Crossing the line: migratory and homing behaviors of Atlantic bluefin tuna

Abstract

Assessment and management of Atlantic bluefin tuna Thunnus thynnus populations is hindered by our lack of knowledge regarding trans-Atlantic movement and connectivity of eastern and western populations. Here, we evaluated migratory and homing behaviors of bluefin tuna in several regions of the North Atlantic Ocean and Mediterranean Sea using chemical tags (δ 13 C and δ 18 O) in otoliths. Significant emigration of bluefin tuna from their place of origin was inferred from otolith δ 13 C and δ 18 O, with both eastern and western bluefin tuna commonly 'crossing the line' (45° W management boundary) in the Central North Atlantic Ocean and mixing with the other population. Several western migrants were also detected in Moroccan traps off the coast of Africa, indicating that trans-Atlantic movement occurs for members of the western population; however, the degree of mixing declined with proximity to the eastern spawning area (Mediterranean Sea). The origin of bluefin tuna collected at the entrance to the Strait of Gibraltar and from several regions within the Mediterranean Sea (Balearic Islands, Malta, and Sardinia) was essentially 100% eastern fish, demonstrating that natal homing is well developed by the eastern population, with western migrants rarely entering the Mediterranean Sea.

Otolith shape variation provides a marker of stock origin for north Atlantic bluefin tuna (Thunnus thynnus)

“…The estimates of stock composition in the mixed samples that were obtained using otolith shape descriptors are within the ranges reported by Rooker et al (2014) for Atlantic bluefin collected from the same locations (albeit in different years and from a wider size range of fish). According to our estimates, the sample from the central north Atlantic collected in 2012 comprised 94 AE 7% (mean AE s.d.)…”

“…eastern origin fish. Although this is higher than the 63.9 AE 9.6% reported by Rooker et al (2014) for the same location in 2010, it is consistent with their estimates of 90.7 AE 5.3% for 2011. Our estimate is based on just 10 fish within a restricted size range and should therefore not be taken as representative of the entire fishery for that year.…”

“…Proper integration of the information provided by multiple stock markers requires careful consideration and coordination of sampling strategies (Abaunza et al 2008). A limitation of the present study lies in the sampling design; although there is some overlap in the years and locations used in the otolith shape analysis and in previous analyses of otolith stable isotopes Rooker et al 2014), because the otolith shape study was initiated after most of the sampling had been conducted, the majority of otoliths were not analysed using both methods. The simultaneous collection of otolith shape and chemistry data is logistically very feasible because otoliths can be photographed without interfering with subsequent chemical analysis.…”

“…Young-of-the-year from the main spawning areas in the Gulf of Mexico and Mediterranean show differences in otolith trace elements (Rooker et al 2003) and stable isotopes (Rooker et al 2008), as well as concentrations of organochlorines and polychlorinated biphenyls (PCBs) in tissues (Dickhut et al 2009). Stock-specific differences in otolith stable isotopes have facilitated the estimation of stock composition in potentially mixed assemblages (Schloesser et al 2010;Fraile et al 2014;Rooker et al 2014). However, the degree of uncertainty associated with this method of assignment warrants the investigation of other stock-identification approaches to corroborate estimated mixing rates and ultimately to develop a multimarker approach to stock assignment for greater precision in mixed-stock projections.…”

“…However, the assessment is subject to uncertainty, particularly with regard to stock-mixing rates (Fromentin et al 2014a;ICCAT 2014)). Improving the accuracy of mixing rate estimates is acknowledged by ICCAT as a research priority (ICCAT 2008(ICCAT , 2013 The 458 meridian is the assumed boundary between the eastern and western stocks, although trans-Atlantic movements are known to occur (Block et al 2005;Rooker et al 2014). Although stock discreetness is maintained by high rates of natal homing, there is considerable overlap in the distribution of the two stocks outside of spawning time (Carlsson et al 2006;Rooker et al 2008).…”