Basking shark (Cetorhinus maximus) in Canadian Pacific Waters: recovery strategy: chapter 13
5. Population and Distribution
Global Population and Distribution
Basking Sharks are found circumglobally in temperate coastal shelf waters but are characterized by localized occurrences (Froese and Pauly 2010). The IUCN Red List assessment has categorized Basking Sharks as Vulnerable globally and Endangered in the north Pacific (Fowler 2005). In the North Pacific, they are observed in China, Japan, Alaska, and British Columbia down through the United States and Mexico (Compagno 2001). Basking Sharks in Canadian Pacific waters are considered to be part of a North American population which migrates into British Columbia waters in spring and summer and winters off California. The northeast Pacific population of Basking Shark occurs primarily in waters under the jurisdiction of Canada and the United States. There are reported catches in international waters and at least two recent catches within Mexican waters. Information on basking sharks in Mexican waters is poorly understood but sharks in these waters are likely part of the same population being addressed in this Recovery Strategy. Basking Sharks in U.S. Pacific waters have undergone a decline of at least the same magnitude as that seen off the coast of British Columbia. Since 1994, Basking Shark sightings have been extremely rare along the Pacific coast for both the Canadian and U.S. waters. Further, since 1993, Basking Sharks have not been observed in groups of more than two or three individuals within Pacific Canadian and U.S. waters. The best estimate of current abundance of Basking Sharks within this range (Canadian and U.S. Pacific waters) is between 321 to 535 individuals (McFarlane et al. 2009). This study estimated the range-wide pre-exploited population (e.g., prior to 1920) to be between 2,790 and 5,116 individuals. It is estimated that the decline from pre-exploited numbers exceeds 90%3. Over the time period of three generations (66-99 years), Basking Sharks have all but disappeared from all areas where they were historically abundant.
Canadian Pacific Waters Population and Distribution
The early historic and scientific record for Basking Sharks is limited but consistently describes the Pacific population of Basking Shark as abundant and widely spread, with a minimum historic population of 750 individuals off the coast of British Columbia (COSEWIC 2007, McFarlane et al. 2009)4. At present, Basking Sharks appear infrequently in Canadian Pacific waters with only 13 confirmed sightings since 1996. These sightings are detailed in Table 2 below, and are mapped in Figure 2 on page 11. Current abundance in Canadian Pacific waters is unknown, but all evidence indicates it is much reduced. It is estimated that some proportion up to the full range-wide population (321-535 individuals) utilizes Canadian Pacific waters on a seasonal, annual, and decadal scale (McFarlane et al. 2009). Further monitoring and research will assist in closing this knowledge gap. A combination of historic entanglement, eradication, and directed harvest is responsible for the decline in abundance (COSEWIC 2007).
Prior to 1970 large aggregations of Basking Sharks were seasonally common and widely distributed in Canadian Pacific waters (Wallace and Gisborne 2006; COSEWIC 2007). Historical records dating back to the early 1900’s have been used to identify three areas with large aggregations of Basking Sharks within the waters off of British Columbia: (1) Rivers Inlet / Queen Charlotte Sound; (2) Clayoquot Sound; and (3) Barkley Sound (see Figure 2) (Wallace and Gisborne 2006). Unconfirmed or smaller groups have been observed historically in numerous other locations in the Strait of Georgia and of the southern banks of Vancouver Island.
It is important to note that sightings of Basking Sharks are only those that are visible at the surface. The percentage of the species’ time spent at the surface is varied and unknown. Surface activity is likely influenced by prey distribution, weather conditions, and reproductive behaviours. Abundance estimates based on daytime surface sightings may under or over-estimate shark abundance by at least 10-fold (Sims et al. 2005). Decline in abundance of Basking Sharks may also be obscured by historical unpredictability in the occurrence and numbers visiting the coastal areas in which they are seen. Further, effects of climate driven changes such as sea surface temperature may also be a factor (Cotton et al. 2005).
|Year||Location||No. sharks observed|
|May 1996||Queen Charlotte Sound||1|
|1999||SW Vancouver Island (Pacific Rim NPR, near mouth of Nitinat)||1|
|Feb 2000||Queen Charlotte Sound||1|
|Mar 2000||Queen Charlotte Sound||1|
|Jul 2002||30 miles SW of Rose Spit (Haida Gwaii)||1|
|Aug 2004||Rennell Sound, west coast Haida Gwaii||1|
|May 2008||Between East Pt. and Pt. Roberts||1|
|Sept 2008||2 mi NW McGinnis Lighthouse (30 mi NW of Bella Bella near Millbanke)||1|
|Sept 2008||Cape Beale (Pacific Rim NPR, SW of Bamfield)||1|
|July 2009||Strait of San Juan de Fuca 1 nautical mile S/SW of Salmon Banks, San Juan Island||2|
|Aug 2009||Mouth of San Juan River, Port Renfrew||1|
|May 2010||Starling Point near entrance to Sydney Inlet, Clayoquot Sound||1|
|Total number of Basking Sharks Observed, 1996-2010||13|
A recovery goal is to see positive growth in the Pacific population of Basking Shark, perhaps reaching the number of observations (average annual kills) recorded for the 1945-1970 period, i.e., 40 per year. A long term recovery goal is to promote the long-term viability of a naturally-producing population. The Basking Shark is a long lived species with a low rate of increase (i.e., generation time of 22-33 years). Using best estimates of current abundance and stock decline, it is assumed that if a breeding population currently exists in the northeast Pacific Ocean, and no further human-induced mortality and changes to existing habitat occurs, that approximately 200 years are needed before population numbers will return to their unexploited rates (McFarlane et al. 2009).
The following population and distribution objectives will guide recovery efforts for Basking Sharks within Canadian Pacific waters:
- Maintain the current abundance of Basking Sharks.
- Attain positive population growth of Basking Sharks within 15-20 years.
- Attain increase in Basking Shark aggregations (two or more sharks).
- Maintain distribution of Basking Sharks.
Recovery potential for the Pacific population of Basking Shark was assessed in 2008 (see McFarlane et al. 2009). Three sets of recovery objectives were presented, which include a) rebuild to 1000 breeding pairs, b) attain 30, 40, 50, and 99% of carrying capacity (assumed equal to pre-exploitation numbers), and c) attain 30, 40, 50 and 99% of initial biomass (assumed to be biomass prior to exploitation). Canada’s efforts, including the activities outlined in this recovery strategy to meet the above objectives, will contribute towards the survival and recovery of Basking Sharks range-wide; however, coordination of research and management activities with the U.S. and Mexican governments is required to meet these range-wide recovery objectives.
3 Based on current estimate of 321-535 individuals for the Pacific population of Basking Shark within Canada and the U.S., measured against the catch history ranges from 3,725-5,925 individuals killed between 1920 and 1979 (McFarlane et al. 2009).
4 Reconstructed from the estimated annual removals from 1945-1970 (40 individuals, i.e., 1000/25 years) coupled with the estimate of annual productivity (r=0.023) (COSEWIC 2007; Smith et al. 1998). 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 or trends in abundance to corroborate this inference (McFarlane et al. 2009).
5 Thirteen confirmed sightings as reported to the Basking Shark Sightings Network, from 1996 through July 2010.
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