White shark (Carcharodon carcharias) COSEWIC assessment and status report: chapter 7
Population Sizes and Trends
There are no surveys for white sharks in Canadian waters. Most records in Canadian waters come from opportunistic stranding reports, published historical observations, investigative work by the authors, and the occasional reported incidental catch. Observer databases from both coasts were examined for records as were all available fisheries surveys. Effort was made to examine surveys in adjacent jurisdictions.
Information on the global population size of white sharks is sparse, but most sources agree the species is relatively rare compared with sympatric shark species. As an apex predator occupying a mean trophic level of 4.5, population size in a given region is necessarily low (Cortez 1999). Based on a low encounter rate, it is suspected that abundance of white sharks in Canadian waters is less than in neighbouring southern regions.
Off Pacific Canada between 1961 and 2004, 14 white shark records have been collected (Table 1). Most of these records are from stranded animals. Given the low frequency of sightings in Canada’s Pacific waters, it is not possible to estimate a population but it is assumed to be very low. For the eastern North Pacific, from northern Washington to central Mexico, between 1936 and 1984, 116 white sharks were documented (Klimley 1985). In Monterey Bay, California, white sharks were sighted during spotter aircraft surveys between 1948 and 1950; 104 white sharks were seen in 445 flights (average 0.23 per flight); a maximum of 27 white sharks were recorded in any one month (Squire 1967). There are no indicators of abundance anywhere in the North Pacific at present time.
Off Atlantic Canada between 1874 and 2004, 32 white shark observations have been recorded, with only one record over the last decade (Table 1). There are only two records from the Scotia Fundy Observer Program which indicates that white sharks are not regularly caught in Atlantic Canadian waters. In U.S. Atlantic waters, longlining has resulted in significant catches. Baum et al. (2003) reported 6,087 records of white shark in the U.S. pelagic longline database from 1986-2000 extracted from over 200,000 sets. Most of these records (80%) were from areas south of Florida (Areas 2-4; Figure 4). In northern waters adjacent to Canada’s 200 mile limit (Areas 6 and 7; Figure 4), there have been no reported white sharks since the early 1990s. There are no estimates of white shark abundance in Canadian waters but given the low encounter rate in commercial and recreational fisheries, the abundance is likely much lower than in adjacent U.S. waters, particularly in southern latitudes.
Fluctuations and trends
Recent North American Trends
There is a general lack of white shark population trend information worldwide, reflecting their rarity. International longline fishing effort for pelagic species occurring outside of Canada has increased exponentially in the North and South Atlantic over much of the last fifty years but uncertainty remains about the extent of the impact on white shark (Figure 5). The only published study examining trends in white shark abundance is that of Baum et al. (2003), which covers only 14 years (less than one generation) in the northwest Atlantic Ocean.
Figure 4. Map of the Northwest Atlantic showing the distribution of effort in the U.S. pelagic longline fishery between 1986 and 2000, categorized by number of sets (0 to 800+), within the nine areas assessed: 1, Caribbean; 2, Gulf of Mexico; 3, Florida East Coast; 4, South Atlantic Bight; 5, Mid Atlantic Bight; 6, Northeast Coastal; 7, Northeast Distant; 8, Sargasso/North Central Atlantic; 9, Tuna North/Tuna South. Areas were modified from the U.S. National Marine Fisheries Service classification for longline fisheries. The 1,000-m coastal isobath (dotted line) is given for reference. From Baum et al. 2003.
Figure 5. Trend in effort for the North Atlantic longline fleet (1956-1997). Source: ICCAT 2005.
Baum et al. (2003) calculated trend information based on catch per unit of effort (CPUE) logbook data from the U.S. pelagic longline swordfish and tuna fleets in the Northwest Atlantic from 1986 to 2000 (Figure 6). They estimated a decline of 79% in CPUE during this period (95% CI: 59 to 89%) which was based on 6,087 records primarily from the southeastern seaboard of the United States and Caribbean (Areas 1-4; see Figure 4).
The white shark analysis by Baum et al. (2003) has been challenged recently by ten shark biologists in the U.S. (Burgess et al. 2005). Burgess et al. (2005) criticized the Baum et al. (2003) analysis on the basis of several potential shortcomings of the longline data set for estimating trends in shark abundance; however, in a reply Baum et al. (2005) rebutted the main points of the critique and concluded that their trend estimate was robust. For example, a major concern of Burgess et al. (2005) was that that “white sharks” in the pelagic longline logbooks may have included misidentified oceanic whitetip sharks (Carcharhinus longimanus) or any other light-coloured sharks; however, Baum et al. (2005) noted that if the southern area where the mis-identifications were most likely to have occurred was omitted from the analysis, the magnitude of the estimated decline in white shark abundance would actually increase. Burgess et al. (2005) further suggested that pelagic longlining does not adequately sample coastal shark species such as the white shark. Contrary to this statement, Boustany et al. (2002) found that white sharks are more pelagic than was previously thought.
Figure 6. (A) Relative abundance of white shark from 1986-2000 in the entire Northwest Atlantic indicated by an analysis of U.S. commercial longline fishery logbook (decline of 79%); (B) estimated annual rate of change for nine assessment areas and total. From Baum et al. 2003.
White shark populations in several regions show marked reduction from historical abundance. Catches of white sharks in the protective beach meshing program off New South Wales, Australia, show an almost unbroken decline since the commencement of meshing in 1937; CPUE over all meshed regions dropping over 80% between 1950 and 1990 (Reid and Krogh 1992). Pepperell (1992) expressed concern at the disappearance of white sharks from game fishing catches off southeastern Australia in the 1980s and Bruce (1992) noted a decline in white shark numbers in some areas off South Australia. A tagging-resighting study of white shark population in South Australia during five expeditions over 2.5 years revealed a low estimated probability of survival between samples of 0.20 (Strong et al. 1996). Catches in the beach meshing program off Natal, South Africa, are among the highest in the world and CPUE data show a long-term, irregular decline between 1974 and 1988 (Cliff et al. 1989). Finally, the killing of four white sharks off Southeast Farallon Island, California in 1982 resulted in a drastic reduction in shark sightings by researchers stationed at the Island during 1983, 1984, and 1985 (Ainley et al. 1985; Pyle et al. 1996). These results suggest that there may be small localized populations which are less migratory and consequently removal of only a few individuals may have a noticeable effect on local populations.
The relationship between white sharks found in Canadian waters with those found in adjacent jurisdictions is unknown. It is suspected that Canadian waters comprise part of the range of North Pacific and North Atlantic populations and, as such, an increase in the total population will likely result in increased abundance of white sharks in Canadian waters.
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