340
The Biology of Sea Turtles, Vol. II
In 1988, the U.S. Congress mandated a study of the causes and significance of
turtle mortality in the coastal waters of the country. A study team was convened by
the National Research Council’s Board on Environmental Studies and Toxicology
and Board on Biology. After a comprehensive review, the study team concluded that
the largest human-associated source of mortality was incidental capture in shrimp
trawls, associating that activity with more turtle deaths than all other human activities
combined (Magnuson et al., 1990). The team estimated that as many as 44,000 turtles
were killed annually by the U.S. fleet. Although other fishing gears used worldwide,
including longlines, gill nets, and pots and traps, also are significant sources of
mortality (Allen, 2000; Castroviejo et al., 1994; Gerosa and Casale, 1999; Gribble
et al., 1998; Julian and Beeson, 1998; Lagueux, 1998), this chapter will focus on
the interaction of turtles with trawl fisheries, especially those for shrimp.
13.2 THE PROBLEM
Trawls are used worldwide to catch many species of aquatic invertebrates and
vertebrates. In warm waters, where turtles are most likely to occur, shrimps, or
prawns, are the main species sought with trawls. Prior to the twentieth century,
shrimp harvesting probably did not significantly impact turtles because the main
gear, haul seines, which allow turtles to surface and breathe, was pulled by hand in
very shallow coastal waters (Klima et al., 1982). Trawling for shrimp is relatively
recent, beginning with the introduction of the otter trawl in the early 1900s. Trawls
allowed the fishery to expand beyond shallow coastal waters, and enabled fewer
workers to efficiently harvest much more than a haul seine crew. The fishery, at least
in the U.S., expanded in earnest after World War II (Klima et al., 1982). The
relationship between trawling effort and sea turtle mortality has been well documented (Caillouet et al., 1991; 1996; Henwood and Stuntz, 1987; Poiner and Harris,
1996; Robins, 1995). Trawls forcefully submerse the air-breathing turtles and are
responsible for the drowning deaths of many; as tow duration increases so does
mortality (Henwood and Stuntz, 1987).
13.3 SOLUTIONS
Mortality of sea turtles may be decreased by closing an area to trawling or by
reducing tow times. Area closures impact the fishery to the greatest extent. Decreased
tow times also may impact the fishery, but reduce turtle mortalities only if there is
compliance with regulations. With shortened tow times, the number of hauls within
a given time increases, providing shorter respites for the crew and increasing wear
on deck machinery. In the U.S., tow times have not been regulated under most
conditions because they are difficult to enforce, and when compliance has been
evaluated, it generally has been poor. (National Marine Fisheries Service [NMFS],
2001;* Epperly et al., 1995b).
* National Marine Fisheries Service. Unpublished data. Southeast Regional Office. 9721 Executive
Center Dr., St. Petersburg, FL 33702.
1123 book.book Page 340 Thursday, November 14, 2002 11:50 AM
The Biology of Sea Turtles, Vol. II
In 1988, the U.S. Congress mandated a study of the causes and significance of
turtle mortality in the coastal waters of the country. A study team was convened by
the National Research Council’s Board on Environmental Studies and Toxicology
and Board on Biology. After a comprehensive review, the study team concluded that
the largest human-associated source of mortality was incidental capture in shrimp
trawls, associating that activity with more turtle deaths than all other human activities
combined (Magnuson et al., 1990). The team estimated that as many as 44,000 turtles
were killed annually by the U.S. fleet. Although other fishing gears used worldwide,
including longlines, gill nets, and pots and traps, also are significant sources of
mortality (Allen, 2000; Castroviejo et al., 1994; Gerosa and Casale, 1999; Gribble
et al., 1998; Julian and Beeson, 1998; Lagueux, 1998), this chapter will focus on
the interaction of turtles with trawl fisheries, especially those for shrimp.
13.2 THE PROBLEM
Trawls are used worldwide to catch many species of aquatic invertebrates and
vertebrates. In warm waters, where turtles are most likely to occur, shrimps, or
prawns, are the main species sought with trawls. Prior to the twentieth century,
shrimp harvesting probably did not significantly impact turtles because the main
gear, haul seines, which allow turtles to surface and breathe, was pulled by hand in
very shallow coastal waters (Klima et al., 1982). Trawling for shrimp is relatively
recent, beginning with the introduction of the otter trawl in the early 1900s. Trawls
allowed the fishery to expand beyond shallow coastal waters, and enabled fewer
workers to efficiently harvest much more than a haul seine crew. The fishery, at least
in the U.S., expanded in earnest after World War II (Klima et al., 1982). The
relationship between trawling effort and sea turtle mortality has been well documented (Caillouet et al., 1991; 1996; Henwood and Stuntz, 1987; Poiner and Harris,
1996; Robins, 1995). Trawls forcefully submerse the air-breathing turtles and are
responsible for the drowning deaths of many; as tow duration increases so does
mortality (Henwood and Stuntz, 1987).
13.3 SOLUTIONS
Mortality of sea turtles may be decreased by closing an area to trawling or by
reducing tow times. Area closures impact the fishery to the greatest extent. Decreased
tow times also may impact the fishery, but reduce turtle mortalities only if there is
compliance with regulations. With shortened tow times, the number of hauls within
a given time increases, providing shorter respites for the crew and increasing wear
on deck machinery. In the U.S., tow times have not been regulated under most
conditions because they are difficult to enforce, and when compliance has been
evaluated, it generally has been poor. (National Marine Fisheries Service [NMFS],
2001;* Epperly et al., 1995b).
* National Marine Fisheries Service. Unpublished data. Southeast Regional Office. 9721 Executive
Center Dr., St. Petersburg, FL 33702.
1123 book.book Page 340 Thursday, November 14, 2002 11:50 AM