Temporal shift in the isotopic niche of female Antarctic fur seals from Bouvetøya

Figures & data

Fig. 1  Circumpolar distribution map of all known breeding sites (triangles) of the AFS (Arctocephalus gazella). Filled triangles indicate the study sites mentioned in Table 4. Baseline mapping data are from Scambos et al. (2007) and bathymetric data are from Amante & Eakins (2009).

Table 1 Sample sizes for tissues, by collection year, from female AFS (Arctocephalus gazella) during the breeding season at Bouvetøya (54°23′S 3°47′E).

Tissue	1997	2001	2002	2015
Plasma	50	31	15	0
Whole blood	41	0	0	36

Fig. 2  Interannual variation in isotopic ratios in whole blood (N₁₉₉₇=41 and N₂₀₁₅=36) of female AFS (Arctocephalus gazella) from Bouvetøya. Isotopic biplots show the interannual variation in mean (± SD, empty symbols and error bars) carbon (δ ¹³C) and nitrogen (δ ¹⁵N) isotopic ratios for 1997 (orange) and 2015 (black). The estimated 40% (inner, representing the core niche region) and 95% (outer, representing the global niche region) random ellipses are also shown for each year. Ten thousand random ellipses were generated for each year and each Nr threshold, but for clarity only the first 1000 are shown in this figure. Density curves for each isotope and each year are also drawn marginally along the corresponding axis. Discrimination factors were applied to δ ¹³C and δ ¹⁵N values.

Fig. 3  Shift in isotopic niche region areas between 1997 and 2015 for (a) the 95% (global niche region) and (b) 40% (core niche region) thresholds, based on carbon and nitrogen isotopic ratios of female AFS (Arctocephalus gazella) from Bouvetøya. Sample size is 1000 for each group, based on the random elliptical projections of Nr that were generated from this study (see methods for details; Swanson et al. 2015).

Table 2 Interannual variation in isotopic ratios. Comparison of linear models explaining the shift in isotopic ratios (δ ¹³C and δ ¹⁵N) in whole blood of female AFS (Arctocephalus gazella) from Bouvetøya, between 1997 (intercept level) and 2015 (see Fig. 2). Boldface indicates P values significant at the 0.05 statistical level.

	δ ^13C			δ ^15N
Response	β	95% CI	P	β	95% CI	P
(Intercept)	−25.36	(−26.20; −24.51)		6.12	(5.39; 6.84)
Julian day	−0.01	(−0.03; 0.01)	0.499	−0.00	(−0.02; 0.01)	0.601
Year 2015	1.21	(0.69; 1.72)	<0.001	1.28	(0.83; 1.72)	<0.001

Fig. 4  (a) Nitrogen (δ ¹⁵N) and (b) carbon (δ ¹³C) isotopic ratios in plasma (N=96) of female AFS (Arctocephalus gazella) from Bouvetøya displayed across the timing of the breeding season using cross-sectional sampling in three years. The black lines and grey-shaded areas show the mean and standard error predicted from a simple linear model on the data pooled over three years (Y ~ Julian day). The orange lines and shaded areas show the same information but from a linear model where year is included as a covariate (Y ~ Julian day + Year). Discrimination factors were applied to δ ¹³C and δ ¹⁵N values (see methods for details).

Table 3 Intra-seasonal variation in isotopic ratios. Comparison of linear models predicting δ ¹³C and δ ¹⁵N in plasma of female AFS (Arctocephalus gazella) from Bouvetøya. For each response variable, two models are compared, one with and one without an effect of sampling year (see Fig. 4). Boldface indicates P values significant at the 0.05 statistical level.

	δ ^13C						δ ^15N
Response	β	95% CI	P	β	95% CI	P	β	95% CI	P	β	95% CI	P
(Intercept)	−24.65	(−24.82; −24.48)		−25.11	(−25.77; −24.44)		4.12	(3.78; 4.45)		5.86	(4.65; 7.08)
Julian day	−0.02	(−0.03; −0.02)	<0.001	−0.01	(−0.03; 0.00)	0.121	0.04	(0.02; 0.05)	<0.001	−0.01	(−0.04; 0.02)	0.597
Year 2001				0.15	(−0.24; 0.54)	0.445				−0.79	(−1.50; −0.08)	0.030
Year 2002				0.53	(0; 1.07)	0.051				−1.74	(−2.73; −0.76)	0.001

Table 4 Comparative summary of the results from isotopic studies on AFS (Arctocephalus gazella), including the present study. Temporal trends in isotopic values for carbon and nitrogen are indicated, along with a plausible interpretation when a positive (↑) or negative (↓) trend was detected. See Fig. 1 for a map of the study sites.

	Study site	Sampling			Δ^13C		Δ^15N
Reference	Name	Year	Tissue	Timescale	Trend	Plausible interpretation	Trend	Plausible interpretation
Polito & Goebel 2010	South Shetland Islands	2007	Milk	Seasonal (Jan–Feb)	→		↑	Increased trophic level (more fish in diet)
		2008	Milk	Seasonal (Jan–Feb)	↓	Foraging farther offshore	↑	Increased trophic level (more fish in diet)
Huang et al. 2011	South Shetland Islands	2002	Hair	Interannual (1924–1997)	→		↑	Increased trophic level (more fish in diet)
Cherel et al. 2007	Crozet Islands	2002	Whole blood	Seasonal (summer/winter)	→		→
Kernaléguen et al. 2015	Crozet Islands	2002	Whiskers	Interannual (1997–2002)	→		→
				Seasonal (Dec–Mar)	↑	Increased trophic level (more fish in diet)	↑	Increased trophic level (more fish in diet)
	Kerguelen Archipelago	2002	Whiskers	Interannual (1998–2002)	→		→
				Seasonal (Dec–Mar)	↑	Increased trophic level (more fish in diet)	↑	Increased trophic level (more fish in diet)
Hanson et al. 2009	South Georgia	1971–2006	Teeth	Interannual (1964–2004)	↓	Foraging farther offshore and/or decreased trophic level (more krill in diet)	↓	Decreased trophic level (more krill in diet)
this study	Bouvetøya	1997–2015	Whole blood	Interannual (1997–2015)	↑	Increased trophic level (more fish in diet)	↑	Increased trophic level (more fish in diet)
			Plasma	Seasonal (Jan–Feb)	↓	Foraging farther offshore	↑	Increased trophic level (more fish in diet)

Scambos T.A., Haran T.M., Fahnestock M.A., Painter T.H., Bohlander J. MODIS-based Mosaic of Antarctica (MOA) data sets: continent-wide surface morphology and snow grain size. Remote Sensing of Environment. 2007; 111: 242–257.

 Web of Science ®Google Scholar

Amante C., Eakins B. ETOPO1 1 Arc-Minute global relief model: procedures, data sources and analysis. 2009. NOAA Technical Memorandum NESDIS NGDC-24. Boulder: National Geophysical Data Center, Marine Geology and Geophysics Division, National Oceanic and Atmospheric Administration..

 Google Scholar

Swanson H.K., Lysy M., Power M., Stasko A.D., Johnson J.D., Reist J.D. A new probabilistic method for quantifying n-dimensional ecological niches and niche overlap. Ecology. 2015; 96: 318–324.

 Web of Science ®Google Scholar