Seasonality of decadal-scale trophic declines and nutrient regime shifts in the Laysan albatross and Newell’s shearwater

Ke Morra,S Rossman,Hf James,Ae Wiley,Ph Ostrom,Y Chikaraishi

doi:10.3354/meps13504

Abstract

Stable isotope analyses of archival specimens have revealed trophic declines over the past 150 yr in a growing number of predatory pelagic seabirds that breed in the Hawaiian Islands. However, they have not examined whether isotopic shifts occurred primarily during a specific phase of the annual cycle, which could allow us to better identify the causes of trophic declines and potential consequences for affected populations. We evaluated seasonal (breeding versus nonbreeding season) foraging habits of 2 ecologically distinct species, Newell’s shearwater Puffinus newelli and Laysan albatross Phoebastria immutabilis, and extended this analysis back 50 and 100 yr, respectively. Our assessment relied on amino acid δ15N proxies for nutrient regime use (δ15NPhe) and trophic position (Δδ15NGlu-Phe) from 2 tissues (feather and bone collagen) reflecting different time scales. Both study species exhibited season-specific isotopic shifts resulting in more pronounced seasonality in modern populations. We also identified inter-species differences in nutrient regime use, regardless of season. Laysan albatross experienced a trophic decline exclusive to the breeding season, while their nonbreeding season foraging ecology has remained constant over the past century. In contrast, the nutrient regime at the base of the food chain for Newell’s shearwaters during the nonbreeding season underwent a shift within the last 50 yr, and the trophic decline they experienced was heavily weighted toward the nonbreeding season. Efforts to mitigate potential fitness consequences of future trophic declines might benefit from focusing on fisheries management near the Hawaiian Islands, where susceptible seabirds forage during winter/spring, rather than the entire North Pacific.

Highlights

Trophic declines during the past 150 yr have been detected in 8 ecologically diverse pelagic seabirds that breed in the Hawaiian Islands (Wiley et al 2013, Ostrom et al 2017, Gagne et al 2018, Morra et al.2019)
In contrast to Newell’s shearwaters, modern Laysan albatross feathers had lower δ15NPhe values and higher Δδ15NGlu-Phe values relative to collagen (Fig. 3c,d, Tables 1a & S1A; p > 99% and p = 99%, respectively), suggesting greater reliance on food webs supplied by nitrogen fixation and a higher trophic level during the nonbreeding season relative to the breeding season
Seasonal shifts in trophic position that were absent in historical populations emerged in modern ones for both species, and seasonal shifts in nutrient regime use extend back 50 yr for Newell’s shearwaters but are unique to modern populations of Laysan albatross

Summary

Introduction

Trophic declines during the past 150 yr have been detected in 8 ecologically diverse pelagic seabirds that breed in the Hawaiian Islands (Wiley et al 2013, Ostrom et al 2017, Gagne et al 2018, Morra et al.2019). Because these declines occurred in wide-ranging, generalist predators, they suggest pervasive ecosystem shifts within the world’s largest ocean during the past 150 yr. Whether each species’ trophic decline has manifested throughout the entire year or influenced a species during a specific phase of the annual cycle (i.e. the breeding or molting period) is unknown and could differ among species, depending on their breeding and feeding strategies. To our knowledge, changes in foraging seasonality have never been addressed by pelagic seabird studies at the scale of decades

Methods

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Discussion

Conclusion