Identifying patterns in foraging-area origins in breeding aggregations of migratory species: Loggerhead turtles in the Northwest Atlantic.

Joseph B Pfaller,Mariela Pajuelo,Dubose B Griffin,Breanna L Ondich,Kimberly M Andrews,Mark G Dodd,Hannah B Vander Zanden,Kristina L Williams,Karen A Bjorndal,Campbell J Nairn,S Michelle Pate,Matthew H Godfrey,Alan B Bolten,Brian M Shamblin,Mariana M P B Fuentes

doi:10.1371/journal.pone.0231325

Abstract

Population assessments conducted at reproductive sites of migratory species necessitate understanding the foraging-area origins of breeding individuals. Without this information, efforts to contextualize changes in breeding populations and develop effective management strategies are compromised. We used stable isotope analysis of tissue samples collected from loggerhead sea turtles (Caretta caretta) nesting at seven sites in the Northern Recovery Unit (NRU) of the eastern United States (North Carolina, South Carolina and Georgia) to assign females to three separate foraging areas in the Northwest Atlantic Ocean (NWA). We found that the majority of the females at NRU nesting sites (84.4%) use more northern foraging areas in the Mid-Atlantic Bight, while fewer females use more proximate foraging areas in the South Atlantic Bight (13.4%) and more southerly foraging areas in the Subtropical Northwest Atlantic (2.2%). We did not find significant latitudinal or temporal trends in the proportions of NRU females originating from different foraging areas. Combining these findings with previous data from stable isotope and satellite tracking studies across NWA nesting sites showed that variation in the proportion of adult loggerheads originating from different foraging areas is primarily related differences between recovery units: individuals in the NRU primarily use the Mid-Atlantic Bight foraging area, while individuals from the three Florida recovery units primarily use the Subtropical Northwest Atlantic and Eastern Gulf of Mexico foraging areas. Because each foraging area is associated with its own distinct ecological characteristics, environmental fluctuations and anthropogenic threats that affect the abundance and productivity of individuals at nesting sites, this information is critical for accurately evaluating population trends and developing effective region-specific management strategies.

Highlights

Reproductive sites where migratory species congregate offer valuable opportunities to conduct population assessments of threatened species [1]
Among 24 published articles identified in our literature search (S3 Table), we found 1,398 loggerheads from 13 nesting sites that were assigned to an Northwest Atlantic Ocean (NWA) foraging area using stable isotope analysis (Fig 4) and 408 loggerheads from 16 nesting/breeding sites that were tracked to an NWA foraging area using satellite telemetry (Fig 5)
Variation in the proportion of females assigned to different foraging areas was subtle based on isotopic data with the majority of females at all eight nesting sites originating from the northern Mid-Atlantic Bight (MAB) foraging area (Fig 4)

Summary

Introduction

Reproductive sites where migratory species congregate offer valuable opportunities to conduct population assessments of threatened species (e.g., fish spawning sites, bird rookeries, whale and ungulate calving grounds, and marine turtle nesting beaches) [1]. Loggerhead sea turtles (Caretta caretta) nesting in the Northwest Atlantic Ocean (NWA) are separated into five recovery units based on phylogeographic isolation and geopolitical boundaries [3,4,5]: (1) Northern Recovery Unit (NRU) nest on beaches from Maryland to North Florida, (2) Peninsular Florida Recovery Unit (PFRU) nest on beaches from central eastern Florida to central western Florida, (3) Dry Tortugas Recovery Unit (DTRU) nest on islands west of the Florida Keys, (4) Northern Gulf of Mexico Recovery Unit (NGMRU) nest in beaches from the Florida Panhandle through Texas, and (5) Greater Caribbean Recovery Unit (GCRU) nest in Mexico and throughout the rest of Greater Caribbean Turtles from these recovery units display differential use of six broad foraging areas ranging from the waters off Nova Scotia in the north to the Yucatan Peninsula in the south [6,7,8,9] (Fig 1). Determining the foraging-area origins of female loggerheads at different nesting sites across the NWA is critical for evaluating trends in breeding population abundance and developing region-specific management strategies

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Conclusion