Summer foraging behaviour of shallow-diving seabirds and distribution of their prey, Arctic cod (Boreogadus saida), in the Canadian Arctic

Figures & data

Fig. 1  The study area in Allen Bay, Cornwallis Island, Nunavut, with an inset showing the Canadian Arctic Archipelago. The zigzagging lines illustrate the hydroacoustic transects undertaken on 29 and 31 August 2010. Circles show the locations of shore-based observations.

Table 1 The Akaike information criterion with small-sample bias adjustment (AIC_c) optimal linear model selection for environmental variables influencing the presence of schools of Arctic cod (Boreogadus saida) during shore-based observations (N=15). The best eight models are presented based on the smallest AIC_c differences. Statistically significant variables are indicated with asterisks.

Number of variables in model	R ^2	AICc	Error sum of squares	Variables in model	P
2	0.532	-35.507	0.882	Air temperature	0.004*
				Presence of ice^a	0.081
2	0.514	-34.941	0.916	Air temperature	0.004*
				Presence of ice^b	0.106
1	0.391	-34.237	1.148	Air temperature	0.013*
3	0.568	-33.529	0.814	Air temperature	0.003*
				Presence of ice^a	0.265
				Presence of ice^b	0.358
4	0.664	-33.473	0.633	Air temperature	0.002*
				Presence of ice^a	0.034*
				Beaufort state^c	0.119
				Wind speed	0.076
3	0.566	-33.462	0.817	Air temperature	0.004*
				Presence of ice^a	0.075
				Wind speed	0.372
3	0.556	-33.122	0.836	Air temperature	0.007*
				Presence of ice^a	0.086
				Time of day	0.455
3	0.556	-33.100	0.837	Air temperature	0.004*
				Presence of ice^a	0.095
				Visibility	0.461

^aIndicates the estimated presence of ice in the study area.

^bIndicates the estimated presence of ice in Allen Bay.

^cBeaufort state is a measure of wave action.

Fig. 2  Representation of visible schools (A–F) observed from elevated positions during shore-based observations. Inset: Letters correspond to a school which is shown in its approximate size and shape. Size was estimated based on surface area and standardized using a qualitative scaling grade of 0–5 (0≤30 m²<1≤200 m²<2≤1000 m²<3≤6000 m²<4≤10 000 m²<5).

Fig. 3  Location and shape and size and duration (inset) of schools (G–N) and number of birds observed during large feeding events (>100 birds). Note schools G and H consisted of larval Arctic cod (Boreogadus saida) and size was not estimated. Schools G and H were observed from our boat before surveys commenced. The size of schools was not correlated with the number of birds. Species names are abbreviated as follows: glaucous gull (Larus hyperboreus) is GLGU, northern fulmar (Fulmarus glacialis) is NOFU and black-legged kittiwake (Rissa tridactyla) is BLKI.

Fig. 4  Demersal school (black arrow) and nearby shallow “satellite” schools (white arrows). The dashed area represents the blanking range of about 1.75 m. Note this echogram was recorded independent of bird survey on 29 August 2010.

Table 2 Summary of results of hydroacoustic surveying during 29 and 31 August 2010. Fish per unit area (areal density) is abbreviated to FPUA, fish per unit area (volume density) to FPCM and target strength to TS (decibels).

		Targets (n)	FPUA (fish m^-2)	FPCM max (fish m^-3)	FPCM average (fish m^-3)	Average TS (dB)	Target depth upper limit (m)	Target depth deepest limit (m)
29 Aug 2010	Average	118.1	82.0	61.9	9.6	-52.83	7.2	13.1
	Max	497	718	614	92.4	-48.64	11.1	17.0
	Min	7	0.1	0.0	0.0	-55.33	1.8	3.6
	SD	144.8	169.1	145.4	23.0	2.23	2.6	3.4
31 Aug 2010	Average	197.0	209.8	93.0	25.4	-53.54	6.2	18.4
	Max	800	521.3	223.6	73.3	-49.64	8.2	24.8
	Min	9	1.4	1.0	0.2	-55.33	2.9	12.0
	SD	223.0	166.6	71.5	21.7	1.90	1.5	4.1

Fig. 5  (a) The density (birds km^-2) of black-legged kittiwakes (Rissa tridactyla), northern fulmars (Fulmarus glacialis) and glaucous gulls (Larus hyperboreus) during the survey on 31 August 2010. (b) Schools detected in Allen Bay during the same survey. Solid bars represent length of school.

Fig. 6  Density of seabirds counted in each 250 m long section (N=87) on 29 August 2010 and 31 August 2010 partitioned when schools were present and absent. Species names are abbreviated as follows: glaucous gull (Larus hyperboreus) is GLGU, northern fulmar (Fulmarus glacialis) is NOFU and black-legged kittiwake (Rissa tridactyla) is BLKI. Significant differences (P<0.05) are indicated with asterisks.

Fig. 7  (a) Arctic cod per unit area (FPUA; fish m^-2), (b) transect abundance (TA) and (c) minimum depth of Arctic cod (Boreogadus saida) schools (N=35) when below-average (absent) and above-average (present) densities of each seabird species occurred. Average densities were as follows: glaucous gull (GLGU; Larus hyperboreus) 34.08 birds km^-2; northern fulmar (NOFU; Fulmarus glacialis) 61.61 birds km^-2; and black-legged kittiwake (BLKI; Rissa tridactyla) 71.12 birds km^-2. All comparisons are at the 95% confidence level.

Table 3 Association between seabird densities in each 250 m long section (N=87) surveyed on 29 and 31 August 2010 partitioned when schools were present and absent. Species names are abbreviated as follows: glaucous gull (Larus hyperboreus) is GLGU, northern fulmar (Fulmarus glacialis) is NOFU and black-legged kittiwake (Rissa tridactyla) is BLKI. R² is Spearman rank correlation and Phi is Phi coefficient of association. Significant differences (P<0.05) are indicated with asterisks.

	NOFU–BLKI	NOFU–GLGU	BLKI–GLGU
	R ^2=0.333	R ^2=0.665	R ^2=0.313
School present	P=0.047*	P<0.001*	P=0.063
	Phi=0.265	Phi=0.422	Phi=0.056
	P=0.156	P=0.027*	P=0.999
	R ^2=0.263	R ^2=0.404	R ^2=0.268
School absent	P=0.062	P=0.003*	P=0.058
	Phi=0.129	Phi=0.326	Phi=0.211
	P=0.407	P=0.037*	P=0.221