Vol. 414: 249–256, 2010 MARINE ECOLOGY PROGRESS SERIES Published September 13 doi: 10.3354/meps08718 Mar Ecol Prog Ser Shark aggregation in coastal waters of British Columbia Rob Williams1, 2,*, Thomas A. Okey3, 4, S. Scott Wallace5, Vincent F. Gallucci6 1Marine Mammal Research Unit, Room 247, AERL, 2202 Main Mall, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada 2The Henry M. Jackson School of International Studies, University of Washington, Box 353650, Seattle, Washington 98195-3650, USA 3School of Environmental Studies, University of Victoria, PO Box 3060 STN CSC, Victoria, British Columbia V8W 3R4, Canada 4West Coast Aquatic, #3 4310 10th Avenue, Port Alberni, British Columbia V9Y 4X4, Canada 5David Suzuki Foundation, 2211 West 4th Avenue, Vancouver, British Columbia V6K 4S2, Canada 6School of Aquatic and Fishery Sciences, University of Washington, Box 355020, Seattle, Washington 98195-5020, USA ABSTRACT: A concentration of pelagic sharks was observed in an area of western Queen Charlotte Sound, British Columbia, during systematic shipboard line-transect surveys conducted (2004 to 2006) for marine mammals throughout coastal waters of British Columbia. Surveys allowed only brief observations of sharks at the surface, providing limited opportunity to confirm species identity. Observers agreed, however, that salmon sharks Lamna ditropis (Lamnidae) were most common, fol- lowed by blue sharks Prionace glauca (Carcharhinidae). Both conventional and model-based dis- tance sampling statistical methods produced large abundance estimates (~20 000 sharks of all species combined) concentrated within a hotspot encompassing ~10% of the survey region. Neither statisti- cal method accounted for submerged animals, thereby underestimating abundance. Sightings were made in summer, corresponding with southern movement of pregnant salmon sharks from Alaska. The previously undocumented high density of these pelagic sharks in this location has implications for understanding at-sea mortality of returning Pacific salmon Oncorhynchus spp. (Salmonidae) and for assessing conservation status of sharks in Canada and beyond. We recommend that a dedicated Canada-US sightings and biological sampling programme be considered, perhaps under the UN Transboundary Species Fishery programme. KEY WORDS: Abundance · Density · Distance sampling · Distribution · Elasmobranch · Line transect · Salmon mortality · Shark · Spatial model Resale or republication not permitted without written consent of the publisher INTRODUCTION caught in unmanaged global fisheries (Stevens 2005). As a generalisation, stock assessments of shark popula- Shark populations in Canada’s Pacific waters are tions are usually not practical except for those few spe- poorly understood, and information on their population cies for which catch-effort data exist. Risk analyses may size and status is required to understand the role of top nevertheless be possible (Aires-da-Silva & Gallucci predators in marine ecosystems and to inform conser- 2007) as a substitute to estimate the probability of pop- vation and management efforts. Status assessments ulation declines below e.g. 50% of pre-exploitation lev- have not been pursued in Canada for many species due els or the estimation of a reference point of interest. Es- to insufficient information. On a global scale, salmon timates of abundance are especially difficult to obtain sharks Lamna ditropis are considered ‘data deficient’ and any information towards this end is valuable. by the International Union for Conservation of Nature Salmon sharks are distributed throughout the sub- (IUCN), and blue sharks Prionace glauca are currently arctic North Pacific Ocean and north temperate Pacific listed as ‘near threatened’ due to large numbers being Ocean waters. They inhabit the Gulf of Alaska and *Email: [email protected] © Inter-Research 2010 · www.int-res.com 250 Mar Ecol Prog Ser 414: 249–256, 2010 even parts of the Bering Sea at least during summer marine mammals and sharks, but predation effects and autumn (Hulbert et al. 2005, Weng et al. 2005, have received relatively little attention due to the 2008, Okey et al. 2007, Gallucci et al. 2008). Individu- challenges in studying them (Groot & Margolis 1991). als tagged in Alaska during July and August migrated A systematic, shipboard, sightings survey was from summer coastal habitat (about 60° N latitude) designed (Thomas et al. 2007) and conducted southward throughout the northeast Pacific Ocean to (Williams & Thomas 2007) to estimate summertime about 28° N latitude and as far south as 22° N latitude abundance and distribution of marine mammals in near Hawai’i (Hulbert et al. 2005, Weng et al. 2005, coastal waters of British Columbia. Line-transect sur- 2008), though migration timing and routes varied con- veys are widely used to estimate abundance of wildlife siderably. Northward return apparently occurs during (Buckland et al. 2001), and have been used recently to spring through autumn along the North American estimate abundance of basking sharks Cetorhinus coast via British Columbia (BC). Tagged migrants were maximus off Canada’s Atlantic coast to inform a con- all female and likely to pup in productive waters south servation status assessment for the species (Campana of the oceanic transition zone (Goldman 2003, Hulbert et al. 2008). Sightings of sharks were recorded in the et al. 2005, Gallucci et al. 2008), including California BC marine mammal survey (Williams & Thomas 2007, coastal areas and offshore areas. In California waters, 2009), but have not been reported previously. Here we neonates apparently feed and grow for 1 or 2 yr, after describe an area of high shark density identified from which they begin to travel northward towards coastal that survey. We present 2 methods for generating ten- BC and Alaska. Little is known about this stage of the tative abundance estimates for sharks from the effort life cycle or the true distribution of pupping. It is possi- and sightings data, and consider hypotheses for this ble that salmon shark populations have increased in observed aggregation including resting, reproductive recent decades in the North Pacific, at least in particu- activity and feeding. Improving our understanding of lar areas (Okey et al. 2007), and they have the potential abundance and diet of marine predators, including to consume large numbers of returning Pacific salmon sharks, is fundamental for conservation and ecosys- (Nagasawa 1998, Hulbert & Rice 2002, Hulbert et al. tem-based management of Pacific marine ecosystems. 2005, DeMaddalena et al. 2007, Okey et al. 2007). Despite the recent opening of a recreational salmon shark fishery in Alaska, basic demographic and related MATERIALS AND METHODS information about the species is unknown, some of which may be more easily or cost-effectively collected Study design and field methods. Shipboard surveys in areas in which sharks are found reliably in high for marine mammals for coastal waters of British Co- numbers. lumbia were conducted during the summers of 2004 to Some BC Coho and Chinook salmon stocks (On- 2006 and spring 2007. Field protocols required ob- corhynchus kisutch and O. tshawytscha, respectively) servers to record all sightings of sharks. The survey fol- appear to have declined relatively suddenly (i.e. since lowed a systematic design, which has been described the early 1990s), while stocks of these species in Wash- previously (Thomas et al. 2007), and observers followed ington, Oregon and California have declined gradually standard line-transect survey field protocols (Williams (i.e. over an 83 yr time period) (based on catch data in & Thomas 2007). Unlike mark-recapture methods, dis- Eggers et al. 2005). Sockeye salmon O. nerka have also tance sampling methods make no assumption about the recently exhibited signs of distress in BC; e.g. the sock- identity of individuals, but rather estimate the average eye returns in the Fraser River were unexpectedly low number of animals in the study area at the time the sur- in 2009. Explanations for declines in BC salmon stocks vey was being conducted. In the unlikely event that a include changes in climate and oceanography (e.g. shark was seen on 2 transects, it is assumed that there is BCMWALP 2002, Brodeur et al. 2006, Battin et al. an equal probability of another shark being missed on 2007, BCME 2007, Crozier et al. 2008), increased both transects (Buckland et al. 2001, p. 253; Thomas et hatchery production (e.g. Hilborn 1992, NRC 1996, al. 2010). Waples 1999), increased open-net farming of Atlantic The survey was completed as planned in the sum- salmon in coastal BC and the associated transmission mers of 2004 and 2005, but not in 2006 due to funding of parasitic salmon lice (e.g. Lepeophtheirus salmonis, constraints. The survey was also completed in May and Caligidae) (e.g. Krkosek et al. 2006, 2007), degradation June 2007, but no sharks were seen during the spring of spawning habitat, poorly managed fisheries, and in- survey, and so that season’s effort was excluded from creased predation. A better understanding of factors the analysis. Consequently, the field effort resulted in a influencing at-sea survival of salmon is needed; this is ‘design-unbiased’ (Buckland et al. 2001) sample for the central question of the ongoing Pacific Ocean Shelf 2004 and 2005, but an unrepresentative sampling de- Tracking Project. At-sea predators of salmon include sign for 2006. Consequently, (1) a conventional distance Williams et al.: Summertime shark