'Turtle guards': A method to reduce the marine turtle mortality occurring in CERTAIN seismic survey equipment

During 2007 Ketos Ecology released a document entitled 'Reducing the fatal entrapment of marine turtles in towed seismic survey equipment' (Ketos Ecology, 2007), which outlined the issue of accidental mortality of endangered marine turtles occurring in seismic tail buoys.  The document contained information on the mechanism of turtle entrapment in this equipment and made recommendations for minimising the problem via the use of 'turtle guards' that could be fitted to the front of seismic tail buoys to prevent turtles from becoming trapped.  During October 2007, this document was distributed to the IAGC, the OGP and various seismic regulatory authorities with the aim of raising awareness and finding a long-term solution to the problem. 

During 2008/09, it has become apparent that the turtle guards designed and implemented by certain seismic contractors do not eliminate turtle mortality in tail buoys.  It also became apparent that not all designs of seismic tail buoy trap turtles; the problem is restricted to particular designs used by certain seismic contractors.  Consequently, an industry-wide review of this situation is required to assess which designs of tail buoy and turtle guard are effective in reducing mortality and to exchange information on appropriate design and implementation of turtle guards.  Updated information on the turtle guards used to date, problems encountered and recommendations for future implementation has been added to the existing 2007 document and is presented on the following web pages and as a 2009 download.

1. INTRODUCTION

Seven species of marine turtle occur worldwide, five of which are found throughout tropical and subtropical waters while a further two species have more restricted geographical ranges (Table 1). The IUCN classifies three species as Critically Endangered, a further three as Endangered and a single species as Data Deficient. All marine turtle species are therefore of conservation concern due to declining worldwide populations and increasing anthropogenic impacts both offshore and at their nesting beaches and adjacent coastal zones. Identified anthropogenic threats to marine turtles include deliberate hunting for meat, exploitation of eggs, high levels of accidental capture in fishing nets, trawls and on long-lines, loss of nesting habitat, loss of foraging habitat, marine litter, pollution and disease.

Table 1. Status of marine turtles worldwide (IUCN = International Union for Conservation of Nature and Natural Resources)

One impact on marine turtles that has been undocumented to date, is the accidental entrapment and mortality of turtles within geophysical seismic survey equipment. Marine turtles are frequently encountered during seismic surveys conducted in tropical/subtropical waters (Figure 1), for example off Brazil (de Gurjão et al., 2005), the Canary Islands (Pierpoint and Fisher, 2003) and Angola (Weir, 2007).

Figure 1. Olive ridley and leatherback turtles photographed from 3D seismic survey vessels off Angola

In recent years, the increased presence of Marine Mammal Observer's (MMOs) onboard seismic survey vessels has raised awareness of marine fauna amongst seismic crews and oil companies. One conservation issue that has become apparent over this period is that of turtles becoming caught in seismic tail buoys and other towed equipment. Marine seismic surveys are usually conducted using specialised vessels that tow up to 16 seismic cables, each of up to 10 km in length, with a typical separation of 50 to 100 m between the cables. Maintaining the cable separation and monitoring their position requires a considerable amount of equipment to be towed astern of seismic ships, including paravanes (or 'doors'), mono-wings, dilt floats (located at the head of each cable) and tail buoys (located at the end of each cable) (Figure 2).

 Figure 2. Schematic (not to scale) showing a simplified configuration

for a 3D marine seismic vessel towing six cables

The following pages provide information on the fatal entrapment of turtles within seismic tail buoys, aiming to raise awareness of this issue and provide some potential solutions to the problem.