COSEWIC Assessment and Status Report on the Basking Shark (Pacific population) in Canada
- Assessment Summary
- Executive Summary
- COSEWIC History, Mandate, Membership and Definitions
- Lists of Figures, Tables and Appendices
- Species Information
- Population Sizes and Trends
- Limiting Factors and Threats
- Special Significance of the Species
- Aboriginal Knowledge
- Existing Protection or Other Status Designations
- Technical Summary
- Acknowledgements, Authorities Consulted, and Information Sources
- Biographical Summary of Report Writers
- Appendix 1: Headlines and Titles of Articles Pertaining to Basking Sharks from Non-scientific Sources
Population Sizes and Trends
- Search Effort
- Abundance and Mortality
- Fluctuations and Trends
- Estimated Total Mortality and Decline
- Rescue Effect
It is important to note that basking sharks have only been enumerated while visible at the surface but the percentage of time spent at the surface is unknown. Surface activity is likely influenced by prey distribution (see Habitat section), weather conditions, and reproductive behaviours (e.g., nose to tail circling). There is no basis to speculate about inter-annual trends in the amount of time a basking shark population spends at the surface.
Although no comprehensive survey data exist for basking sharks in Canada’s Pacific region, extensive boat-based surveys of marine mammals have been conducted in coastal habitat suitable for basking sharks for more than 20 years. Since 2002, the coastal surveys were augmented by offshore surveys along the west coast of Vancouver Island, west and east coasts of the Queen Charlotte Islands, and central coast of the mainland (Ford pers. comm. 2004). In addition, surveillance flights (8 with a marine mammal observer) were conducted mostly in 2002 and 2003, covering all parts of the coast out to 200 nautical miles offshore (Ford pers. comm. 2005). No basking sharks have been observed in any of these surveys.
The Juan de Fuca Express operates along the southwest coast of Vancouver Island between Port Renfrew and Bamfield. Since 1996, at least 1900 passes have been made through these waters during summer months with no records of basking sharks (Gisborne pers. comm. 2006). The water taxi service is operated by a trained marine mammal observer.
In British Columbia there is an extensive marine tourism industry that overlaps with historic areas of basking shark occupancy. An informal telephone and email survey of marine operators (including ferry, transport, and tourism operators), researchers, and educators was undertaken by one of the authors. From these correspondences there were only six recorded sightings since 1973 (not including Clayoquot Sound) (Table 2). The B.C. Cetacean Sightings Network has no records of basking sharks (Pinnell pers. comm. 2004). The main historical areas of large aggregations of basking sharks are Rivers Inlet, Barkley Sound and Clayoquot Sound. If surface-active basking sharks were using these areas they would certainly be noticed and reported due the presence of tourism operators, a biological station, fishing operations, and extensive marine transport networks. Darling and Keogh’s (1994) paper provides a comprehensive list of reliable sightings in Clayoquot Sound and includes 54 basking shark observations from a commercial pilot’s flight log (1973-1992, observations in all but 5 years), and six other observations (1988-1991). All observations were from channels and inlets.
|1973||Gisborne, B. (Juan de Fuca Express water taxi)||Head of Bamfield Inlet, Barkley Sound|
|1979||Stewart, Anne (BMSC, Public Education)||Trevor Channel, Barkley Sound|
|1982||Stewart, Anne (BMSC, Public Education)||Trevor Channel, Barkley Sound||35’ long|
|1984||Watson, Jane (Malaspina University College)||Trevor Channel, Barkley Sound||Present for a week|
|1999||Mitchell, Jim (DFO, South Coast Division)||48 39 50N, 124 50.8W; southwest coast Vancouver Island (off Nitnat)||12’, in 8 m of water|
|2002||Kattler, D. (BC Ferries 2nd Officer)||30 miles SW of Rose Spit (53 43.1 131 18.95)||July|
|2005||Lloyd, Kitty (Naturalist, Bluewater Adventures)||Queen Charlotte Islands|
Since 1996, the groundfish bottom trawl fishery (outside waters-‘Option A’) has been monitored intensively (100% observer coverage on all trips). The number of separate bottom trawl tows has averaged about 18 000 per year yet the database includes only four reliable records of encounters with basking sharks (PacHarvTrawl) (Table 3). There are no known basking shark records in other fisheries surveys, surveillance flights, or catch databases. If basking sharks were present, they would be caught as bycatch in the commercial Pacific hake fishery because both species typically prefer zooplankton-rich waters, yet again no encounters have been recorded from this fishery despite comprehensive observer coverage.
|Fishing Year||Latitude||Longitude||Estimated Weight (kg)||Month||Fishing Depth (m)|
*Record confirmed from photograph taken onboard, not yet available in database.
Source: Pactrawl database.
Considerable effort was made to comprehensively examine historical records to develop an understanding of past abundance (this study; Wallace and Gisborne 2006). Records examined were from scientific sources, newspapers, government records pertaining to the eradication program, commercial harvest, and sport fishing of basking sharks. These records are listed in Appendix 1. The conclusions of this report do not depend on the specifics of the anecdotal and newspaper reports, but these accounts are expected to provide a reliable indication of general abundance and distribution.
Historic and Scientific Record
The early historic and scientific record for the Pacific population is limited but consistently describes basking sharks as an abundant species. In a newspaper article in 1862 J.G. Swan describes basking sharks as “plentiful” and describes the waters along the shores of Juan de Fuca Strait (near Port Renfrew) as “alive” with basking sharks (Appendix 1). Swan (1868) describes basking sharks as “very abundant during the summer and fall” in a journal entry referring to Neeah Bay, Washington. Dawson (1880) published a report on the Queen Charlotte Islands and states that “large sharks abound on the northern and western coast”. Although he does not mention the species of shark, he wrote that the livers produce “a large quantity of oil”, and therefore, it is likely that he is referring to basking sharks. In 1905 there is a brief mention of basking sharks in the British Columbia Fisheries Commission report stating that they are common in Queen Charlotte Sound during the summer months and that they are harmless and can be touched by hand. The first scientific account was by Clemens and Wilby (1935) who describe the basking shark as “common along the British Columbia coast.” There is also one correspondence between Dr. W.A. Clemens with the Chief Supervisor of Fisheries, J.A. Motherwell in April 1935 who noted, while on a fur seal survey, that there were “numerous humpback whales and basking sharks” approximately 25 miles south of Pachena (southwest Vancouver Island coast). Darling and Keogh’s (1994) paper is the only scientific study on basking sharks on Canada’s Pacific coast.
The current abundance of basking sharks in Canada’s Pacific waters is unknown but all evidence indicates it is much reduced. Evidence from historical records shows a wide distribution with several areas supporting localized aggregations numbering in the hundreds or possibly thousands (Wallace and Gisborne 2006). At present, basking sharks appear infrequently in Pacific waters with only six confirmed sightings since 1996 and only ten since 1973 (not including Clayoquot Sound), of which four are from trawl observer records and were likely killed (Tables 2 and 3). Thus, there is no reliable way to estimate the current population size.
To assess basking sharks over three generations requires going back at least 66 years. From 1900 to 1970, basking sharks were regularly found in numerous locations along British Columbia’s coast (see Table 1). Throughout this period they were subject to a commercial harvest, a directed eradication program, incidental catch, and sport harpooning. This study has summarized all known historical records of basking sharks in Canada’s Pacific waters and conservatively concludes that over 1000 sharks were killed between 1945 and 1970 (Appendix 1).
Most information on the commercial fishery for basking shark livers is qualitative from newspaper reports. A newspaper article from 1921 quotes the head of Consolidated Whaling Company who describes how “schools of thousands [basking sharks]” in Alberni Canal were so dense that in July “one of the coastal steamers ran into such a solid school of these big fellows that, packed tightly against the sides of the boat and around her bow, they stopped her completely” (Port Alberni News, August 31, 1921). Fisheries statistics from this era make no reference to basking shark landings as the products of the basking sharks were likely sold and categorized as reduction products such as fish oils, fish meal or fish fertilizer.
Based on economic data and newspaper sources, it appears that the war-era commercial fishery for basking shark liver was likely limited to the years between 1941 and 1947. Newspaper articles from 1946 reported that “several fishboats [sic] in the Bamfield area” were utilizing harpooning techniques in the pursuit for basking sharks (Vancouver Sun 1946). Unfortunately all basking shark landings were lumped together and reported as “Mixed Shark” which comprised brown cat, blue, sleeper, and salmon sharks. Between 1941 and 1945 there was 379 t (841 600 pounds) of mixed shark liver reported in the annual catch summaries. According to newspaper records, each basking shark yielded approximately 450 kg (1000 pounds) of liver. If, for example, 10% of the liver landings were from basking sharks, then approximately 80 sharks may have been processed. There is no reasonable basis to speculate about the numbers of sharks killed.
Fisheries Interactions and Eradication
For much of the last century, basking sharks were considered a nuisance to commercial salmon fishing operations, both gillnetting and trolling. Basking sharks appeared to favour habitats similar to those of salmon (e.g., areas of dense aggregations of zooplankton) and consequently interacted with salmon fishing fleets. Mortality was incurred from both entanglement and directed eradication aimed to reduce the nuisance factor. Reports of four basking sharks in commercial groundfish trawl fisheries since 1996 suggests that bycatch of basking sharks has likely occurred at some level throughout the 70-year history of the trawl fleet.
Following are some brief descriptions taken from newspapers and other reports to assist in describing the interactions and for estimating mortality.
A photograph taken in 1901 in Rivers Inlet is the first verified interaction between a basking shark and the salmon gillnet fleet (BC Archives 2004). In 1942, “hundreds of huge basking sharks” were reported to have caused “thousands of dollars” worth of damage to gillnets in the Rivers Inlet district (Province 1942). In 1943, B.C. Packers responded to this loss by designing the “razor-billed shark slasher”, a specially fitted boat “with a sharp steel ram [that] cuts the sleeping monsters down as they lay on the surface” (Province 1943). Only six sharks were reported in the media to have been killed by this device. In June of 1944 it was reported that “giant sharks [basking] are again annoying sockeye salmon fishermen at Namu” and that the sharks are “much bigger than in other years” (Colonist 1944). In 1947, it was reported that “numbers of huge sharks” were inflicting “heavy damage” on the 100 boats fishing in Rivers Inlet (Province 1947). In July 1947, it was reported that “along the mainland coast…thousands of basking sharks have invaded the waters in the past week” (Colonist 1947). It is unknown if the slashing device was utilized subsequent to 1943. Since 1948 there have been no further records of basking sharks in the Rivers Inlet area. It should be noted that in some years, up to 1200 boats fished the Rivers Inlet fishing grounds. There are several other newspaper references to this area listed in Appendix 1.
Anecdotal and newspaper reports describe locations in the Barkley Sound region with hundreds if not thousands of basking sharks (Appendix 1). Between 1945 and 1969, and possibly earlier, basking sharks were a well known nuisance to Barkley Sound gillnetters (Appendix 1; Table 4). Despite annual calls by fishermen for an eradication program between the years of 1948-1954, it was not until 1955 that the Federal Fisheries Department of Canada actively engaged in an eradication program. From 1955-1969, 413 sharks were killed by a large blade mounted on the bow of a fisheries patrol vessel (Table 4). Prior to the blade method of eradication, shooting and harpooning by patrol vessels was also tried. Concurrent to the blade method, other patrol vessels along the coast were under directive to opportunistically ram basking sharks which may account for an additional 200-300 kills (Fletcher, pers. comm. 2004).
Entanglement was likely the largest source of historical mortality, but the number of kills cannot be accurately quantified. Once a basking shark became entangled in a gillnet, sharks either died or were killed by fishermen in an attempt to salvage their nets (Peterson 1999). In November 1952, Western Fisheries reported that a single gillnetter caught seven basking sharks in that one season alone. One Barkley Sound fishermen recounted “destroying” seven or eight basking sharks over several years (circ. 1950s) that were caught in his net. The same fisherman estimated that there were approximately 150 gillnetters operating in Barkley Sound during this period and therefore many mortalities would have gone unrecorded (Peterson 1999). There have also been reports of entanglement in trolling gear (Table 4). In any given year between 1942-1969) several hundred gillnetters fished in these core areas. Based on historical interpretation by the authors, it is suspected that several hundred sharks (400-1500) may have been killed from entanglement in Rivers Inlet and Barkley Sound from 1942-1969.
|Year||Comments Transcribed from Reports||# killed|
|1949||Basking sharks appeared in Barkley Sound at the start of the sockeye season and did some damage to fishermen’s nets. This year however they did not remain in the area as long as usual and damage was much lighter than it has been for the past few years.|
|1950||Basking sharks appeared in large numbers during the sockeye season and did a great deal of damage to fishermen’s nets.|
|1952||Basking sharks did not appear so numerous in Barkley Sound this year and consequently damage to Sockeye gill-nets was not too serious.|
|1955||Predators as usual inflicted their toll on fish and fishermen, with the basking shark again in the limelight. These sharks appeared in Barkley Sound in late February and remained a menace to gill-nets until June, at which time the bulk of them moved offshore where they hampered trolling operations. After rather futilely attempting to reduce their numbers by harpooning, permission was granted by the Department to have a knife-like weapon installed on the bow of the patrol vessel. This device, after a few strengthening modifications, proved very effective and a total of 65 sharks were killed during the year, evoking many favorable comments from fishermen.|
Basking sharks were again present in large numbers in and off Barkley Sound, causing considerable damage to trolling gear and nets. The knife installed on the FPC “Comox Post” to help combat this menace proved successful, with 65 being destroyed.
|1956||Basking sharks were again present in large numbers in and off Barkley Sound. By use of the shark knife mounted on the FPC “Comox Post”, 105 were destroyed, following which very few reports of net damage were received by fishermen.||105|
|1957||Basking Sharks: Were again present in and off Barkley Sound in quite large numbers, although evidently decreased from the previous year judging by the lighter net damage. Only 7 were destroyed by the use of the knife on the bow of the F.P.C. “Comox Post” due to the fact that the boat was in refit during the time the sharks were most prevalent.||7|
|1958||Basking Sharks: Were again present in quite large numbers but did not show on the surface very often during the hot summer. During October when the sharks were showing the “Comox Post” was in refit. However, during the season the “Comox Post” destroyed a total of 52 with the knife mounted on the bow. Considerable net damage was caused by the sharks during October, and during the sockeye fishing in summer.||52|
|1959||Basking Sharks: Were as usual present in quite large numbers in Barkley Sound during the Spring, Summer, and Fall, and they were destroyed by means of the knife mounted on the FPC “Comox Post” whenever seen. During 1959 a total of 47 were destroyed in this manner. Considerable damage to salmon gillnets were reported throughout the season, mainly during the summer Sockeye fishery in Alberni Inlet.||47|
|1960||Basking Sharks: Were again very numerous in Alberni Inlet and Barkley Sound, causing considerable damage to gillnets. However, as they did not often show on the surface, only eleven were destroyed by the knife mounted on the bow of the FPC “Comox Post”.||11|
|1961||Basking Sharks: Were as usual very numerous in Barkley Sound but, except for May and part of June, they did not often show, whle still doing considerable damage to salmon gillnets. From May 9th to August 10th 32 were destroyed by means of the knife mounted on the FPC “Comox Post”. This was the total for the year.||32|
|1962||Basking Sharks: Were numerous in Barkley Sound, and 20 were destroyed by the knife on the FPC “Comox Post”.||20|
|1963||Basking sharks were destroyed by the knife on the FPC “Comox Post”, and in this manner 37 were destroyed during the year, compared to 21 last year, 32 in 1961, 11 in 1960, 47 in 1959, and 52 in 1958.||37|
|1964||Basking Sharks: Were quite numerous in Barkley Sound during the summer, but none were destroyed due to the absence of the refit of the FPC “Comox Post”, which is the only vessel adapted to carry the shark knife.||0|
|1965||The destruction of basking sharks by the knife-equipped F.P.C. “Comox Post” was only 8, compared to none in 1964, 37 in 1963, 20 in 1962 and 32 in 1961.||8|
|1966||The destruction of basking sharks in the Barkley Sound subdistrict this year was nil. Although the FPC “Comox Post” has the knife located at Ecoole for quick attachment there were no basking sharks reported. For some reason this past year they were not showing at the surface.||0|
|1967||21 basking sharks were destroyed in Barkley Sound by the Departmental personnel using the Comox Post Shark Knife attachment. Three nets were destroyed by these fish in 1967. One was a total loss, and the other two were 60% losses.|
Several gillnets were damaged by basking sharks in the early part of the season. 21 sharks were destroyed in two days by the FPC “Comox Post”. No damage to nets was reported after that date, and sightings of the animals decreased considerably.
|1968||Trollers and one gillnetter reported basking sharks tangling up and destroying their gear on May 17th. The shark knife was installed and eight basking sharks were destroyed on May 22nd. The sharks then moved out of the area.||8|
|1969||No control program was carried out on the basking shark population. Six reports were received of nets being damaged and two nets were completely destroyed. One shark was strangled in a gillnet. The shark knife was installed on the FPC “Comox Post” with “nil” results.||0|
Darling and Keogh (1994) provide a thorough report of basking shark behaviour, abundance and distribution in Clayoquot Sound using observations from 1973-1992. Aerial sightings and other anecdotal reports indicate that basking sharks were in the Sound throughout most of the 20-year period investigated. A single summer of photographic identification work undertaken in 1992 resulted in 27 individuals being photo-identified. Many of the sharks had wounds from propellers which was supported by observations of the sharks seemingly being attracted to boat propellers. The following summer resulted in a few sporadic sightings (not reported) and since 1994 there have been no confirmed sightings from Clayoquot Sound (Darling pers. comm. 2003). Their disappearance coincides with the rapid development of salmon aquaculture in the region but there is no formal evidence to link these two events (Darling pers. comm. 2003). In Clayoquot Sound the salmon farms make large netted pens perpendicular to the coastline within the areas where basking sharks were common for centuries. According to Darling (pers. comm. 2003) basking sharks are easily entangled in anything, even a single line going to a prawn trap, and therefore would be vulnerable to entanglement in the net pens of a salmon farm. Official replies from salmon farm companies state that entanglements have not occurred but unofficial communications from employees suggest otherwise (Darling pers. comm. 2005).
It is possible that their sudden disappearance from Clayoquot Sound was caused by a shift in distribution but there are no records of their reappearance in other locations. Jordan (1887), who wrote extensively on whaling, noted that in Monterey Bay the basking shark was sometimes not seen for 20 years.
It is not possible to estimate the number of basking sharks killed for sport in the 1940s through to the mid-1960s as the only written records are from newspaper stories (Table 5). In the 1940s, the sport of harpooning basking sharks had acquired enough interest that the Canadian Pacific Railway promoted fishing for BC basking sharks in publicity releases (Colonist, September 27, 1953). One newspaper article describes a person harpooning ten and landing five in a single day around the waters off Texada Island in June (Province, June 7, 1947). Sport kills for basking sharks, which includes all forms of ‘recreational’ killing and harassment leading to death was likely in the multiple hundreds (50-400).
There is great uncertainty associated with trying to interpret and quantify historical information. In this report it has been estimated that the total number of sharks killed (1945-1970) by eradication is 413, other patrol/eradication methods (200-300), entanglement (400-1500), and sport kills (50-400). This results in a range of kills between 1000-2600. Note that this estimate does not include commercial kills for liver oil.
|1947||Texada Island||5 killed, 5 misses, 2 unknown||June||Province, June 7 1947 p. 5|
|1952||Brentwood Bay||School of basking sharks||July||Times Colonist, July 5 1952 p. 11.|
|1952||West Coast VI||3 killed||July||Times Colonist, July 5 1952 p. 11|
|1956||Saanich Inlet||1 killed||April||Colonist, April 20 1956 p. 1|
|1957||Qualicum Beach||“Considerable numbers”||June||Colonist, June 5 1956 p. 13|
A minimum historical population of 750 individuals can be reconstructed from the estimated annual removals from 1945 to 1970 (Nm= 40 individuals, i.e., 1000/25 years) coupled with the estimate of annual productivity (r=0.023, see Lifecycle and Reproduction section). In other words, at a mortality rate of 40 animals per year, it would take 25 years for an initial population of 750 to be diminished to zero assuming r=0.023. Note that there is no reliable information on trends in abundance to corroborate this inference.
There is little doubt that basking sharks once frequented British Columbia’s coast in numbers and distribution much larger than found today (Wallace and Gisborne 2006). For most of the historical record, basking sharks were regularly encountered by mariners. They were described as being plentiful and common in several early descriptions. The disappearance of local aggregations coincides with known sources of human-caused mortality, which is consistent with experiences from other regions in the world. Where basking shark populations have been observed, annual number of records is at most in the low thousands (Squire 1967; Compagno 2001). The small local occurrence of basking sharks in Clayoquot Sound was the last known aggregation in British Columbia.
There are very few recent surface (i.e., visual observations) or subsurface (trawl observer data) records, suggesting that basking sharks are presently rare in British Columbia waters. Overall, it appears that the population coastwide has declined from a minimum of 750 individuals to virtually none within a span of 60 years (2-3 generations).
The only other area known for basking shark aggregations in the eastern north Pacific is central and southern California. Basking sharks in British Columbia and California sharks may belong to one population based on convincing data for the seasonal disappearance of basking sharks from California waters between May and July, just as they reappear in relative abundance in waters off BC (Squire 1967, 1990). Within California waters, two areas in particular were known to have high concentrations, Monterey Bay and Pismo Beach (Squire 1967). Squire (1990) notes that there was a greater abundance of basking sharks observed prior to 1970 based on extensive aerial survey data. Humans, both in Canada and California, have inflicted significant mortality upon the Pacific populations. In California, the basking shark fishery developed in earnest in 1924 as a sport. Between 1924 and 1938 an average of 25 sharks per year were killed with close to a 100 being landed in one year. Starting in 1946 the fishery targeted basking sharks for their livers, 300 sharks were taken in the first season. Efficiency in capturing sharks was improved by using aerial surveillance to spot the animals (Phillips 1948). The number of California kills from the late 1940s is estimated at 200/year (Roedel and Ripley 1950). It is generally accepted that the fisheries of the 1940s and 1950s reduced the populations substantially, and that the species has never recovered (UK CITES proposal 2002, Van Sommeran pers. comm. 2004). The concurrent disappearance in waters off both California and British Columbia is further evidence of a single population.
Overall, the likelihood of a rescue effect from U.S. waters is considered low. First, abundance in US waters is also depleted. Second, no rescue effect has been evident to date, 30 years after directed killing ceased. The population structure of basking sharks may be more complicated than is presently inferred from seasonal migration. For example, a similar inference about population structure in Pacific salmon based only on the seasonality of individual movements would be seriously in error.
- Date Modified: