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Article Excerpt
ABSTRACT. The frequency distributions of bowhead whale (Balaena mysticetus) size classes were studied as functions of year, location, water depth, and date. Whales were classified by size and status as calves, small subadults (non-calves 13 m). Adults include mothers with calves, which were also counted separately. During mid-August to early October of 1982, 1984-86, and 1998-2000, calibrated vertical photography was used to obtain known-scale images of 901 different whales in waters up to 200 m deep between Flaxman and Herschel islands (146[degrees] to 139[degrees] W) in the central Beaufort Sea. Age composition of the whales photographed over all years of our study was calves 6.2%, small subadults 31.4%, large subadults 33.3%, and adults 29.1%. We found proportionally more subadults and fewer adults than are estimated to be in the overall population, and this result was found both before and after making allowance for reduced effort to obtain photographs early and late in the migration period. Thus parts of the central Beaufort Sea up to 200 m deep appear to be more heavily used by subadult bowheads than by adults in most years. Significant interannual variation existed in length-frequency distributions of whales among years, geographic subdivisions of the study area, water depth categories, and time periods. This variation was due to variable use of the study area by each size class in different years, differences in the water depths used by different size classes, and different migration timing by each size class. In all years, small subadult whales were the dominant group in shallow (> 20 m) nearshore habitats, and the size of the whales increased with increasing water depth. Timing of movements into and through the study area were also related to size class: small subadults arrived first in late August and departed in late September, and adults arrived last in late September. Mothers and calves arrived in early September and were common until at least early October.
Key words: bowhead whale, Balaena mysticetus, habitat use, photogrammetry, lengths, feeding, autumn migration, water depth, date, annual variation, productivity
RESUME. La repartition des frequences de la baleine boreale (Balaena mysticetus) en fonction des classes de dimensions a ete etudiee a la lumiere de criteres tels que l'annee, l'emplacement, la profondeur de l'eau et la date. Les baleines etaient classees d'apres leurs dimensions et leur etat, comme suit: baleineaux, petites baleines immatures (non-baleineaux 13 m). Les adultes comprenaient les meres avec leurs baleineaux, qui etaient aussi comptes separement. De la mi-aout au debut octobre 1982, 1984 a 1986 et 1998 a 2000, nous nous sommes servide photographies verticales calibrees pour obtenir des images d'echelle connue de 901 baleines differentes dans des eaux pouvant atteindre une profondeur de 200 m entre lesiles Flaxman et Herschel (146[degrees] a 139[degrees]0), dans le centre de la mer de Beaufort. La composition par age des baleines photographiees au cours de toutes les annees visees par l'etude s'etablissait comme suit: 6,2 % de baleineaux, 31,4 % de petites baleines immatures, 33,3 % de grosses baleines immatures et 29,1 % de baleines adultes. De maniere proportionnelle, nous avons repere plus de baleines immatures et moins de baleines adultes comparativement aux estimations de telles baleines au sein de la population generate, resultat qui a ete trouve tant avant qu'apres avoir tenu compte de l'effort reduit pour obtenir des photographies vers le debut et vers la fin de la periode de migration. Par consequent, certaines parties du centre de la mer de Beaufort ou l'eau atteint une profondeur allant jusqu'a 200 m semblent plus utilisees par les baleines boreales immatures que par les baleines adultes pendant la plupart des annees. Par ailleurs, il existait une variation interannuelle importante sur le plan de la repartition des frequences de longueur des baleines en fonction des annees, des subdivisions geographiques de la region a l'etude, des categories de profondeur de l'eau et des periodes. Cette variation etait attribuable a l'utilisation variable de la region visee par l'etude par chaque classe de dimension au cours des differentes annees, aux differences de profondeur de l'eau utilisee par les differentes classes de dimension ainsi qu'aux periodes de migration differentes de chaque classe de dimension. Dans le cas de toutes les annees, les petites baleines immatures dominaient les habitats peu profonds (< 20 m) en zone cotiere, et la taille des baleines augmentait en fonction de la profondeur de l'eau. Le moment des deplacements vers la region a l'etude et dans celle-ci dependait egalement de la classe de dimension: les petites baleines immatures arrivaient en premier, vers la fin aout et repartaient vers la fin septembre, tandis que les baleines adultes arrivaient en dernier, vers la fin septembre. Les meres et leurs baleineaux arrivaient au debut septembre et y restaient au moins jusqu'au debut octobre.
Mots cles: baleine boreale, Balaena mysticetus, utilisation de 1'habitat, photogrammetrie, longueurs, alimentation, migration automnale, profondeur de l'eau, date, variation annuelle, productivite
Traduit pour la revue Arctic par Nicole Giguere.
INTRODUCTION
Habitat segregation of different size classes of bowhead whales (Balaena mysticetus) in the Bering-Chukchi-Beaufort (BCB) population has been documented in summering areas in the eastern (i.e., Canadian) and central Beaufort Sea and in Amundsen Gulf (Cubbage and Calam-bokidis, 1987; Koski et al., 1988). Those studies, and observations by local residents (Galginaitis and Koski, 2002), indicate that small subadult whales tend to occupy shallow nearshore areas along the Yukon and eastern Alaskan coasts. Large subadults tend to be found farther offshore along those coasts and in shallow waters farther east (i.e., north of the Mackenzie Delta and the Tuktoyaktuk Peninsula). Adults tend to be found even farther east. Bowheads found in Amundsen Gulf are almost exclusively adults (Koski etal., 1988).
Temporal, as well as spatial, segregation by size class has been seen in parts of the BCB bowhead range. Whalers from Kaktovik, along the coast of the central Beaufort Sea, have stated that in late summer, small bowheads arrive in nearshore waters near Kaktovik earlier than larger whales, but that all sizes of whales are seen near Kaktovik over the autumn season (Braham et al., 1984; Galginaitis and Koski, 2002). However, harvest data from Kaktovik do not show any seasonal trend in the proportion of small whales harvested, presumably because whalers selectively harvest small whales (Koski et al., 2005). Temporal segregation also occurs during spring migration past Barrow (Zeh et al, 1993; Angliss et al., 1995; Koski et al., 2006). A large proportion of the whales passing Barrow early in the spring migration period are subadults, and a large proportion of those passing near the end of the period are adults. Females accompanied by recently born calves are among the last whales to pass Barrow in spring (Angliss et al., 1995; Koski et al., 2006).
For the Baffin Bay-Hudson Bay bowhead stock, there is also evidence that in summer, different components of the stocks concentrate in different parts of their overall ranges (Finley, 1990, 2001; Cosens and Blouw, 2003).
In some years, the central Beaufort Sea may be an important feeding area for some individual bowhead whales during late summer and autumn (Lowry, 1993; Lowry et al., 2004). At this time of year bowheads commence their westward migration to overwintering areas (Moore and Reeves, 1993; Mate et al., 2000; Koski et al, 2005), either stopping to feed at times or feeding while traveling (Wtirsig et al., 2002).
This study examines bowhead size data collected in the central Beaufort Sea during late August through early October of 1982, 1984 to 1986, and 1998 to 2000 in relation to year, location within the study area, water depth, and date. It assesses the size classes of bowhead whales found in this area and evaluates whether there was habitat or seasonal segregation (or both) by size class (small subadults vs. large subadults vs. adults) and status (mothers vs. other adults). This information is needed to correct aerial survey data for size-related variation in detectability in different regions and at different times of the late summer-fall period. It is also relevant in evaluating the importance to the different components of the bowhead whale population of specific parts of the study area that may be developed by the oil and gas industry. In addition, a traditional bowhead hunt occurs near Kaktovik, and interactions among the proposed developments, the traditional harvest, and future changes in bowhead use of different parts of the central Beaufort Sea may need to be evaluated. Also, recent studies have found a relationship between sea-ice cover and bowhead whale distribution (Moore et al., 2000; Treacy et al., 2006), so reductions in summer and autumn sea-ice cover due to global warming may result in changes to bowhead distribution. Finally, the bowhead whale population is increasing (George et al., 2004; Zeh and Punt, 2005) and includes a...
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