Science Advisory Report 2010/042

Recovery Potential Assessment for Loggerhead Sea Turtles (Caretta caretta) in Atlantic Canada

Summary

• There are currently no estimates of loggerhead sea turtle abundance in Atlantic Canadian waters. A paucity of data precludes estimation of the overall Northwest Atlantic population abundance in oceanic habitat. However, because females exhibit nesting site fidelity, trends in nests can be used as a proxy for trends in mature female abundance. The total estimated number of nests on Northwest Atlantic nesting beaches has fluctuated between 47,000 and 90,000 nests per year over the last decade. There appears to have been a decline in the number of nests since 1998, notably in the largest breeding unit in the Atlantic (Peninsular Florida).
• The estimated range of loggerhead sea turtles in Atlantic Canadian waters extends from Georges Bank, along the edge of the Scotian Shelf and Grand Banks, to the limits of the Exclusive Economic Zone, with occasional forays into waters on the shelf.
• Atlantic Canadian loggerhead sea turtle habitat appears to be defined geographically and temporally, in part, by sea surface temperature. They are encountered in waters greater than 15°C, especially between 20-25°C, favouring a distribution in thermally dynamic waters along the shelf break and offshore. The primary use of habitat in Atlantic Canadian waters is thought to be for foraging.
• In the absence of population indices in Canada, a recovery target for abundance could not be established at this time. A reasonable recovery target for abundance would be an increase in population size over three generations. A practical management strategy (secondary target) could be to ensure that human-induced harm and mortality rates due to Canadian activities do not exceed levels that would impede population recovery (and be kept at or below current levels until the levels that would not impede population recovery are determined). A reasonable recovery target for distribution would be to maintain current distribution, as there has been no evidence that loggerhead sea turtle distribution has been reduced.
• The only documented source of human-induced harm or mortality of loggerhead sea turtles, considered to be primarily oceanic and neritic juveniles (see sources of uncertainty), in Canadian waters is the Canadian tuna and swordfish longline fishery. Based on a ratio estimation model, there were an estimated 1,200 loggerhead sea turtles (95% confidence range of 700-1,800) caught annually in Canadian tuna and swordfish longline fisheries between 2002-2008. Assuming that post-hooking mortality ranges between 20-45%, this would result in roughly 200-500 deaths of oceanic/neritic juveniles annually from this fishery.
• In the USA Recovery Plan, mortalities of oceanic and neritic juvenile loggerhead sea turtles in the Northwestern Atlantic from all sources (primarily fisheries by-catch, but including vessel strikes, dredging activities, entanglement, and oil pollution) were estimated to be in the order of roughly 10,000-100,000 oceanic juvenile turtles and roughly 14,000-140,000 neritic juvenile turtles annually over the last decade.
• Potential mitigation measures and alternatives to minimize the threat posed by the tuna and swordfish longline fishery have been identified (e.g., hook type and size, set time, bait type). Further study is required to evaluate their effectiveness.
• Published population modeling studies suggest that the Northwest Atlantic loggerhead sea turtle population is likely to continue to decline given current estimates of population growth rates and the effects of human-induced mortality. However, these studies also indicate scope for recovery if total mortality is reduced.
• Reduction or elimination of mortality in Canadian waters alone is highly unlikely to be sufficient to achieve recovery. In addition to minimizing threats to loggerhead sea turtles in Canadian waters, international cooperation to reduce threats to the population as a whole is needed to achieve recovery of this species.

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