Foraging responses of black-legged kittiwakes to prolonged food-shortages around colonies on the Bering Sea shelf

Abstract

We hypothesized that changes in southeastern Bering Sea foraging conditions for black-legged kittiwakes (Rissa tridactyla) have caused shifts in habitat use with direct implications for population trends. To test this, we compared at-sea distribution, breeding performance, and nutritional stress of kittiwakes in three years (2008-2010) at two sites in the Pribilof Islands, where the population has either declined (St. Paul) or remained stable (St. George). Foraging conditions were assessed from changes in (1) bird diets, (2) the biomass and distribution of juvenile pollock (Theragra chalcogramma) in 2008 and 2009, and (3) eddy kinetic energy (EKE; considered to be a proxy for oceanic prey availability). In years when biomass of juvenile pollock was low and patchily distributed in shelf regions, kittiwake diets included little or no neritic prey and a much higher occurrence of oceanic prey (e.g. myctophids). Birds from both islands foraged on the nearby shelves, or made substantially longer-distance trips overnight to the basin. Here, feeding was more nocturnal and crepuscular than on the shelf, and often occurred near anticyclonic, or inside cyclonic eddies. As expected from colony location, birds from St. Paul used neritic waters more frequently, whereas birds from St. George typically foraged in oceanic waters. Despite these distinctive foraging patterns, there were no significant differences between colonies in chick feeding rates or fledging success. High EKE in 2010 coincided with a 63% increase in use of the basin by birds from St. Paul compared with 2008 when EKE was low. Nonetheless, adult nutritional stress, which was relatively high across years at both colonies, peaked in birds from St. Paul in 2010. Diminishing food resources in nearby shelf habitats may have contributed to kittiwake population declines at St Paul, possibly driven by increased adult mortality or breeding desertion due to high foraging effort and nutritional stress.

Figure 1. Foraging tracks of chick-rearing black-legged kittiwakes at the Pribilof Islands.

A) GPS tracking (St. Paul  =  red; St. George  =  blue) during 2008 (n = 34 tracks), 2009 (n = 58) and 2010 (n = 70). B) Number of foraging trips to the basin or shelf habitats made by birds from St. Paul and St. George each year. C): close-ups of first maps showing nearshore (St. Paul) and shelf trips.

Figure 2. Core feeding areas (50% contours of foraging locations) of black-legged kittiwakes from St. Paul and St. George Islands during trips to marine habitats.

Trips to the shelf (A), and to the basin (B). Patterns: red  =  St. Paul; black  =  St. George; Years: 2008 =  dots, 2009  =  crossed lines, 2010  =  diagonal lines.

Figure 3. Frequency histograms of foraging locations of black-legged kittiwakes from St. Paul and St. George Islands according to time of day and marine habitat (shelf and basin) of the southeastern Bering Sea.

The rectangle indicates the dark (black), twilight (grey), and daylight (white) periods.

Figure 4. Examples of individual tracks and foraging locations (circles) overlaid on eddy fields (blue  =  cyclonic eddies, orange  =  anticyclonic; generated using AVISO Products) matched by date (July 23±7 days 2008; July 7±3 days 2010) of data collection.

Figure 5. Frequencies of foraging and random locations according to distance to nearest eddy perimeter (vertical line); negative values indicate locations inside features.

Figure 6. Prey species consumed by tracked kittiwakes during trips to shelf and basin habitats.

Percentages are based on the total number of samples (n) of trips to each habitat. “Other fish”: eelpout, rockfish, and gadids; “Crustaceans”: amphipods and euphausiids; “Others”: amphipods and sea nettle. Empty samples from stomach lavages were only found from birds that traveled to the shelf in 2008.

Figure 7. Diet composition of chick-rearing kittiwakes from St. George (n = 41 samples in 2008; n = 26 in 2009; n = 51 in 2010) and St. Paul Islands (n = 32 in 2008; n = 29 in 2009; n = 35 in 2010).

Categorization of prey by domain location (shelf and basin) was based on diet of tracked birds (Fig. 6).

Figure 8. Acoustically measured biomass density (g/m²) of juvenile pollock integrated from 100 m to the surface.

Data were collected from mid-July to mid-August in each year over 110 transects of 10 km in length that were randomly placed within 200 km of St. George Island. Map surfaces were generated using minimum curvature interpolation.

Figure 9. Breeding performance and physiological stress of black-legged kittiwakes at St. Paul and St. George islands, and eddy kinetic energy in the basin area during 2008-10.

Chick-feeding frequency (A), fledging success (B), adult nutritional stress levels (C), and July-mean eddy kinetic energy (D). Decreasing stress levels of St. George birds coincide with the increase in EKE between 2008 and 2010.