Influence of small vessel operation and propulsion system on loggerhead sea turtle injuries

Abstract

Collisions with marine vessels represent a significant threat to many marine vertebrates, including whales, dolphin, manatees, and sea turtles. Many recovered turtles display injuries that appear to result from interactions with marine vessels and their associated propulsion systems, yet the details of these interactions and the potential for reducing their lethality have not been previously investigated. This paper describes a series of experiments designed to investigate the type and severity of damage inflicted on full-scale model loggerhead sea turtles struck by small vessels, and the potential for reducing the likelihood of fatal interactions by modifying either the vessel propulsion system or operational characteristics. Artificial carapaces were developed to mimic selected material properties of a natural loggerhead carapace. These were secured to frames to yield models with size, mass, and density representative of real animals. The model turtles were subjected to impact by small (3–6 m) vessels with traditional outboard, jet outboard, and inboard jet propulsion systems under controlled conditions in the field. Dependence of catastrophic injury on vessel speed, model depth below the water surface, and propulsion system was investigated via repeated tests. The effectiveness of two commercial propeller guards was also investigated. Results indicate that vessel speed does significantly influence the likelihood of catastrophic damage, whereas depth in the water column does not. Propeller guards were ineffective at planing speed and only slightly helpful at idle speed. Both tested jet propulsion systems yielded dramatic improvements in animal safety compared to a traditional outboard and propeller arrangement. Results thus suggest that appropriate changes in both vessel operation and configuration can reduce threats to sea turtles, and likely other marine organisms.