Owyn E. Snodgrass

Stable Isotope Analysis Challenges Wasp-Waist Food Web Assumptions in an Upwelling Pelagic Ecosystem

Eastern boundary currents are often described as ‘wasp-waist’ ecosystems in which one or few mid-level forage species support a high diversity of larger predators that are highly susceptible to fluctuations in prey biomass. The assumption of wasp-waist control has not been empirically tested in all such ecosystems. This study used stable isotope analysis to test the hypothesis of wasp-waist control in the southern California Current large marine ecosystem (CCLME). We analyzed prey and predator tissue for δ13C and δ15N and used Bayesian mixing models to provide estimates of CCLME trophic dynamics from 2007–2010. Our results show high omnivory, planktivory by some predators, and a higher degree of trophic connectivity than that suggested by the wasp-waist model. Based on this study period, wasp-waist models oversimplify trophic dynamics within the CCLME and potentially other upwelling, pelagic ecosystems. Higher trophic connectivity in the CCLME likely increases ecosystem stability and resilience to perturbations.

Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus

A cold-water fish with a warm heart Mammals and birds warm their entire bodies above the ambient temperature. Generally, this ability is lacking in other vertebrates, although some highly active fish can temporarily warm their swim muscles. Wegner et al. show that the opah, a large deepwater fish, can generate heat with its swim muscles and use this heat to warm both its heart and brain. This ability increases its metabolic function in cold deep waters, which will help the fish compete with other, colder-blooded species. Science, this issue p. 786 Unlike other fish, opah distribute warmed blood throughout their bodies, enhancing physiological performance in the deep ocean. Endothermy (the metabolic production and retention of heat to warm body temperature above ambient) enhances physiological function, and whole-body endothermy generally sets mammals and birds apart from other animals. Here, we describe a whole-body form of endothermy in a fish, the opah (Lampris guttatus), that produces heat through the constant “flapping” of wing-like pectoral fins and minimizes heat loss through a series of counter-current heat exchangers within its gills. Unlike other fish, opah distribute warmed blood throughout the body, including to the heart, enhancing physiological performance and buffering internal organ function while foraging in the cold, nutrient-rich waters below the ocean thermocline.

Reconstructing transoceanic migration patterns of Pacific bluefin tuna using a chemical tracer toolbox

Large pelagic predators play important roles in oceanic ecosystems, and may migrate vast distances to utilize resources in different marine ecoregions. Understanding movement patterns of migratory marine animals is critical for effective management, but often challenging, due to the cryptic habitat of pelagic migrators and the difficulty of assessing past movements. Chemical tracers can partially circumvent these challenges by reconstructing recent migration patterns. Pacific bluefin tuna (Thunnus orientalis; PBFT) inhabit the western and eastern Pacific Ocean, and are in steep decline due to overfishing. Understanding age-specific eastward transpacific migration patterns can improve management practices, but these migratory dynamics remain largely unquantified. Here, we combine a Fukushima-derived radiotracer (134Cs) with bulk tissue and amino acid stable isotope analyses of PBFT to distinguish recent migrants from residents of the eastern Pacific Ocean. The proportion of recent migrants to residents decreased in older year classes, though the proportion of older PBFT that recently migrated across the Pacific was greater than previous estimates. This novel toolbox of biogeochemical tracers can be applied to any species that crosses the North Pacific Ocean.

Radiocesium in Pacific bluefin tuna Thunnus orientalis in 2012 validates new tracer technique.

The detection of Fukushima-derived radionuclides in Pacific bluefin tuna (PBFT) that crossed the Pacific Ocean to the California Current Large Marine Ecosystem (CCLME) in 2011 presented the potential to use radiocesium as a tracer in highly migratory species. This tracer requires that all western Pacific Ocean emigrants acquire the (134)Cs signal, a radioisotope undetectable in Pacific biota prior to the Fukushima accident in 2011. We tested the efficacy of the radiocesium tracer by measuring (134)Cs and (137)Cs in PBFT (n = 50) caught in the CCLME in 2012, more than a year after the Fukushima accident. All small PBFT (n = 28; recent migrants from Japan) had (134)Cs (0.7 ± 0.2 Bq kg(-1)) and elevated (137)Cs (2.0 ± 0.5 Bq kg(-1)) in their white muscle tissue. Most larger, older fish (n = 22) had no (134)Cs and only background levels of (137)Cs, showing that one year in the CCLME is sufficient for (134)Cs and (137)Cs values in PBFT to reach pre-Fukushima levels. Radiocesium concentrations in 2012 PBFT were less than half those from 2011 and well below safety guidelines for public health. Detection of (134)Cs in all recent migrant PBFT supports the use of radiocesium as a tracer in migratory animals in 2012.

Insights into the life history and ecology of a large shortfin mako shark Isurus oxyrinchus captured in southern California

In June 2013, a record-breaking female Isurus oxyrinchus (total length 373 cm, mass 600 kg) was captured by rod and reel off Huntington Beach, California, where it was subsequently donated to research and provided a rare opportunity to collect the first data for a female I. oxyrinchus of this size. Counts of vertebral band pairs estimate the shark to have been c. 22 years old, depending upon assumptions of band-pair deposition rates, and the distended uteri and spent ovaries indicated that this shark had recently given birth. The stomach contained a c. 4 year-old female California sea lion Zalophus californianus that confirmed the high trophic position of this large I. oxyrinchus, which was corroborated with the high levels of measured contaminants and tissue isotope analyses.

Basking Shark (Cetorhinus maximus) Movements in the Eastern North Pacific Determined Using Satellite Telemetry

To fill data gaps on movements, behaviors and habitat use both near- and offshore, two programs were initiated to deploy satellite tags on basking sharks. Basking sharks are large filter feeding sharks that are second in size only to whale sharks. Similar to many megafauna populations, available data suggest that populations are below historic levels. In the northeast Pacific Ocean, the limited information on basking sharks comes from nearshore habitats where they forage. From 2010-2011, four sharks were tagged with pop-off satellite archival tags with deployments ranging from 9-240 days. The tags provided both transmitted and archived data on habitat use and geographic movement patterns. Nearshore, sharks tended to move north in the summer and prefer shelf and slope habitat around San Diego, Point Conception and Monterey Bay. The two sharks with 180 and 240 day deployments left the coast in the summer and fall. Offshore their paths diverged and by January one shark had moved to near the tip of the Baja Peninsula, Mexico and the other to the waters near Hawaii, USA. Vertical habitat use was variable both within and among individuals and changed as sharks moved offshore. Nearshore, most time was spent in the mixed layer but sharks did spend hours in cold waters below the mixed layer. Offshore vertical movements depended on location. The shark that went to Hawaii had a distinct diel pattern, with days spent at ~450-470 m and nights at ~250-300 m and almost no time in surface waters, corresponding with the diel migration of a specific portion of the deep scattering layer. The shark that moved south along the Baja Peninsula spent progressively more time in deep water but came to the surface daily. Movement patterns and shifts in vertical habitat and use are likely linked to shifts in prey availability. Data collected indicate the potential for large-scale movements and the need for international dialogue in any recovery efforts.

Mercury Stable Isotopes Reveal Influence of Foraging Depth on Mercury Concentrations and Growth in Pacific Bluefin Tuna

Pelagic ecosystems are changing due to environmental and anthropogenic forces, with uncertain consequences for the oceans top predators. Epipelagic and mesopelagic prey resources differ in quality and quantity, but their relative contribution to predator diets has been difficult to track. We measured mercury (Hg) stable isotopes in young (<2 years old) Pacific bluefin tuna (PBFT) and their prey species to explore the influence of foraging depth on growth and methylmercury (MeHg) exposure. PBFT total Hg (THg) in muscle ranged from 0.61 to 1.93 μg g-1 dw (1.31 μg g-1 dw ±0.37 SD; 99% ± 6% MeHg) and prey ranged from 0.01 to 1.76 μg g-1 dw (0.13 μg g-1 dw ±0.19 SD; 85% ± 18% MeHg). A systematic decrease in prey δ202Hg and Δ199Hg with increasing depth of occurrence and discrete isotopic signatures of epipelagic prey (δ202Hg: 0.74 to 1.49‰; Δ199Hg: 1.76-2.96‰) and mesopelagic prey (δ202Hg: 0.09 to 0.90‰; Δ199Hg: 0.62-1.95‰) allowed the use of Hg isotopes to track PBFT foraging depth. An isotopic mixing model was used to estimate the dietary proportion of mesopelagic prey in PBFT, which ranged from 17% to 55%. Increased mesopelagic foraging was significantly correlated with slower growth and higher MeHg concentrations in PBFT. The slower observed growth rates suggest that prey availability and quality could reduce the production of PBFT biomass.

Assessing Fukushima-Derived Radiocesium in Migratory Pacific Predators

The 2011 release of Fukushima-derived radionuclides into the Pacific Ocean made migratory sharks, teleosts, and marine mammals a source of speculation and anxiety regarding radiocesium (134+137Cs) contamination, despite a lack of actual radiocesium measurements for these taxa. We measured radiocesium in a diverse suite of large predators from the North Pacific Ocean and report no detectable (i.e., ≥ 0.1 Bq kg-1 dry wt) Fukushima-derived 134Cs in all samples, except in one olive ridley sea turtle (Lepidochelys olivacea) with trace levels (0.1 Bq kg-1). Levels of 137Cs varied within and across taxa, but were generally consistent with pre-Fukushima levels and were lower than naturally occurring 40K by one to one to two orders of magnitude. Predator size had a weaker effect on 137Cs and 40K levels than tissue lipid content. Predator stable isotope values (δ13C and δ15N) were used to infer recent migration patterns, and showed that predators in the central, eastern, and western Pacific should not be assumed to accumulate detectable levels of radiocesium a priori. Nondetection of 134Cs and low levels of 137Cs in diverse marine megafauna far from Fukushima confirms negligible increases in radiocesium, with levels comparable to those prior to the release from Fukushima. Reported levels can inform recently developed models of cesium transport and bioaccumulation in marine species.