Foraging ecology of ringed seals (Pusa hispida), beluga whales (Delphinapterus leucas) and narwhals (Monodon monoceros) in the Canadian High Arctic determined by stomach content and stable isotope analysis

Figures & data

Fig. 1  Map of the eastern Canadian Arctic. Inset indicates the sampling location in Allen Bay and Resolute Bay, off Cornwallis Island.

Table 1 Sample collection information of ringed seals, belugas and narwhals in Allen Bay and Resolute bay in 2010 and 2011.

	Ringed seals
Collection date	Aug 9–Sep 14 2010	Jun 15–Sep 31 2011	Belugas Sep 1–Sep 16 2010	Narwhals Sep 16–Sep 17 2010
Sample size (n)	21	6	13	3
Males	17	2	8	3
Adult	9	1	6	3
Juvenile	4	1	2	–
Pup	4	–	–	–
Females	4	2	5	–
Adult	4	2	3	–
Juvenile	–	–	2	–
Pup	–	–	–	–
Unknown	–	2	–	–
Total stomachs with contents	17 (2)^a	–	5 (7)^a	3 (0)^a

^aThe numbers in parentheses represent empty stomachs.

Fig. 2  Frequency distribution of Arctic cod size (fork length [FL], mm) found in the stomachs of ringed seals, belugas and narwhals. The symbol O represents the number of otoliths sampled, and S represents the number of stomachs collected. Arctic cod fork length was extrapolated from otolith length (OL) using FL=24.20*OL – 4.29 (Matley et al. 2013).

Fig. 3  Smoothing functions (k=3) of general additive modelling on the size of Arctic cod consumed by ringed seals in relation to age (a) and length (b). The y axes represent the smoothing function centred at zero and the estimated degrees of freedom for each explanatory variable. The adjusted R ² for this model is 0.47. The dark line represents the overall mean, shaded areas are 95% confidence intervals, and small lines on the x axes are observations at each predictor level.

Table 2 Number of prey, frequency of occurrence (FO_i) and percent composition (N_i) of ringed seal, beluga and narwhal food items.

	Number of prey^a	FOi	Ni
Ringed seals
Juveniles and pups (n=8)
Arctic cod	160	87.5	23.6
Amphipods	519^b	50	76.4
Adult (n=11)
Arctic cod	578	100	91.9
Amphipods	51	36.4	8.1
Belugas (n=12)
Arctic cod	556	41.7	93.6
Amphipods	34	16.7	5.7
Squid	4	25	<1
Narwhals (n=3)
Arctic cod	257	100	81.6
Amphipods	55	66.7	17.5
Squid	3	66.7	1

^aNumber of Arctic cod calculated by dividing the total number of otoliths by 2 (e.g., 1476/2=738 individuals). ^bTwo individuals (a juvenile and a pup) contributed ca. 98% of amphipods collected.

Table 3 Model selection from generalized additive modelling (GAM) examining the effect of ringed seal age and length on the size of consumed Arctic cod (FL). Akaike information criterion accounting for small sample size (AICc) was used to select the best models (i.e., lower AICc), and the relative importance of each model—its weight—is indicated.

	Model	AICc	ΔAICc	Weight
M1	FL~s(Age)	140.3	0	0.52
M2	FL~s(Length)	141.8	1.52	0.25
M3	FL~s(Age) + s(Length)	142.9	2.64	0.14
M4	FL~1 (null model)	143.8	3.57	0.09

Table 4 Sample number (n), mean±1 standard error (SE) of liver and muscle δ¹⁵N (‰) and δ¹³C (‰) of ringed seals, belugas and narwhals collected near Resolute in 2010 and 2011.

		Liver		Muscle
Species	n	δ^15N	δ^13C	δ^15N	δ^13C
Ringed seals (2010)	21	17.45±0.18	−18.56±0.10	17.21±0.18	−19.35±0.14
Ringed seals (2011)	6	16.50±0.17	−19.37±0.14	16.96±0.43	−19.76±0.23
Belugas	12	17.00±0.12	−19.14±0.10	16.33±0.20	−19.28±0.09
Narwhals	3	16.97±0.12	−19.57±0.14	16.34±0.07	−19.59±0.11

Table 5 Summary of ANCOVA results testing the effect of ringed seal maturity (adult ≥6 years old, juvenile/pup < 6 years old) and length (covariate) on muscle and liver δ¹³C and δ¹⁵N values. Significant relationships (p≤0.05) are in boldface.

	Parameter	F value	P value
Muscle δ^13C	Length	<0.01	0.93
	Maturity	<0.01	0.99
Muscle δ^15N	Length	3.86	0.07
	Maturity	1.12	0.30
Liver δ^13C	Length	0.22	0.64
	Maturity	<0.01	0.98
Liver δ^15N	Length	5.80	0.03
	Maturity	1.22	0.29

Fig. 4  Isotope biplot of potential prey sources (mean±SD) of ringed seals, belugas and narwhals, corrected for trophic discrimination by subtracting 3.1 and 0.6‰ from ringed seal values for liver δ¹⁵N and δ¹³C, respectively, as well as 2.4 and 1.3‰ for muscle (Hobson et al. 1996). The discrimination factors used for the toothed whales were 2.78 and 1.27‰ for liver δ¹⁵N and δ¹³C, respectively, and 1.23 and 1.26‰ for muscle (Caut et al. 2011). Prey were collected from Allen Bay in 2010, except for the following species: Gonatus fabricii and Reinhardtius hippoglossoides collected in Davis Strait in 2011 (Watt & Ferguson 2015) and Mallotus villosus collected in Cumberland Sound 2008 (Dennard et al. 2009).

Fig. 5  Contribution of common prey sources based on isotopic mixing model output for ringed seal and toothed whale (belugas and narwhals combined) liver and muscle from 2010. Prey sources used in the models were Arctic cod and/or Reinhardtius hippoglossoides (AC), Themisto libellula (TL), Onisimus sp. (OS), Gammarus sp. (GM), Gonatus fabricii (GF) and Mallotus villosus (MV). The boxplots represent probability densities for credibility intervals of 95% (lighter boxes), 75%, and 50% (darker boxes).

Matley J.K., Fisk A.T., Dick T.A. The foraging ecology of Arctic cod (Boreogadus saida) during open water (July–August) in Allen Bay, Arctic Canada. Marine Biology. 2013; 160: 2993–3004.

 Web of Science ®Google Scholar

Hobson K.A., Schell D.M., Renouf D., Noseworthy E. Stable carbon and nitrogen isotopic fractionation between diet and tissues of captive seals: implications for dietary reconstructions involving marine mammals. Canadian Journal of Fisheries and Aquatic Sciences. 1996; 53: 528–533.

 Web of Science ®Google Scholar

Caut S., Laran S., Garcia-Hartmann E., Das K. Stable isotopes of captive cetaceans (killer whales and bottlenose dolphins). The Journal of Experimental Biology. 2011; 214: 538–545.

 Web of Science ®Google Scholar

Watt C.A., Ferguson S.H. Fatty acids and stable isotopes (δ13C and δ15N) reveal temporal changes in narwhal (Monodon monoceros) diet linked to migration patterns. Marine Mammal Science. 2015; 31: 21–44.

 Web of Science ®Google Scholar

Dennard S.T., McMeans B.C., Fisk A.T. Preliminary assessment of Greenland halibut diet in Cumberland Sound using stable isotopes. Polar Biology. 2009; 32: 941–945.

 Web of Science ®Google Scholar