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Recovery Strategy for the Basking Shark (Cetorhinus maximus) in Canadian Pacific Waters

4. Threats

The Pacific population of Basking Shark is threatened by various anthropogenic sources. Four classes of current threats have been identified in this Recovery Strategy, which are entanglement, collision with vessels, harassment from marine based activities, and prey availability. Historic threats included entanglement, eradication efforts, and directed fisheries.  The influence of some or all of these current threats may affect normal behaviour, habitat use, or result in direct mortality. Both past and present threats identified in Canadian waters are also relevant in the U.S. and Mexico component of their range. Basking Sharks elsewhere in the North Pacific, including the coastal regions of Asia, have also experienced decline thought to be primarily due to direct exploitation. Basking Sharks were also vulnerable to entanglement in the North Pacific high seas drift net fisheries; while the squid fishery was banned in 1992, Basking Sharks may continue to be susceptible to other high seas drift fishing (McFarlane et al. 2009). It is unknown how much the northeast Pacific population is linked to the mid and northwest Pacific populations. As this population is migratory throughout Canada, U.S. and Mexican Pacific waters, the need for international collaboration is apparent.

4.1 Threat Classification

Table 1: Threat Classification Table

The following threats have been identified and ranked in terms of significance, with the greatest threat to the survival of the species appearing at the top of the table.  Current and historic threats have been identified under separate headings.  It is to be noted that only current threats were ranked.  Historical threats are identified due to the impact on the population, but have not been included in the ranking system as they currently have no level of concern to the present population.  Threats have been identified based on best available data, such as the COSEWIC Status Report (COSEWIC 2007), the Recovery Potential Assessment (McFarlane et al. 2009), as well as new or updated information.  A detailed description of each threat is provided following the table. Appendix C provides further explanation of threat attributes.

Current Threats
 1 EntanglementThreat Attributes
Threat
Category
Accidental mortality ExtentWidespread
 LocalRange-wide
General
Threat
Fishing activity/aquacultureOccurrence Current
Frequency Recurrent
Specific
Threat
Entanglement, bycatch, change in behaviourSeverity High
Causal Certainty High
StressReduced population size/viability, local extinctions, small populationLevel of ConcernHigh
 2 Collision with vesselsThreat Attributes
Threat
Category
Accidental mortalityExtentWidespread
 LocalRange-wide
General
Threat
Marine transportationOccurrence Current
Frequency Recurrent
Specific
Threat
Collision with vessels, behavioural or life cycle disruption, damage or injury to individualsSeverity Unknown
Causal Certainty Low
StressReduced population size/viability, reduced fitness, injury, mortalityLevel of ConcernMedium
 3 HarassmentThreat Attributes
Threat
Category
Disturbance or harmExtentWidespread
 LocalRange-wide
General
Threat
Marine based activitiesOccurrence Current
Frequency Recurrent
Specific
Threat
Behavioural or life cycle disruption, damage or injury to individualsSeverity Unknown
Causal Certainty Low
StressBehavioural changes, reduced productivity, increased natural mortalityLevel of ConcernLow
 4 Prey availabilityThreat Attributes
Threat
Category
Climate and natural disastersExtentWidespread
 LocalRange-wide
General
Threat
Climate and oceanographic changeOccurrence Unknown
Frequency Unknown
Specific
Threat
Reduced foraging habitat and prey availabilitySeverity Unknown
Causal Certainty Medium
StressReduced productivity, increased natural mortalityLevel of ConcernLow
Historical Threats
 - Entanglement (historical)Threat Attributes
Threat
Category
Accidental mortalityExtentWidespread
 LocalRange-wide
General
Threat
FishingOccurrence Historic
Frequency Continuous
Specific
Threat
EntanglementSeverity High
Causal Certainty High
StressReduced population size/viability, local depletion, small populationLevel of ConcernN/A
 - Eradication effortsThreat Attributes
Threat
Category
Disturbance or harmExtentLocalized
 LocalRange-wide
General
Threat
Discriminate killingOccurrenceHistoric 
FrequencyOne-time 
Specific
Threat
Eradication programSeverityHigh 
Causal CertaintyHigh 
StressReduced population size/viability, local extinctions, small populationLevel of ConcernN/A
 - Directed fishingThreat Attributes
Threat
Category
Biological resource useExtentWidespread
 LocalRange-wide
General
Threat
Commercial fishery, recreational fisheryOccurrence Historic
Frequency Continuous
Specific
Threat
HarvestingSeverity High
Causal Certainty High
StressReduced population size/viability, local extinctions, small populationLevel of ConcernN/A


4.2 Description of Threats

Current Threats

Entanglement

Known present day human-induced mortality of the Pacific population of Basking Shark is primarily from continued interactions with fishing gear. Historically, known areas of high occurrence coincided with productive fishing areas which likely holds true today. The overlapping use of productive areas increases the probability for interaction with fishing gear and vessels. Globally, Basking Sharks have been found entangled in gillnets, trawls (bottom, midwater, and shrimp), longlines, trap lines, and seines (Wallace and Gisborne 2006). Basking Sharks are also likely susceptible to entanglement with fish farming equipment such as nets and lines (COSEWIC 2007). Since 1996 the only recorded mortalities of Basking Sharks in Canadian Pacific waters are from four records observed on groundfish trawl vessels (COSEWIC 2007). These four incidental capture records represent a high proportion of the 13 confirmed2 sightings in Canadian Pacific waters within this time period.  In known areas of historical high occurrence (see Figure 2), entanglement threats are from trap lines, gillnets, trawls, and aquaculture operations. Range-wide there are recent accounts of Basking Sharks caught in gillnets in Baja Mexico (Sandoval-Castillo et al., 2005) and California. The Recovery Potential Assessment for this population suggested an annual range-wide mortality rate of 10-17 individuals may be acceptable (McFarlane et al. 2009). Documented mortality in Canadian waters is less than one per year; mortality elsewhere in their range is unknown. Furthermore, Basking Sharks have been observed in close aggregations and are therefore susceptible to a multiple shark entanglement event.  Because of their low and uncertain population size, historical susceptibility to entanglement, high mortality rates when entangled, and uncertainty around actual entanglement rates elsewhere in their range, this threat is considered a ‘high’ level of concern.

Collision with vessels

Basking Sharks often feed by slowly moving along the surface, and therefore altercations between boats (hulls and propellers) and Basking Sharks are probable. Vessel strikes, as reported through photo identification projects, are not uncommon in areas where Basking Sharks are found in greater numbers, such as the Northeast Atlantic (Gore et al. 2010). Basking Sharks have an apparent attraction to boat propellers; possibly to the sound they generate (Darling and Keogh 1994). Several Basking Sharks in Clayoquot Sound in the early 1990s had scarring indicative of interactions with boat propellers (Darling and Keogh 1994). There are no recent reported vessel strikes in Canadian Pacific waters or elsewhere in their range. The extent and population consequence of vessel collisions is unknown. This threat is considered to be a ‘medium’ level of concern.

Harassment

Historically harassment of Basking Sharks was commonplace along the BC coast (e.g., shooting, ramming with small boats) (Wallace and Gisborne 2006).  While the extent of this threat is currently unknown along the BC Coast, it is anticipated that if Basking Sharks increase in abundance, harassment in the form of vessel based disturbance by ecotourism operators or individuals may impact normal surface feeding behaviour as has been observed in the northeast Atlantic (Gore et al. 2010). The impacts of underwater noise through seismic, explosives, or otherwise on sharks in general has not been well documented. Overall, harassment is considered to be a ‘low’ level of concern.   

Prey availability

Evidence from throughout their global range indicates that Basking Sharks prey primarily on calanoid copepods while feeding in surface waters (Sims 2008); however, diet preference and composition for the Pacific population of Basking Shark is unknown. Prey sources in deep waters are poorly understood but may include larger zooplankton. A single stomach sample of a Basking Shark caught in subsurface water (~100 m) off the east coast of Japan was found to contain only the pelagic shrimp, Sergestes similis, suggesting that larger prey (e.g., euphausiids) may be an important part of their diet (Sims 2008). The ability of Basking Sharks to switch to alternate prey during periods of low copepod abundance is also not known. Basking Sharks are highly adapted to prey on small (~2mm) zooplankton and have a demonstrated preference for a narrow range of prey in surface waters. At small spatial and temporal scales, Basking Shark distribution and occurrence appears strongly linked to zooplankton abundance (Sims 2008). As well, in the northeast Atlantic, long term trends in surface sightings are correlated with sea surface temperature that may also influence zooplankton abundance and distribution (Sims 2008). A long term downward shift in prey availability from either natural or human causes will certainly influence the behaviour of the Basking Shark and could threaten the population as a whole, considering their low abundance. Trends in prey availability in the context of Basking Shark recovery have not been thoroughly studied. This threat is considered to be a ‘low’ level of concern.

Historical Threats

Entanglement

Beginning as early as 1901 there are records of Basking Sharks being caught in salmon gillnets in Canadian Pacific waters. The productive salmon fishing areas of Barkley Sound and Rivers Inlet are known areas with several reported interactions (Wallace and Gisborne 2006).  It is estimated that between 400 and 1500 Basking Sharks may have been killed by entanglement between 1942 and 1969 (COSEWIC 2007). Entanglement has also been reported in the U.S. component of its range during the 1880s and the 1920s (Thomas 2004).  This threat resulted in a reduction in the population and contributed to local depletion.

Eradication efforts

Between 1943 and 1954, Basking Sharks were increasingly being reported ensnarled in salmon gillnets within Canadian Pacific waters (Wallace and Gisborne 2006).  To assist fishers, a directed eradication effort was established which operated from 1955-1969. Basking Sharks were killed by the use of a large blade mounted on the bow of a fisheries patrol vessel. Government records reported 413 kills using this method (Wallace and Gisborne 2006). In addition to the blade method, patrol vessels were also under directive to opportunistically ram Basking Sharks (COSEWIC 2007). There was also a small eradication effort in 1943 in Rivers Inlet where six sharks were reported to have been killed (Wallace and Gisborne 2006).

Directed fishing

The historic record suggests that most of the commercial Basking Shark fishery in Canadian Pacific waters occurred between 1941 and 1947 in Barkley Sound with some additional fisheries possibly in the 1920s (Wallace and Gisborne 2006).  This fishery was targeting the Basking Shark primarily to extract oil from its massive liver.  There is insufficient information to accurately estimate the number of sharks commercially killed for their oil. In addition to commercial fishing, recreational harpooning of Basking Sharks was widespread between 1940 and the mid-1960s (COSEWIC 2007). In Monterrey Bay, Basking Sharks were the focus of a commercial fishery beginning in the mid 1920s and continued until 1952 (Thomas 2004). During this period, U.S. landings ranged between 25 and 200 sharks per year (McFarlane et al. 2009). A recreational harpoon fishery also occurred at this time.

2 Within the context of this recovery strategy, ‘confirmed’ is based on photo/video identification, or from an experienced source (e.g., on-board observers).