Sockeye salmon population dynamics over the past 4000\xa0years in Upper Russian Lake, south-central Alaska

Molly D Mccarthy,Bruce P Finney,Daniel J Rinella

doi:10.1007/s10933-018-0024-1

Abstract

Stable nitrogen isotope (δ15N) data from sediment cores taken in clear-water Upper Russian Lake (Kenai River Watershed, Alaska, USA) indicate that sockeye salmon (Oncorhynchus nerka) populations varied significantly over the past 4000 years, with a prominent ~ 650-year period of lower salmon abundance from ~ 100 BCE to 550 CE. Sediment characteristics during this ~ 650-year interval reflect glacial sediment input, which may have contributed to the salmon decline by degrading spawning habitat and reducing carrying capacity. The decline, however, coincides with large reductions in sockeye salmon abundance identified previously in Karluk and Akalura lakes on Kodiak Island, > 400 km southwest, supporting the possibility of regionally synchronous, multi-centennial production regimes that may originate from shifts in oceanographic conditions such as biological productivity in the northeastern Pacific Ocean. Under such a scenario, coincidence with watershed glacial activity indicates a common driver, i.e. regional climate change. Climate conditions that led to significant glacial advances in this part of the Kenai Peninsula (cold and/or wet conditions) may have also created unfavorable ocean conditions during critical periods in the marine phase for these stocks of Gulf of Alaska sockeye salmon. Future climate projections and management strategies should focus on how climate regimes impact not only prey availability for salmon at sea, but also local conditions for spawners and juveniles.

Highlights

Paleolimnological analyses have led to reconstructions of sockeye salmon (Oncorhynchus nerka) abundance spanning hundreds or thousands of years in many nursery lakes (Finney 1998, 2000, 2002; Gregory-Eaves et al 2003; Rogers et al 2013)
UW stands for unidentified wood and T stands for tephra a 2-cm-thick deposit centered at a core depth of 17 cm, which correlated with a 505 ± 25 year BP tephra layer found in a peat core from Seldovia, Alaska (AT3508, unpublished data)
Sediment d15N in Upper Russian Lake has fluctuated around a mean of 4.5% for most of the * 4000 years represented in our core, suggesting that spawner abundance has been relatively consistent on timescales similar to or greater than sampling resolution over much of this time period

Summary

Introduction

Paleolimnological analyses have led to reconstructions of sockeye salmon (Oncorhynchus nerka) abundance spanning hundreds or thousands of years in many nursery lakes (Finney 1998, 2000, 2002; Gregory-Eaves et al 2003; Rogers et al 2013). A synthesis of paleolimnological data spanning * 500 years from 20 nursery lakes in southwestern and south-central Alaska, the center of wild sockeye salmon abundance, reveals substantial shifts in salmon productivity within individual lakes that often lasted for decades or centuries (Rogers et al 2013). These productivity regimes were often asynchronous among lakes, presumably related to differential responses of habitats and salmon populations to regional climatic conditions, resulting in a ‘‘portfolio effect’’ that stabilizes regional salmon returns over time (Rogers et al 2013). Examination of temporal variation at long timescales in distant river systems will provide insights into the temporal and spatial scales over which population trends of different stocks are synchronized or not

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