Aggregative responses of marine predators to a pulsed resource

Pulsed resources resulting from animal migrations represent important,
transient influxes of high resource availability into recipient
communities. The ability of predators to respond and exploit these large
increases in background resource availability, however, may be constrained
when the timing and magnitude of the resource pulse vary across years. In
coastal Newfoundland, Canada, we studied aggregative responses of multiple
seabird predators to the annual inshore pulse of a key forage fish
species, capelin (Mallotus villosus). Seabird aggregative responses to
fish biomass were quantified from weekly hydroacoustic and seabird surveys
during July-August within an annually persistent foraging area (10 km2)
associated with a cluster of capelin spawning sites across 10 years
(2009-2010, 2012, 2014-2020). Seabird predators included breeding members
of the families Alcidae (Common Murres Uria aalge, Razorbills Alca torda,
Atlantic Puffins Fratercula arctica) and Laridae (Great Black-backed Gulls
Larus marinus, American Herring Gulls L. argentatus smithsonianus) and
Northern Gannets Morus bassanus, along with non-breeding, moulting members
of the Family Procellariidae (Sooty Shearwaters Ardenna griseus, Great
Shearwaters A. gravis). The inshore migration of spawning capelin resulted
in 5-619 times (mean ± SE, 146 ± 59 times) increase in coastal fish
biomass along with a shift toward more, larger and denser fish shoals.
Within years, seabird abundance did not increase with inshore fish biomass
but rather peaked near the first day of spawning, suggesting that seabirds
primarily respond to the seasonal resource influx rather than short-term
variation in fish biomass. Across years, the magnitude of the seabird
aggregative response was lower during low-magnitude resource pulse years,
suggesting that predators are unable to perceive low-magnitude pulses,
avoid foraging under high competitor densities, and/or shift dietary
reliance away from capelin under these conditions. The seabird response
magnitude, however, was higher when the resource pulse was delayed
relative to the long-term average, suggesting that predators increase
exploitation during years of minimal overlap between the resource pulse
and energetically demanding periods (e.g., breeding, moulting). This
long-term study quantifying responses of multiple predators to a pulsed
resource illustrates the ability of natural systems to tolerate natural
and human-induced disturbance events.