Action

Install exclusion devices on fishing gear: Tortoises, terrapins, side-necked & softshell turtles

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

Study locations

Key messages

  • Thirteen studies evaluated the effects of installing exclusion devices on fishing gear on tortoise, terrapin, side-necked & softshell turtle populations. Ten studies were in the USA, two were in Canada and one was in Australia.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (1 STUDY)

  • Survival (1 study): One replicated, randomized, paired, controlled study in the USA found that fewer turtles died in hoop nets with an exclusion device than in unmodified traps.

BEHAVIOUR (1 STUDY)

  • Use (1 study): One randomized, controlled trial in the USA found mixed effects of crab pot exclusion devices on use of pots by diamondback terrapins depending on the device design.

OTHER (13 STUDIES)

  • Unwanted catch (13 studies): Eight of 13 controlled studies (including seven replicated, paired studies) in the USA, Australia and Canada found that crab pots, fyke nets, hoop nets and eel traps with exclusion devices caught fewer turtles, diamond back terrapins and short-necked turtles than unmodified gear. Two studies also found that modified gear caught smaller short-necked turtles and diamondback terrapins than unmodified gear. Three studies found mixed effects of exclusion devices on unwanted catch of turtles and diamondback terrapins depending on the device design. The other two studies found that that crab pots with wire exclusion devices or magnetized exclusion devices caught a similar number of diamondback terrapins compared to unmodified pots. One study also found that crab pots with wire exclusion devices caught larger diamondback terrapins than pots with plastic exclusion devices.

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, controlled study in 1996–1997 in an estuarine river in Maryland, USA (Roosenburg & Green 2000) found that after fitting rectangular exclusion devices (‘bycatch reduction device’) to crab pots, unwanted catch of diamondback terrapins Malaclemys terrapin tended to be lower. In 1996, no terrapins were caught in crab pots modified with a 4 × 10 cm exclusion device, compared to 21 terrapins in unmodified pots. In 1997, fourteen terrapins were caught in pots with a 4.5 × 12 cm exclusion device and 56 in pots with a 5 × 10 cm exclusion device, compared to 105 in unmodified pots (results were not statistically tested). Blue crab Callinectes sapidus catch was 2 crabs/pot/day lower when 4 x 10 cm devices were used compared to unmodified pots. Neither the 4.5 x 12 cm nor the 5 x 10 cm excluder device affected crab catch. Three sizes of 11-gauge galvanised wire exclusion devices were tested on modified and unmodified crab pots (standard: 60 cm square, tall: 60 x 60 x 180 cm). In 1996, fourteen unmodified and 14 pots modified with a 4 × 10 cm exclusion device were used (50 days total fishing). In 1997, ten unmodified and 20 pots modified with either 4.5 x 12 cm or 5 x 10 cm exclusion devices were used (10 pots/excluder type, 42 total fishing days). Traps were checked and baited daily.

    Study and other actions tested
  2. A replicated, controlled study (years not provided) in two upper tidal creeks in New South Wales, Australia (Lowry et al. 2005) found that using 100 mm exclusion rings on eel Anguilla spp. traps reduced unwanted catch of short-necked turtles Emydura macquarii. Fewer turtles were caught in traps modified with exclusion rings (8 individuals) compared to unmodified traps (54). Most turtles caught in modified traps were smaller than those caught in unmodified traps (see paper for details). Commercially-targeted eel catch (numbers and size of eels) was similar between modified (49 individuals caught overnight, 21,005 g total catch weight) and unmodified traps (25 individuals, 8,535 g). Standard commercial eel traps (50 cm wide x 40 cm high x 90 cm long mesh traps) had 100 mm PVC rings placed in the entrance funnel. In one site three traps with exclusion rings and three unmodified traps were fished overnight and in a second site, four traps with rings and four unmodified traps were fished for 5 h during the day, cleared, and then fished overnight (12 h). Traps were baited with frozen pilchards Sardinops neopilchardus.

    Study and other actions tested
  3. A replicated, randomized, controlled, paired study in 2006 along a river in Missouri, USA (Fratto et al. 2008) found that hoop nets modified with an excluder device caught fewer turtles and fewer target fish than unmodified hoop nets in a catfish fishery. Modified hoop nets caught fewer turtles (18 turtles caught in 11 of 50 nets) than unmodified nets (166 turtles caught in 33 of 50 nets). Ten of 18 turtles (56%) died in modified nets compared to 101 of 166 turtles (61%) in unmodified nets (results were not statistically tested). Fewer target catfish species were caught in modified nets (15 individuals) compared to unmodified nets (70 individuals). Unmodified hoop nets (six hoops, 90 cm maximum hoop with 38 mm mesh, 3.7 m long) and modified hoop nets (addition of a tight mesh covering the net entrance to reduce the entrance to 30 cm diameter) were deployed in pairs along four river stretches in May–July 2006 (50 nets/type) using a randomized block design. Nets were set for 48 hours at a time over nine weeks. The catch of turtle and commercially targeted catfish species was recorded.

    Study and other actions tested
  4. A replicated, paired, controlled study in 2005–2006 in 34 sites in three river systems in Missouri, USA (Fratto, Barko & Scheibe 2008) found that modifying fyke nets (a ‘bycatch reduction device’) by tying ropes across the entrance resulted in fewer turtles being captured compared to when nets were not modified. Fewer turtles were caught in modified nets (331) compared to unmodified nets (1,355). The average number of turtles caught/night was lower for three of nine species in modified (0–0.2 turtles/night) compared to unmodified nets (0.6–1.4 turtles/night) and similar in modified and unmodified nets for the remaining six species (see paper for details). There was no significant difference in the number of fish caught (modified: 478; unmodified: 415), the number of fish species caught (modified: 23; unmodified: 29), or average catch/night (modified: 0–5 fish/night; unmodified: 0–3 fish/night) in modified compared to unmodified nets. The fyke net was modified by tying four braided ropes (3 mm) vertically (38 mm apart) and three horizontally across the entrance. In 2005–2006, pairs of modified and unmodified nets were deployed ≥100 m apart at 34 sites, including rivers, side channels, backwaters and floodplains. Nets were deployed for 24 hours at each site, and all turtles and fish were counted, identified to species and released.

    Study and other actions tested
  5. A replicated, paired, controlled study in 2008 in tidal creeks in Virginia, USA (Rook et al. 2010) found that using a plastic rectangular device (a ‘bycatch reduction device’) to reduce the size of entry holes to crab pots reduced the unwanted catch of diamondback terrapins Malaclemys terrapin in a blue crab Callinectes sapidus fishery. Crab pots with devices caught fewer terrapins (0.01 terrapins/pot/day, 2 individuals) compared to pots without devices (0.20 terrapins/pot/day, 46 individuals). Terrapins caught in traps with devices were smaller on average (5.1 cm shell depth) than terrapins caught in traps without devices (4.3 cm shell depth). Commercially-targeted blue crabs caught in pots with devices had 1.5–2.0 mm wider shells than crabs caught in pots without devices. Catch rates and weight of commercially-targeted crabs were similar between pots with and without devices (see original paper for details). Devices were 4.5 x 12 cm plastic rectangles that were fitted on each of the four entrances of a recreational-style crab pot with chimney (see original paper for details). Crab pots were deployed in shallow-water in 10 pairs in two creeks in summer 2008 (one with and one without devices fitted). Traps were baited once a week for four weeks and checked after 48 h. Terrapin catch was only monitored on one creek.

    Study and other actions tested
  6. A replicated, paired, controlled study in 2000–2004 in coastal waters in North Carolina, USA (Hart & Crowder 2011) found that after rectangular devices were placed at crab pot entrances (a ‘bycatch reduction device’), the unwanted catch of diamondback terrapins Malaclemys terrapin tended to be lower in a commercial blue crab Callinectes sapidus fishery. No terrapins were caught during hard-shell crab fishing and five terrapins were caught during peeler crab fishing in pots modified with excluders (peeler crab 4.3 cm excluder: 0 individuals, 5.0 cm excluder: 2, vertical ties: 3). Hard shell crab catch was lower in pots with smaller excluders (4.0 cm excluder: 1,002 individuals, 4.5 cm excluder: 459) compared to unmodified pots (625–1,270), but similar when pots with the largest excluder were used (365 individuals) compared to unmodified pots (386). Peeler crab catch was similar in modified pots (372–376 individuals) compared to unmodified pots (374). In May–June 2000–2001 and September–November 2000, hard crab were fished for using pots (60 x 60 x 60 cm) in 21 pairs (with and without excluder devices). Three rectangular excluder devices were tested/season: 16 x 4 cm, 16 x 4.5 cm, and 16 x 5 cm (75 fishing days, 3,150 crab pot days). In April–May 2004, peeler crabs were fished in blocks of four pots with either unmodified, or one of three excluders: 16 x 4.3 cm rectangle, 15.2 x 5.1 cm rectangle, or two vertical wire ties/entrance set 7.8 cm apart (19 fishing days, 1,672 total crab pot days).

    Study and other actions tested
  7. A replicated, randomized, paired, controlled study in 2010 in a shallow freshwater lake in Ontario, Canada (Larocque et al. 2012, same experimental location as Cairns et al. 2013) found that adding exclusion bars to fyke nets (a ‘bycatch reduction device’) reduced turtle bycatch but that adding exclusion rectangles did not. Fyke nets modified with exclusion bars captured fewer turtles (0.03 turtles/hour) compared to unmodified nets (0.1 turtles/hour). In separate trials, nets modified with an exclusion rectangle captured similar numbers of turtles (0.02 turtles/hour) compared to unmodified nets (0.04 turtles/hour). Catch rates of target fish species were similar in exclusion bar nets (2.9 fish/hour), exclusion rectangle nets (2.6 fish/hour) and unmodified nets (2.9 fish/hour). Standard commercial hooped fyke nets (see original paper for details) were set in a shallow freshwater lake (788 ha) in April–June and September–October 2010 in pairs of modified and unmodified nets. Nets were either modified with exclusion bars made of wooden dowels (8 x 1.3 cm spaced 8 cm apart; set at 30 sites in April–June) or an exclusion rectangle made by attaching a hose clamp at the first funnel of the fyke net (18 x 7.5 cm rectangle; set at 15 sites in September–October). Tandem modified and unmodified nets were set fully submerged within 15 m of each other for 8–48 hours at a time.

    Study and other actions tested
  8. A replicated, paired, controlled study in 2011–2012 in a shallow freshwater lake in Ontario, Canada (Cairns et al. 2013, same experimental location as Larocque et al. 2012) found that modifying fyke nets with a rectangular excluder device (‘bycatch reduction device’) reduced unwanted catch of turtles in a freshwater fishery. In a first-year smaller scale trial, nets modified with exclusion devices caught statistically similar numbers of turtles (3–4 turtles) compared to unmodified nets (11). The catch of target and non-target fish was also statistically similar between modified (109–144 individuals) and unmodified nets (224). However, in the second year larger scale trial, unwanted catch of turtles was lower in modified nets (0.03 turtles/trapping effort) compared to unmodified nets (0.13). Target species catch was also lower in modified nets (0.64 individuals/trapping effort) compared to unmodified nets (0.95). In September 2011, two fyke nets connected by an entrance net (7 hoops/net, 0.91 m diameter) were deployed in a shallow lake (2.8 m average depth, 780 ha total area) in threes: unmodified net, modified with a 22.5 x 5 cm copper rectangle, or modified with 5 cm spaced vertically-oriented bars across the mouth of the net (nine groups of nets in one site). In April–June 2012, the set up was repeated twice at 11 sites, but did not include nets with the barred exclusion device. All target and non-target catch was identified, counted and released.

    Study and other actions tested
  9. A randomized, controlled study (years not provided) in a brackish water experimental enclosure in South Carolina, USA (McKee et al. 2016) found that using vertically-oriented rectangular devices to limit the size of entry holes on crab pots (a ‘bycatch reduction device’) reduced the number of entries, increased the time taken to enter and reduced the proportion of successful entry attempts by diamondback terrapins Malaclemys terrapin to crab pots. A vertically-oriented device reduced the number of entries into the pot (2 entries/terrapin) compared to horizontally-oriented devices and no device, which produced similar results (horizontal: 5; no device: 6 entries/terrapin). Vertically-oriented devices increased the average time taken to enter a pot (58 seconds before entry) compared to no device, whereas time to enter horizontally-oriented devices was similar to no device (horizontal: 32; no device: 19 seconds before entry). The proportion of terrapins that entered a pot after investigating it was reduced when a vertically-oriented device was used (0.1 terrapins entered/investigation), compared to a horizontally-oriented device (0.2 terrapins entered/investigation). Both types of device reduced the rate of terrapins entering pots compared to no device (0.3 terrapins entered/investigation). In total, 38 wild terrapins were caught to take part in the study, all of a size where they could enter a crab pot when an opening limiting device was present. Each terrapin participated in three randomly ordered trials: vertically-oriented device fitted to entry holes, horizontally-oriented device fitted to entry holes and no device. Devices were 5.1 x 15.2 cm. Crab pots with chimneys baited with mackerel were used. Terrapins were monitored by webcam in 3 h videos (27 h total footage, 3 h/treatment/study group).

    Study and other actions tested
  10. A replicated, randomized, controlled study in 2014–2015 in three brackish tidal creeks and a captive setting in Virginia, USA (Corso et al. 2017) found that modifying crab pots with red-painted rectangular funnels to reduce the size of entry holes (a ‘bycatch reduction device’) reduced the unwanted catch of diamondback terrapins Malaclemys terrapin compared to unmodified pots in a blue crab Callinectes sapidus fishery. Crab pots with red-painted funnels set in three tidal creeks caught fewer terrapins (10 individuals) than unmodified pots (58 individuals; no statistical tests were carried out). Trials in both a captive setting and in two tidal creeks found that red-painted funnels also reduced unwanted terrapin catch compared to unmodified pots and that orange, green and blue-painted funnels caught a similar number of terrapins to unmodified pots (see original paper for details). For crab pots set in three creeks, commercially-targeted legal-size blue crab catch was similar in pots with red-painted funnels (622 individuals) compared to unmodified pots (630 individuals). In a captive setting, crabs stayed in pots with funnels for longer (4 h, 45 individuals) compared to unmodified pots (1 h, 76 individuals). Red plastic rectangular funnels (5.1 x 15.2 cm) were fitted horizontally to each of the four entry points on 15 commercial-style crab pots with chimneys. The 15 modified pots were deployed paired with 15 unmodified pots in June–July 2015 in three creeks (587 trap nights, 3–6 pairs of pots/creek). All pots were baited. In separate trails in two creeks (June–July 2014) and in a captive setting (June 2015), orange, black, blue, green-painted and magnetized funnels were also tested (see original paper for details). The captive setting was a seawater tank and crabs and terrapins were monitored by video.

    Study and other actions tested
  11. A replicated, controlled study in 2014 in two brackish tidal creeks in Virginia, USA (Corso et al. 2017) found that modified crab pots with wire rectangular funnels to reduce the size of entry holes (a ‘bycatch reduction device’) caught similar numbers of diamondback terrapins Malaclemys terrapin compared to unmodified pots and larger-sized terrapins compared to plastic rectangular funnels in a blue crab Callinectes sapidus fishery. Crab pots with wire funnels caught similar numbers of terrapins (22 individuals) compared to unmodified pots (20 individuals; no statistical tests were carried out). Terrapins caught in pots with wire funnels were larger (5.5 cm shell height) compared to terrapins caught in pots with plastic funnels (4.8 cm shell height). Commercially-targeted legal-size blue crab catch was similar in pots with wire funnels compared to unmodified pots (see original paper for details). Copper wire or plastic rectangular funnels (5.1 x 15.2 cm) were fitted horizontally to each of the four entry points on commercial-style crab pots with chimneys (five with copper wire funnels, 15 with plastic funnels, five with plastic magnetized funnels, and five unmodified pots). In total five groups of modified and unmodified pots were deployed in June–July 2014 at least 50 m apart in two locations (327 trap days). All pots were baited.

    Study and other actions tested
  12. A replicated, randomized, controlled study in 2014–2015 in brackish tidal creeks and a captive setting in Virginia, USA (Corso et al. 2017) found that modified crab pots with magnetized rectangular funnels to reduce the size of entry holes (a ‘bycatch reduction device’) caught similar numbers of diamondback terrapins Malaclemys terrapin compared to unmodified pots in a blue crab Callinectes sapidus fishery. Crab pots set in two creeks with magnetized funnels caught similar numbers of terrapins (15 individuals) compared to unmodified pots (20 individuals; no statistical tests were carried out). Trials in a captive setting found similar results (see original paper for details). Commercially-targeted legal-size blue crab catch was similar in pots with magnetized funnels compared to unmodified pots (see original paper for details). Rectangular funnels (5.1 x 15.2 cm) were fitted horizontally to each of the four entry points on commercial-style crab pots with chimneys (four with magnetized funnels, 5 unmodified pots). Modified and unmodified pots were deployed in June–July 2014 at least 50 m apart in two creeks (327 trap days, 1–3 pots of each type/creek). All pots were baited. Separate trials in a captive setting were carried out in June 2015 and took place in a seawater tank and crabs and terrapins were monitored by video.

    Study and other actions tested
  13. A replicated, controlled, paired study in 2012–2013 in five estuarine sites in North Carolina, USA (Chavez & Williard 2017) found that crab pots fitted with one of two different sized wire rectangles (‘bycatch reduction devices’) limiting the size of the pot opening reduced the numbers of diamondback terrapins Malaclemys terrapin caught, compared to unmodified pots. No statistical analyses were carried out due to a small sample size. Pots modified to have small-sized openings and large-sized openings caught zero and one terrapin respectively, compared to 13 terrapins in unmodified pots. None of the terrapins caught were small enough to enter pots with the small-sized opening. One terrapin died in an unmodified pot. Standard commercial crab pots (61 × 61 × 61 cm) were modified with galvanized wire to create large-sized openings (5.1 × 15.2 cm, 10 pots), small-sized openings (3.8 × 15.2 cm, 10 pots) or were unmodified (20 pots). Pots were deployed June–July in pairs (one modified pot and one unmodified pot in 4 sites) in 2012 or in triplicate (1 of each size of modified pot with a single unmodified pot in 2 sites) in 2013. Pots were baited and submerged for 48 hours at a time.

    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.

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