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The Effects of Pelagic Longlining on Pacific Sea Turtle Populations

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The Effects of Pelagic Longlining on Pacific Sea Turtle Populations ;;;;;;;;;;;;;;;;;;;;;; Such interactions can have potentially devastating Bycatch: The Effects effects on species like sea turtles. Of the seven species of sea turtles found in our oceans today, all of Pelagic Longlining are considered threatened or endangered, with six listed on the IUCN redlist of endangered species. on PaciÞc Sea Turtle Two species, the loggerhead (Caretta caretta) and leatherback (Dermochelys coriacea) have been identiÞed Populations as being at particular risk of population decline as a result of incidental take by longline pelagic Þsheries (Lewison et al. 2004b). In fact, incidental catch by by Frances Kinslow Þsheries is considered to be one of the most important causes of anthropogenic mortality for sea turtles Each day the worldÕs human population expands (DeFlorio et al 2005). to record size. The ultimate apex predator, humans As with many other marine megafauna, the have put pressure on nearly every ecosystem in the life history characteristics of sea turtles make their world to provide food and other resources. That populations particularly vulnerable to collapse. pressure not only affects the species that is being Turtles have a long lifespan and take years to reach hunted, but can sometimes have unintentional sexual maturity. There is a very high rate of mortality effects on other species. In Þshing, this type of among young offspring, and population stability accidental interaction with non-target species is requires a high survival rate for those few individuals called Òincidental take,Ó with the non-target species which do reach adulthood. Therefore, turtles suffer becoming Òbycatch.Ó signiÞcant population decline when adult and Many populations of marine animals have been sub-adult age classes endure higher-than-average negatively impacted as a result of Þshing practices mortality: the loss of even a few individuals can have which incur large amounts of bycatch. Generally, signiÞcant effects (Lewison et al. 2004a). Longlines are bycatch species are not of economic value and documented to cause selective mortality among these therefore bycatch incidents have been largely ignored older age classes in sea turtles (Lewison et al. 2004b). and under reported. However, studies of threatened Like turtles, longliners are found in every ocean and endangered marine vertebrates have shown that in the world. With the banning of high seas driftnets many already at-risk species face an even greater by the United Nations in 1991, many industrialized decrease in survival rates due to incidental take from Þsheries turned their efforts to longlining. Boats Þsheries (DeFlorio et al. 2005). from 40 nations set approximately 3.8 million hooks One such Þshery, pelagic longlining, is on 100,000 miles of longline every day, with more responsible for hundreds of thousands of tons of than half of that Þshing effort concentrated in the bycatch every year (Lewison et al. 2004b). Longlines PaciÞc. In the year 2000, conservative estimates put target commercially valuable pelagic Þsh using numbers of global bycatch for loggerheads at over heavy duty monoÞlament. A surface longline will 200,000 and for the critically endangered leatherback have a mainline suspended by ßoats with weighted at 50,000. Nesting populations of PaciÞc leatherbacks vertical lines attached which have baited hooks hung have experienced a 95% decline in just two decades at regular intervals. Longlines can stretch for tens (Lewison et al. 2004b), leading scientists to speculate of kilometers (several miles) and are suspended at that the PaciÞc leatherback faces imminent extinction different depths depending on the location and type in the next ten years (James et al. 2005). Loggerhead of Þsh being targeted (Deßorio et al. 2005). In the turtles showed over an 80% population decline in the PaciÞc Ocean, longliners primarily target two species: same period (Lewison et al. 2004b). bigeye tuna (Thunnus obesus) and swordÞsh (Xiphius Although these numbers seem to indicate a gladius). However, non-target animals accidentally correlation between longlining and population caught on longlines are extensive and include decline, the actual effects of longlining are difÞcult dolphins, whales, seabirds, sharks, and sea turtles, to quantify. When analyzing the effects of Þsheries among others (Lewison et al. 2004a). bycatch, two things are crucial: determine how many individuals are being removed from the population, 1 and then determine the effects of this removal fronts (Polovina et al. 2000), one indication that the (Lewison et. al. 2004a). Data to accurately answer juvenile turtles are feeding at those sites. More recent these questions in many cases is simply unavailable. advances in technology have allowed researchers to Although the existence of bycatch is acknowledged track turtles in the wide open ocean using satellite for all Þsheries, the extent and magnitude is transponders. Studies that tracked loggerhead turtles sporadically recorded. The most reliable data on and olive ridley turtles for months at a time in the bycatch numbers comes from onboard observers PaciÞc conÞrmed that those species did in fact migrate (Lewison et al. 2004b). However, out of 40 nations along ocean fronts (Polovina et al. 2000). A single with active longlining Þsheries, only 15 have an loggerhead tracked in 2001 spent over three months onboard observer program and of those, only a small following the edge of an oceanic front (Polovina et al. percentage of trips are actually required to have an 2004). observer onboard (Lewison et al. 2004a). Logbooks Determining the nature of the turtleÕs pelagic kept by shipÕs crew commonly under-report bycatch, habitat can be crucial in Þnding ways to limit turtle and in most cases there is no reporting at all. In interactions with longlines. For instance, satellite addition, much longline Þshing is illegal and even tracking studies that recorded the depths of turtlesÕ the Þshing itself is unreported (Lewison et al. 2004b), dives found that loggerheads spent 90% of their time making extrapolating data to draw conclusions an at depths of less than 40m (Polovina et al. 2004). This even more difÞcult task. would explain why although Þshing effort for tuna in In addition, relatively little is known about the PaciÞc is up to six times greater than Þshing effort the behavior of sea turtles while in their pelagic for swordÞsh, swordÞsh longliners have bycatch habitat. As adults, turtles will often Þnd foraging ten times that of tuna (Lewison et al. 2004b). The grounds near land masses, and adult female turtles longlines for tuna are known as Òdeep-setÓ because will travel thousands of miles to the beaches where they are weighted to depths below 100m, while the they were born to lay their eggs. Hatchlings emerge swordÞsh lines are Òshallow-setÓ at less than 100m several months later and enter the sea on their own. (Polovina et al. 2004), where turtles are much more However, during the years in between hatchling likely to encounter them. and adulthood, which vary depending on the turtle Another factor that needs to be considered is how species, the turtles ÒdisappearÓ and their habits are the turtles are interacting with the longlines. A study virtually unknown. of loggerheads in the Ionian Sea showed that of 200 This lack of comprehensive knowledge has turtles caught in longlines, 87% were hooked, while hampered conservation efforts to save the sea turtles 13% were entangled (Deßorio et al. 2005). This is a (Lewison et al. 2004a, James et al. 2005). Recent deÞnite indication that the bait is a primary attractant studies have focused on strengthening scientiÞc data for the turtle, and that the turtles were caught when in these areas. For instance, turtle biologists have long trying to feed. However, this did not hold true for hypothesized that juvenile turtles spend much of their all species or all areas. A study of leatherbacks off developmental years associating with oceanic fronts. the Atlantic Coast of Canada found that 95% of those In the PaciÞc, these fronts occur when warmer water turtles caught by Þxed gear longlines (a speciÞc from a subtropical ocean gyre meets cold water from type of longline set in shallower coastal waters and a subartic ocean gyre, causing a weak downwelling. attached to the bottom) were entangled, not hooked. These fronts are characterized by rich chlorophyll A Þfth of these turtles drowned (James et al. 2005). density caused by the abundance of phytoplankton These are important distinctions to make when in the cold water. Predators are attracted to the rich discussing conservation efforts as type of interaction convergence of the phytoplankton, and a complete can have major implications: snared or hooked turtles pelagic food web develops at these sites. JellyÞsh, may be discovered before they drown and released, such as the wind sailor, Vellela vellela, are among but often with hooks embedded deeply in their mouth the Þrst predators, and also among the staple foods or digestive system, or with Þshing line entangling a of loggerhead turtles (Polovina et al. 2000) and limb which may become severed. The implications of leatherbacks (Lewison et al. 2004b), giving rise to the these post-hooking scenarios are simply not known theory that oceanic fronts can provided an abundance (James et al. 2005). of resources for turtles in their pelagic stages. U.S. based Þsheries are monitored by the National Studies support this theory. Stomach analyses of Marine Fisheries Service (NMFS), a division of the 55 juvenile and sub-adult loggerheads drowned in National Oceanic and Atmospheric Administration driftnets in 1993 showed that all of the prey consumed (NOAA). In 2001, the Hawaiian based longline by the turtles were species found at these convergent Þshery was ordered closed, based on data collected 2 by the NMFS that showed that the incidental take of SST of 20oC. Further research found a correlation endangered species by pelagic longliners exceeded between sea surface temperatures at these levels that allowed by the Endangered Species Act (NMFS and high incidence of accidental take with turtles.
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