Research Document - 2010/088

An Examination of Loggerhead Sea Turtle (Caretta caretta) Encounters in the Canadian Swordfish and Tuna Longline Fishery, 2002-2008

By S.D. Paul, A. Hanke, S.C. Smith, and J.D. Neilson

Abstract

Loggerhead sea turtles (Caretta caretta) were listed as threatened under the USA Endangered Species Act in 1978 and listed endangered on the International Union for Conservation of Nature (IUCN) Red Listing in 1996 and by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 2010. Estimates of loggerhead sea turtle encounters by the Canadian swordfish and tuna longline fleet were derived from commercial landings and observer data from Maritimes and Newfoundland regions for 2002-2008.

Set-level encounter rates of the swordfish component of the Canadian fishery (2005-2007) were comparable with published estimates from the USA pelagic longline fishery for sets made in the same area and season, whereas the tuna-targeted Canadian fishery had a higher median encounter rate than did the swordfish-targeted component of the Canadian fishery.

Delta-lognormal and ratio estimation approaches, with stratifying and prorating variables such as quarter, region, commercial landings and the number of hooks per trip, were used to estimate fleet-wide loggerhead sea turtle encounters. Overall, these approaches showed the same trends when similarly stratified, notably a decrease in encounters from 2006 to 2008. However, a major difference was that the delta-lognormal approach predicted almost 50% fewer loggerhead sea turtle encounters compared to the ratio estimation approach. It was resolved that the ratio estimation approach best represented loggerhead sea turtle encounters by the longline fleet.  Using that approach, it was estimated 1,200 loggerhead sea turtles (95% confidence range of 700-1,800) were caught annually in the Canadian swordfish and tuna longline fishery during the period 2002-2008.

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