(Tteds) for Bycatch Reduction in Suriname's Seabob Shrimp Trawl Fishery

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Sustainable Management of Bycatch in Latin America and Caribbean Trawl Fisheries

REBYC-II LAC - SURINAME

Evaluating Trash-and-Turtle Excluder Devices (TTEDs) for bycatch reduction in Suriname’s seabob shrimp trawl fishery

- December 2017 -

By Pieter Meeremans1, Tomas Willems2 and Yolanda Babb-Echteld3

1Technical Project Assistant REBYC-II LAC 2National Project Coordinator REBYC-II LAC 3Act. Director of Fisheries Ministry of Agriculture, Animal Husbandry and Fisheries (LVV), Fisheries Department Cornelis Jongbawstraat 50, Paramaribo, Suriname Contact: [email protected]

1. Introduction

Atlantic seabob shrimp (Xiphopenaeus kroyeri) is a relatively small species of penaeid shrimp with a short life span, living in shallow coastal waters on a high mud content bottom (e.g. Freire et al., 2011). The species has a wide geographical range, occurring along the Central Western Atlantic coast from Sao Paulo (Brazil) in the south up to Florida (USA) in the north. In Suriname, seabob is traditionally caught with fyke nets in estuaries (Babb-Echteld, 2008).

Since 1996, industrial trawl fisheries started targeting seabob as well (Chin-A-Lin and IJspol, 2000; LVV Fisheries Department, 2013). Industrial seabob fisheries use ‘Florida-type’ demersal outrigger trawlers, equipped with paired trawls in ‘quad rig’ configuration (Southall et al., 2011). The trawls are similar to those employed for the marine shrimp (Penaeus spp.) fishery, but they are geared in top-off position. The number of licenses issuable for the seabob trawl fishery is 22 (LVV Fisheries Department, 2013). Seabob shrimp are landed on ice at one of the two Surinamese processing companies (SAIL and Heiploeg Suriname) were the shrimp are peeled, graded, and frozen for export to Europe and the US. The industrial seabob shrimp fishery lands between 6,000 and 10,000 tons annually, making it the third highest producer of Atlantic seabob shrimp in the world (FAO, 2014).

In the seabob trawl fishery off Suriname, as many as 40 different species of non-target catch might occur in a single haul (Southall et al., 2011). To reduce this unwanted bycatch, the fishery currently uses Turtle Excluder Devices (TEDs) with a 4” bar spacing and square-mesh panel Bycatch Reduction Devices (BRDs) with 150 mm stretched mesh size (LVV Fisheries Department, 2010) (Fig 1). The TEDs and BRDs were introduced in the seabob fishery in 1999 and 2009, respectively, and have proven to be quite effective in reducing bycatch. An evaluation of the square-mesh panel BRD has shown that it causes an average 34%-reduction in overall fish bycatch (by weight) (Polet et al., 2010). Bycatch of marine turtles is assumed to be negligible due to the use of TEDs (S. Hall, pers. comm.).

Figure 1. Sketch of a trawl codend fitted with a square-mesh panel Bycatch Reduction Device (BRD) and super-shooter Turtle Excluder Device (TED) (source: Willems et al., 2016)

Despite the positive effects of TEDs and BRDs, an analysis of the catch composition in the seabob fishery has shown that bycatch still accounts for ca. 40% of the catch (by weight) in the seabob fishery (Willems et al., 2016b). Bycatch is dominated by fish (31 ± 14 % of the total catch), followed by jellyfish (8 ± 10 %) and benthos (benthic invertebrates; 2 ± 3 %). Only a small proportion of the bycatch (4% of the total catch) is utilized, with three fish species representing the bulk of the retained bycatch: Macrodon ancylodon (local name: bangamary/dagutifi), Cynoscion virescens (trout/kandratiki) and Nebris microps (butterfish/botrofisi). Whenever caught, prawns (Penaeus subtilis = brown shrimp) are also retained.

Most of the bycatch in the seabob fishery, however, is discarded. Small “trash fish”, including juvenile individuals of the commercial species Cynoscion similis/jamaicensis (witwitie), C. virescens, M. ancylodon, N. microps and Ariidae spp. (catfishes) represent an important part of the discards. Discards also include several species of elasmobranchs (cartilaginous fishes) such as Dasyatis geijskesi (sharpsnout stingray), Dasyatis guttata (longnoze stingray), Urotrygon microphthalmum (smalleyed round stingray), Gymnura micrura (smooth butterfly ray), Dasyatis americana (southern stingray), Rhinoptera bonasus (cownose ray), Rhinobatos percellens (guitarfish) and Narcine bancroftii (electric ray) (Willems et al., 2016a; Willems et al., 2016b). Due to their life cycle characteristics, elasmobranchs are generally known to be vulnerable to fishing mortality (e.g. Graham et al., 2001; Goodwin et al., 2002; Fisher et al., 2013), and several species are therefore categorized as Endangered, Threatened or Protected (ETP) species. According to the IUCN Red List of Threatened Species, D. geijskesi and R. bonasus classify as “Near Threatened”, while the N. bancroftii is listed as “Critically Endangered” (IUCN, 2016).

The impact of the seabob fishery on these stingray species has raised conservation concerns in the management of the fishery. Because it was unclear how frequently the rays occurred in the bycatch of the fishery, and to what degree the TED actually reduced their capture, a catch- comparison study was conducted in 2012-2013 (Willems et al., 2016a). It appeared that TEDs were efficient in excluding large-sized rays from the trawls, while smaller rays passed through the bars of the TED and ended up being caught. Hence, high escape ratios were observed for D. geijskesi (77%), a large-sized species, while exclusion of the small species U. microphthalmum was not significant. A size-dependent escape was observed for the two most abundant mid-sized ray species D. guttata and G. micrura. The study concluded that ray bycatch in the Suriname seabob fishery might be further reduced by introducing TEDs with reduced bar-spacing (Willems et al., 2016a).

TEDs with reduced bar spacing (2” instead of 4”) have been successfully tested in shrimp fisheries in the Gulf of Mexico (Nalovic, 2014) and French-Guiana, where they are referred to as TTEDs (Trash-and-Turtle Excluder Devices) (Fig. 4 b and c) (Nalovic & Rieu, 2010; WWF, 2010). Based on these positive results, the Suriname Fisheries Department decided to conduct trials with TTEDs in the seabob trawl fishery.

The goal of this study is to evaluate the performance in bycatch reduction of TTEDs with 2” and 3” bar-spacing, against the standard 4” TEDs. More specifically, the study aimed to assess whether these TTEDs are effective in reducing elasmobranch bycatch, while retaining the target seabob shrimp catch. Further, the effect of TTEDs on retained bycatch was addressed. The study therefore aimed to answer the following questions:

(1) Do TTEDs cause a reduction in overall bycatch? (2) Do TTEDs cause a reduction in bycatch of elasmobranch species? (3) Do TTEDs affect catches of seabob shrimp? (4) Do TTEDs affect catches of commercially valuable bycatch species?

The result of this study will help to identify an optimal bar spacing for the TEDs in the Suriname seabob fishery, balancing bycatch reduction with retention of both target catch and valuable bycatch. This approach is in line with the Suriname Fisheries Management Plan 2014 – 2018 (LVV Fisheries Department, 2013), and with the goals of the project ‘Sustainable Management of Bycatch in Latin America and Caribbean Trawl Fisheries (REBYC-II LAC). The Suriname seabob fishery is certified by the Marine Stewardship Council (MSC) in 2011 (Southall et al., 2011). By investigating the potential of TTEDs in reducing bycatch, this study will help the fishery in optimizing the fishing gear and keep improving the fishery to comply with the MSC standards for sustainable fisheries.

2. Material and methods

2.1. Study area

The study was carried out on commercial seabob fishing grounds which are delimited by the 10 and 15 fathoms depth contours and up to 18 fathoms in the eastern part of the shelf (LVV Fisheries Department, 2010). The experimental hauls were mainly performed in the eastern part of the study area (Fig. 2).

Figure 2. Study area with the location of the experimental hauls. Start locations of sampled hauls of July (3” TTED; red circles ) and of August (2” TTED; green squares ).

2.2. Sea trials and gear specifications

The experimental catch-comparison hauls were performed during two fishing trips, on board FV Neptune-6 (1-6 July 2016) and FV Sechong-29 (9-12 August 2016). Both vessels are typical ca. 20m-long, 425 hp ‘Florida-type’ outrigger trawlers of the seabob shrimp trawling fleet. They are equipped for quad-rig bottom trawling, dragging a double trawl at both starboard and port side (Fig. 3). The trawls had a vertical opening of ca. 2 m and tickler chains attached to the footrope. Mesh size of the trawls ranged from 57 mm in the body and wings of the trawl to 45 mm in the codend (Willems et al., 2016b).

Figure 3. The quad-rig trawl configuration as used in the Suriname seabob fishery. The outer codends (1 and 4) are used for the side-by-side catch comparisons. The inner two codends (2 and 3) are not used because the try-net (5) might interfere with the catches in these trawls. Figure adapted from Eayrs, 2012.

Besides the four main trawls, the seabob fishery uses a try-net to assess potential shrimp catches before and during fishing operation. The try-net, however, might interfere with the catches of the inner codends (2 and 3). Therefore, only the outer codends (1 and 4) were used in the catch comparison experiments (Fig. 3). One of the outer codends was modified to fish as experimental codend by replacing the standard TED with a TTED. Because port and starboard trawls rarely fish with an identical efficiency, the experimental codend was regularly switched between trawl 1 and 4 to cancel out these port/starboard effects. An equal amount of hauls was completed with the experimental codend fitted to trawl 1 and 4. Apart from the replacement of the standard TED by a TTED, the sea trials were done under commercial fishing circumstances, implying that aspects like vessel type, fishing gear, trawling speed, etc. were identical to normal fishing operation.

The control TED was the standard TED used in the commercial seabob fishery: an aluminum super-shooter TED with eight round vertical bars with a diameter of 1.6 cm and a bar spacing of 4” (10.2 cm) . The experimental TTEDs had bar spacings of 3” (7.6 cm) and 2” (5.1 cm). These were flat-bar TEDs, with flattened vertical bars (0.5 cm wide and 3.8 cm deep) to reduce drag through the water. The 13 (3” TTED) or 18 (2” TTED) vertical bars were supported by a horizontal pipe near the middle of the device. Both TEDs and TTEDs measured 130 x 107 cm and were installed in a downward-excluding configuration with a double net flap covering the bottom escape opening. Codends fitted with TED or TTED were referred to as ‘control’ or ‘experimental’ codend respectively.

a) TED (4”) b) TTED (3”) c) TTED (2”)

Figure 4. a) Turtle Excluder Device (TED) with a bar spacing of 4” (Willems et al., 2016a). b) Trash-and-Turtle Excluder Devices (TTEDs) with a bar spacing of 3” (adapted from Nalovic, 2014) and c) 2” (Nalovic 2014). A is the aluminium pipe of the frame, B the bar spacing, C the flat bars of the grid, D the structural support pipe and E the height and width of the frame of the TTEDs (see text for the dimensions).

Figure 5. From left to right: 4” TED, 3” TTED and 2” TTED © Tomas Willems.

2.3. Sampling

At the end of every haul the nets were brought on deck. To assure separation of the catches, codends 2 and 3 were emptied together near the ice hole on the middle of the deck, while experimental and control codends (i.e. codend 1 and 4) were emptied each in an opposite corner of the deck, near the stern of the ship. While the crew processed the catch of codend 2 and 3, the content of experimental and control codend was shoveled separately into baskets and weighted. In each case, half a basket was set aside as a catch subsample. Consequently, the ‘total catch’ (TC) of both experimental and control codend consisted of the ‘subsample’ (SS, 1/2 basket) and the ‘main catch’ (MC, the remainder of the TC) (Fig. 6).

MAIN CATCH (MC) cmlkjlkcddddddCAT CHcmlkjmlkqdmflkj TOTALqmdlkqjmflkq(qsfdf CATCH (TC) qd(MC) SUBSAMPLE (SS)

Figure 6. Picture of catch (of codend 1 or 4) put into baskets, indicating the difference between total catch, main catch and subsample ©Nick Hopkins/NOAA

The different catch components were processed as follows. From the MC all elasmobranchs and four commercial fish species (M. ancylodon, C. jamaicensis/similis, C. virescens and N. microps) larger than 25 cm (minimum retention size; hereafter termed adults), were selected, counted and weighed. The SS was emptied on the sorting table and broken down in the following components:

- Seabob: the target species X. kroyeri - Prawns: larger shrimps of the genus Penaeus - Commercial fish species: M. ancylodon, C. jamaicensis/similis, C. virescens, N. microps (all sizes) - Elasmobranchs - Trash: all other fish, jellyfish, debris, etc.

The bulk weight of the fractions seabob, prawns and trash were taken. Elasmobranchs and commercial fish were sorted out per species and measured (total length for finfish and sharks, disk width for rays). When species occurred abundantly, thirty randomly selected individuals were measured and the remainder was counted. Altogether, the following raw data were collected on board, for both the experimental and control codend:

- TC weight - Weight and number of elasmobranchs in the MC - Weight and number of four commercial fish species (adults) in the MC - Weight of seabob, prawns and trash in SS - Number and length of elasmobranchs and commercial fishes (all sizes) in SS

2.4. Data preparation

All data were raised to represent weights in the TC. Therefore, (1) length-frequency distributions (LFDs) were extrapolated to the SS level in case of abundant (>30) individuals in the SS, (2) LFDs were converted to weight in the SS and (3) weights in the SS were raised to weight in the TC. Length-weight conversions were done using the common formula W = a * Lb, where W is fish wet- weight and L the length (standard length or fork length). The parameters a and b were obtained from FishBase (www.fishbase.org). The length-weight conversion was done for each length class of each species, in each subsample. The total weight of each length class was then obtained by multiplying the outcome of the formula (W = a * Lb) with the total count in that length class. Finally, the weights of the different length classes were summed to obtain the total weight per species in the subsample. Raising of SS weight data to the TC level was done using the subsample factor SF, which was calculated as:

SF = weight TC / weight SS

In summary, the following data were prepared at the TC level, for both experimental and control codend in each haul:

• Weight of seabob, prawns and trash (raised weight from SS)

• Weight of elasmobranchs (weight in MC + weight in SS (after extrapolating LFDs to SS and L-W conversion)) • Weight of commercial fish species >25 cm (weight in MC + weight in SS (after extrapolating LFDs to SS and L-W conversion)) • Weight of juvenile commercial species (raised weight from SS, after extrapolating LFDs to SS and L-W conversion)

For the data analysis, ‘total bycatch’ was defined as ‘total catch (TC) minus seabob (the target catch)’. ‘Total bycatch’ was further subdivided in ‘discarded bycatch’ (trash + elasmobranchs + juvenile commercial fishes) and ‘retained bycatch’ (prawns + adult commercial fishes).

2.5. Data analysis

Average catch composition

To get a general overview of catch composition during the sea trials, the average weight of the different catch components was calculated. This was done for the catch of control and experimental codend together.

CPUE analysis

Weight data were converted to catch-per-unit-effort (CPUE; expressed as kg per trawling hour) by dividing the weights by the haul duration (in hours). The CPUEs of all catch components were compared between the control and the experimental codend. Paired t-tests were used to assess whether observed differences between average CPUEs in experimental and control codends were statistically significant. Prior to testing, a Sharipo-Wilk test was used to check for normality in the differences between paired data. Whenever data transformation (log transform) did not improve the distribution, the non-parametric Wilcoxon signed-rank test was used instead of the paired t-test.

Length analysis

Length analyses were used to evaluate the size-selectivity of the TTEDs. This was done by calculating the proportion of fish, of a certain species and length, that is retained in the control versus experimental codend. This proportion can be expressed (for each size and each haul) as:

φ(S) = NS,exp / (NS,exp + NS,control), where φ(S) is the probability of catching an individual at length

S in the experimental codend, and NS,exp and NS,control are the numbers of fishes at size S retained in the experimental and control codends respectively. Selectivity curves were made by plotting the probability φ(S) against the size S.

Length analysis was performed based on the subsample, as only the length of these individuals were measured. To assure sufficient statistical power, selectivity curves were only produced for the samples of those species containing more than 20 individuals.

Data were analyzed using R version 3.2.5 (R Development Core Team 2016). In the results section, mean values are reported together with their standard deviation (SD).

3. Results

3.1 . 3” TTED

3.1.1. General test results

Trials with the 3” TTED were conducted in July 2016. Thirty-two comparative hauls were conducted, of which 31 were sampled. Tow duration varied between 1.7 and 2.5 hours (2.1 ± 0.1 h; Mean ± SD). Total catch of the control and experimental combined averaged 211.1 ± 80.0 kg. Seabob shrimp represented on average 47% (93.7 ± 35.7 kg) of this catch, followed by trash (33%), juvenile commercial fish (9%), prawns (6%), adult commercial fishes (4%) and elasmobranchs(1%) (Table 1; Fig.7). Only four small sharks were caught during the trials, which were omitted from the analyses. Elasmobranchs therefore constituted only of stingsray species (D. guttata, D. geijskesi, G. micrura and U. microphthalmum) and are further referred to as “rays”.

Table 1. Average catch composition during the 3” TTED trials, expressed as mean weight per catch component of the control and experimental codend combined Species Average catch per haul (kg; mean ± SD) Seabob 93.7 ± 35.7 Prawns 10.9 ± 26.3 Adult commercial fishes 9.4 ± 10.7 Rays 2.0 ± 3.2 Juvenile commercial fishes 17.6 ± 14.3 Trash 65.1 ± 26.4 Total 211.1 ± 80.0

Figure 7. Average catch composition during the 3” TTED trials , represented as percentages (by weight) of the combined total catch of control and experimental codend. Discarded bycatch is represented by shaded areas.

3.1.2. CPUE analysis

Total catch-per-unit-effort (CPUE) in control (51.5 ± 20.9 kg/h) and experimental (49.1 ± 19.4 kg/h) codends did not differ significantly (p > 0.05) (Table 2). Total bycatch was lower in the experimental codend (22.6 ± 12.4 kg/h) compared to the control codend (27.4 ± 14.2 kg/h) (p = 0.04314). Looking separately at discarded and retained bycatch, no significant differences were observed. The catch rates of trash, prawns and commercial fish species (both adults and juveniles) did not differ either. The experimental codend, however, caught 44 % less rays (0.3 ± 1.0 kg/h) than the control codend (0.5 ± 0.8 kg/h). Further, the experimental codend showed a 16 % higher catch rate of seabob (24.0 ± 10.8 kg/h) than the control codend (20.6 ± 8.9 kg/h) (p = 0.04765) (Table 2, Fig. 8, Fig. 9).

Table 2. Comparison of catch-per-unit-effort (CPUE; in kg/h) between control codend (4” TED) and experimental codend (3” TTED). Mean and standard deviation of the different catch components are displayed, together with the p-value of the paired t-test or Wilcoxon signed-rank test. Asterisks indicate significant differences (* = p < 0.05). Discarded bycatch includes trash, juvenile commercial fish species and rays. Retained bycatch consists of adult commercial fish species and prawns. Overall bycatch is the sum of the discarded and retained bycatch.

Control 4” TED Experimental 3” TTED p test (kg/h) (kg/h) Mean SD Mean SD Total catch 51.5 20.9 49.1 19.4 0.086 Paired t-test Total bycatch 27.4 14.2 22.6 12.4 0.043* Paired t-test

Discarded bycatch 21.7 12.3 18.6 9.8 0.176 Paired t-test Retained bycatch 5.7 9.3 4.0 5.3 0.236 Wilcoxon test Seabob 20.6 8.9 24.0 10.8 0.048* Paired t-test Prawns 3.3 8.9 1.9 4.6 0.455 Wilcoxon test Trash 16.7 8.9 14.3 7.7 0.225 Paired t-test Rays 0.5 0.8 0.3 1.0 0.015* Wilcoxon test Commercial fish species 9.4 11.1 7.4 9.4 0.085 Paired t-test ALL SIZES M. ancylodon 5.4 8.9 4.6 9.1 0.244 Paired t-test N. microps 1.0 1.7 0.5 0.9 0.163 Wilcoxon test C. virescens 0.9 2.3 0.5 1.0 0.487 Wilcoxon test C. jamaicensis/similis 2.2 2.4 1.7 2.1 0.278 Paired t-test Commercial fish species 2.4 2.8 2.1 2.4 0.173 Paired t-test ADULTS M. ancylodon 0.7 1.4 0.5 0.9 0.230 Wilcoxon test N. microps 0.7 0.9 0.7 0.8 0.788 Wilcoxon test C. virescens 1.0 1.8 0.9 1.7 0.459 Paired t-test C. jamaicensis/similis 0.0 0.1 0.0 0.0 0.093 Wilcoxon test Commercial fish species 4.5 4.3 3.9 3.6 0.622 Wilcoxon test JUVENILES M. ancylodon 2.6 3.2 2.2 3.0 0.537 Wilcoxon test N. microps 0.0 0.1 0.0 0.0 0.295 Wilcoxon test C. virescens 0.09 0.2 0.03 0.06 0.068 Wilcoxon test C. jamaicensis/similis 1.8 2.1 1.7 2.0 0.697 Paired t-test

CPU E (kg/ h)

Figure 8. Mean (+SD) catch-per-unit effort (CPUE)of the different catch components in control codend (4” TED; green) and experimental codend (3” TTED; blue). Percentages denote reduction or increase in mean CPUE in the experimental codend. Asterisks indicate significant differences (paired t-test or Wilcoxon test; * = p < 0.05; ns = not significant).

CPU E (kg/ h)

Figure 9. Mean (+SD) catch-per-unit-effort (CPUE) of adult commercial fish species (kg/h) in control codend (4” TED; green) and experimental codend (3” TTED; blue). Percentages denote reduction or increase in mean CPUE in the experimental codend. Asterisks indicate significant differences (paired t-test or Wilcoxon test; * = p < 0.05; ns = not significant).

3.1.3. Length analysis

Length-frequency plots were made for the four commercial fish species (M. ancylodon, N. microps, C. virescens, C. jamaicensis/similis) and for the rays. Visual inspection of the distributions of the different species showed that, except for N. microps and C. virescens, the largest part of the individuals were below 25 cm (size from which these species are generally retained). Further, the distributions of M. ancylodon, N. microps and C. jamaicensis/similis indicated that the largest individuals were not retained by the experimental codend. In contrast, the largest individuals of C. virescens and the rays were found in the experimental codend.

Figure 10. Length-frequency plots of M. ancylodon (a), N. microps (b), C. virescens (c), C. jamaicensis/similis (d) and rays (e). CTR means control codend (4” TED) and EXP experimental codend (3” TTED). The red arrow indicates the minimum retention size (25 cm).

Due to small sample sizes, a selectivity curve was only produced for the commercial fish species M. ancylodon. Visual inspection revealed that most of the dots were situated around the 0.50 value indicating little effect of size on the probability of being retained in either codend (Fig.11).

Figure 11. Proportion M. ancylodon retained in the experimental codend (NS,exp / (NS,exp + NS,control)) (X-axis) in function of its length (Y-axis).

3.2. 2” TTED

3.2.1. General test results

Trials with the 2” TTED were conducted in August 2016. Thirty-four comparative hauls were conducted, of which 31 were sampled. Total catch of the control and experimental combined averaged 155.1 ± 106.9 kg. Seabob shrimp represented on average 43% (65.5 ± 65.2 kg) of this catch, followed by trash (33%), juvenile commercial fish (14%), adult commercial fishes (7%), prawns (2%) and elasmobranchs (1%) (Table 3; Fig. 12). Besides rays, elasmobranch species like sharks, electric rays and guitar fishes were also caught, but the data were not included in the analyses due to the lack of comparable data for the 3” TTED. Elasmobranchs therefore only included stingray species (D. guttata, D. geijskesi, G. micrura and U. microphthalmum) and are further referred to as “rays”.

Table 3. Average catch composition during the 2” TTED trials, expressed as mean weight per catch component of the control and experimental codend combined Species Average catch per haul (kg; mean ± SD) Seabob 65.5 ± 65.2

Prawns 3.3 ± 5.7 Adult commercial fishes 9.8 ± 9.2 Rays 2.1 ± 4.4 Juvenile commercial fishes 21.2 ± 35.7 Trash 49.8 ± 60.7 Total 155.1 ± 106.9 kg

Figure 12. Average catch composition during the 2” TTED trials , represented as percentages (by weight) of the combined total catch of control and experimental codend. Discarded bycatch is represented by shaded areas.

3.2.2. CPUE analysis

Total catch-per-unit-effort (CPUE) in control (38.4 ± 28.3 kg/h) and experimental (39.2 ± 26.2 kg/h) codends did not differ significantly (p > 0.05) (Table 4). The experimental codend (16.6 ± 20.3 kg/h) did not cause a significant reduction (p > 0.05) in total bycatch (control codend: 20.0 ± 21.8 kg/h). Considering discarded and retained bycatch separately, a significant reduction (p < 0.05) was, however, observed for the retained bycatch, with lower CPUEs in the experimental (3.0 ± 3.2 kg/h) compared to the control (3.6 ± 2.6 kg/h) codend. This decline could be attributed to the reduction of adult commercial fish species, in particular to C. virescens and N. microps, whose catch rates were significantly lower (p < 0.05) in the experimental (0.6 ± 0.9 kg/h and 0.5 ± 0.3 kg/h, respectively) compared to the control codend (1.1 ± 0.8 kg/h and 0.7 ± 0.6 kg/h, respectively). Catch rates of trash, prawns, juvenile commercial fishes and rays did not differ. Although the experimental codend caught 20 % more seabob than the control codend, this increase was not significant (Table 4).

Table 4. Comparison of catch-per-unit effort (CPUEs; in kg/h) between control codend (4” TED) and experimental codend (2” TTED). Mean and standard deviation of the different catch components are displayed, together with the p-value of the paired t-test or Wilcoxon signed-rank test. Asterisks indicate significant differences (* = p < 0.05). Discarded bycatch includes trash, juvenile commercial fish species

17 and rays. Retained bycatch consists out of adult commercial fish species and prawns. Overall bycatch is the sum of discarded and retained bycatch.

Control 4” TED Experimental 2” TTED p test (kg/h) (kg/h) Average SD Average SD Total catch 38.4 28.3 39.2 26.2 0.695 Paired t-test Total bycatch 23.1 28.1 20.0 21.8 0.130 Wilcoxon test Discarded bycatch 19.5 27.0 17.0 20.5 0.256 Wilcoxon test Retained bycatch 3.6 2.6 3.0 3.2 0.005* Wilcoxon test Seabob 14.9 13.9 17.9 19.2 0.099 Wilcoxon test Prawns 0.8 1.5 0.8 1.4 0.248 Wilcoxon test Trash 13.9 20.2 11.0 10.9 0.141 Wilcoxon test Rays 0.6 1.5 0.4 0.8 0.943 Wilcoxon test Commercial fish species 7.9 10.8 9.1 12.8 0.794 Wilcoxon test ALL SIZES M. ancylodon 4.6 5.8 4.9 6.4 0.992 Wilcoxon test N. microps 0.8 1.0 0.5 0.6 0.154 Wilcoxon test C. virescens 0.6 1.4 0.7 1.0 0.588 Wilcoxon test C. jamaicensis/similis 1.9 8.6 2.9 10.3 0.891 Wilcoxon test Commercial fish species 2.8 1.9 2.1 2.8 0.001* Wilcoxon test ADULTS M. ancylodon 0.9 1.2 1.0 1.9 0.780 Wilcoxon test N. microps 0.7 0.6 0.5 0.3 0.013* Paired t-test C. virescens 1.1 0.8 0.6 0.9 0.004* Paired t-test C. jamaicensis/similis 0.1 0.2 0.1 0.2 0.096 Wilcoxon test Commercial fish species 5.0 9.2 5.6 9.9 0.572 Wilcoxon test JUVENILES M. ancylodon 3.1 4.5 3.0 3.7 0.773 Wilcoxon test N. microps 0.2 0.3 0.2 0.2 0.127 Wilcoxon test C. virescens 0.0 0.1 0.1 0.1 0.485 Wilcoxon test C. jamaicensis/similis 1.6 7.5 2.4 8.0 0.855 Wilcoxon test

CP UE (kg /h)

Figure 13 Mean (+SD) catch-per-unit effort (CPUE)of the different catch components in control codend (4” TED; green) and experimental codend (2” TTED; blue). Percentages denote reduction or increase in mean CPUE in the experimental codend. Asterisks indicate significant differences (paired t-test or Wilcoxon test; * = p < 0.05; ns = not significant).

CP UE (kg /h)

Figure 14 Mean (+SD) catch-per-unit-effort (CPUE) of adult commercial fish species (kg/h) in control codend (4” TED; green) and experimental codend (2” TTED; blue). Percentages denote reduction or increase in mean CPUE in the experimental codend. Asterisks indicate significant differences (paired t-test or Wilcoxon test; * = p < 0.05; ns = not significant).

3.2.3. Length analysis

Lengthfrequency plots were made for the four commercial fish species (M. ancylodon, N. microps, C. virescens, C. jamaicensis/similis) and for the rays (Fig. 15). Except for C. virescens, most fishes were below 25 cm (size from which these species are generally retained). Further, the experimental codend did not retain the largest individuals of all four species (Fig. 15).

Figure 15. Length-frequency plots of M. ancylodon (a), N. microps (b), C. virescens (c), C. jamaicensis/similis (d) and rays (e). CTR means control codend (4” TED) and EXP experimental codend (2” TTED). The red arrow indicates the minimum retention size (25 cm).

Due to small sample sizes, a selectivity curve was only produced for the commercial fish species M. ancylodon. Figure 15 shows that most of the dots are situated around the 0.50 value indicating little effect of size on the probability of being retained in either codend.

Figure 16. Proportion M. ancylodon retained in the experimental codend (NS,exp / (NS,exp + NS,control)) (X-axis) in function of its length (Y-axis).

4. Discussion

This is the first study to evaluate the potential of TEDs with reduced bar in the seabob shrimp trawl fishery in Suriname. We tested the performance of two types of so-called Trash-and-Turtle Excluder Devices (TTEDs): flat-bar TEDs with bar spacings of resp. 3” and 2”. Compared to the standard 4” round-bar TEDs, the TTEDs caused significant reductions in bycatch, and had positive effects on the target catch.

4.1 Reduction of bycatch

The 3” TTED caused a significant 17% reduction in total bycatch. A reduction was expected, because more organisms are likely to escape through the TED’s escape opening when bar spacing is reduced. Many studies have indeed found that reducing the bar-spacing of a sorting grid effectively reduces bycatch in shrimp fisheries by enhancing the mechanical sorting properties of the grid (e.g. Hataway, 2017). In that respect, it was surprising that the reduction in total bycatch was less pronounced (a non- significant 13% decrease) in the 2” TTED.

To minimize spatial and temporal variability in the catches, the two data collection trips took place in the same fishing area, over a period of 6 weeks. Nevertheless, the catch composition in the seabob fishery is rather variable between fishing days or even hauls (Willems, 2016). This is indeed reflected in the somewhat different catch composition observed during both trips. Escape through a TED is species- and size dependent (e.g. Willems et al., 2016). Changes in the catch composition will therefore affect bycatch reduction caused by a TED. For that reason, comparing the ‘total bycatch’

22 reduction between the two types of TTEDs is this study is probably biased. The results on the different catch components, based on species and sizes, are more valid and relevant.

One of the main aims of the current study was to explore the potential of TTEDs in reducing elasmobranch bycatch. Because sharks were uncommon during the sea trials, we were only able to assess the bycatch of rays. The 3” TTEDs reduced ray bycatch by 44 %. This is a positive result, given the conservation status of some of the ray species (IUCN, 2017). In other fisheries, TTEDs have also proven to effectively reduce ray bycatch (Nalovic & Rieu, 2010, Nalovic, 2014; Hataway et al., 2017). While we would assume that the reduction is caused by larger rays escaping through the TTED opening (Willems et al., 2016), the length frequency distribution did not support a clear size-dependent escape. Using the 3” TTED, some of the largest individuals were still retained by the experimental codend and the mean disc width was similar in the control (21 ± 8 cm) and experimental (21 ± 14) codend.

During both sea trips, rays accounted for ca. 1% of the catch. This is low compared to other samples from the fishery (Willems, 2016). The fact that no clear size selection was seen in the 3” TTED, and that the 32%-reduction in ray bycatch in the 2” TTED was not statistically significant, mostly likely relates to the relatively low numbers that were captured during the trials. Nevertheless, Nalovic (2014) assessed similar low catch rates and did find a significant reduction for rays and skates (7 smooth butterfly rays and 15 Raja spp. were found in the TED, none in the TTED). Likewise, is the study of Garstin et al. (2017), that investigated the use of a 1.75” TED for the Atlantic seabob industrial trawl fishery of Guyana and which resulted in a significant reduction of longnose and butterfly rays despite relatively low catch rates.

The 3” nor the 2” TTED affected the capture of prawns, ‘trash’ or juvenile commercial fishes. This was not surprising, given the small average size of the organisms in these groups. The latter are considered an unsustainable component of the bycatch in this fishery. To reduce the bycatch of small sized (commercial) fishes, the effectiveness of the current square-mesh panel BRD should further be evaluated and improved, and other behavior-type BRDs could be tested (e.g. Eayrs, 2012).

The 3” TTED had no effect on the capture of ‘adult commercial fishes’: individuals of four species of the Sciaenidae family that are retained in the seabob fishery from a total length of 25 cm onwards. The fact that the larger commercial fish species were retained by the 3” TTED is surprising, as for example Boopendranath et al. (2010) reviewed that in Indian waters trawlers geared with TEDs with bar spacing smaller than 3.5 ” did not perform well in retaining the larger commercial fish species. On the other hand, a significant 23%-reduction in adult commercial fish was observed in the 2” TTED. Looking at the different species, catches of adult N. microps and C. virescens were reduced by resp. 34 % and 41 % using the 2” TTED. Among the four retained bycatch species, these fishes indeed had the largest average size, causing their escape through the TTED. No reduction in the capture of other two commercial fish species was observed, which was also seen in the selectivity curve for M. ancylodon. Unfortunately, insufficient specimens were captured and measured to construct selectivity curves for the other species (e.g. Eayrs, 2012).

4.2 Target catch retention

The overall catch rate of seabob shrimp in this study showed a significant 16 % increase in the 3” TTED, and a 20 % (but non-significant) increase in 2” TTED. In the Gulf of Mexico, Nalovic (2014) observed no change in shrimp capture using a 2” TTED, while Hataway et al. (2017) observed a 10% reduction. Indeed, reducing the bar spacing of a TED increases the contact probability and interaction of the shrimp with the TED, and therefore possibly also the escape through the TEDs escape opening.

It remains unclear through what mechanism the introduction of TTEDs caused an increase in shrimp catches in the seabob fishery. Most likely, however, elements other the TED’s bar spacing played a role. The TTEDs were fitted with their standard amount of floatation to balance out the weight of the grid. In the Suriname seabob fishery, however, it is common practice to load the TEDs with excessive floatation to avoid that the TEDs drag along the bottom under the weight of the shrimp catch (S. Hall, pers. comm.). Potentially, this causes the control 4” TEDs to float up too much, inducing shrimp loss through the bottom escape flap (N. Hopkins, pers. comm.). During the study, the control TEDs were used as they are normally used in the fishery, i.e. with a large number of floats (8 hard floats). Clearly the mechanism that caused the observed increase in shrimp capture in the TTEDs, or the loss of shrimp in the standard 4” TED, warrants further study.

Maintaining the catch of the target species is of one of the most important factors that determines the effectiveness of a TED or BRD, as it will finally also determine the acceptance of the TTED by the fishermen (Nalovic, 2014; Schroeder et al., 2016). As such, the increase in target catch observed in the current study, whatever the underlying reason, is a positive result.

4.3 Conclusions

Both experimental TTEDs performed very good in the reduction of bycatch, while retaining and even improving the target catch. The 3” TTED caused a significant reduction in ray bycatch, while the 2” TTED reduced the bycatch of fish. The 3” TTED increased the capture of seabob shrimp by 16 %, and had no effect on capture of retained fish. Choosing the adequate bar spacing for the TTED should result in bycatch reduction, but should not be at the expense of the bycatch users (their income and food security; FAO, 2015). Given the economic value of fish bycatch in the seabob fishery, the 3” TTED will probably gain easier acceptance in the fishery. To confirm the results observed in this study, more sea trials should be carried out, across larger spatial and temporal scale.

5. Acknowledgements The authors which to thank anyone who contributed in some way to this study: staff and observers from the Ministry of LVV - Fisheries Department, boat captains and crew, and CRPMEM (French Guiana). This research could only take place due to the financial and logistic support of Heiploeg Suriname N.V., SAIL Ltd, WWF Guianas and the regional FAO/GEF REBYC-II LAC project.

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Annex

CPUE 3” TTED – number of individuals per hour

Table X. Comparison of CPUEs (in #/h) between control codend (4” TED) and experimental codend (3” TTED). Mean and standard deviation of the different catch components are displayed, together with the p-value of the paired t-test or Wilcoxon signed-rank test. Asterisks indicate significant differences (* = p < 0.05).

Control 4” TED Experimental 3” p test (#/h) TTED (#/h) Mean SD Mean SD Ray 1.0 1.6 0.5 1.5 0.037* Wilcoxon test Commercial fish species 182.7 190.2 144.0 116.5 0.098 Wilcoxon test ALL SIZES M. ancylodon 139.3 175.0 107.7 107.8 0.133 Wilcoxon test N. microps 3.0 4.9 1.7 2.8 0.151 Wilcoxon test C. virescens 4.3 8.6 2.5 4.1 0.281 Wilcoxon test C. jamaicensis/similis 36.2 35.2 32.2 33.3 0.446 Wilcoxon test Commercial fish species 5.7 8.0 5.0 6.9 0.153 Paired t-test ADULTS M. ancylodon 2.5 5.0 2.0 4.1 0.212 Wilcoxon test N. microps 1.5 1.8 1.3 1.2 0.550 Wilcoxon test C. virescens 1.6 3.8 1.8 3.6 0.381 Paired t-test C. jamaicensis/similis 0.1 0.2 0.0 0.1 0.115 Wilcoxon test Commercial fish species 163.1 171.9 128.7 93.7 0.133 Wilcoxon test JUVENILES M. ancylodon 126.6 159.3 96.0 83.4 0.150 Wilcoxon test N. microps 0.5 1.4 0.1 0.6 0.142 Wilcoxon test C. virescens 1.6 2.9 0.7 1.5 0.156 Wilcoxon test C. jamaicensis/similis 34.4 34.0 31.8 32.8 0.759 Wilcoxon test

CPUE 2” TTED – number of individuals per hour

Table X. Comparison of CPUEs (in #/h) between control codend (4” TED) and experimental codend (2” TTED). Mean and standard deviation of the different catch components are displayed, together with the p-value of the paired t-test or Wilcoxon signed-rank test. Asterisks indicate significant differences (* = p < 0.05).

Control Experimental p test (# / h) (# / h) Average SD Average SD Ray 1.2 1.7 1.2 1.9 0.895 Wilcoxon test Commercial fish species 176.6 188.7 213.2 225.4 0.019* Wilcoxon test ALL SIZES M. ancylodon 146.0 152.9 169.5 172.0 0.058 Wilcoxon test N. microps 4.6 5.4 3.1 4.0 0.118 Wilcoxon test C. virescens 2.5 3.9 2.8 3.6 0.611 Paired t-test C. jamaicensis/similis 23.5 94.5 37.8 120.4 0.187 Wilcoxon test Commercial fish species 7.5 6.9 7.0 9.3 0.077 Wilcoxon test ADULTS M. ancylodon 3.0 4.3 3.6 7.2 0.943 Wilcoxon test N. microps 2.3 2.4 1.6 1.2 0.047* Wilcoxon test C. virescens 1.9 1.9 1.4 1.9 0.043* Paired t-test C. jamaicensis/similis 0.3 0.9 0.3 0.9 0.518 Wilcoxon test Commercial fish species 166.1 184.1 199.9 215.2 0.021* Wilcoxon test JUVENILES M. ancylodon 139.8 148.1 162.4 167.3 0.051 Wilcoxon test N. microps 2.7 3.3 1.8 2.8 0.103 Wilcoxon test C. virescens 0.8 1.2 0.7 1.7 0.485 Wilcoxon test C. jamaicensis/similis 22.8 91.5 35.0 109.7 0.269 Wilcoxon test