Visual predators tend not to hunt during periods when efficiency is compromised by low light levels. Yet common murres, a species considered a diurnal visual predator, frequently dive at night. To study foraging of murres under different light conditions, we used a combination of archival tagging methods and astronomical models to assess relationships between diving behaviour and light availability. During diurnal and crepuscular periods, murres used a wide range of the water column (2–177 m), foraging across light intensities that spanned several orders of magnitude (103–10−10 Wm−2). Through these periods, they readily dived under conditions equivalent to ambient moonlight (∼10−4 Wm−2) but rarely under conditions equivalent to starlight (∼10−8 Wm−2). At night, murres readily foraged during both moonlit and starlit periods, and diving depth and efficiency increased with nocturnal light intensity, suggesting that night diving is at least partially visually guided. Whether visually guided foraging is possible during starlit periods is less clear. Given the dense prey landscape available, random-walk simulations suggest that murres could benefit from random prey encounters. We hypothesise that murres foraging through starlit periods rely either on close-range visual or possibly nonvisual cues to acquire randomly encountered prey. This research highlights the flexibility of breeding common murres and raises questions about the strategies and mechanisms birds use to find prey under very low light conditions.

Limited knowledge of year-round seabird distributions hinders efforts to assess consequences of anthropogenic threats and climate-induced changes in the marine environment. In particular, there is urgent need to understand how populations from different breeding colonies share and partition ocean habitat. Using geolocators, we identified winter habitat use patterns of 115 adult murres Uria spp. from 7 colonies, spanning the eastern Canadian coast from the high Arctic to Newfoundland, during 2007 to 2010. Thick-billed murres U. lomvia dispersed throughout the region (Davis Strait, Labrador Sea, Orphan Basin, Grand Bank) with 0 to 45% overlap of core wintering areas (50% kernel home range) among breeding populations. Common murres U. aalge concentrated on the Grand Bank and Orphan Basin, with 50 to 67% overlap among breeding populations. For both species, most individuals (up to 70%) wintered offshore, in shelf (≤500 m deep) and oceanic zones (> 500 m); fewer than one-third (30%) of individuals used nearshore zones (≤50 km to shore). Tracked common murres representing > 80% of the eastern Canadian breeding population converged in winter in areas of high risk from hydrocarbon exploration and extraction activity. In contrast, tracked thick-billed murres, representing ~34% of the eastern Canadian population, dispersed over a larger area and displayed more variable wintering strategies. Thus population vulnerability to spatially constrained risks may be greater for common than thick-billed murres. Populations from several colonies of both species converged on the Grand Bank and Orphan Basin, with the implications for each breeding population depending on its particular dispersal pattern. We demonstrate the utility of tracking data for highlighting areas of risk, and improving the targeting of broad-scale marine conservation efforts.

No abstract

Chromosome structural variation may underpin ecologically important intraspecific diversity by reducing recombination within supergenes containing linked, coadapted alleles. Here, we confirm that an ancient chromosomal rearrangement is strongly associated with migratory phenotype and individual genetic structure in Atlantic cod (Gadus morhua) across the Northwest Atlantic. We reconstruct trends in effective population size over the last century and reveal declines in effective population size matching onset of industrialized harvest (after 1950). We find different demographic trajectories between individuals homozygous for the chromosomal rearrangement relative to heterozygous or homozygous individuals for the noninverted haplotype, suggesting different selective histories across the past 150 years. These results illustrate how chromosomal structural diversity can mediate fine-scale genetic, phenotypic, and demographic variation in a highly connected marine species and show how overfishing may have led to loss of biocomplexity within Northern cod stock.

The spatial and temporal distribution of prey directly influences the foraging and feeding behaviour of predators. To investigate predator-prey interactions through the diel cycle, we examined continuous records of diving activity by a pursuit-diving seabird, the common murre Uria aalge, in conjunction with fine-scale data on the vertical distribution of their main prey, capelin Mallotus villosus, off the northeast Newfoundland coast, Canada. Diurnal patterns in the diving activities of murres closely reflected changes in the vertical distribution and movements of capelin. During daylight hours, 43% of murre dives were deep (≥ 50 m), bringing murres into sub-0°C water in the Cold Intermediate Layer (CIL; ~40 to 240 m), when 82% of capelin biomass was located within or below the CIL. At night, murres concentrated diving activity at shallower depths (94% of dives were < 50 m) when 86% of capelin biomass was in the upper water column. Capelin migrated through the water column during twilight periods, moving up at dusk and down at dawn. In response, murres' diving frequency increased and diving depths were graduated, becoming shallower through dusk and deeper through dawn. Crepuscular habits indicate that capelin are more accessible during twilight periods. In summary, though murres are constrained by commuting costs, they show exceptional behavioural flexibility in their efforts to access capelin throughout their diel vertical migration (DVM). The various trade-offs involved in such predator-prey interactions are discussed, as are the ecological consequences of the DVM pattern across trophic levels.