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Interannual Changes in Body Fat Condition Index of Minke Whales in the Antarctic

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Interannual Changes in Body Fat Condition Index of Minke Whales in the Antarctic MARINE ECOLOGY PROGRESS SERIES Published December 17 Mar Ecol Prog Ser Interannual changes in body fat condition index of minke whales in the Antarctic Taro ~chii'~*,Narimasa Shinohara2, Yoshihiro ~ujise~,Shigetoshi Nishiwaki3, Koji ~atsuoka~ 'National Research Institute of Far Seas Fisheries, 5-7-1 Orido, Shimizu, 424-8633 Japan 2Faculty of Marine Science and Technology, Tokai University, 20-1, Orido, Shimizu, 424-8610 Japan 3The Institute of Cetacean Research, 4-18, Toyorni-cho, Chuo-ku, Tokyo 104-0055, Japan ABSTRACT To study whether or not wide-rang~ngpelagic predators should be affected by localized changes in prey availability, interannual variab~lityin body fat condition index (assessed from girth measurements) of minke whales Balaenoptera acutorostrata was analyzed in relation to their distribu- tion, stomach-content mass and sea-ice extent during the austral summer in the Antarctic Ocean between 130°E and 170°W. The research area comprised offshore, ice-edge and Ross Sea areas. Of the 3 years (1990/91. 1992/93 and 1994/95) included in the study, 1994/95 was a year of significantly poor body fat condition compared with the other 2 years. The 1994/95 year was characterized by extensive sea-ice conditions, covering the usually krill-rich slope region throughout the season. Since minke whales were scarce and their stomach-content mass small in the ice-edge area during 1994/95, food availability in the area during the season was considered to be poor as a result of the high sea-ice extent. Antarctic krill Euphausja superba was regularly the dominant prey species throughout the sur- vey area, although on the Ross Sea shelf E. crystallorophias was the codominant prey with E. superba. The spatial distribution of body size and maturity of E. superba Indicated apparent unsuitability of the Ross Sea area as a nursery and spawning ground for that species. The Ross Sea area was considered to be an area of low food supply throughout the study period, considering the lower level of stomach con- tents. Paradoxically, this area usually contained numerous minke whales, especially pregnant females. In conclusion, minke whales appear to have been affected by considerable interannual variability in food availability in the ice-edge area, i.e. potentially the most krill-rich area along their migratory route. KEY WORDS: Mlnke whale . Balaenoptera acutorostrata Feeding ecology . Body fat condition index Antarctic krill . Euphausia superba . Sea ice - Ross Sea INTRODUCTION There is, however, little information available on this matter in the case of pelagic predators, such as whales, In the Antarctic Ocean, penguins, seals and baleen although speculation exists that they would be less whales depend substantially on Antarctic krill Eu- affected by localized changes in food availability phausia superba for food. Of these animals, land-based because of their greater mobility (SC-CAMLR 1997). predators, such as breeding penguins and fur seals, the Even substantial declines in the overall abundance of foraging ranges of which are restricted to their breed- krill might not present a problem for whales, as long as ing colonies, have been shown to have their foraging at least 1 high concentration of krill existed along their performance, body condition and reproductive success migratory route in the Antarctic (Anon 1997). change in relation to local krill availability (e.g. Bengt- The minke whale Balaenoptera acutorostrata has son 1988, Croxall et al. 1988, Watanuki et al. 1993). been estimated as consuming 95 % of the total biomass of krill consumed by baleen whales in the Antarctic (Armstrong & Siegfried, 1991).Considering that minke whales store energy as fat tissue during the feeding 0 Inter-Research 1998 Resale of full article not permitted 2 Mar Ecol Prog Ser 175: 1-12. 1998 165"E no' W I region between 60" and 71"s was 7--.-S defined as the east offshore area, / : Fast offshore area and the entire Ross Sea region south of 71"s as the Ross Sea area [Fig. l). The JARPA survey was con- ducted from early December to late March. In 1990/91, the research area was surveyed twice (i.e.Legs I and Il), but the Ross Sea area was not covered during Leg 1 due to blockading sea-ice preventing entry (Fig. 2a). In 1992/93 and 1994/95, the entire research area was surveyed once (i.e. Leg 11), with parts of the research area also be- Fig. 1. Map showing the subdivision of the entire research area. (-) Ice-edge line ing surveyed before (i.e. Leg I) and after (i.e. Leg 111) Leg I1 (Fig. 2b, c). season, body fat condition could be a sensitive indicator A saw-tooth type trackline design was applied to of the integrated effects of krill availability during the ensure that there was an equal probability of coverage austral summer. According to Lockyer (1987),the single of every point in each area (Fig. 2). Minke whales were most useful indicator of body fat condition in whales is taken at random (Fujise et al. 1997), their initial sight- girth circumference, measured approximately mid- ing positions being shown in Fig. 2. Samples were length at the position of the umbilicus, because varia- taken only from primary sightings. The '2-whale sam- tions in such reflect not only changes in blubber thick- pling' scheme was used in 1990/91 and the 'l-whale ness, which are relatively small, but also internal sampling' scheme was used in 1992/93 and 1994/95. quantitative changes in muscle and visceral fat. Under the former scheme, all solitary animals detected Under the auspices of the Japanese Whale Research were taken, and 2 animals were taken from schools of Program under Special Permit in the Antarctic 2 or more animals. Under the latter scheme, only 1 (JARPA), girth data of minke whales have been col- whale was taken from any school. The whale sampled lected every year since 1989/90, such data not having was determined by using a random-numbers table. been collected during earlier commercial whaling. In Although 4 and 3 dwarf minke whales were taken in this paper, interannual variations in girth of minke 1990/91 and 1992/93, respectively, only specimens whales are analyzed in relation to their stomach-con- having an ordinary form were used for this study. (The tent mass and distribution, and sea-ice extent, in order dwarf form minke whale is different from the typical to examine whether or not wide-ranging pelagic minke whale in body size, characters of the baleen predators should be affected by localized changes in plates and the dorsal fin [Best 1985, Arnold et al. prey availability. 19871.) A summary of the specimens taken is given In Table 1. Data collected included sex, maturity, body length, girth circumference at the position of the MATERIALS AND METHODS umbilicus, body weight, and stomach-content weight and prey species' composition. The Japanese Whale Research Program under Spe- Females were considered sexually mature if at least cial Permit in the Antarctic (JARPA) covers the entire 1 corpus luteum or albicans was present in the area south of 60"s in IWC Whaling Management Areas ovaries; otherwise they were classified as immature. IV (70" to 130°E) and V (130°E to 170°W), being de- The presence of a foetus was taken to indicate preg- signed so as to repeat surveys in those areas in alter- nancy. The detection of a milk-colored fluid was used nate years. Since Area V showed significant interan- to classify whales as lactating. Sexual maturity for nual changes in sea-ice extent, compared with Area IV, males was determined by examination to histological surveys in the former area (i.e. 1990/91, 1992/93 and examination of testis tissues collected from the center 1994/95 surveys) were chosen for this study. The re- of right testis. Males having seminiferous tubules over search area was divided into west and east areas along 100 pm in diameter (average of 15 to 20 measure- the 165 "E longitude line (Fig. l).In the west area, off- ments) or sperm in the tubules were determined to be shore and ice-edge regions were separated by a line sexually mature; the others were classified as being 45 n miles from the ice-edge limit. In the east area, that immature (Kato et al. 1990). Leg II I '~.~, M)' S \. ,,S' Leg I Fig. 2. Left and right: (a) Cruise tracks during Legs I and I1 in 1990/91. Survey periods were 21 Dec to 31 Jan (Leg I) and 2 Feb to 20 Mar (Leg 11). (b) Cruise tracks during Legs I, I1 and 111 in 1992/93. Survey periods were 3 to 12 Dec (Leg l), 30 Dec to 6 Mar (Leg 11) and 10 to 24 Mar (Leg 111). (c)Cruise tracks during Legs I, I1 and III in 1994/95. Survey periods were 3 to 17 Dec (Leg I), 18 Dec to 14 Mar (Leg 11) and 13 to 18 Feb (Leg 111). (0) Initial sighting positions of minke whales taken. (m)Ice-edge line Mar Ecol Prog Ser 175 1-12, 1998 Table 1 Balaenoptera acutor-ostrata. Reproduct~vestatus of mlnke whales taken by area In each season Season Leg Area Male - Female Total Immature Mature Immature Ovulating Restlng Pregnant Pregnant Unknown and lactLitlnq mature - 1990/Y 1 Leg l [Vest oftshore 5 29 3 1 6 1 4 4 West ~ce-edge 3 3 0 9 1 2 '3 0 1 7 6 East offshore 1 16 2 1 2 2 2 Leg 2 West offshore 3 19 1 1 24 West ~ce-edge 2 1 7 1 14 4 3 East offshore 4 24 1 4 2 3 5 Ross Sea 9 5 9 53 7 9 Total 12 148 28 18 108 323 1992/93 Leg l West offshore 3 9 5 2 19 West lce-edge 3 2 2 3 29 5 8 Leg 2 West offshore 11 2 13 West ~ce-edge 2 3 7 10 24 7 3 East offshore 2 28 10 13 5 4 Ross Sea 1 18 6 4 2 68 Leg 3 West ~ce-edge 1 30 3 8 42 Total 12 155 3 7 120 327 1994/95 Leg l East offshore 2 4 6 1 13 Leg 2 West offshore 7 35 8 4 55 West ~ce-edge 3 68 5 8 85 East offshore 18 4 6 15 3 8 90 Ross Sea 17 9 16 45 87 Total 3 0 170 4 3 19 66 2 330 Body length was measured to the nearest 10 cm from (31 samples in 1990/91, 116 in 1992/93 and 147 in the tip of the upper jaw to the deepest part of the fluke 1994/95) (The initial low sample number reflects notch along a straight line parallel to the body axis.
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