Turtle Guards 2

2. MECHANISM OF TURTLE ENTRAPMENT

2.1. ENCOUNTERS BETWEEN TURTLES AND SEISMIC EQUIPMENT

Clearly, a marine turtle must have to come into very close proximity with seismic equipment in order to become trapped. While regular contact between turtles and seismic equipment might initially seem unlikely, in particular geographic areas and at specific times of the year high densities of marine turtles can be present in the vicinity of a seismic survey. For example, off West Africa large numbers of turtles have been noted during offshore seismic surveys prior to the turtle nesting season (Weir et al., 2007), and close proximity of animals to the seismic vessel and towed equipment is frequently noted (e.g. Figure 3). Weir (2007) reports 'near miss' collisions between basking turtles and dilt floats (Figure 3). The same potential for collision occurs with tail buoys, although these are usually located several kilometres astern of the ship where such interactions cannot easily be monitored.

2.2. SEISMIC TAIL BUOYS

The piece of seismic equipment that almost all reported turtle entrapments have been associated with is the tail buoy. A tail buoy is a large float attached to the end of each seismic cable (Figure 4), which is used to monitor the location of the cables. The upper surface of the tail buoy is fitted with radar reflectors and Global Positioning System (GPS) receivers, and some designs also have solar panels for powering the equipment. Tail buoy designs vary, and not all seismic contractors utilise the same type. However, the tail buoys used by several of the main seismic contractors have a subsurface structure ('undercarriage') consisting of a 'twin-fin' design (Figure 5), which is used for: (a) counter-balancing the upper structure to ensure stability in the water; and (b) facilitating easy upright storage on deck. A propeller unit is housed within the undercarriage of some buoy designs to provide additional power to the unit. Towing points are located on the leading edge of each side of the undercarriage, and these are attached by chains to a swivel which leads to the stretch at the end of the seismic cable.

Figure 3. Basking turtle on a direct collision path with a seismic dilt float off Angola. The turtle ‘startle dived' immediately (<1 m) prior to contact

Figure 4. A seismic tail buoy with the tow cable visible subsurface

Figure 5. A tail buoy on deck, showing the twin-fin design, swivel and tow chains

Figure 6. Two dilt floats on deck (upside down) showing the twin-fin subsurface structure

2.3. HOW DO TURTLES BECOME ENTRAPPED?

It is not clear exactly how turtles become trapped within tail buoys. Two potential theories are: (a) as a result of 'startle diving' in front of towed equipment, and (b) as a result of foraging along seismic cables.

(A) Startle dives are usually observed when a turtle is basking at the water surface for metabolic purposes during hot, calm weather. Basking behaviour appears to make turtles slow to react to approaching threats, with startle dive reactions occurring only at close range to approaching objects and apparently based principally on visual detection. Turtles have been observed startle diving in reaction to towed seismic equipment (paravanes and dilt floats) and at the bow of seismic vessels themselves (Weir, 2007), usually when less than 1 m from the approaching object. A turtle that startle dives in response to an approaching tail buoy will be in a prime position for entrapment (Figure 7).

(B) Foraging by turtles on barnacles and other organisms growing along seismic cables is suggested by the frequent observations reported by workboat crews of turtles swimming immediately over seismic cables. Furthermore, 'noise' on the hydrophones within seismic cables has been attributed to turtles sliding along the cables while foraging. It is plausible that a turtle feeding along a seismic cable may travel to the end of the cable and along the stretch, before surfacing to breathe. At that point it would be immediately in front of the tail buoy giving rise to a possible entrapment situation. Not all species of turtle feed on barnacles and other invertebrates. Loggerhead, olive ridley and Kemp's ridley turtles would be the main species likely to be attracted to seismic cables for foraging purposes.

Figure 7. Schematic of a turtle that has startle-dived in response to an approaching tail buoy

2.4. WHERE DO TURTLES BECOME TRAPPED?

Seismic personnel have reported two areas of a tail buoy where turtles become trapped: (A) in front of the undercarriage in the area between the buoy and the towing chains; and (B) inside the 'twin-fin' undercarriage structure:

(A)

The attachment of the tow chains to the tail buoy undercarriage results in the creation of an angled gap between the chains and the underside of the buoy. Seismic crew have reported turtles becoming stuck within this angle, lying across the top of the chains and underneath the float. In all reported cases these turtles have been trapped on their sides, with their ventral (under-) surface facing the oncoming water, resulting in considerable drag and causing the turtle to be held firmly in position and the tail-buoy to tow awkwardly.

(B)

The gap below a typical tail buoy (e.g. Figure 5) extends to 0.8 m below water level, and is approximately 0.6 m in width. The potential for a turtle to become stuck within this gap will therefore depend on the size of the animal. It would need to be small enough to enter the gap, but too big to pass all the way through the undercarriage.

The presence of the propeller in some buoy designs prohibits turtles that have entered the undercarriage from travelling out of the trailing end of the buoy. It is unclear at this stage in what position turtles trapped here are orientated, i.e. whether they enter the structure head-first or tail-first and whether they are upright or upside-down.

Unfortunately no photographs are currently available to confirm these potential entrapment sites. However, workboat crews using underwater cameras have reported that most turtles are trapped in position A above, i.e. across the tow chains and in front of the tail buoy undercarriage.

Since the twin-fin design is also used as the undercarriage of some dilt floats (Figure 6), entrapment is also a possibility in that equipment. However, on dilt floats the twin-fin is positioned towards the rear of the float rather than at the front, and turtles probably have sufficient time to move out of the way and avoid becoming trapped.

2.5. FOLLOWING ENTRAPMENT

Once stuck inside or in front of a tail buoy, a turtle would be unable to escape due to the angle of its body in relation to the forward movement of the buoy. The 4–5 knot water speed of a seismic vessel would result in considerable water pressure against a trapped turtle, acting to hold the animal against/inside the buoy with little chance of manoeuvring away.

For a trapped turtle this situation will be fatal, since marine turtles are air-breathing reptiles with lungs and must regularly reach the surface to breathe. Although resting turtles may remain submerged for several hours due to their inactivity and the large stores of oxygen maintained in their blood and muscle tissues, eventually they must return to the surface to breathe. It is likely that the added stress of being trapped subsurface would result in a turtle's oxygen supplies diminishing more rapidly than usual, and suffocation would potentially occur quite soon after entrapment.

3. GEOGRAPHICAL AREAS AND REGULARITY OF TURTLE ENTRAPMENT

The frequency and distribution of turtle mortality within tail buoys is mostly unknown at present due to lack of a rigorous reporting of these events by seismic contractors (see Section 5). However, informal discussions with seismic personnel indicate that turtle entrapment is certainly not uncommon, and happens with frequency in some areas (pers. comms.). For example a survey off West Africa in 2003 caught 'numerous turtles', and a survey off India in 2007 was reported to have killed 'several turtles every week'. Seismic personnel have also indicated that turtle entrapment has occurred in the Gulf of Mexico and off Australia, and it is likely to be a problem whenever 2D/3D/4D seismic surveys and turtles co-occur.

Clearly, this problem is far less likely to impact significantly upon turtle populations than other anthropogenic threats such as the thousands of marine turtles caught incidentally in trawls, on long-line hooks and in fishing nets every year. For example there have been many sightings of turtles trapped in discarded fishing gear offshore of West Africa, and it is recognised that certain seismic crews do take the opportunity to rescue turtles entrapped in fishing gear (Figure 8) therefore making a positive contribution to turtle conservation (visit the Turtle Rescue page for more information on releases of turtles from netting by seismic survey crews).

However, in contrast to the far larger issue of fisheries bycatch the entrapment of turtles in tail buoys is relatively straightforward to prevent and seismic contractors, tail buoy manufacturers and oil/gas companies could eliminate this mortality completely without expending significant time or resources. The following pages describe some potential solutions.

Figure 8A. Several olive ridley and green turtles trapped in fishing net, photographed from a seismic vessel off Equatorial Guinea (© C. Weir). This situation is fatal for turtles.

Return to the top of the page