Loggerhead Turtles (Caretta caretta) in the California Current: Abundance, Distribution, and Anomalous Warming of the North Pacific

Tomoharu Eguchi,Robin A Leroux,Scott R Benson,Jeffrey A Seminoff,Sam Mcclatchie,Cara Wilson

doi:10.3389/fmars.2018.00452

Abstract

Environmental variability affects distributions of marine predators in time and space. With expected changes in the ocean climate, understanding the relationship between species distributions and the environment is essential for developing successful management regulations. Here we provide information on an ephemeral but important habitat for North Pacific loggerhead turtles (Caretta caretta) at the northeastern edge of their range. North Pacific loggerhead turtles nest on Japanese beaches and juveniles disperse throughout the North Pacific; some remain in the high seas of the central North Pacific whereas others transition to the eastern Pacific and forage near Baja California, Mexico. Loggerheads have also been reported along the U.S. west coast, with the majority of sightings off southern California. Here we describe their demography and distribution in the area, based on two aerial surveys (2011, 2015), at-sea sightings, and stranding records. Our aerial survey during fall 2015 determined density, abundance, and distribution of loggerheads in the area, when anomalous warming of the North Pacific and El Nino conditions co-occurred. Using line-transect analysis, we estimated ca. 15,000 loggerheads at the sea surface (CV=21%) and more than 70,000 loggerheads when accounting for those that were submerged and not available for detection. Our survey during fall 2011 resulted in no loggerhead sightings, demonstrating a high variability of loggerhead density in the region. We encourage further research on loggerheads in the area to determine the mechanisms that promote their occurrence. These studies should include regular surveys throughout their foraging areas along the west coast of the North America as well as assessments of prey availability and local oceanographic conditions.

Highlights

Spatial distributions of marine predators are strongly affected by the environmental conditions they are exposed to via physiological constraints and shifts in prey distribution (Pörtner and Knust, 2007; Hazen et al, 2012)
Some juveniles transition to the eastern Pacific, along the west coast of the Baja California Peninsula (BCP) (Bowen et al, 1995; Peckham et al, 2007), where they reside for decades until they mature (Turner Tomaszewicz et al, 2015)
In fall 2014, ocean conditions along the United States west coast were significantly different from the long-term average, a trend that lasted throughout 2015 (Leising et al, 2015) due to a combination of anomalous North Pacific warming and interaction with developing El Niño conditions

Summary

Introduction

Spatial distributions of marine predators are strongly affected by the environmental conditions they are exposed to via physiological constraints and shifts in prey distribution (Pörtner and Knust, 2007; Hazen et al, 2012). To predict the expected changes of distributions, it is essential to understand the environmental conditions that influence a species’ whereabouts. Such understanding, requires dedicated study over large spatial and temporal scales. Small loggerheads depart Japanese nesting beaches and disperse eastward following the Kuroshio and its extensions, and eventually disperse throughout the central North Pacific (CNP; Polovina et al, 2006; Kobayashi et al, 2008). Other juveniles remain in oceanic habitats of the North Pacific where their movements have been studied extensively (Polovina et al, 2000; Kobayashi et al, 2008; Abecassis et al, 2013; Briscoe et al, 2016a). At the onset of maturity, loggerheads depart these areas and return to their natal nesting beaches in Japan for reproduction and remain in the western Pacific for the remainder of their lives (Resendiz et al, 1998; Nichols et al, 2000; Hatase et al, 2002)

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