Influence of small vessel operation and propulsion system on loggerhead sea turtle injuries
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Loggerhead sea turtles (Caretta caretta) can be found worldwide, inhabiting tropical and subtropical coastal waters. The loggerhead was classified as an endangered species and placed on the International Union for Conservation of Nature and Natural Resources (IUCN) Red List in 1996 (IUCN 2006).The problem of sea turtle mortality as a result of collisions with vessels is of increasing concern, especially in the southeastern United States, where increased development along the coasts results in increased recreational boat traffic. In the United States, the percentage of strandings that were attributed to vessel strikes has increased from approximately 10% in the 1980's to a record high of 20.5% in 2004 (NMFS 2007). This report presents results from field experiments designed to investigate the ways in which loggerhead sea turtles are injured in boat collisions, and the effectiveness of several mitigation options for reducing the risk of fatal interactions. In order to conduct these field experiments, a synthetic sea turtle carapace was designed and built that approximated the structural behavior of a biological sea turtle carapace. Hodges (2008) quantified the material strength properties of loggerhead sea turtle carapaces. From these results, it was determined that the target parameter for simulating tensile strength in a synthetic carapace should be force per unit width of sample. Hodges designed and constructed an artificial carapace made of composite material for use in controlled experiments. Modifications were made to the design proposed by Hodges (2008) to facilitate rapid construction. Several designs were tested using the force per unit width as the target strength parameter and compared to the strength of the biological carapace. Tests on the design ultimately adopted showed a force per unit width 17.6% stronger than the biological carapace. The composite material being stronger than the biological carapace means the testing will result in conservative reports of damage. Once the design and construction methods were finalized, approximately 60 artificial carapaces were fabricated for field testing. A frame, weighting scheme and buoyancy unit were designed and fabricated so that each test carapace floated at proper draft and had realistic specific gravity and weight. Field testing procedures were designed to investigate the influence of a) boat speed, b) animal position in the water column, and c) vessel propulsion system on the severity of vessel collisions on turtles. All experiments were done with small (<6 m in length) vessels. Boat/sea turtle collisions were simulated by placing a test specimen (a synthetic carapace attached to a test frame) in the water column and striking it with the vessel. The speeds considered were idle (7 km/h), sub-planing (14 km/h), and planing (40 km/hr). The two animal positions in the water column were 1) at the water surface and 2) at "prop depth" (depth to the center of the propeller hub on the standard outboard motor). Five propulsion options were tested: 1) a standard outboard motor, 2) a standard outboard motor with Hydroshield® propeller guard 3) a standard outboard motor with Prop Buddy® propeller guard, 4) a jet outboard motor and 5) a jet-propelled personal watercraft, often referred to generically as a "jet ski". The experiments typically included five trials per test configuration. Catastrophic (presumably fatal) damage was defined to occur when any damage penetrated the carapace. Small wounds (< 4 cm in length) along the sides or rear of the artificial carapace, where the shell and bone extend beyond the edge of the body cavity, were not classified as catastrophic This definition was used to classify the effectiveness of the various mitigation options. Results indicate that reducing the speed of the vessel reduces the odds of severe damage to the animals. Of all of the tests performed with the standard outboard motor (including tests with propeller guards installed), 25% of those performed at idle speed resulted in catastrophic damage, compared to 100% for planing speed tests. The two tested propeller guards both modified the type of damage to the animal when compared to similar tests with the standard motor configuration, but they only slightly reduced the risk of catastrophic damage. At idle speed, with propeller guard installed, 10% of the tests resulted in catastrophic damage. The corresponding number for the standard motor was 40%. At planing speed, 100% of the tests resulted in catastrophic damage, with or without the propeller guard. No catastrophic injuries were observed during testing of both jet propulsion systems (jet outboard and jet ski) at any speed or depth in the water column. Both feature a much smaller draft than the standard outboard, which results in little chance of striking an animal below the surface. And both the jet outboard and the jet-powered watercraft feature water intakes that are relatively smooth and appeared to slide across the animal with minimal damage to the carapace when the model animal was floating on the surface. The experiments described here involved a limited range of hull configurations; results may be different for hulls or propulsion systems drastically different than those tested here. But the results obtained indicate that equipment, in the form of the boat's propulsion system, and the mode in which it is used both play a role in defining the risk of boats to turtles in the field.