Decline of the Sea Turtles Causes and Prevention (1990) / Chapter Skim
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6. Sea Turtle Mortality Associated with Human Activities
6. Sea Turtle Mortality Associated with Human Activities
Pages 74-117
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From page 74... ...
They are vulnerable to diverse potentially lethal interactions with human activities, situations including direct predation and habitat modification, incidental capture or entanglement in fishing gear, and physical damage caused by dredging of shipping channels, collisions with ships and boats, and oil-rig removal or other underwater explosions. Each species in the pelagic environment is vulnerable to ingestion of plastics, debris, and petroleum residues.
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From page 75... ...
The most important identifiable source of mortality for loggerhead and Kemp's ridleys is incidental capture in shrimp trawls (Table 6-21; other fisheries and fishery-related activities are also important, but collectively only one-tenth as important as shrimp trawling. Dredging, collisions with boats, and oilrig removal are also important, but only one-hundredth as important as shrimp trawling.
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From page 77... ...
Beach armoring can result in permanent loss of a dry nesting beach through accelerated erosion and prevention of natural beach and dune accretion, and it can prevent or deter nesting females from reaching suitable nesting sites. Clutches deposited seaward of the structures can be inundated at high tide or washed out by increased wave action near the base of them.
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From page 78... ...
Increased human activity on a project beach at night might cause further disturbance to nesting females. Artificial lights along a project beach and in the nearshore area of the borrow site might deter nesting females and disorient emergent hatchlings on adjacent nonproject beaches.
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From page 79... ...
found that adult green turtles avoided bright areas on nesting beaches. Raymond (1984)
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From page 80... ...
can deter nesting attempts and interfere with incubating egg clutches and the seaward journey of hatchlings. The documentation of false crawls near such obstacles is increasingly common as more recreational equipment is left in place all night on nesting beaches.
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From page 81... ...
Beach Vehicles The operation of motor vehicles on turtle nesting beaches is still permitted in many areas of Gulf of Mexico and Atlantic states (e.g., Florida, North Carolina, and Texas)
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From page 82... ...
indicated that dense stands of Australian pines in southwestern Florida affect nest-site selection and cause increased nesting in the middle beach area and higher ratios of false crawls to nests compared with areas of native vegetation. MORTALllY OF SEA TURTLE JUVENILES AND ADULTS Shrimp Fishing Description of the Fishery The shrimp fishery has the highest product value of any fishery in the United States.
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From page 83... ...
During some years, catches of brown shrimp exceed those of white shrimp in South Carolina as well. The peak of the brown shrimp harvest occurs during the summer in all four south Atlantic states.
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From page 84... ...
The offshore fishery provides the basis for the adult brown shrimp fishery. The pink shrimp fleet off Florida uses a variety of vessels of different sizes but is associated primarily with larger offshore boats.
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From page 85... ...
Brown and pink shrimp are important fisheries in the Gulf; therefore, more shrimping effort is expended in deeper waters of the gulf than in the Atlantic, where white shrimp dominate the fishery. Statistical reporting procedures vary between the Atlantic and gulf data bases (pers.
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From page 86... ...
This pattern largely represents that of the western gulf; local variations from this pattern occur. Off Georgia and the Carolinas, little fishing takes place in winter, whereas off the Atlantic coast of northern Florida, effort is more uniform through the year and includes significant winter fishing.
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From page 87... ...
Seasonal Changes in Stranding, Shrimp-Fishing Effort, and Turtle Abundance The recent abundance of stranded sea turtles and the intensity of shrimp fishing vary from the western Gulf of Mexico along the coast to the Gulf of Maine and from season to season. The distributions of strandings are complex interactions between trawling intensity and the abundance of sea turtles and other factors.
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From page 88... ...
, and Georgia and South Carolina (zones 31-32? ; the largest number of turtles per unit of shrimp-fishing effort was for Atlantic north Florida (zones 28-301.
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From page 89... ...
That area has the most turtles per unit of shrimping effort among the four locations examined and would be the most likely to support a direct relation between stranding and effort over an extended period with modest levels of fishing effort relative to the standing stock of sea turtles. We cannot eliminate the alternative hypotheses from the existing data on turtle migration and ocean currents.
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From page 90... ...
We consider below five observations that, when taken together, constitute a compelling demonstration that incidental capture during shrimp trawling is the proximate cause of mortality of substantial numbers of sea turtles. Relation Between Sea Turtle Mortality in Trawls and Tow Time The most convincing data available to assess whether shrimp trawling is responsible for sea turtle deaths come from NMFS studies relating the time that a trawl was allowed to fish (tow time)
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From page 91... ...
The exact mechanism of sea turtle drowning is not known, but a diagnostic condition of the wet-drowning syndrome the exudation of copious amounts of white or pink froth from the mouth or nostrils has been observed in trawl-captured turtles. Turtles captured in shrimp trawls might be classified as alive and lively, comatose or unconscious, or dead.
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From page 92... ...
used the data on sea turtle stranding in South Carolina in 1980-1986 to seek a temporal relation between the opening of the ocean shrimp fishery and the rate of stranding. In South Carolina, the Sea Turtle Stranding and Salvage Network (STSSN)
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From page 94... ...
(Compiled from NMFS data9 MARINE TURTLE CARCASSES FROM SOUTH CAROLINA BEACHES 160 In .
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From page 95... ...
No other contrast between successive 2-week intervals had a probability of less than 0.10. This analysis thus implies that shrimp trawling was indeed responsible for the increase in turtle strandings.
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From page 96... ...
Consequently, although a large and statistically significant decline in loggerhead stranding had occurred after closing of the Texas brown shrimp fishery, the decline continued to occur between the last two periods. Given the uncertainty as to how long it takes for dead turtles to reach the beach, those results are consistent with either an effect of brown shrimping on sea turtle stranding or a general decline in sea turtle stranding during the period for other reasons.
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From page 97... ...
provide the most complete assessment of sea turtle capture and mortality for the south Atlantic and Gulf of Mexico shrimp fisheries. Their study, based on more than 27,000 hours of observed trawling, estimated an annual incidental capture of approximately 47,000 sea turtles, with an estimated mortality of about 11,000.
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From page 98... ...
This produces an underestimate, because not all comatose sea turtles do survive. Captures in inside waters were not included, thus reported estimates are low.
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From page 99... ...
. This local area harbors large concentrations of sea turtles throughout the year, and high turtle catch rates there do not reflect those occurring outside the Canaveral area (Henwood and Stuntz, 19871.
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From page 100... ...
With its assumption that all comatose turtles survive and its omission of all turtle capture and mortality estimates for inside waters, the approach taken by Henwood and Stuntz results in a marked underestimate of total sea turtle mortality associated with the shrimp fishery. Relation Between Sea Turtle Stranding and Spatiotemporal Paltern of Shrimp Trawling in North Carolina The northern limit of the geographic zone of ocean shrimp trawling occurs at Ocracoke Inlet, North Carolina.
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From page 101... ...
The assessment of sea turtle mortality attributed to entanglement in stationary or fixed fishing gear is difficult, because of the disparity and discontinuity of reliable data. It is fair to assume that in some localities and with some types of fishing gear, entrapment and entanglement occur fairly often, but the resulting turtle deaths might not be as consistent.
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From page 102... ...
Other stationary or passive fishing gear that has caused deaths of turtles includes pound nets, long lines, sturgeon nets, and nylon and monofilament gill nets (Van Meter, 19831. Leatherbacks and green turtles are prone to entangling their front flippers and heads in buoy ropes or discarded twine (O'Hare et al., 1986; O'Hara and Iudicello, 19871.
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From page 103... ...
New York Bight Turtle mortalities have resulted from lobster-pot lines and pound nets. Between 1979 and 1988, 58 stranded sea turtles reported in the New York Bight exhibited signs of entanglement with debris or inactive or fixed fishing gear.
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From page 104... ...
An estimated 50-200 sea turtles strand from all causes in and around the Chesapeake Bay each year (Keinath et al., 1987; personal communication., D Barnard and I
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From page 105... ...
could be definitely attributed to entanglement in pound or gill nets, or other fishing gear (Barnard et al., 19891. South Atlantic Sea turtle deaths other than those caused by shrimp fishing have occurred in the south Atlantic in association with oceanic gill nets, large ocean set nets, and tuna and billfish long lines.
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From page 106... ...
, whereas loggerheads are prone to entanglement in the ganglion lines attached to the main line. In Florida, there were five recent confirmed sightings by divers of sea turtles entangled in monofilament fishing line on reefs and wrecks (pers.
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From page 107... ...
A comprehensive survey of records and project reports recognized 149 confirmed incidents of sea turtles entrained by hopper dredges working in two shipping channels from 180 to 1990 (Table 6-6) (pers.
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From page 108... ...
Nearly all sea turtles entrained by hopper dredges are dead or dying when found, but an occasional small green turtle has been known to survive. Entrapment and death of turtles by hopper dredges first became an issue of concern at the Port Canaveral Entrance Channel, Florida, in 1980 after unusually high concentrations of loggerheads were noted in the area (Carr et al., 19819.
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From page 109... ...
If we assume that half the boat-collision injuries documented by the STSSN were the primary causes of death of the stranded sea turtles in 1986-1988, and only about 20% of the dead turtles wash ashore, about 400 turtles are killed by boat collisions each year along the gulf and Atlantic coasts of the United States outside of coastal beaches. That estimate might be low, because the strandings include only the ocean beaches (boat collisions with turtles also occur in inside waters)
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From page 110... ...
Obtaining a permit requires use of qualified observers to monitor sea turtles near platforms and in some cases to remove turtles to a safe location away from the potential impact of explosive charges. Data collected by NMFS since 1986 support an association between turtles and some offshore platforms.
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From page 111... ...
Although information on association of sea turtles with energy platforms is sparse, the potential for mortality must be considered genuine. It is difficult to document a cause-effect relation between turtle deaths and offshore explosions, because no dead animals have been recovered at removal sites and freshly killed turtles sink and might drift a long way by the time putrefaction causes them to float.
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From page 112... ...
~ 1 ~ in: .. ~ ~ Entrainment of Sea Turtles in Power-Plan' Intake Pipes Sea turtles can become entrained in intake pipes for cooling Upstater at coastal power plants.
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From page 113... ...
, the committee was unable to quantify the extent of the problem. Toxicology Tissues and eggs from several species of sea turtles in the southeastern United States, Ascension Island in the South Atlantic, the coast of France, and other geographic regions have been analyzed for organochlorine compounds, heavy metals, hydrocarbons, and radionuclides (Hillestad et al., 1974; Thompson et al., 1974; Stoneburner et al., 1980; Clark and Krynitsky, 1980, 1985; Witkowski and Frazier, 1982; Bellmund et al.,
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From page 114... ...
An estimated 1-2 million birds and more than 100,000 marine mammals and sea turtles die from eating or becoming entangled in plastic debris each year, including netting, plastic fishing line, packing bands, and Styrofoam (Welch, 1988; McGavern, 1989; Sanders, 19899. Sea turtles ingest a wide variety of synthetic drift items, including plastic bags, plastic sheeting, plastic particles, balloons, Styrofoam beads, and monofilament fishing line.
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From page 115... ...
They were significantly affected by ingestion of, and to a smaller extent entanglement in, marine debris. Necropsies of Kemp's ridleys, loggerheads, and green sea turtles revealed that the intestines of at least 65 of 237 turtles examined contained marine debris, such as plastic bags, Styrofoam, monofilament fishing line, polyethylene beads, aluminum foil, tar, glass, and rubber.
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From page 116... ...
The most important source of mortality for juveniles to adults in the coastal zone is shrimp trawling. Other factors judged to be of significance for juveniles and adults are other fisheries and entanglement in lost fishing gear and marine debris.
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From page 117... ...
The strong evidence that shrimp trawling is the primary agent for sea turtle mortality caused by humans comes from five lines of analysis and information First, the proportion of sea turtles caught in shrimp trawls that are dead or comatose increases with an increase in tow time from 0% during the first 50 minutes to about 70% after 90 minutes. Second, the numbers of turtles stranding on the coastal beaches consistently increased in a steplike fashion when the shrimp fishery opened in South Carolina and Texas and decreased when the fishery closed in Texas.
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