BIOLOGICAL OBSERVATIONS ON A DWARF MINKE WHALE , BALAENOPTERA ACUTOROSTRATA , CAUGHT IN SOUTHERN BRAZILIAN WATERS , WITH A NEW RECORD OF PREY FOR THE SPECIES

In the late austral spring, a 3.43m long immature male dwarf minke whale (Balaenoptera accutorostrata) was accidentally caught in a gillnet set in waters 143m deep, close to the continental slope off southern Brazil. A brief description of the specimen, including external measurements, colour patterns, reproductive status and skull measurements, is presented. The stomach was full of the euphausiid, Euphausia similis, and was heavily infested by anisakid nematode parasites. Euphausia similis is reported for the first time as a prey species for dwarf minke whales. Resumo – Durante a primavera austral, um macho imaturo de baleia-minke-anã, Balaenoptera accutorostrata, medindo 3,43m, foi capturado acidentalmente em uma rede de emalhe armada em águas de 143m de profundidade, nas proximidades do talude continental sul do Brasil. Se descreve, brevemente, aspectos biológicos do exemplar, incluindo medidas externas, padrões de coloração, estágio reprodutivo e medidas cranianas. O estômago estava repleto de eufausiáceos, Euphausia similis, e infestado por parasitas nematóides. Este é o primeiro registro de Euphausia similis como presa de baleia-minke-anã.

Until recently, the minke whale was considered to be a single species, Balaenoptera acutorostrata, originally described from the northeastern Atlantic Ocean but considered to have a circumglobal distribution in northern and southern hemispheres.In the mid 1980s, two forms were recognised in the Southern Hemisphere based primarily on body and baleen colouration as well as some biological parameters (Best, 1985;Arnold et al., 1987).The first form corresponded to Balaenoptera bonaerensis, described from Argentina by Burmeister (1865), and documented from commercial whaling catches in the Antarctic (Williamson, 1959;Kasuya and Ichihara, 1965;Doroshenko, 1979), South Africa (Best, 1982) and Brazil (da Rocha and Braga, 1982).Despite earlier skepticism, B. bonaerensis has now generally been recognised as a valid species, referred to as the Antarctic minke whale (Rice, 1998;IWC, 2001).The second form was a previously unrecognised diminutive form, now more popularly called the dwarf minke whale.Based on both morphological (Best, 1985;Arnold et al., 1987;Zerbini and Simões-Lopes, 2000) and molecular (Wada et al., 1991;Pastene et al., 1994) data, the dwarf minke whale has been considered more closely related to the Northern Hemisphere B. acutorostrata than to the Antarctic minke whale, but its taxonomic status remains unclear, pending further study (IWC, 2001).
Although some biological data were collected from individuals caught during commercial (da Rocha and Braga, 1982;Best, 1985) and scientific whaling (Kato et al., 1990), or through accidental capture (Marsh, 1985;Arnold et al., 1987), the biology of the dwarf minke whale remains poorly documented.In this report, we present new information based on a fresh specimen, accidentally caught in fishing gear.

Specimen description and discussion
On 25 November 1992, a 3.43m long young male dwarf minke whale (Fig. 1) was incidentally caught in a gillnet set for sharks at a depth of 143m.The net was 6,000m long and 4m high, with 30cm stretched mesh size.The capture occurred south of Rio Grande (33 o 35'66"S -51 o 29'80"W), Rio Grande do Sul State, southern Brazil.It is not known, however, whether the animal was caught when the net was set at the bottom or during the setting/ hauling procedure.The animal was brought by fishermen and landed while still fresh at a fishing facility of Rio Grande.Subsequently, the whale was transported to the Museu Oceanográfico "Prof.Eliézer C. Rios", where it was dissected.External colour patterns were examined, organs were collected and weighed, stomach contents, including parasites, were preserved.After cleaning and preparation, the skeleton was deposited in the marine mammal collection (MORG 0096) of the Museu Oceanográfico.
External morphology and colouration: External measurements and some organ weights are presented in Table 1.Measurements were comparable to dwarf minke whales from other regions (Best, 1985;Arnold et al., 1987).The more anterior position of the dorsal fin observed in the present specimen support the hypothesis that this feature may have taxonomic value to distinguish the dwarf minke whale from other minke whale types.Colour patterns were also similar to the ones described for the dwarf minke whale by Best (1985) and Arnold et al. (1987).The base of the flippers was white and connected to a nearly rounded white blaze on the shoulder (Fig. 1a).The dark pigmentation extended from the back onto the area between the eyes and the anterior insertion of the flipper, continuing as a dark throat patch which covered the ten uppermost ventral grooves just under and behind the angle of the mouth (Fig. 1a).However, this specimen presented a striking dark lateral wave-like patch located in the flank of the whale (Fig. 1a), extending from just before the anterior insertion of the dorsal fin almost to the ventral grooves (Fig. 1b).A similar feature was observed in another young dwarf minke whale collected in Australia (Paterson, 1994).
Not all baleen plates were kept so an accurate count can not be provided.The colour of the preserved plates was similar to the ones reported for dwarf minke whales collected in South Africa and Australia (Best, 1985;Arnold et al., 1987).Most plates were light yellowish and a few presented a narrow external dark border.This border's breadth corresponded to up to 5% of the width of the plate.
Skeleton: Cranial measurements are presented in Table 2, while pictures of the skull were illustrated by Zerbini et al. (1996, Fig.3).The following cranial characters were observed in the present specimen: the parietals were incorporated in the vertex and were in contact with the interparietal.The latter was exposed and presented a somewhat rhomboidal shape.The anterior border of the supraoccipital was curved posteriorly in the midline of the skull and the hamular process of the pterigoyds was narrow and elongated.These were previously described as diagnostic for dwarf minke whales (Arnold et al., 1987;Zerbini and Simões-Lopes, 2000).The vertebral formula was C7-T11-L10-Ca18.The vertebral epiphyses were not fused.
Reproductive status: The testes were very small.Both measured 9.3cm of length and were clearly undeveloped.The size of the specimen, i.e. 3.43m, and external analysis of the testes indicated its immature status.Information from animals caught in whaling activities suggests that male dwarf minke whales reach sexual maturity at approximately 6m (e.g.Best, 1985;Kato, 1991).
Parasite infestation: Despite the small size of this animal, its stomach was heavily infested by at least two nematode species of the genera Anisakis (about 3%) and Pseudoterranova (about 97%).Further analyses, including molecular methods, may help determine the species identification of these nematodes.No parasites were found either in the intestines or any other organ though no fine search was performed.The genus Anisakis has also been found in the stomach of an adult dwarf minke whale stranded in New Zealand (Dawson and Slooten, 1990).Studies on North Pacific minke whales (Araki et al., 1997) and Antarctic minke whales (Dailey and Vogelbein, 1991) indicated only Anisakis in the stomachs.There is an early record of Pseudoterranova decipiens from Northern Hemisphere minke whales (from Delyamure, in Tomilin, 1967) but the accuracy of this information is uncertain given the complex taxonomy of Anisakidae parasites.Nematode parasites of the family Anisakidae, including the genera Anisakis, Contracaecum, Pseudoterranova (= Terranova, Phocanema, Porrocaecum) are composed of several complexes of sibling species and are the nematodes most commonly reported from the stomachs of cetaceans (Dailey, 2001).Heavy infections of these parasites rarely cause clinical signs, but can result in gastritis and ulceration (Dailey, 2001).

Diet:
The stomach was almost full of Euphausia similis, suggesting that dwarf minke whales may wean at much smaller sizes than Antarctic minke whales, given that the latter measure approximately 2.8m at birth.This euphausiid is considered an important prey for balaenopterid whales (Kawamura, 1980).Despite the extensive review on the food of baleen whales around the world (Kawamura, 1980) and specifically in the Southern Ocean (Kawamura, 1994), this euphausiid species has not previously been reported in the diet of dwarf minke whales.Dwarf minke whales taken in the Antarctic fed primarily on myctophid fishes (e.g.Kato et al., 1989).
In the western South Atlantic E. similis seems to occur from the Abrolhos Bank region (ca 16 o to 19 o S) northeastern Brazil to the Uruguayan continental shelf (ca 34 o to 35 o S) (Ramirez, 1971;1973;Tôha, 1981;Freire, 1991;Gorri, 1995;Muxagata, 1999).According to Gorri (1995), this euphausiid has its centre of dispersion in warm waters and is an indicator species of the influence of the warm Brazil Current in the southern Rio Grande do Sul State, southern Brazil (Gorri, 1995).In this area, E. similis is abundant, representing 40-60% of the zooplankton captured in sampling nets in the spring and summer (Gorri, 1995).E. similis occurs in neritic waters (Tôha, 1981;Muxagata, 1999), where sightings of dwarf minke whales have been reported in recent years (Hassel et al., 2003;Zerbini et al., 1997).The more neritic distribution pattern of dwarf minke whales had previously been well documented off South Africa (Best, 1985) and off eastern Australia (Arnold, 1997).Since E. similis typically inhabits mild and warm waters, its lack in the checklist of prey species in the minke whales' diet probably is because most data come from animals caught in high latitudes.However, the species is not included among the euphausiids found in the stomach of minke whales in medium latitudes from waters off Durban, South Africa (Best, 1982), which were: E. diomedea, E. lucens and E. recurva.The "krill" (certainly not E. superba) found in at least 3% of the analysed stomachs of minke whales caught during whaling activities in northeastern Brazil (Williamson, 1975) is probably not E. similis as the whales were killed mostly in oceanic waters and further beyond the northern limit of this eupahusid range.Although it is already known that balaenopterids may feed outside their regular feeding grounds in high latitudes whenever plenty of prey is available (e.g.Kawamura, 1975;1980), this record of E. similis is the only data available on food of dwarf minke whales in mid latitudes.No records exist for the South African specimens (P.Arnold, pers. commn) and whales seen at low latitudes in Australia have not been seen feeding (Birtles et al., 2002).Records of juveniles of dwarf minkes throughout much of the year along the Brazilian coast suggest that at least some individuals do not migrate to Antarctic waters (Zerbini et al., 1996;1997) and may explore locally high productive coastal areas off eastern South America (e.g.Hassel et al., 2003).

Figure 1 .
Figure 1.External colour patterns (A) and details of the wave-like flank patch (B) of a dwarf minke whale incidentally captured in southern Brazil.

Table 2 .
Skull measurements of a juvenile male dwarf minke whale, Balaenoptera acutorostrata, incidentally caught in a gillnet in southern Brazil.