Vol. 26: 103–113, 2014 ENDANGERED SPECIES RESEARCH Published online December 1 doi: 10.3354/esr00630 Endang Species Res OPENPEN ACCESSCCESS Sei whale movements and behaviour in the North Atlantic inferred from satellite telemetry Rui Prieto1,*, Mónica A. Silva1,2, Gordon T. Waring3, João M. A. Gonçalves1 1Departamento de Oceanografia e Pescas, IMAR and LARSyS Associated Laboratory, Universidade dos Açores, 9901-862 Horta, Portugal 2Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543-1049, USA 3NOAA Fisheries, Northeast Fisheries Science Center, 166 Water Street, Woods Hole, Massachusetts 02543-1026, USA ABSTRACT: The stock structure of the sei whale Balaenoptera borealis in the North Atlantic is unknown, despite years of commercial hunting. New and up-to-date data on distribution and movements are essential for the creation of plausible hypotheses about the stock structure of this species. Between 2008 and 2009 satellite tracks of 8 sei whales were obtained, 7 during spring and 1 in late September. Using a hierarchical switching state-space model we investigated the move- ments, behaviour and the role of distinct areas in their life history. Two distinct phases correspon- ding to migratory and foraging movements were identified. A migratory corridor between the Azores and the Labrador Sea is clearly identifiable from the data. Behaviour consistent with for- aging was observed frequently in the Labrador Sea, showing that it constitutes an important feed- ing ground. A link between the Labrador Sea and other feeding grounds to the east is deemed likely. The data also support a discrete feeding ground in the Gulf of Maine and off Nova Scotia. A possible link between the feeding grounds in the Labrador Sea and wintering grounds off north- western Africa is proposed. KEY WORDS: Migration · Satellite tracking · Marine mammal · Stock structure · Labrador Sea · Azores · Whale ecology · Sei whale INTRODUCTION taken in the North Atlantic, all of them under scien- tific and aboriginal subsistence permits (Prieto et al. The sei whale Balaenoptera borealis can be found 2012). in all oceans, predominantly in deep waters or near Currently there are no abundance estimates for the the continental slopes of temperate to subpolar entire sei whale population in the North Atlantic. waters. The species was heavily hunted worldwide, Existing estimates from restricted areas indicate that especially in the North Pacific and Southern Ocean the present population in the North Atlantic may be (Horwood 1987), leading to a drastic population de- over 10 000 animals (Prieto et al. 2012). cline (Reilly et al. 2008). After the introduction of the moratorium on com- In the North Atlantic nearly 17 000 sei whales were mercial whaling, the Scientific Committee of the IWC killed in the period between 1885 and 1986 (Prieto et developed a new robust management procedure, al. 2012). In 1982 the International Whaling Commis- coupling politically set management goals with the sion (IWC) adopted a moratorium on commercial scientific aspects of management (Punt & Donovan whaling that came into effect in 1986 (Gambell 1993). 2007, Cooke et al. 2012). This management proce- Since that date fewer than 100 sei whales have been dure is called the ‘revised management procedure’ in © Prieto, Silva and Gonçalves 2014. Open Access under Creative *Corresponding author: [email protected] Commons by Attribution Licence. Use, distribution and repro- duction are un restricted. Authors and original publication must be credited. Publisher: Inter-Research · www.int-res.com 104 Endang Species Res 26: 103–113, 2014 IWC jargon, but is also known as ‘management 1991). Following the adoption of those boundaries the strategy evaluation’ in other wildlife management discussion about the stock structure of the North applications (Smith et al. 1999, Milner-Gulland et al. Atlantic sei whale within the IWC ceased, despite re - 2010). cognition that the issue was not settled (Donovan One of the primary features of the revised manage- 1991). ment procedure is that it explicitly takes into account Data on migration behaviour are also deficient. Ex- uncertainty from several sources (Punt & Donovan isting theories suggest that migration on a north-south 2007). Performance of a management strategy in the axis, on both sides the ocean basin. In the western face of uncertainties is evaluated through simulation North Atlantic, Mitchell & Chapman (1977) suggested trials considering, e.g. plausible alternative hypo - that whales belonging to the purported ‘Nova Scotia theses about the underlying resource dynamics (Punt stock’ migrate along the North American shelf-break & Donovan 2007, Milner-Gulland et al. 2010). A con- from and to unidentified wintering grounds at lower siderable part of that uncertainty may reside in issues latitudes. Ingebrigtsen (1929) suggested that sei of stock structure and the temporal and spatial varia- whales in the eastern North Atlantic migrated be- tion in the mixing of stocks in the areas being evalu- tween wintering grounds somewhere off northwest- ated (Punt & Donovan 2007). ern Africa and feeding grounds around Iceland and Thus, application of the revised management pro- Scotland and in the Norwegian Sea. To the best of our cedure depends on identifying a set of plausible knowledge, there is no migration theory for the pur- alternative hypotheses of stock structure and on a ported ‘Labrador stock’. Olsen et al. (2009) presented good understanding of the patterns of migration, dis- data on 1 sei whale equipped with a satellite tag off persal and mixing of animals from different breeding the Azores islands that moved to the Labrador Sea, stocks on the feeding grounds. showing for the first time that North Atlantic sei Recently, the Scientific Committee of the IWC started whales are capable of cross-ocean movements. to review the data for North Atlantic sei whales to In 2008 and 2009, we equipped sei whales with determine whether sufficient data are available for satellite tags off the Azores islands with the goal of application of the revised management procedure investigating (1) medium- to large-scale movements, (Anonymous 2014). (2) migratory routes and migratory destinations and Evidence for biological stocks or sub-stocks (sensu (3) habitat use. We used a Bayesian hierarchical Jackson & Pampoulie 2012) of sei whales in the North switching state-space model (hSSSM) (Block et al. Atlantic is feeble (Donovan 1991). It has been hypo- 2011, Jonsen et al. 2013) to model surface positions thesized that 2 stocks occurred simultaneously in the and estimate movement parameters in order to infer summer in the Canadian Atlantic, termed the ‘La bra - migratory and foraging behaviours within whale dor stock’ and the ‘Nova Scotia stock’ (Mitchell & tracks. The results of this study are reported here. Chapman 1977). Sei whales from the eastern North Atlantic were considered to belong to another stock (Donovan 1991). A link between the whales from the MATERIALS AND METHODS ‘Labrador stock’ and the Denmark Strait was held to be unlikely but possible by Mitchell & Chapman Data collection (1977); however it is not clear if they thought the whales could belong to the same biological stock. Satellite-monitored platform transmitter terminals Schmidly (1981) suggested that whales belonging to (PTT) housed in surgical grade stainless steel (Model that purported stock were limited to waters around SPOT5-implantable; Wildlife Computers) were im- Newfoundland and Labrador and also suggested that planted in 16 sei whales off Faial and Pico Islands, in another stock could exist in the Caribbean/Gulf of the Azores archipelago, Portugal (38°N, 28°W; Fig. 1). Mexico, but no supporting evidence was provided for Whales were tagged from May to June in 2008 and either of those theories. 2009, except for 1 whale that was tagged in late Sep- To address management needs, 3 management tember 2009 (Table 1). Tagging procedures were units were adopted by the IWC for the North Atlantic similar to those described in Olsen et al. (2009) and sei whale in 1977 (Fig. 1). The boundaries of those Silva et al. (2013). Tags were deployed from a 12 m management units were apparently designed to fiberglass boat using a compressed air gun (Model accommodate existing whaling operations and to ARTS/RN; Restech), similar to that described in match boundaries of ICES fishing areas rather than Heide-Jørgensen et al. (2001), and set at 10 to 13 bar trying to reflect actual population structure (Donovan pressure. All tags were cleaned with 70% ethanol to Prieto et al.: Sei whale movements in the North Atlantic 105 Fig. 1. Balaenoptera borealis. (A) Sei whale tracks derived from raw ARGOS locations. Tagging locations in the Azores are shown in the inset. International Whaling Commission stock boundaries for the species in the North Atlantic are shown as nar- row lines. (B) Hierarchical switching state-space model-derived locations of sei whales showing inferred behavioural modes. The thick, blue line is a schematic representation of the main branch of the North Atlantic Current, showing the quasi-stationary large meander known as the ‘Northwest Corner’ and referred to in the text. ARS: area-restricted search; NS: Nova Scotia; NF: Newfoundland; FC: Flemish Cap remove contaminants from manufacture and han- In 2008 the tags were programmed to transmit dling and then sterilized under UV light and stored in every hour of the day, every other day, in an effort to sterilized sealed bags until use. Prior to implantation, prolong battery life. In 2009 no duty cycle was the tip was covered with gentamicin sulphate cream applied to the tags, since the experience gained in to act both as antibiotic and lubricant. Tags were the previous year demonstrated that battery duration attached to the back of the individuals, anterior to the was not critical for deployment longevity. All tags dorsal fin, with a 4-bladed tip and held in place with were programmed to transmit a maximum of 500 4 sets of barbs and 6 backward-facing petals.