Feeding ecology of harbour porpoises: stable isotope analysis of carbon and nitrogen in muscle and bone

Figures & data

Figure 1.  Porpoise (Phocoena phocoena) stranding locations and numbers along the Dutch coast analysed in this study (2003–2008). The dark grey area indicates the Eastern Scheldt.

Table I. Porpoise (Phocoena phocoena) length and weight measurements, grouped by age-class (adult/juvenile/neonate) and sex (M/F/U) with number of samples (n), mean, standard deviation (SD) and range (Min/Max).

			Length (cm)				Weight (kg)
Age-class	Sex	n	Mean	SD	Min	Max	Mean	SD	Min	Max
Neonate	Female	6	76.0	4.3	71	80	7.2	1.2	6	9
	Male	6	81.7	4.2	75	86	8.3	0.8	7	9
Juvenile	Female	45	112.2	10.9	92	133	19.7	6.2	9	41
	Male	63	109.1	12.0	87	141	18.6	5.5	8	37
	Unknown	3	105.7	15.5	88	117	–	–	–	–
Adult	Female	20	152.1	7.3	140	165	41.8	6.7	31	58
	Male	14	141.1	6.7	131	153	38.4	7.3	28	49

Table II. Range of latitude and longitude of the fishing localities of prey samples (n=624).

		Latitude (°)		Longitude (°)		δ^15N (‰)		δ^13C (‰)
Area	n	Min	Max	Min	Max	Mean	SD	Mean	SD
Central North Sea	32	53.91	56.24	1.41	6.16	13.08	1.78	−20.02	1.31
Dutch Coastal Zone	404	35.50	53.91	4.20	6.91	16.14	1.56	−18.26	1.94
Eemsdelta	18	53.29	53.49	7.16	7.47	18.76	1.13	−17.96	0.80
English Channel	28	50.79	50.79	0.86	0.86	15.14	1.09	−18.45	0.75
Firth of Forth	20	56.32	56.71	−1.44	−0.24	14.09	0.54	−18.31	0.48
German Coastal Zone	30	54.00	54.53	8.11	8.32	16.68	1.14	−18.53	1.23
Northern North Sea	22	57.59	58.12	0.55	1.90	12.17	1.10	−20.08	0.88
North-western Moray Firth	18	57.89	58.21	−3.24	−2.70	11.60	1.36	−20.13	0.96
South-eastern UK Coast	8	51.48	53.11	1.73	1.89	15.63	1.25	−18.57	1.09
Southern Bight	27	52.21	53.21	2.12	3.61	15.76	1.19	−18.01	1.10
Outer Delta	17	51.79	51.79	3.52	3.52	18.84	1.24	−18.46	1.82

Figure 2.  Porpoise (Phocoena phocoena) strandings per calendar month (n=157) analysed in this study (2006–2008), separated by age-class. The solid line shows the total number of strandings on the Dutch coast from 2006 to 2008 (www.walvisstrandingen.nl).

Figure 3.  Porpoise (Phocoena phocoena) isotopic composition of muscle; effect of length and sex (black = females, grey = males) on δ¹⁵N (A) and the effect of stranding location (Dutch coast versus Eastern Scheldt) on δ¹³C (B). Solid line presents the mean model estimate and the dotted lines the 95% confidence intervals.

Table III. Porpoise (Phocoena phocoena) isotopic composition (δ¹⁵N and δ¹³C) of muscle samples, grouped by age-class and sex with number of samples (n), C : N ratio (C : N), mean, standard deviation (SD) and range (Min/Max).

Muscle				δ^15N (‰)				δ^13C (‰)
Age-class	Sex	n	C : N	Mean	SD	Min	Max	Mean	SD	Min	Max
Neonate	Female	3	3.50	17.28	1.76	15.46	18.98	−17.99	0.88	−19.00	−17.46
	Male	3	3.51	18.23	0.91	17.27	19.07	−17.76	0.08	−17.82	−17.68
Juvenile	Female	33	3.34	16.66	1.24	14.18	18.73	−18.19	0.51	−19.31	−17.26
	Male	41	3.38	16.41	1.26	13.92	18.56	−18.29	0.49	−19.69	−17.21
Adult	Female	11	3.38	16.17	1.35	13.68	17.65	−18.11	0.61	−19.09	−16.82
	Male	11	3.29	15.15	1.34	13.45	17.52	−18.32	0.57	−19.21	−17.02

Figure 4.  Porpoise (Phocoena phocoena) isotopic composition of bone; effect of length on δ¹⁵N (A) and δ¹³C (B), effect of month on δ¹⁵N (C) and δ¹³C (D). Solid line presents the mean model estimate and the dotted lines the 95% confidence intervals.

Table IV. Porpoise (Phocoena phocoena) isotopic composition (δ¹⁵N and δ¹³C) of bone samples, grouped by age-class and sex with number of samples (n), C : N ratio (C : N), mean, standard deviation (SD) and range (Min/Max).

Bone				δ^15N (‰)				δ^13C (‰)
Age-class	Sex	n	C : N	Mean	SD	Min	Max	Mean	SD	Min	Max
Neonate	Female	5	3.35	19.10	1.89	15.93	20.69	−15.03	0.89	−16.59	−14.46
	Male	7	3.33	17.93	1.32	16.17	19.77	−15.21	0.62	−16.14	−14.56
Juvenile	Unknown	3	3.68	14.75	1.21	13.77	16.11	−16.46	0.21	−16.68	−16.27
	Female	37	3.48	16.24	1.65	12.97	19.21	−15.58	0.68	−17.31	−14.01
	Male	39	3.44	16.08	1.37	12.31	18.67	−15.47	0.66	−17.05	−14.29
Adult	Female	18	3.56	15.64	1.55	11.33	17.49	−14.85	0.63	−15.77	−13.81
	Male	9	3.47	15.87	1.17	13.69	17.72	−14.77	0.48	−15.68	−14.29

Figure 5.  Porpoise (Phocoena phocoena) isotopic composition, effect of length on Δδ¹⁵N_{bone–muscle} (A) and Δδ¹³C_{bone–muscle} (B). Solid line presents the mean model estimate and the dotted lines the 95% confidence intervals.

Table V. Isotopic composition (δ¹³C and δ¹⁵N) of prey species collected from the North Sea between 2002 and 2008.

				δ^13C (‰)		δ^15N (‰)
Species		n	C : N	Mean	SD	Mean	SD
Alloteuthis subulata	Common squid	19	3.6	−18.52	0.52	13.85	0.74
Ammodytes tobianus	Lesser sandeel	10	3.2	−18.76	0.80	15.08	1.02
Arnoglossus laterna	Scaldfish	20	3.3	−18.28	0.83	15.98	1.03
Buglossidium luteum	Solenette	20	3.3	−17.93	0.77	16.51	0.59
Callionymus lyra	Dragonet	15	3.1	−19.26	1.32	15.75	0.61
Ciliata mustela	5bearded-rockling	20	3.2	−17.14	0.56	16.96	0.63
Clupea harengus	Herring	20	3.2	−18.59	0.37	15.82	1.18
Dicentrarchus labrax	Seabass	21	3.3	−15.13	1.22	18.18	1.14
Gadus morhua	Cod	33	3.1	−18.75	1.50	15.68	2.54
Gobius niger	Black goby	13	3.3	−18.34	0.60	19.46	0.48
Hippoglossoides platessoides	Long rough dab	19	3.2	−18.70	1.00	14.40	3.06
Hyperoplus lanceolatus	Greater sandeel	20	3.2	−18.07	0.95	15.19	0.63
Limanda limanda	Dab	31	3.3	−18.47	0.65	14.21	1.74
Loligo forbesi	Veined squid	20	3.5	−20.54	0.95	12.46	0.45
Merlangius merlangus	Whiting	30	3.1	−18.19	0.86	16.38	1.60
Osmerus eperlanus	Smelt	20	3.4	−18.50	0.97	18.48	0.28
Platichthys flesus	Flounder	30	3.2	−19.98	3.44	17.20	0.67
Pleuronectes platessa	Plaice	27	3.1	−18.06	0.80	15.53	0.68
Pomatoschistus microps	Common goby	20	3.5	−15.66	0.72	16.73	0.48
Pomatoschistus minutus	Sand goby	20	3.3	−16.61	1.00	16.96	0.41
Scomber scombrus	Mackerel	20	6.4	−22.42	0.63	13.45	1.50
Scophthalamus maximus	Turbot	20	3.6	−17.63	0.74	16.71	0.38
Solea solea	Sole	15	3.2	−17.72	1.12	16.34	1.56
Sprattus sprattus	Sprat	20	4.4	−20.57	0.89	14.96	0.71
Syngnathus acus	Greater pipefish	5	3.3	−18.33	0.83	15.21	0.81
Syngnathus rostellatus	Nilsson's pipefish	21	4.3	−19.05	1.21	15.97	0.51
Trachurus trachurus	Scad	20	3.7	−18.91	1.30	16.18	0.73
Trisopterus esmarkii	Norway pout	20	3.3	−19.68	0.83	11.56	1.34
Trisopterus luscus	Bib	20	3.2	−17.65	0.49	18.02	1.96
Trisopterus minutus	Poor cod	35	3.1	−18.54	0.63	14.58	0.77

Figure 6.  Isotopic composition δ¹⁵N and δ¹³C of prey species grouped by fishing localities. VD, Voordelta; ED, Eemsdelta; GCZ, German Coastal Zone; DCZ, Dutch Coastal Zone; SB, Southern Bight; SE UKC, South-eastern UK Coast; EC, English Channel; FF, Firth of Forth; CNS, Central North Sea; NW MF, North-western Moray Firth.