A note on stomach contents of minke whales (Balaenoptera acutorostrata) in Icelandic waters

There is limited available information on food habits of minke whales (Balaenoptera acutorostrata) in coastal Iceland. Sixty-eight minke whales were examined for stomach contents; 51.5% contained fish only, 22.1 % krill (Euphausiacea) only, 25.0% fish and krill together, and one animal (l.5%) had no food remains in the stomach. The fish species identified were capel in (Mallotus villosus) , sandeel, (Ammodytidae), cod (Gadus morhua) and herring (Clupea harengus) . Two species of krill were identified; Thysanoessa raschii and Meganyctiphanes norvegica . Sandeel was the dominant prey species in the western and southwestern areas, while capelin and krill were more frequently found in animals sampled in North Iceland. Sigutj6nsson, 1., Galan, A. and Vikingsson, G.A .. 2000. A note on stomach contents ofminke whales (Balaenoptera acutorostrata) in Icelandic waters. NAMMCO Sci. Publ. 2: 82-90


Introduction
The minke whale (Balaenoptera acutorostrata) is generally regarded as opportunistic in feeding habits. A large variety of food items have been found in the stomachs of minke whales caught in the North Atlantic (see review by Horwood 1990). While minke whales feed predominantly on krill in the Southern Hemisphere (Kawamura 1994), the species appears to be the most piscivorous of the Balaenopterids in the Northern Hemisphere. In Norwegian and adjacent waters, large temporal and spatial variations have been found in the diet of minke whales (Jonsgard 1951, 1982, Haug et al. 1995, 1996. In the North Western Atlantic capelin appears to be the most important prey species, although a variety of other species is also taken (Jonsgard 1982, Larsen and Kapel 1981, 1982. Minke whales near the coast of Iceland have been exploited since early this century (SigUlj6nsson 1982) (Fig. 1). Although they are widely distributed in the coastal waters, particularly off the southwest, southeast and northeast coasts (see e.g. Gunnlaugsson et al. 1988, Sigurj6nsson et al. 1989, SigUlj6nsson et al. 1991, NAMMCO 1998, catches have been taken mainly in the sheltered northwest to northeast Icelandic waters (Sigurj6nsson 1982). According to the most recent abundance estimate, around 56,000 minke whales inhabit the Icelandic continental shelf area during summer (NAMMCO 1998).
Systematic studies on the biology of minke whales off Iceland were initiated in the late 1970's. These studies were mainly focused on age and reproductive biology (SigUlj6nsson  1980, MS 1988, Vikingsson and SigUlj6nsson 1998. Although minke whales are by far the most important cetacean predator in Icelandic coastal waters (S igmj6nsson and Vikings son 1997) information on their diet in the area is very limited.

MATERIALS AND METHODS
This paper summarizes all avai lable data on the diet of minke whales in Icelandic waters. Most of the material was collected in connection with the commercial minke whaling operations, while the remainder was derived from stranded or bycaught animals. As a part of a routine sampling program conducted by the Marine Research Institute (MRI), Reykjavik, minke whales were examined for stomach contents during the whaling seasons 1977 and 1978. The whales were usually landed at the nearest landing site to the catch position some 10 hours after being caught (Sigmj6nsson 1982), or at the two fixed localities where special freezing and flensing facilities are located, i. e. one in West Iceland at Blj{ll1slrekur (see Fig. 2), and the other at Arsk6gssandur, North Iceland. In total, 56 whales were examined in the two seasons July 1977 and June-July 1978 (Table 1),22 males and 34 females. In addition, one female from the 1984 season was sampled by a whaler. After the temporary stop in whaling operations, starting in 1986, minke whales have been sampled by MRI staff as a part of the Institute 's efforts to monitor and sample stranded and by caught cetaceans. Included in the pre ent analysis are 11 minke whales (4 males, 6 fema les and one of unknown sex) that were bycaught in fishing gear or found dead on beaches during 1988-1997.
Usually only the forestomach and the fundic chamber were examined, but sometimes the oesophagus and the mouth were also sampled. Samples were most often taken for later examination, labe ll ed, f ixed and stored in 4% formaldehyde solution. The amount of food was assessed visually as one of six categories (empty, very little, little, much, very much, full) and the prey species preliminarily identified. In some cases, the first author cou ld positively identify the species and no sample was taken , but in most cases the samples were taken to the laboratory for 'close exam ination under magnification. Bones and otoliths were used to identify fish remains when intact animals were not available.
As the stomach analysis from the period 1977-1984 consisted only of species identification, an evaluation of relative importance of different food species based on reconstructed weight is not possible. Therefore the relative contributions of prey items are expressed in terms of: a) The frequency of occurrence i. e. the proportion of the 67 exam ined non-empty whale stomachs in which a particular prey species was found. b) A rough weighted frequency index (WF) giving equal weight to each prey species (group) in stomachs containing more than one prey species. Thus, for each prey item WF was calculated as: where C) is the number of stomachs containing only that particular prey, C 2 is the number of stomachs containing the prey item together with one other prey type and C 3 is the number of stomachs containing two other prey types. The total number of non empty stomachs was 67.

RESULTS AND DISCUSSION
In total 68 minke whales were examined for -  stomach contents. While some animals were caught/found in most parts of Icelandic coastal waters, the majority of the present sample (73.5%) was derived from the area north of Iceland, mostly from the 1978 season ( Fig. 3 and Table 1), The material sampled in connection with the whaling operation was obtained during summer (12 June -26 July), while stranded animals were sampled in the period 12 September - 24 April (Fig. 3). There was also a geographic difference between the caught vs stranded minke whales. Fifty out of the 57 harvested animals were caught off northern Iceland while 9 out of 10 strandedlbycaught animals originated from the waters southwest and west of Iceland.   small sample, a significantly larger proportion of the diet consisted of crustaceans in the northern areas than in the western and southwestern areas, where fish was the dominant food (x 2 = 8.0012; df= 2; P= 0.02). This geographical difference is, however difficult to interpret because of the difference in sampling period (Table 1), sampling season (Fig. 3) and sampling methods (catch vs. stranding).
The food composition is further detailed in Tables 3 and 4. Only 4 species of fish could be  In Icelandic waters the former species is usually confined to fjord and bay areas while the latter is more oceanic in distribution (Astthorsson and Gislason 1990). To our knowledge, M norvegica has only once before been reported as a prey item of minke whales (Nord0y and Blix 1992), but it is predominant in the diet of fin whales (Balaenoptera physalus) off Iceland (R0rvik et al. 1976, Vikingsson 1997. No temporal or spatial differences in occurrence were apparent between minke whales containing the two euphausiid species.  Table 4 shows the frequency of occurrence of the different food species found in the 67 non-empty stomachs, and gives a rough weighted frequency index (WF). If these indices reflect the relative importance of different prey groups, nearly two thirds of the diet consists of fish, while around one third is krill.

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The data are in agreement with other studies showing opportunistic, and predominantly piscivorous feeding habits of minke whales in the North Atlantic, as has been demonstrated off Norway and in the Barents Sea (Christensen 1974, Jonsgard 1982, Haug et al. 1995, 1996,  (Christensen 1974, Jonsgard 1982, Larsen and Kapel 1981, 1982 and off the eastern coast of Canada (Mitchell 1974, 1975, NAMMCO 1998. Among the fish prey in this sample, . sandeel and capelin stand out as predominant species (Table 4). Sandeel was found in 25 stomachs, but distinction between the three Icelandic species of sandeel could be made only in one sample, where the species was identified as A. marinus . Sandeel was by far the dominant prey in the western and southwestern areas, while capelin and krill were more frequently found in animals sampled in North Iceland (Fig. 4). The absence of capelin from stomachs sampled in the western and southwestern areas was to be expected, as the sampling period did not overlap with the migration time of capelin in that area (Vi lhj almsson 1994). Large temporal and spatial variations in diet, apparently related to capelin migration, have been demonstrated for another cetacean species in Icelandic waters, the harbour porpoise (Phocoena phocoena) (Vikingsson and SigUljonsson 1996). One minke whale stomach was almost full of cod (G. morhua) and another contained cod or cod-like species (Gadiformes) together with Euphausiids.
Capelin also seems to be among the most important prey species off Greenland (Christensen 1974, Jonsgard 1982 and Canada (NAMMCO 1998), whereas in Norway, herring is the dominant fish prey (Haug et al. 1995(Haug et al. , 1996. Herring was found in only one stomach in the present study, although this may be an artifact resulting from the lack of overlap of sampling areas/time with herring distribution. Evidently, fish constitutes a major part of the food of minke whales in Icelandic waters. Fish or fish remains were found in 76% of the examined stomachs , while the corresponding frequency for krill was 47%. However, because of the limitations of the sample, both with regard to total sample size, and sampling distribution in time and space, it is difficult to assess the rela-tive importance of different prey spec ies with any confidence. As Kasamatsu and Tanaka (1992) demonstrate for North Pacific minke whales, major changes in the food habits of minkes can take place in a few years time, whi ch they were able to associate with drastic changes in the abundance of the pelagic fis h stocks that were important prey species.
Our knowledge of the feeding habits of minke whales in Icelandic waters is very limited. Further research in this fie ld is of particular importance in light of the large numbers of minke whales in the area, which have been estimated to consume around 2 million tonnes annually (SigUljonsson and Vikingsson 1997). Depending, among other things, on the composition of the diet, this consumption may significantly influence the yield of commercially important fish stocks in Iceland (Stefansson et al. 1997).