Seasonal resource pulses and the foraging depth of a Southern Ocean top predator

Seasonal resource pulses can have enormous impacts on species interactions. In marine ecosystems, air-breathing predators often drive their prey to deeper waters. However, it is unclear how ephemeral resource pulses such as near-surface phytoplankton blooms alter the vertical trade-off between predation avoidance and resource availability in consumers, and how these changes cascade to the diving behaviour of top predators. We integrated data on Weddell seal diving behaviour, diet stable isotopes, feeding success and mass gain to examine shifts in vertical foraging throughout ice break-out and the resulting phytoplankton bloom each year. We also tested hypotheses about the likely location of phytoplankton bloom origination (advected or produced in situ where seals foraged) based on sea ice break-out phenology and advection rates from several locations within 150 km of the seal colony. In early summer, seals foraged at deeper depths resulting in lower feeding rates and mass gain. As sea ice extent decreased throughout the summer, seals foraged at shallower depths and benefited from more efficient energy intake. Changes in diving depth were not due to seasonal shifts in seal diets or horizontal space use and instead may reflect a change in the vertical distribution of prey. Correspondence between the timing of seal shallowing and the resource pulse was variable from year to year and could not be readily explained by our existing understanding of the ocean and ice dynamics. Phytoplankton advection occurred faster than ice break-out, and seal dive shallowing occurred substantially earlier than local break-out. While there remains much to be learned about the marine ecosystem, it appears that an increase in prey abundance and accessibility via shallower distributions during the resource pulse could synchronize life-history phenology across trophic levels in this high-latitude ecosystem.

Ecological consequences of Great Lakes salmon subsidies for stream-resident brook and brown trout

Introduced Pacific salmon (Oncorhynchus spp.) deliver novel, pulsed resource subsidies to Great Lakes streams. We explored interactions between native brook trout (Salvelinus fontinalis) and non-native brown trout (Salmo trutta) in the context of this resource pulse. Diets surveyed before and during salmon spawning showed that, regardless of species, trout consumed 4.5-fold more biomass during than before salmon runs. Brook trout grew more quickly than brown trout under controlled feeding regimes due, in part, to their higher food conversion efficiency of 36% compared with 21%. Bioenergetics model simulations explored the influence of temperature on the exploitation of resource pulses and found 35% lower growth rates and increased gorging at colder temperatures. Overall, we found evidence that brook trout and brown trout foraging and growth are modulated by the salmon resource pulse, especially through gorging on eggs. However, these species exhibit distinct physiological adaptations and environmental preferences that may influence their ultimate capacity to exploit resource pulses. The effects of environmental conditions and salmon subsidies on stream-resident trout have broader consequences for fisheries management and conservation efforts.