Limit vessel speeds

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • Three studies evaluated the effects of limiting vessel speeds on reptiles. One study was in each of Australia, Costa Rica and the USA.




  • Survival (2 studies): One replicated, site comparison study in Costa Rica found that in waterways with enforced speed limits, fewer spectacled caiman were found dead with boat-related injuries compared to waterways with no speed limits. One replicated study in the USA found that vessels travelling at lower speeds caused fewer catastrophic injuries to artificial loggerhead turtle shells, though vessels with jet motors caused no catastrophic injuries at any speed tested.
  • Condition (1 study): One replicated, site comparison study in Costa Rica found that in waterways with enforced speed limits, fewer spectacled caiman were found with boat-related injuries compared to waterways with no speed limits.


  • Behaviour change (1 study): One replicated study in Australia found that green turtles were more likely to flee from vessels travelling at lower speeds.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated study in 2004 in shallow oceanic water off the coast of Queensland, Australia (Hazel et al. 2007) found that green turtles Chelonia mydas were more likely to flee a vessel driven at slower speeds. Of 1,819 turtle encounters when the turtle was on the sea floor, 60% were able to flee a slow-moving boat (416 of 694 turtles fled), 22% at a moderate speed (136 of 620 turtles fled) and 4% at a fast speed (20 of 505 turtles fled). This trend was statistically significant only when turtles were within 6 m offset of the vessel (see original paper). Turtles in the water column or on the surface also tended to show a reduced flight response at faster vessel speeds (small sample size precluded statistical analysis, see original paper). A 6 m aluminium boat with a 40 hp outboard motor was driven at three speeds (4, 11 and 19 km/h) on a transect 5 km parallel to the shoreline about 200–450 m from the shore in <5 m of water in the 3 hours before and after noon. The behaviour of turtles sighted within 10 m of the boat were recorded by a spotter.

    Study and other actions tested
  2. A replicated, site comparison study in 2006 in a canal system in Limon Province, Costa Rica (Grant & Lewis 2010) found that waterways with enforced speed limits had lower numbers of injured spectacled caiman Caiman crocodilus fuscus. No injured spectacled caiman were caught in waterways with enforced speed limits (injured: 0 individuals; non-injured: 24 individuals), whereas 37% of spectacled caiman caught in waterways without enforced speed limits had boat-related injuries (injured: 11 individuals; non-injured: 19 individuals), of which two died. Caiman were surveyed in April–June 2006 in three waterways with enforced speed limits (idle–slow) and three without enforced speed limits (high speeds up to 40 km/hour). Adult caiman (1.0–2.5 m long) were caught at night and checked for scars or injuries. Mortalities from boat propellers were recorded.

    Study and other actions tested
  3. A replicated study in 2009 in an abandoned sand quarry in Georgia, USA (Work et al. 2010) found that lower vessel speeds reduced catastrophic injuries to artificial loggerhead turtle Caretta caretta shells. Catastrophic injuries to artificial turtle shells occurred less frequently at lower speeds than higher speeds on unmodified vessels (7 km/hr: four of 10 turtles; 40 km/hr: 10 of 10 turtles) and vessels with propeller guards (7 km/hr: one of 10; 40 km/hr:  nine of nine). With a jet outbound motor or an inboard jet motor (on a jet ski) none of 40 turtles were damaged at 7 or 40 km/hr. Injury rates were similar regardless of the position of the artificial shell in the water. A 90 hp, 4-stroke outboard motor with a three-bladed stainless-steel propeller was mounted on a 5.8 m skiff and driven at 7 or 40 km/h over surface level or propeller-depth (48 cm) fibreglass model loggerhead turtle shells (5 trials/speed/turtle depth). One of two propeller guard designs were then added to the same skiff: a horizontal-fin (Hydroshield®), or a stainless-steel cage (Prop Buddy®) and driven over propeller-depth artificial turtle shells (7 km/h: 5 trials/guard; 40 km/h: 4–5 trials/guard). A personal watercraft (jet ski) with an inboard jet motor and the 5.8 m skiff modified with an 80 hp, jet-drive outboard motor were also both driven at 7 and 40 km/h over surface-level or propeller-depth artificial loggerhead turtle shells (5 trials/speed/vessel/turtle depth). Injuries to artificial shells were classified as catastrophic if they would have killed a real sea turtle.

    Study and other actions tested
Please cite as:

Sainsbury K.A., Morgan W.H., Watson M., Rotem G., Bouskila A., Smith R.K. & Sutherland W.J. (2021) Reptile Conservation: Global Evidence for the Effects of Interventions for reptiles. Conservation Evidence Series Synopsis. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

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Reptile Conservation

This Action forms part of the Action Synopsis:

Reptile Conservation
Reptile Conservation

Reptile Conservation - Published 2021

Reptile synopsis

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