PIFSC Sea Turtle Longline Research EA June 1, 2009
1.2.3 Worldwide Interactions of Longline Sets and Sea Turtles
The extent of international longline fisheries grew rapidly worldwide since the 1950s, then leveled off at relatively high levels during the last 50 years. Longline fishing increased four-fold from 113 million hooks to 680 million hooks annually between 1952 and 2002 in the western central Pacific Ocean (Molony 2004, in: Beverly and Chapman 2007). The combined catch of bigeye and yellowfin tuna doubled from 100,000 metric tonnes to 200,000 metric tonnes in the same time span. Since the 1980s, 50% of the fleet fishing in the western and central Pacific Ocean has been Japanese, Taiwanese, and Korean flagged. Other fleets have grown since the 1980s, including the U.S. fleet, the mainland Chinese fleet, and smaller fleets in several Pacific Island countries and territories. The growth in global catch since 1952 is striking in that the combined fleets fishing in the western central Pacific Ocean and Indian Ocean (the only fleets operating at that time) caught a total of 68,000 metric tonnes of albacore, bigeye, and yellowfin tuna. By 2002, the combined global fleets fishing in the western central Pacific Ocean, Indian Ocean, eastern Pacific Ocean and the Atlantic Ocean caught 643,000 metric tonnes of the same three species (WCPFC 2006, in: Beverly and Chapman 2007).
Longline vessels are known to fish throughout the Pacific Basin from 50oN to 50oS latitude, as well as in the Atlantic Ocean, Indian Ocean, and Mediterranean Sea (Carocci and Majkowski 1996). Many sea turtle species also exhibit broad distribution north and south of the equator, with apparent affinity for certain oceanographic conditions indicative of available food and favorable currents for migrations. Relevant conditions for sea turtles include converging oceanic fronts, suitable salinity, sea temperature, and surface chlorophyll levels, all of which interact to create highly productive foraging areas for both sea turtles and the larger fish targeted in the longline fleets, such as swordfish and tuna (Polovina et al. 2000, Polovina et al. 2004, Beverly and Chapman 2007). This increases the potential for sea turtle bycatch in the pelagic longline fisheries worldwide.
Sea turtle tracking studies indicate that loggerhead turtles spend a majority of their time at depths less than a maximum of 40 m (130 feet), with much of their time spent at shallower depths (Polovina et al. 2003, Polovina et al. 2004). Even when hooks are intended to fish at depth, they must pass through the near surface waters frequented by sea turtles, and hooks near the floats also tend to be in the near-surface waters. Observer data from the Hawaii-based longline fleet (Kleiber and Boggs 2000) and from experiments in the Atlantic Ocean (Watson et al. 2004a) show that a higher-than-expected proportion of leatherback turtles are taken on the shallower branch lines closest to floats than on other deeper branch lines. Despite even the deepest hooks briefly passing through near-surface waters during set and haul operations, deep-set pelagic longline fisheries have been shown to have lower turtle interaction rates than shallow-set fisheries in the U.S., Japan, Spain, Costa Rica, and the western tropical Pacific (WPRFMC 1993, SPC 2001, NMFS 2001, NMFS 2002, Gilman et al. 2006).
Turtles may become hooked by biting the bait, by being snagged by the hook in passing, or by becoming entangled in the line. Leatherbacks cannot back up, so they are especially vulnerable to becoming entangled. If the branch line is not long enough to allow the hooked or entangled turtle to reach the surface to breathe, it will drown. Sea turtles feeding on hooked bait may swallow the hook as well as the bait, with the hook becoming snagged in the esophagus or stomach. Turtles tend to bite pieces off of fish used as bait until the fish is completely removed