this index is currently high. This studyinvestigated whether some of the variability in larvalblueﬁn tuna abundances was related to environmentalconditions, by deﬁning associations between larvalblueﬁn tuna catch locations, and a suite of environ-mental variables. We hypothesized that certain habitattypes, as deﬁned by environmental variables, would bemore likely to contain blueﬁn tuna larvae. Favorablehabitat for blueﬁn tuna larvae was deﬁned using aclassiﬁcation tree approach. Habitat within the LoopCurrent was generally less favorable, as were warm-core rings, and cooler waters on the continental shelf.The location and size of favorable habitat was highlyvariable among years, which was reﬂected in thelocations of larval blueﬁn tuna catches. The modelsuccessfully placed blueﬁn tuna larvae in favorablehabitat with nearly 90% accuracy, but many negativestations were also located within theoretically favor-able habitat. The probability of collecting larvalblueﬁn tuna in favorable habitat was nearly twice theprobability of collecting blueﬁn tuna larvae across allhabitats (35.5 versus 21.0%). This model is a usefuladdition to knowledge of larval blueﬁn tuna distribu-tions; however, the incorporation of variablesdescribing ﬁner-scale features, such as thermal fronts,may signiﬁcantly improve the model’s predictivepower.Key words: blueﬁn tuna, Gulf of Mexico, habitatmodelingINTRODUCTIONAtlantic blueﬁn tuna (Thunnus thynnus; blueﬁn tunahereafter) is a large and wide-ranging species, foundthroughout the Atlantic Ocean (Mather et al., 1995). Adult ﬁsh are highly migratory, and are capable oftrans-Atlantic crossings (Restrepo, 1996; Block et al.,2001; Stokesbury et al., 2007; Dickhut et al., 2009). Inthe western North Atlantic, adult ﬁsh are found fromcool-water foraging grounds in New England, to tem-perate coastal waters of the south-eastern US, andopen ocean environments (Stokesbury et al., 2004;Block et al., 2005; Teo et al., 2007b). Spawning in thewestern North Atlantic has been recorded predomi-nantly within the tropical waters of the Gulf ofMexico (GOM), in spring (April–June) (Richards,1976; Scott et al., 1993). Tagging data suggest that notall sexually mature ﬁsh make this migration every year(Lutcavage et al., 1999; Block et al., 2001; Stokesburyet al., 2004), and it has been suggested that somespawning activity may take place elsewhere in theAtlantic (McGowan and Richards, 1989; Lutcavageet al., 1999; Rooker et al., 2007). Historically, blueﬁntuna have been heavily exploited, and they arecurrently considered to be undergoing overﬁshing(McAllister and Carruthers, 2008). A steep decline instocks was recorded in the 1970s, with stocks esti-mated to have stabilized at low levels since this time(Rooker et al., 2007). Adult blueﬁn tuna can tolerate ambient tempera-tures from approximately 3 to 31 C, which is thebroadest thermal niche of any scombrid ﬁsh (Careyand Lawson, 1973; Block et al., 2001). Their uniquephysiology allows them to tolerate colder watersthan tropical tunas, and to maintain their body