Diet composition and biomass consumption of harbour seals in Telemark and Aust-Agder, Norwegian Skagerrak

Figures & data

Figure 1. Harbour seal scat sampling locations in red. Kragerø is located in the county of Telemark, while Risør, Tvedestrand and Grimstad are located in Aust-Agder.

Table I. Number of samples and number of otoliths per site. The location ‘Kragerø’ consists of Knallaren, Ødegårdskilen and Gjesskjæra.

Location	Otoliths	Samples
Kragerø	257	45
Risør	149	18
Tvedestrand summer/autumn	234	36
Tvedestrand winter/spring	48	17
Grimstad	69	5
Total	757	121

Table II. Frequency of occurrence (FO_i), relative frequency (NUM_i), percentage biomass (PB_i) and combined index (Q_i) for all species and pooled groups. The group ‘other prey’ include the following: Labridae: goldsinny wrasse (Ctenolabrus rupestris, Linnaeus, 1758), cuckoo wrasse (Labrus mixtus, Linnaeus, 1758), ballan wrasse (Labrus bergylta, Ascanius, 1767) and unidentified species from the Labridae family; Gobiidae: black goby (Gobius niger, Linnaeus, 1758), painted goby (Pomatoschistus pictus, Malm, 1865), common goby (Pomatoschistus microps, Krøyer, 1838), Fries’s goby (Lesuerigobius friesii, Malm, 1874) and unidentified species from the Gobiidae family; and Bothidae: Norwegian topknot (Phrynorhombus norvegicus, Günther, 1862) and unidentified species from the Bothidae family.

Family and species	FOi (%)	NUMi (%)	PBi (%)	Qi (%)
Clupeidae
Herring (Clupea harengus)	15.7	5.8	11.1	10.0
Gadidae
Cod (Gadus morhua)	5.8	0.9	2.3	0.7
Haddock (Melanogrammus aeglefinus)/ pollack (Pollachius pollachius)/saithe (Pollachius virens)	29.8	9.5	19.3	32.7
Whiting (Merlangius merlangus)	25.6	12.7	4.8	7.0
Blue whiting (Micromesistius poutassou)	3.3	0.5	0.2	0.03
Genus Trisopterus	33.9	27.3	6.5	12.5
Four-bearded rockling (Enchelyopus cimbrius)	1.7	2.2	0.7	0.1
Unidentified Gadidae	21.5	6.1	3.7	4.5
Merlucciidae
Hake (Merluccius merluccius)	6.6	1.1	0.4	0.2
Pleuronectidae
Plaice (Pleuronectes platessa)	9.1	3.2	24.1	12.4
Flounder (Platichthys flesus)	0.8	0.1	1.0	0.05
Dab (Limanda limanda)	9.1	2.8	4.8	2.5
Lemon sole (Microstomus kitt)	9.1	2.8	6.9	3.6
Long rough dab (Hippoglossoides platessoides)/witch (Glyptocephalus cynoglossus)	18.2	13.7	7.3	7.6
Unidentified Pleuronectidae	15.7	5.3	5.6	5.0
Scombridae
Mackerel (Scomber scombrus)	1.7	0.3	0.2	0.02
Other prey	19.8	4.2	1.1	1.3

Figure 2. Combined index (Q_i) for all species and species groups in harbour seal scat samples in all locations, Kragerø (summer), Risør (summer), Tvedestrand (summer-autumn and winter-spring) and Grimstad (spring). ‘Norway pout/poor cod/bib’ refer to genus Trisopterus. ‘Other prey’ includes four-bearded rockling, hake, mackerel, Cottidae, Labridae, Gobiidae and Bothidae. ‘Flatfish’ includes plaice, flounder, dab, lemon sole, long rough dab/witch and unidentified Pleuronectidae.

Figure 3. Length (mm) and weight (g) distribution (median, 25% and 75% quartiles) for fish species found in the harbour seal diet. Red lines indicate minimum allowed landing size (Lovdata 2005). Grey numbers indicate outliers. Black numbers on right side indicate number of otoliths. (Color online)

Table III. Annual fish consumption by harbour seals in Telemark and Aust-Agder. Annual commercial fisheries landings of cod, haddock, saithe and mackerel (FD 2018), and annual consumption of these species by harbour seals as percentages of commercial fisheries landings (in parentheses) are also given. All amounts in tons. Biomass for Cottidae, Labridae, Gobiidae and Bothidae is summarized in ‘other prey’. *Pollack not included, as it is not a target species for local fisheries.

Prey items	Telemark and Aust-Agder
	Seal consumption Telemark	Seal consumption Aust-Agder	Total seal consumption	Fisheries landings 2016
Clupeidae
Herring (Clupea harengus)	28.5	6.7	35.1
Gadidae
Cod (Gadus morhua)	5.8	1.4	7.1	141 (5.1)
Haddock (Melanogrammus aeglefinus)/ pollack (Pollachius pollachius)/saithe (Pollachius virens)	49.3	11.6	60.9	45* (135.3)
Whiting (Merlangius merlangus)	12.4	2.9	15.2
Blue whiting (Micromesistius poutassou)	0.5	0.1	0.6
Genus Trisopterus	16.5	3.9	20.4
Four-bearded rockling (Enchelyopus cimbrius)	1.8	0.4	2.2
Unidentified Gadidae	9.4	2.2	11.6
Total Gadidae	95.6	22.4	118.0
Merlucciidae
Hake (Merluccius merluccius)	1	0.2	1.3
Pleuronectidae
Plaice (Pleuronectes platessa)	61.5	14.4	75.9
Flounder (Platichthys flesus)	2.6	0.6	3.2
Dab (Limanda limanda)	12.3	2.9	15.2
Lemon sole (Microstomus kitt)	17.6	4.1	21.7
Long rough dab (Hippoglossoides platessoides)/witch (Glyptocephalus cynoglossus)	18.7	4.4	23.1
Unidentified Pleuronectidae	14.3	3.3	17.6
Total Pleuronectidae	126.9	29.7	156.7
Scombridae
Mackerel (Scomber scombrus)	0.5	0.1	0.7	21 (3.1)
Other prey	2.9	0.7	3.5
Total	255.5	59.9	315.4

Lovdata: Forskrift om utøvelse av fisket i sjøen. [Norwegian Legislations and Regulations: Regulations on the Exercise of Fisheries in the Sea]. 2005. Oslo, Norway: Nærings- og fiskeridepartementet [Ministry of Trade, Industry and Fisheries], [accessed: autumn 2018] https://lovdata.no/dokument/SF/forskrift/2004-12-22-1878?q=Forskrift%20om%20ut%C3%B8velse%20av%20fisket.

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