Deploy fishing gear at different depths

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

Study locations

Key messages

  • Three studies evaluated the effects of deploying fishing gear at different depths on reptile populations. One study was in each of Canada, off the coast of Mexico and the Atlantic.



  • Survival (1 study): One replicated, randomized, paired study in Canada found that no turtles died in floated nets, but some died in submerged nets.
  • Condition (1 study): One replicated, randomized, paired study in Canada found that turtles caught in floated nets were less at risk of drowning than those caught in submerged nets.



  • Unwanted catch (3 studies): Two of three studies (including two replicated studies) in Canada, Mexico and the Atlantic found that bottom-set fishing nets with fewer buoys caught fewer sea turtles than standard nets or that fewer loggerhead turtles were caught when longline hooks were set below 22 m deep, but the number of leatherback turtles caught was unaffected by hook depth. The other study found that floated and submerged nets caught a similar number of turtle species.

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, randomized, paired study in 2009–2010 in a freshwater lake in Ontario, Canada (Larocque et al. 2012) found that using floated nets did not reduce levels of unwanted catch but did reduce drowning risk (measured using blood lactate levels) and mortality in turtles caught in fyke nets. Turtle catch rates and species composition were similar between floated (0.06 turtles/hour, 35 individuals) and submerged nets (0.10 turtles/hour, 48 individuals). Blood lactate levels (a measure of drowning risk) were reduced in turtles tested in floated nets (1.3–3.5 mmol/L) compared to submerged nets (13.2–16.4 mmol/L). Turtle mortality only occurred in submerged nets (3 individuals, no statistical tests were carried out). Species composition was similar between net types (data presented as statistical model outputs). Turtle species caught included painted turtles Chrysemys picta, eastern musk turtles Sternotherus odoratus, northern map turtles Graptemys geographica, and common snapping turtle Chelydra serpentina. Target fish catch rates were similar between floated (2.5 fish/hour) and submerged nets (3.1 fish/hour). In August 2009, experimental tests of blood lactate and pH levels of turtles placed in submerged, floated and semi-submerged nets were carried out initially to test whether turtles used air spaces provided by elevating nets in the water (9–10 trials/net, see original paper for details). In April–June 2010, submerged nets (without floats) and nets with floats were deployed in pairs (two submerged nets deployed within 15 m of two floated nets) for 8–48 hours in 30 locations (1–2 m deep). Blood samples were taken from all turtles and the number caught (including mortalities) was recorded.

    Study and other actions tested
  2. A controlled study in 2007–2009 on the sea floor in Baja California Sur, Mexico (Peckham et al. 2016) found that reducing the number of buoys attached to bottom-set fishing nets reduced unwanted catch of sea turtles. Reduced-buoy nets caught fewer sea turtles (0.06 turtles/100 m of net/day) compared to standard nets (0.19 turtles/100 m of net/day). Unwanted catch included loggerhead Caretta caretta, green turtle Chelonia mydas and olive ridley turtles Lepidochelys olivacea. Average catch of target fish species was similar in both net types (reduced-buoy: 10; standard: 12 kg/100 m of net/day) although the market value of target fish was lower in reduced-buoy nets ($18/trip) compared to standard nets ($25/trip). Reduced buoy nets (1 buoy/8.5 m net) and standard nets (1 buoy/1.7 m net; both net types were 111–120 m long and 4–6 m high) were deployed in pairs for 21–25 hours at a time during summer 2007 (40 deployments), 2008 (40 deployments) and 2009 (96 deployments). The market value of target catch species was calculated based on the catch composition.

    Study and other actions tested
  3. A replicated study in 1992–2015 in pelagic longline fisheries in the Atlantic (Swimmer et al. 2017) found that longlines with deeper hooks caught fewer loggerhead turtles Caretta caretta, but bycatch of leatherbacks Dermochelys coriacea was not affected by hook depth. The chance of catching loggerheads was lower when hooks were around 22 m deep or more (data presented as statistical model results), but leatherback catch was unaffected by hook depth. Maximum hook depth was calculated by adding up the length of the floatline, branchline and dropline. Pelagic Observer Program data from (1992–2015) was used to determine the number of turtles caught/1,000 hooks. Variation in practices relating to hook depth was used to test its effect on bycatch.

    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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