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A Top Predator in Hot Water: Effects of a Marine Heatwave on Foraging and Reproduction in the Northern Elephant Seal

  • Author(s): Holser, Rachel Rose
  • Advisor(s): Costa, Daniel P
  • et al.
Creative Commons 'BY-ND' version 4.0 license

All organisms face resource limitations that will ultimately restrict population growth, but the controlling mechanisms vary across ecosystems, taxa, and reproductive strategies. As climate change continues to alter ecosystem processes across the globe, organisms are confronted with new challenges to their ability to survive. Marine heatwaves are prolonged warm water events that are increasing in frequency and magnitude due to rising global temperatures. The Northeast Pacific Blob was a multi-year marine heatwave that affected ecosystems across the Northeast Pacific, from producers to top predators. The northern elephant seal (Mirounga angustirostris) is a top predator that forages on the abundant biomass of the mesopelagic Northeast Pacific Ocean. Northern elephant seals are both generalist predators and capital breeders, which may buffer the effect of environmental changes on their population.

The goal of my research was to quantify the subsurface extent of the Blob and assess its effect on the foraging and reproductive success of adult female northern elephant seals. I used a combination of telemetry data collected by instrumented elephant seals (temperature, salinity, location, depth), body composition and energy gain metrics, and pup weaning mass to examine the population-level effects of the Blob. I found that there were significant warm anomalies throughout the top 1000m of the water column during the Blob, and that northward advection of warm, salty water at the base of the pycnocline likely played an important role in the sustained accumulation of warm water. Comparing foraging behavior during 2014 and 2015 to our 15-year tracking time series, I found evidence of a plastic behavioral response. Females increased their use of the Alaska Gyre, increased their daytime foraging effort, and increase their use of deep water (>800m depth) during the summer months, all suggesting that the prey field changed relative to previous years. Using a four-decade weaning mass time-series, we observed density-dependent effects on both weaning mass and male offspring-biased allocation of resources. Furthermore, maternal age was more important than oceanographic conditions or maternal mass in determining offspring weaning mass. While elephant seals did show reduced reproductive output during the Blob, they did not experience the mass mortality or reproductive failures that were seen in other species in the region, suggesting that they are more resilient to environmental change.

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