Chemical tracers elucidate trophic and migratory dynamics of Pacific pelagic predators [electronic resource]
Keywords: Biology.

Pelagic predators face many challenges and uncertain futures. Much work has revealed their decline due to overfishing, alteration of ecosystems, trophic cascades, and climate change. In the face of these challenges there has been a dearth of information regarding fundamental life history and ecology of these species due to the difficulty of studying them in their natural environment. Electronic tagging studies have greatly expanded our knowledge of pelagic organisms, but in many cases they raise as many questions as are answered. From the moment of animal capture, we can find out where a tagged animal migrates to, but from where did it emigrate before capture? What are the ecological roles of these predators on their various regions of high use? What is the timing and origin of migrations? These questions have implications for management, both using movement data (to understand where, and to what an extent, a species may need protection from exploitation) and ecological data (to understand how over-exploitation of different species or trophic levels may affect other organisms in a pelagic ecosystem). Various chemical tracers have served as new tools to answer some of these questions. Environmental contaminants, radioactive isotopes, and stable isotopes all have been used to examine movement patterns and trophic interactions in marine animals. Complementary techniques and laboratory-based studies are sometimes necessary to interpret data and provide the necessary parameters to use chemical analyses to understand the timing and origin of migrations and the roles migratory predators play in their various oceanic environments. In Chapter 1, I use archived, frozen Pacific bluefin tuna Thunnus orientalis (PBFT) tissues to study the turnover and trophic fractionation of two stable isotope ratios ([delta]13C and [delta]15N) in two PBFT tissues (white muscle and liver). Captive bluefin with demonstrably lower white muscle and liver [delta]13C and [delta]15N values than their captive diet were kept in captivity for 1-2914 days, allowing for a long-term experiment of stable isotope dynamics (specifically, [delta]13C and [delta]15N) in PBFT. This experiment tuna reveals isotopic turnover rates and trophic discrimination factors (TDFs) for PBFT in the size range commonly encountered in the California Current Large Marine Ecosystem (CCLME). TDFs can then be used to investigate tuna trophic ecology, and the long turnover times of tuna muscle (t1/2 for PBFT WM = 167 days) demonstrate that tunas take more than a year to reflect local isotopic prey conditions, and turnover rates can be applied to multiple tissues using isotopic clock techniques. These parameters are necessary for the interpretation of trophic ecology of pelagic predators in Chapter 2 and the timing and origin of PBFT migrations in Chapter 4. In Chapter 2, I assess trophic dynamics in the CCLME using stable isotope analysis (SIA), utilizing isotope turnover parameters from Chapter 1. Using [delta]13C and [delta]15N values of primary consumers (plankton), secondary consumers (small squids and forage fish), and mid-upper trophic level predators (n = 17 predator and 13 prey species; 292 predator and 181 prey samples), I categorize organisms into trophic groups and estimate food inputs between trophic groups. I reveal higher connectivity in the pelagic food web of the CCLME than is predicted by the generally-accepted wasp-waist model of upwelling pelagic food webs, which assumes that most pelagic predators in upwelling, eastern boundary current systems feed primarily on one or few species of planktivorous secondary consumers, such as sardine or anchovy, which in turn feed on highly diverse and abundant zooplankton. Chapter 3 and Chapter 4 examine PBFT migration using different chemical tracer techniques. In 2011 the accident at the Fukushima Daiichi nuclear plant caused a massive spill of radionuclides into the Pacific Ocean in the waters off eastern Japan. This presented the possibility that migratory animals that forage in this region and subsequently migrate to distant ecoregions could be identified as emigrants from western Pacific waters. To test this new tracer we measured radioactive cesium (134Cs and 137Cs) in 15 PBFT that were caught in the CCLME and were between 1-2 years of age, making them definitive recent (previous year) migrants from waters around Japan. All 15 PBFT had elevated 134+137Cs compared to pre-Fukushima bluefin and post-Fukushima CCLME yellowfin tuna (CCLME migrants), proving that the PBFT had transported radiocesium across the Pacific Ocean and demonstrating the potential use of radiocesium as a tracer of migration. Chapter 4 validates the concept put forth in Chapter 3 to use Fukushima-derived radiocesium to track the movements of PBFT. In 2012, we sampled a larger dataset (n = 350) of PBFT in the CCLME to determine migration status using presence or absence of 134Cs and levels of 137Cs compared to 'background' levels of this radioisotope present in yellowfin tuna Thunnus albacares, residents of the CCLME. Using a sample set of 50, we demonstrate that all small PBFT (n = 28), known from size to be recent migrants from Japan, show measurable levels of 134Cs and elevated levels of 137Cs. This shows that all known migrants carry the radiocesium signal from the Fukushima accident. In contrast, larger fish (n = 22) showed pre-Fukushima levels of radiocesium in 17 fish, and 5 fish showed measurable levels of 134Cs and elevated levels of 137Cs, indicating recent migration from Japan. This study demonstrates that the radiocesium marker is detectable in all recent migrants, and that recent migrants or > 1 year CCLME residents can be discerned using this tracer in larger PBFT, for which recent, retrospective migratory history is unknown. Chapter 5 combines three chemical techniques (SIA, Cs radiotracer, and amino acid compound-specific isotope analysis or AA-CSIA) to elucidate bluefin migration in the CCLME. I used Cs-marked PBFT (definitive Japan migrants) to inform a larger SIA dataset for PBFT sampled between 2008-2010. We revealed that a larger proportion of older PBFT in the CCLME are recent Japan migrants than is generally believed. We also demonstrate that there is a seasonal trend to the arrival of Japan migrants to the CCLME. Finally, we suggest that this complementary chemical tracer toolbox can be applied to many highly migratory pelagic species in the Pacific to further elucidate their migration dynamics. Overall this work develops several tracers for application to PBFT (SIA and AA-CSIA) and presents the discovery and validation of a new tracer for migrations of Pacific pelagic predators (Fukushima-derived radionuclides). New information is supplied on the migratory dynamics of PBFT. These tracers, when used in the context of their model organism, can be applied to other pelagic predators to better understand their movement patterns. These approaches are especially pragmatic for species that are targeted by fisheries, as with the use of chemical tracers novel information can still be obtained from organisms that are no longer alive.