Variable ocean conditions can greatly impact lower trophic level prey assemblages in marine ecosystems, with effects propagating up to higher trophic levels. Our goal was to better understand how varying ocean conditions influence diets and niche overlap among a suite of low- to mid trophic level predators. We studied the diets of common murres (Uria aalge) over 10 contrasting years between 1998 and 2011, a period in which the Northern California Current experienced dramatic interannual variability in ocean conditions. Likewise, murre diets off Oregon varied considerably. Interannual variation in murre chick diets appears to be influenced by environmental drivers occurring before and during the breeding season, at both basin and local spatial scales. While clupeids were an important diet component throughout the study period, in some years murre diets were dominated by Pacific sand lance (Ammodytes hexapterus) and other years by osmerids (likely Allosmerus elongatus and Hypomesus pretiosus). Years in which the Pacific Decadal Oscillation and local sea surface temperatures were above average during summer months also showed elevated levels of clupeids in murre diets, while years with higher winter ichthyoplankton biomass and summer northern copepod biomass anomalies had fewer clupeids and more sand lance and smelts. Years with higher Northern Oscillation Index values during summer months also showed more smelts in the murre diets. Nesting phenology and reproductive success were correlated with diet as well, reflecting demographic consequences of environmental variability mediated through bottom-up food web dynamics.
To examine niche overlap between murres and other marine predators we employed collaborative fisheries research with synoptic observations of a major seabird colony to determine the diets of four predator species on the central Oregon coast during two years of contrasting El Niño (2010) vs. La Niña (2011) conditions. The greatest degree of dietary overlap was observed between Chinook salmon (Oncorhynchus tshawytscha) and common murres, with both smelts (Osmeridae) and clupeids (primarily Clupea pallasii) observed as the dominant prey types. Diets differed between El Niño and La Niña conditions for two predators, murres and black rockfish (Sebastes melanops). During La Niña, smelts decreased, while sand lance increased in common murre diets. Black rockfish had fewer larval Dungeness crabs (Cancer magister) and a greater proportion of crab species associated with the later spring transition. Chinook salmon and Pacific halibut (Hippoglossus stenolepis) diets were similar during El Niño and La Niña conditions.
These findings underscore that the diets of common murres during chick rearing reflect local- and basin-scale biophysical processes in the Northern California Current, and are valuable for understanding the response of upper trophic level organisms to changing oceanographic conditions. Additionally, using multiple predators across several diverse taxa to track changes in prey communities provided a way to detect seemingly subtle changes in prey communities and contributes to a more comprehensive understanding of food web dynamics and ecosystem indicators.
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