Regional growth patterns of juvenile albacore (Thunnus alalunga) in the Eastern North Pacific

Population structure of North Pacific albacore (Thunnus alalunga) may be more complex than the current single-stock hypothesis given the apparent regional differences in size-at-age and movement patterns in the eastern North Pacific. In this study, otolith-based techniques were used to analyze the age structure, daily growth rates and hatch dates of 126 albacore collected from two regions in the eastern North Pacific (northern: >40 ̊N and southern: <40 ̊N) in July through October of 2010 and 2011. Juvenile albacore collected from the southern region had significantly larger size-at-age than those collected from the northern region over a similar age range examined (423–1254 days). No significant regional difference in growth rates of juvenile albacore was detected over the ages examined, suggesting processes occurring in the first year and a half of life may have been driving regional size-at-age differences. Back-calculated hatch dates suggest protracted spawning for fish from both regions ranging from February to September with peak hatch dates between April and July (73% of all fish). Results from this two-year study suggest that juvenile albacore length-at-age estimates in the eastern North Pacific are region-specific and may need to be considered for future life history studies and stock assessments. INTRODUCTION Albacore (Thunnus alalunga) are highly migratory predators with a cosmopolitan distribution (Sund et al. 1981). In the Pacific Ocean, albacore are separated into North and South Pacific stocks due to limited mixing across the equator and isolated spawning grounds (Nakamura 1969). In the North Pacific, albacore spawn in the central and western Pacific where many begin transoceanic migrations to utilize the highly productive California Current in the eastern North Pacific (Clemens and Craig 1965; Wetherall et al. 1981). The distribution and relative abundance of albacore in the eastern North Pacific varies latitudinally (Laurs and Lynn 1977; Laurs and Wetherall 1981; Wetherall et al. 1987; Childers et al. 2011) and it has been suggested that there are two subgroups of albacore: a northern group based offshore of the Pacific Northwest of the United States and Canada and a southern group located offshore of southern California and Baja, Mexico (Laurs and Lynn 1977; Laurs and Wetherall 1981; Wetherall et al. 1987; Laurs and Lynn 1991). The dividing line between these two subgroups is considered to be associated with the Mendocino Escarpment located at 40 ̊N. Albacore from the two subgroups have been reported to differ in size composition, growth rates, movement patterns, and peak spawning periods, which has led to the hypothesis that there may be two substocks of albacore in the eastern North Pacific (Laurs and Wetherall 1981; Laurs and Lynn 1991; Barr 2009; Childers et al. 2011). Stock structure is a key component of management, and current management of North Pacific albacore assumes a single stock in the North Pacific. Growth rates during the juvenile stage can be directly linked to survival and subsequent population dynamics of adult fish populations (Houde 1987). One of the best approaches to retrospectively examine early life history is using otolith-derived age and growth estimates based on the deposition of daily rings. To date, however, few studies have attempted to use daily otolith ageing to examine albacore in the eastern North Pacific (Wetherall et al. 1987) because the technique is technically challenging and labor intensive (Williams et al. 2013). Near-daily deposition of daily growth increments has been validated for North Pacific albacore (Laurs et al. 1985) and allows for relatively high precision in estimates of age, growth rates, and hatch dates. In this study, otolith microstructure was used to examine regional growth rates and length-at-age estimates of juvenile albacore in the eastern North Pacific. The daily ages of fish were then used to back-calculate fish hatch date to estimate albacore spawning periods, which can be used to address the sub-stock hypothesis. Temporal differences in spawning may support the possibility of two sub-stocks because separate stocks are defined by separation in time and/or space. Ultimately, information on the life history and ecology of albacore in the eastern North Pacific will provide updated biological parameters useful for stock-assessment models. RENCK ET AL.: REGIONAL GROWTH PATTERNS OF JUVENILE ALBACORE CalCOFI Rep., Vol. 55, 2014 136 Otolith Age and Growth In the laboratory, albacore were measured to the nearest cm fork length (FL) and sagittal otoliths were removed from each fish, and cleaned following procedures described by Secor et al. 1991. One of the two otoliths was randomly chosen and embedded in low viscosity epoxy resin (fig. 2). After the epoxy resin hardened, the otolith was sectioned transversely using a Buehler isomet low-speed saw equipped with a diamondwafering blade. Longitudinal sections were also performed on a subset of otoliths (n = 20); however, transverse sections produced the best product for counts. The otolith sections were mounted on glass slides with Crystalbond thermoplastic cement and polished with 0. 3μm Alumina Alpha polish using 400 and 600 grit MATERIALS AND METHODS Study Area and Field Collections The study area is located at the eastern extent of the North Pacific Gyre and is focused on two regions within the California Current during the summer and fall when albacore are targeted by local fisheries (Laurs and Lynn 1977) (fig. 1). The two regions are within the California Current and are exposed to coastal, winddriven upwelling and are north and south of 40 ̊N at the Mendocino Escarpment, defined as the northern and southern regions. Albacore samples were collected from the north and south fishing regions from July through October of 2010 and 2011 (table 1). North samples were collected from commercial troll (artificial-jig fishing gears) and poleand-line (live-bait fishing) surface fisheries targeting albacore. South samples were collected from recreational pole-and-line fishing vessels operating off southern California and Baja California, Mexico. All fish collected were frozen and transported to the NOAA Southwest Fisheries Science Center in La Jolla, California, for later processing. 40°N 50°N