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Using composite square-mesh panels and the Nordmøre-grid to reduce bycatch in the Shark Bay prawn-trawl fishery, Western Australia
Fisheries Research 58 (2002) 349–365
Using composite square-mesh panels and the Nordmøre-grid to reduce bycatch in the Shark Bay prawn-trawl fishery, Western Australia Matt K. Broadhursta,b,*, Mervi I. Kangasc, Cristiana Damianod, Scott A. Bickfordc, Steven J. Kennellyb a
Departamento de Pesca, Universidade Federal Rural de Pernambuco, Av. Dom Manuel de Medeiros, s/n, Dois Irma˜os, Recife-PE CEP: 52.171-900, Brazil b NSW Fisheries, Cronulla Fisheries Centre, P.O. Box 21, Cronulla, NSW 2230, Australia c Western Australian Marine Research Laboratories, P.O. Box 20, North Beach, WA 6020, Australia d Projeto Tamar, Alameda do Boldro´ s/n, Fernando de Noronha-PE CEP: 53.990-000, Brazil Received 1 March 2001; received in revised form 7 August 2001; accepted 26 August 2001
Abstract An industry-modified Nordmøre-grid was tested on its own and with a composite square-mesh panel as a secondary bycatch reduction device (BRD) at two different positions (aft and forward) in codends. Compared to the control codend (which had no BRDs), all three combinations (Nordmøre-grid only, Nordmøre-grid and aft composite square-mesh panel and Nordmøre-grid and forward composite square-mesh panel) reduced catches of prawns (mostly western king prawns, Penaeus latisulcatus). However, prawns were found to escape out of the Nordmøre-grid—not through the composite square-mesh panels. The Nordmøre-grid with the aft composite square-mesh panel significantly reduced the weight of bycatch (by 49%) and the numbers and weights of several commercially and non-commercially important bycatch species (by up to 75.7%). No other significant differences were detected. The results are discussed in terms of the likely factors influencing the performance of the various designs, including the behaviour of fish in codends, influences of hydrodynamics on their escape and the importance of the positioning of BRDs in codends. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Bycatch reduction; Prawn trawl; Square-mesh panels; Trawl selectivity
1. Introduction The Shark Bay prawn-trawl fishery mainly targets western king prawns, Penaeus latisulcatus and brown *
Corresponding author. Present address: NSW Fisheries, Cronulla Fisheries Centre, P.O. Box 21, Cronulla, NSW 2230, Australia. Tel.: þ61-2-9527-8411; fax: þ61-2-9527-8576. E-mail address: [email protected] (M.K. Broadhurst).
tiger prawns, P. esculentus and is valued at approx. $A32 million per annum. Like the majority of prawntrawl fisheries throughout the world, trawlers operating in Shark Bay often catch and discard large quantities of a diverse assemblage of non-target organisms (termed bycatch—for reviews see Andrew and Pepperell, 1992; Kennelly, 1995). At times, this bycatch can include sea turtles (i.e. loggerhead, Caretta caretta, green, Chelonia mydas and rarely hawksbill, Eretmochelys imbricata and leatherback turtles,
0165-7836/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 7 8 3 6 ( 0 1 ) 0 0 3 9 0 - 3
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Dermochelys coriacea), but more frequently comprises numerous small fish (<20 cm), including juveniles of commercially and recreationally important species. The mortality of these individuals is of particular concern, since it may contribute to recruitment overfishing of stocks that form the basis of other fisheries. The most common management strategy to reduce bycatches from prawn trawling is to install physical devices (collectively termed bycatch reduction devices—BRDs) in the trawl, which are designed to improve selectivity (for a review see Broadhurst, 2000). These devices can be classified under two categories, according to the methods used to facilitate the escape of bycatch. The first category includes designs that mainly function by mechanically partitioning the catch according to size. These BRDs include some sort of separating panel (usually a rigid grid) terminating in an escape exit anterior to the codend, and are designed to exclude individuals that are larger than the openings in the separating panel. The most well-known example of these sorts of BRDs is the Nordmøre-grid, which has been adopted in several prawn-trawl fisheries throughout the world (e.g. Isaksen et al., 1992; Hickey et al., 1993; Broadhurst and Kennelly, 1996a; Pettovello, 1999). The second category of BRDs includes those that are designed to operate by exploiting behavioural differences between prawns and fish using strategically placed panels and escape exits (Broadhurst, 2000). These devices may be used on their own in a trawl, or as secondary BRDs behind designs that mechanically partition the catch according to size (in this case, the latter are termed primary BRDs). The presence of sea turtles in areas of Shark Bay and concerns over the potential for their capture and mortality by prawn trawlers, resulted in recent drafting of bycatch action plans. These specify the implementation of rigid mechanical-type separating BRDs (with bar spacings no larger than 100 mm) and secondary BRDs to reduce bycatch. Numerous existing BRDs, including the Nordmøre-grid, have already been tested and modified by prawn trawlers working in Shark Bay. Anecdotal information suggests that many of these designs maintain catches of prawns, although the presence of very large quantities of sea grass (Amphibolis antarctica) on the trawl grounds at certain times and locations can negatively affect performances of these devices (i.e. block bars and escape exits and result in loss of prawns).
Continual refinements of grids by industry to address the problems associated with sea grass are likely to result in suitable mechanical-type separating BRDs that also exclude sea turtles and other large animals from trawls. However, such designs do not address the capture and mortality of smaller fish which are unlikely to be excluded in any great quantities from mechanical-type separating BRDs. Because of this, modifications that exclude small fish via differences in behaviour need to be developed and tested as secondary BRDs in this fishery. Further, to facilitate the acceptance and implementation of these sorts of BRDs, they should have no adverse effects on efficiency of the trawl and operate over the range of commercial conditions experienced. More specifically, the problems associated with sea grass in Shark Bay means that any secondary BRDs should be simple and free of components or panels that might become blocked. One such BRD (termed the composite square-mesh panel) has been shown to be very effective in other Australian prawn-trawl fisheries. This device involves different-sized square-shaped meshes located in the top of the posterior section of the codend (Broadhurst and Kennelly, 1996b; Broadhurst et al., 1999a). Previous studies have suggested that anteriorly displaced water in front of the catch can assist fish to maintain position in this area of the codend and facilitate their escape through the strategically located composite square-mesh panel (see Broadhurst et al., 1999b). The composite square-mesh panel has no rigid components or complicated guiding panels, is easy to construct and has been proven to maintain catches of prawns and reduce bycatch from several different designs of trawls over a range of commercial conditions in Australia (Broadhurst and Kennelly, 1996b; Broadhurst et al., 1999a). Our specific objectives in the present study were to: (i) quantify the effectiveness of an industry-modified Nordmøre-grid in maintaining catches of prawns in Shark Bay; (ii) examine the bycatch-reduction capability of the composite square-mesh panel as a secondary BRD. As part of this second objective, we aimed to examine some of the possible effects on fish escape due to displaced water in front of the catch. This was achieved by quantifying the performance of the composite square-mesh panel at two positions (forward and aft) in the codend.
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2. Materials and methods
2.1. BRDs examined
The experiment was done on established prawntrawl grounds in Shark Bay (Fig. 1), Western Australia, in August 2000 using a chartered commercial prawn trawler (FV Cape Conway), rigged to tow two trawls (each with a headline length of 14.6 m) in a conventional twin-rig system. The trawls were identical, made from polyethylene twine and had a stretched mesh size of 52 mm in the body and 47 mm in the codend (NB: unless stated otherwise, all stretched mesh sizes were measured as centre knot to centre knot). All tows were done over a combination of sandy and light mud bottoms in depths ranging from 13.7 to 18.5 m and at speeds between 3.5 and 4.6 kn (mean S:E: of 4:2 0:04 kn per tow).
One primary BRD and two secondary BRDs were used in the experiment. The primary BRD, termed the Nordmøre-grid, consisted of an aluminium grid with the upper third offset at 458 and vertical bars spaced at 100 mm (Fig. 2A). The grid was located at an angle of 458 in a 30-mesh extension piece, measuring 120 meshes in circumference and made from 47 mm, 60 ply, UV-stabilised, high-density polyethylene twine. The anterior end of this extension was laced to the trawl body, and a zipper (Burashi S-146R, pinlock side) attached to the posterior end to facilitate the changing of codends. Unlike original designs of the Nordmøre-grid (e.g. Isaksen et al., 1992; Hickey et al., 1993), the design tested in the present study
Fig. 1. Location of Shark Bay and area fished.
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Fig. 2. (A) The Nordmøre-grid and (B) its location in the trawl.
had no funnel or guiding panel, but included two flexible panels of 47 mm diamond-shaped mesh (60 ply, UV-stabilised, high-density polyethylene twine) that were hung loosely above and below the escape exit and extended to the grid (Fig. 2B). These panels were attached to the net only anteriorly, and were free to lift through and above the escape exit (see also Broadhurst and Kennelly, 1996a). The theory behind these panels was that they would limit the escape of prawns, but still allow organisms larger than the bar spacings to be released from the trawl. To maintain stability, two plastic floats (16 cm in diameter) were attached to either side of the upper edge of the grid (Fig. 2B). Three codends were constructed and rigged with zippers (Burashi S-146R, pinlock side) so that they could be attached posterior to the extension containing the Nordmøre-grid. The first codend was a normal
design (termed the conventional codend) and comprised 47 mm diamond-shaped mesh (60 ply, UVstabilised, high-density polyethylene twine) throughout with a circumference of 120 meshes and a length of 70 meshes (Figs. 3A and 4A). The second and third designs, termed the aft composite square-mesh panel (ACSMP—Figs. 3B and 4B) and forward composite square-mesh panel (FCSMP—Figs. 3C and 4C) codends had the same circumference and length as the codend described above, but included secondary BRDs. These BRDs comprised composite panels made of 47, 94 and 155 mm mesh cut on the bar (Fig. 3D—see also Broadhurst and Kennelly, 1996b) and inserted into the top sections of the codends at distances anterior to the draw strings of 24 and 43 meshes, respectively (Fig. 3B and C). The location of the panel in the ACSMP codend was based on designs developed for use in New South Wales (NSW)
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Fig. 3. Schematic diagram of the (A) conventional, (B) aft composite square-mesh panel, (C) forward composite square-mesh panel codends and (D) composite square-mesh panel. T, transversals; N, normals; and B, bars.
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Fig. 4. The locations of the Nordmøre-grid and the (A) conventional, (B) aft composite square-mesh panel and (C) forward composite squaremesh panel codends in the prawn trawl.
and South Australian prawn-trawl fisheries (e.g. Broadhurst and Kennelly, 1996b; Broadhurst et al., 1999a). Broadhurst et al. (1999b) showed that at this location there is substantial displacement of water anterior to the catch in the codend, that might assist small fish to maintain position under the composite square-mesh panel, increasing their probability of randomly encountering the open meshes and escaping (see discussion below). In the FCSMP codend, the square-mesh panel was located further forward and away from any flow-related effects due to catch (Broadhurst et al., 1999b). Instead, a restricting cord (4 mm diameter polypropylene rope, 1.3 m in circumference—Figs. 3C and 4C) was located at a distance of 10 meshes posterior to the composite square-mesh panel. This cord reduced the circumference of the codend by approx. 35% (calculated assuming a fractional mesh opening of 0:35 the stretched mesh length between the knots the mesh circumference;
Broadhurst et al., 1999a). We hypothesised that reducing codend circumference immediately behind the composite square-mesh panel would create a short but sharp taper in the codend that would displace water forward and thereby assist fish to maintain position beneath the composite square-mesh panel (as is believed for the ACSMP). The control codend represented those normally used commercially and was made entirely of 47 mm diamond-shaped meshes (60 ply, UV-stabilised, high-density polyethylene twine), measuring 120 meshes in circumference and 100 meshes in length. This codend was the same length as each of the codends described above attached to the extension containing the Nordmøre-grid and was laced to the end of the trawl body in a similar manner. Using zippers, the conventional design (i.e. with no secondary BRD), FCSMP and ACSMP codends described above were alternatively attached posterior
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to the Nordmøre-grid (Fig. 4) and the entire assembly tested against the control codend, on each side of the twin-rigged gear (i.e. three separate paired comparisons: Nordmøre-grid only, i.e. with conventional codend vs. control; Nordmøre-grid and FCSMP codend vs. control; and Nordmøre-grid and ACSMP codend vs. control). Two replicate 40 min tows of each paired comparison were made on each night, providing a total of 10 replicate comparisons of each configuration over five nights. The position and order of the three codends attached posterior to the Nordmøre-grid were randomly assigned. However, because the Nordmøre-grid could not be easily removed from the net (i.e. it was not possible to attach a zipper between the trawl body and this BRD), this device was alternated between nets on different nights so that equal numbers of paired comparisons of each treatment against the control were made on port and starboard trawls. Trawls were tested prior to the trials to ensure no biases between the sides of the vessel. 2.2. Data collected and statistical analyses After each tow, the two codends were emptied onto a partitioned tray. Prawns, all individuals of commercially and/or recreationally important species and individuals of the most abundant non-commercial species were separated. The following categories of data were determined for each tow: the total weight of prawns; the weights of individual species of prawns; the numbers of western king prawns and brown tiger prawns and a subsample (3 kg from each codend) of their lengths (to the nearest 1 mm carapace length); the weight of bycatch; the weights and numbers of commercially and/or recreationally important species and the most abundant non-commercial species; and the sizes (to the nearest 0.5 cm) of the most abundant and commercially and/or recreationally important fish. Several species (commercial and non-commercial) were caught in sufficient numbers (i.e. 1 individual in at least 7 replicates) to enable meaningful comparisons (see Table 1). Catch data for all replicates that had sufficient number of each variable were analysed with one-tailed paired t-tests ðP 0:05Þ testing the hypothesis that the BRDs caught less than the control. To examine the relative effectiveness of BRDs, the differences in
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Table 1 List of species caught in sufficient quantities to permit analyses Scientific name
Common name
P. latisulcatus P. esculentus Metapenaeopsis crassissima Sillago maculata Platycephalus endrachtensis Pseudorhombus jenynsii Lethrinus genivittatus Portunus pelagicus Leiognathus leuciscus Pentapodus vitta Paramonacanthus choirocephalus Upeneus asymmetrius Pelates quadrilneatus Sardinella gibbosa Sorsogona tuberculata
Western king prawns Brown tiger prawns Coral prawns Trumpeter whiting Bar-tailed flathead Small-toothed flounder Threadfin Blue swimmer crab Ponyfisha Butterfisha Leatherjacketa Goatfisha Trumpetera Sardinea Heart-headed flatheada
a
Denotes non-commercial/non-recreational species.
catches (between each treatment and their respective controls) for those variables with numbers present in all tows were analysed using a balanced two-factor orthogonal ANOVA (BRDs and nights were treated as fixed and random factors, respectively) after preliminary tests for heteroscedasticity using Cochran’s test. Data with heterogeneous variances were transformed, but if heteroscedasticity persisted, ANOVA was done at a more conservative probability level (0.01) to reduce the likelihood of Type I errors (Underwood, 1981). Means were compared following significant F-tests using Student–Newman–Keuls (SNK) multiple comparisons. With the exception of data for catches of prawns, where analyses provided similar results for weights and numbers of variables, only data concerning numbers were included in Fig. 5 to conserve space. Size-frequencies of prawns and fish retained were combined across all tows and where there were sufficient data (>30 individuals in each codend), they were compared with two-sample Kolmogorov–Smirnov tests ðP 0:05Þ.
3. Results Compared to the control, the Nordmøre-grid only and the Nordmøre-grid with the FCSMP and ACSMP codends significantly reduced the total weight of prawns by similar amounts (e.g. 12.5, 14
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Fig. 5. Differences in mean catch (S.E.) between the BRDs and the control for (A) weight of total prawns, (B) number and (C) weight of western king prawns, (D) number and (E) weight of brown tiger prawns, (F) weight of coral prawns, (G) weight of bycatch and numbers of (H) trumpeter whiting, (I) butterfish, (J) leatherjacket, (K) goatfish, (L) trumpeter, (M) threadfin, (N) sardine, (O) heart-headed flathead, (P) bartailed flathead, (Q) small-toothed flounder and (R) blue swimmer crabs. Histograms in black represent significant reductions. Ng, Nordmøregrid; FCSMP, forward composite square-mesh panel codend; ACSMP, aft composite square-mesh panel codend.
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Fig. 5. (Continued ).
and 12%, respectively) and the numbers (19.5, 19.3 and 16.1%, respectively) and weights (by 14.8, 18.8 and 13.5%, respectively) of western king prawns (Fig. 5A–C and Table 2). The Nordmøre-grid in
combination with the ACSMP codend also significantly reduced the weight of bycatch (by 49%) and the numbers and weights of leatherjacket (by 55.6 and 58.6%), sardine (by 57.4 and 70.2%),
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Fig. 5. (Continued ). Table 2 Summaries of one-tailed paired t-tests comparing three BRDs against the controla Ng only vs. control
Total weight of prawns No. of western king prawns Weight of western king prawns No. of brown tiger prawns Weight of brown tiger prawns Weight of coral prawns Weight of bycatch No. of trumpeter whiting Weight of trumpeter whiting No. of butterfish Weight of butterfish No. of leatherjacket Weight of leatherjacket No. of goatfish Weight of goatfish No. of trumpeter Weight of trumpeter No. of threadfin Weight of threadfin No. of sardine Weight of sardine No. of heart-headed flathead Weight of heart-headed flathead No. of bar-tailed flathead Weight of bar-tailed flathead No. of small-toothed flounder Weight of small-toothed flounder No. of blue swimmer crab Weight of blue swimmer crab
Ng and FCSMP vs. control
Ng and ACSMP vs. control
pt-v
P
n
pt-v
P
n
pt-v
P
n
2.616 3.657 3.801 0.489 0.582 0.785 0.675 – – 1.013 0.725 0.079 0.410 4.088 3.616 1.670 1.582 – – 1.620 1.792 0.838 0.637 – – 1.414 0.367 2.375 2.691
0.014* 0.003** 0.002** 0.318 0.287 0.771 0.258 – – 0.170 0.245 0.469 0.654 0.999 0.997 0.065 0.074 – – 0.925 0.942 0.217 0.274 – – 0.097 0.362 0.972 0.982
10 10 10 10 10 8 10 – – 10 10 10 10 10 10 10 10 – – 8 8 7 7 – – 9 9 7 7
2.263 3.446 2.682 0.303 0.689 0.558 1.666 0.338 0.334 0.634 1.240 1.680 0.921 3.524 1.847 1.284 0.922 1.251 0.539 – – 0.403 1.096 0.584 0.420 1.021 1.321 0.432 0.379
0.025* 0.004** 0.013* 0.616 0.746 0.297 0.065 0.372 0.374 0.273 0.127 0.064 0.191 0.997 0.951 0.123 0.196 0.128 0.305 – – 0.349 0.152 0.712 0.345 0.167 0.109 0.338 0.357
10 10 10 10 10 8 10 8 8 8 8 10 10 10 10 7 7 7 7 – – 9 9 9 9 10 10 10 10
3.132 3.314 3.390 0.996 0.896 0.987 2.431 1.163 1.236 0.393 1.257 3.558 3.515 0.105 0.162 1.668 1.621 1.678 1.833 2.076 2.032 1.318 1.010 3.103 3.500 2.511 2.548 0.043 0.041
0.006** 0.004** 0.004** 0.173 0.196 0.173 0.019* 0.144 0.131 0.352 0.122 0.003** 0.003** 0.459 0.438 0.073 0.078 0.072 0.058 0.038* 0.041* 0.114 0.173 0.007** 0.004** 0.018* 0.017* 0.516 0.484
10 10 10 10 10 10 10 7 7 9 9 10 10 10 10 7 7 7 7 8 8 8 8 9 9 9 9 8 8
a Ng, Nordmøre-grid; FCSMP, forward composite square-mesh panel codend; ACSMP, aft composite square-mesh panel codend; pt-v, paired t-value; n, number of replicates; (–) insufficient data. * Significant at P < 0:05. ** Significant at P < 0:01.
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Table 3 Summaries of F-ratios from two-factor analyses of variance to determine effects on variables due to fishing with three different BRDs on five different nights. The transforms used to stabilise the variances where required are listeda Treatment
Nights BRDs Interaction Residual
d.f.
4 2 8 15
Total weight of prawns
2.17 0.14 0.67
Western king prawns
Brown tiger prawns
No.
Weight
No.
Weight (1=ðx þ 1Þ)
8.57** 0.21 1.32
3.08 1.26 0.44
2.29 0.18 1.74
0.66 0.98 2.16
Weight of bycatch
14.35** 2.83 5.97**
Leatherjacket
Goatfish
No.
Weight
No.
Weight
6.03** 6.16* 1.15
2.68 3.79 1.50
0.93 6.84* 0.76
1.94 1.74 1.85
a
Weight of king prawns was tested using a Type I error rate of a ¼ 0:01 because Cochran’s test of the transformed data was still significant at P ¼ 0:05. * Significant at P < 0:05. ** Significant at P < 0:01.
bar-tailed flathead (by 75.7 and 73.4%) and smalltoothed flounder (by 50 and 46.5%) (Fig. 5G, J, N, P, Q and Table 2). While not significant, this combination of BRDs also reduced the numbers and weights of trumpeter whiting, butterfish, trumpeter, threadfin and heart-headed flathead (differences between 9.4 and 68.2%) (Fig. 5H, I, L, M, O and Table 2). No other significantdifferencesweredetected(Fig.5andTable2), although the Nordmøre-grid only and the Nordmøregrid in combination with the FCSMP codend showed some reduction in the weight of bycatch (by 4.8 and 15.5%, respectively) and the numbers and weights of butterfish, trumpeter, heart-headed flathead and smalltoothed flounder (by between 7.1 and 44.4%) (Fig. 5G, I, L, O, Q and Table 2). Further, the numbers and weights of trumpeter whiting, leatherjacket, threadfin and blue swimmer crab were also reduced by the Nordmøre-grid and FCSMP codend (by between 17.7 and 73.6%), but these catches were not significantly different from the control codend (Fig. 5H, J, M, R and Table 2). ANOVA of the differences in catches between the BRDs and their controls detected significant differences in the numbers of leatherjacket and goatfish for
the main effect of BRDs and an interaction between nights and BRDs for the weight of bycatch (Table 3). Subsequent SNK tests of these means showed that the Nordmøre-grid with the ACSMP codend retained significantly fewer goatfish that the other BRDs, less bycatch on the third night and fewer leatherjackets than the Nordmøre-grid only (Table 4). Two sample Kolmogorov–Smirnov tests comparing size-frequency distributions of western king prawns, brown tiger prawns, butterfish, and heartheaded flathead retained by the codends showed no significant differences in relative size compositions (Figs. 6A, B and 7A, F). No significant differences were detected in size compositions of leatherjacket and trumpeter between the Nordmøre-grid only and control codends or between the Nordmøre-grid with the ACSMP and control codends, but the Nordmøregrid with the FCSMP codend caught proportionally fewer medium and large-sized leatherjacket and trumpeter than did the control (Fig. 7B). Similarly, the Nordmøre-grid with the ACSMP and the Nordmøre-grid only caught fewer large-sized sardine and goatfish, respectively (Figs. 7E, C).
Table 4 Summaries of SNK multiple comparisons of means for the significant interaction between nights and BRDs for weight of bycatch and the main effect of BRDs for number of leatherjacket and goatfisha Weight of bycatch No. of leatherjacket No. of goatfish
Night 3: Ng and ACSMP < Ng and FCSMP ¼ Ng only; other nights: no differences between BRDs Ng and ACSMP ¼ Ng and FCSMP < Ng only Ng and ACSMP < Ng and FCSMP ¼ Ng only
a Ng, Nordmøre-grid; FCSMP, forward composite square-mesh panel codend; ACSMP, aft composite square-mesh panel codend; (<) caught less.
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Fig. 6. Size-frequency distributions of (A) western king prawns and (B) brown tiger prawns retained in the BRD and control codends.
4. Discussion All three modified codends containing the BRDs significantly reduced the catches of western king prawns, contributing to a significant reduction in total weight of prawns. However, because the amounts of these reductions were similar (compared to their controls) and ANOVA failed to detect any significant
differences among the various designs (Table 3), it is likely that these prawns escaped at the Nordmøre-grid and not through the composite square-mesh panels. That is, no additional prawns were lost when using a composite-square-mesh panel behind the Nordmøregrid. This result may be explained by the partial blocking of the Nordmøre-grid due to sea grass encountered during many tows and/or the movement
M.K. Broadhurst et al. / Fisheries Research 58 (2002) 349–365 Fig. 7. Size-frequency distributions of (A) butterfish, (B) leatherjacket, (C) goatfish, (D) trumpeter, (E) sardine and (F) heart-headed flathead retained in the BRD and control codends. * Significant at P < 0:05; ** significant at P < 0:01.
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Fig. 7. (Continued ).
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of this sea grass out through the escape exit. For example, some prawns may have been unable to pass through blocked sections of the grid, or were mixed in with the sea grass and escaped as it was released from the trawl. These results are comparable to those detected by Broadhurst and Kennelly (1996a) during an experiment with a similar design of the Nordmøregrid in the Clarence River, NSW, Australia. In this work, large quantities of debris and weed contributed to a reduction in catches of school prawns, Metapenaeus macleayi by approx. 10%. The large quantities of sea grass encountered in Shark Bay means that, at times, some loss of prawns out through the escape exits of mechanical-type separating BRDs will probably be unavoidable, although the potential for this could be minimised via subtle modifications to the existing design. One possible improvement may be to extend the outer flap of the existing Nordmøre-grid a greater distance past the escape opening. Another option may be to remove the inner and outer panels at the escape exit and insert a long, shallow-angled, guiding panel, starting approx. 2 or 3 m in front of the upper edge of the grid and terminating immediately in front of its base. Broadhurst and Kennelly (1996a) suggested that similar problems of seaweed blocking grids in the Clarence River, NSW, were alleviated using this type of modification, primarily because the geometric angle and contours of the panel rolled the seaweed into a ball before reaching the grid, facilitating its movement out through the large exit. An additional modification that could facilitate the removal of sea grass may be to reduce the bar spacings in the grid (e.g. to approx. 60 mm), thereby increasing the surface area of the grid and possibly the flow of water out through the escape exit. The influences of sea grass on prawn loss also may have contributed to the escape of some larger fish and may partially explain some of the significant reductions in various individuals from the Nordmøre-grid and ACSMP codend and non-significant reductions in number of individuals from the other BRDs (Table 2). For example, significant differences were detected between size-frequency distributions of sardine caught with proportionally fewer large individuals retained in the codend containing the Nordmøre-grid and ACSMP (Fig. 7E). It is possible that some of these larger individuals passed through the escape exit of the grid during the movement of sea grass and other debris. In
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support of this, proportionally fewer large-sized goatfish (13–17 cm) were retained in the Nordmøre-grid only than in the control codend (Fig. 7C). The sizes of these individuals means that they could easily have passed through the 100 mm bar spacings in the grid and into the codend. Similar results were observed for leatherjacket and trumpeter caught in the codend containing the Nordmøre-grid and FCSMP (Fig. 7B, D). Regardless of the potential for this effect, the results showed that most fish escaped through the composite square-mesh panel when it was located closer to the end of the codend (i.e. the ACSMP design). This may be explained in terms of: (i) the swimming ability and behaviour of fish in the trawl; (ii) the influences of water flow anterior to the catch in the codend; (iii) the location of the composite square-mesh panel. Previous studies have shown that fish, unlike benthic invertebrates, have certain characteristic responses to towed trawls (see Chapman, 1964; Wardle, 1975, 1983; Watson, 1989). Small fish that enter the trawl attempt to maintain station, but are quickly fatigued and fall back into the codend. An area of possible escape for fish occurs in the codend because, as they are herded together, the balance of the school is upset which can initiate an escape response upwards and/or to the sides. Further, Broadhurst et al. (1999b) determined that there is substantial displacement of water forwards, owing to twine area and the build-up of catch, that can help fish maintain position up to approx. 1 m in front of the catch in the codend. For example, at a towing speed of 2.5 kn, the displacement of water immediately in front of the catch can reduce relative flow (i.e. for fish attempting to maintain position with the trawl) by up to 40% (Broadhurst et al., 1999b). The inclusion of strategically positioned openings (e.g. a composite-square-mesh panel) above this area can be used to direct some individuals out of the trawl. The critical importance of positioning the composite square-mesh panel correctly is illustrated by the non-significant reductions in bycatch from the FCSMP codend (Table 2). In this codend, we located the panel forward (e.g. 43 meshes or approx. 2.2 m anterior to the draw string) and attempted to induce some flow-related effects by restricting codend circumference immediately behind the panel. Despite the slightly narrower codend, there were no significant reductions of fish using this design (Table 2). These results conflict with those from previous studies that
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have examined different locations of square-mesh panels in crustacean trawls (Hillis et al., 1991; Armstrong et al., 1998). For example, in the Irish Sea, Armstrong et al. (1998) showed that a square-mesh panel located 7 m anterior to the codend extension of a Nephrops trawl reduced more small (<27 cm) whiting, Merlangius merlangus, than an identical panel located 1 m anterior to the codend extension. These results were attributed to a greater width of the square-mesh panel, and therefore more surface area, when located forward in the taper of the trawl (i.e. the trawl stretched the panel laterally). It is unlikely that trawl geometry had any great influence on the results observed in the present study, since both panels were located in the codend (i.e. an area with no taper). A more plausible explanation for the anomaly may involve the relative towing speeds of the trawls. Armstrong et al.’s (1998) experiment involved trawls towed at speeds between 2.2 and 2.8 kn (Mike Armstrong, Pers. Comm.), while in the present study, we towed trawls at up to 4.8 kn. At such speeds, small fish (<20 cm) would not have been able to maintain position in the vicinity of the composite square-mesh panel located in the forward position and probably passed very quickly to the back of the codend. The results presented here have illustrated the utility of simple panels of strategically located square mesh as secondary BRDs for reducing the bycatch of unwanted fish. The composite square-mesh panel is a modification that has no effect on the geometry of the trawl, but provides openings in that area of the codend where fish are able to maintain position (owing to influence of anteriorly displaced water) and either randomly or actively attempt escape. Since the influences of water flow anterior to the catch appear to be the main contributing factor in the escape of fish through this BRD, it may be possible to insert similar other modifications in the same area, provided they do not negatively affect codend geometry. Alternatively, if behavioural-type BRDs are to be located in other parts of the trawl or codend, they need to: (i) be positioned in an area where there are similar reductions in relative water flow; or (ii) contain components that reduce relative water flow to a velocity where small fish are able to maintain position close to the escape exit. Future research in the Shark Bay prawn-trawl fishery should include an assessment of some of these types of modifications, although any designs will have
to be constructed with regard to the possible influences of sea grass on their performance. A concern by fishers in this fishery is that the potential exists for sea grass, that passes through the Nordmøre-grid and into the codend, to build up past an aft composite square-mesh panel and effectively block the square meshes. It may be possible to use the composite square-mesh panel in a more forward position and initiate some displacement of water forward by restricting posterior codend circumference further than that attempted in the present study (i.e. to 70% instead of 35%). Alternatively, it may be appropriate to attach some form of semiporous panel (e.g. fine mesh) on the bottom of the codend, behind the panel and at an angle to the direction of tow (see Broadhurst et al., 1999b for details) to force the required changes in water flow. The willing involvement of Shark Bay prawn-trawl fishers in the design, modification and day-to-day testing of the sorts of modifications discussed above, should result in improvements to the basic designs tested in this study. Further experiments by scientists in this fishery will then be required to validate the performance of any new BRDs.
Acknowledgements Funding for this work was provided by the Fisheries Research and Development Corporation (Grant No. FRDC 2000/189). Thanks are extended to Errol Sporer for technical support and to Mike Moran, Peter Stephenson and Nick Caputi for constructive comments to the manuscript. This work would not have been possible without the advice and assistance of Norm Stevens, netmaster at Nor-west Seafoods. We would also like to thank the staff of Nor-west Seafoods Pty. Ltd., and the Shark Bay prawn-trawl fleet for their ongoing cooperation and support. The Departamento de Pesca, Universidade Federal Rural de Pernambuco in Recife, Brazil is acknowledged for contributing towards the involvement of Matt Broadhurst. References Andrew, N.L., Pepperell, J.G., 1992. The by-catch of shrimp trawl fisheries. Oceanogr. Mar. Biol. Annu. Rev. 30, 527–565. Armstrong, M.J., Briggs, R.P., Rihan, D., 1998. A study of optimum positioning of square-mesh escape panels in Irish Sea Nephrops trawls. Fish. Res. 34, 179–189.
M.K. Broadhurst et al. / Fisheries Research 58 (2002) 349–365 Broadhurst, M.K., 2000. Modifications to reduce bycatch in prawn trawls: a review and framework for development. Rev. Fish Biol. Fisher. 10, 27–60. Broadhurst, M.K., Kennelly, S.J., 1996a. Rigid and flexible separator-panels in trawls that reduce the by-catch of small fish in the Clarence River prawn-trawl fishery, Australia. Mar. Freshw. Res. 47, 991–998. Broadhurst, M.K., Kennelly, S.J., 1996b. Effects of the circumference of codends and a new design of square-mesh panel in reducing unwanted by-catch in the New South Wales oceanic prawn-trawl fishery, Australia. Fish. Res. 27, 203– 214. Broadhurst, M.K., Larsen, R.B., Kennelly, S.J., McShane, P., 1999a. Use and success of composite square-mesh codends in reducing bycatch and in improving size-selectivity of prawns in Gulf St. Vincent, South Australia. Fish. Bull. 97, 434–448. Broadhurst, M.K., Kennelly, S.J., Eayrs, S., 1999b. Flow-related effects in prawn-trawl codends: potential for increasing the escape of unwanted fish through square-mesh panels. Fish. Bull. 97, 1–8. Chapman, C.J., 1964. Importance of mechanical stimuli in fish behaviour, especially to trawls. In: Modern Fishing Gear of the World, Vol. 2. Fishing News Books, London, pp. 537–540.
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Hickey, W.M., Brothers, G., Boulos, D.L., 1993. By-catch reduction in the northern shrimp fishery. Can. Tech. Rep. Fish. Aquat. Sci. No 1964, 41 pp. Hillis, J.P., McCormick, R., Rihan, D., Geary, M., 1991. Squaremesh experiments in the Irish Sea. ICES CM 1991/B:58. Isaksen, B., Valdemarsen, J.W., Larsen, R.B., Karlsen, L., 1992. Reduction of fish by-catch in shrimp trawl using a rigid separator grid in the aft belly. Fish. Res. 13, 335–352. Kennelly, S.J., 1995. The issue of bycatch in Australia’s demersal trawl fisheries. Rev. Fish. Biol. Fish. 5, 213–234. Pettovello, A.D., 1999. By-catch in the Patagonian red shrimp (Pleoticus muelleri) fishery. Mar. Freshw. Res. 50, 123–127. Underwood, A.J., 1981. Techniques of analysis of variance in experimental marine biology and ecology. Oceanogr. Mar. Biol. Annu. Rev. 19, 513–605. Wardle, C.S., 1975. Limit of fish swimming speed. Nature 255, 725–727. Wardle, C.S., 1983. Fish reactions to towed fishing gears. In: MacDonald, A., Priede, I.G. (Eds.), Experimental Biology at Sea. Academic Press, New York, pp. 167–195. Watson, J.W., 1989. Fish behaviour and trawl design: potential for selective trawl development. In: Campbell, C.M. (Ed.), Proceedings of the World Symposium on Fishing Gear and Fishing Vessels. Marine Institute, St. Johns, Canada, pp. 25–29.
Using composite square-mesh panels and the Nordmøre-grid to reduce bycatch in the Shark Bay prawn-trawl fishery, Western Australia Matt K. Broadhursta,b,*, Mervi I. Kangasc, Cristiana Damianod, Scott A. Bickfordc, Steven J. Kennellyb a
Departamento de Pesca, Universidade Federal Rural de Pernambuco, Av. Dom Manuel de Medeiros, s/n, Dois Irma˜os, Recife-PE CEP: 52.171-900, Brazil b NSW Fisheries, Cronulla Fisheries Centre, P.O. Box 21, Cronulla, NSW 2230, Australia c Western Australian Marine Research Laboratories, P.O. Box 20, North Beach, WA 6020, Australia d Projeto Tamar, Alameda do Boldro´ s/n, Fernando de Noronha-PE CEP: 53.990-000, Brazil Received 1 March 2001; received in revised form 7 August 2001; accepted 26 August 2001
Abstract An industry-modified Nordmøre-grid was tested on its own and with a composite square-mesh panel as a secondary bycatch reduction device (BRD) at two different positions (aft and forward) in codends. Compared to the control codend (which had no BRDs), all three combinations (Nordmøre-grid only, Nordmøre-grid and aft composite square-mesh panel and Nordmøre-grid and forward composite square-mesh panel) reduced catches of prawns (mostly western king prawns, Penaeus latisulcatus). However, prawns were found to escape out of the Nordmøre-grid—not through the composite square-mesh panels. The Nordmøre-grid with the aft composite square-mesh panel significantly reduced the weight of bycatch (by 49%) and the numbers and weights of several commercially and non-commercially important bycatch species (by up to 75.7%). No other significant differences were detected. The results are discussed in terms of the likely factors influencing the performance of the various designs, including the behaviour of fish in codends, influences of hydrodynamics on their escape and the importance of the positioning of BRDs in codends. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Bycatch reduction; Prawn trawl; Square-mesh panels; Trawl selectivity
1. Introduction The Shark Bay prawn-trawl fishery mainly targets western king prawns, Penaeus latisulcatus and brown *
Corresponding author. Present address: NSW Fisheries, Cronulla Fisheries Centre, P.O. Box 21, Cronulla, NSW 2230, Australia. Tel.: þ61-2-9527-8411; fax: þ61-2-9527-8576. E-mail address: [email protected] (M.K. Broadhurst).
tiger prawns, P. esculentus and is valued at approx. $A32 million per annum. Like the majority of prawntrawl fisheries throughout the world, trawlers operating in Shark Bay often catch and discard large quantities of a diverse assemblage of non-target organisms (termed bycatch—for reviews see Andrew and Pepperell, 1992; Kennelly, 1995). At times, this bycatch can include sea turtles (i.e. loggerhead, Caretta caretta, green, Chelonia mydas and rarely hawksbill, Eretmochelys imbricata and leatherback turtles,
0165-7836/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 7 8 3 6 ( 0 1 ) 0 0 3 9 0 - 3
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Dermochelys coriacea), but more frequently comprises numerous small fish (<20 cm), including juveniles of commercially and recreationally important species. The mortality of these individuals is of particular concern, since it may contribute to recruitment overfishing of stocks that form the basis of other fisheries. The most common management strategy to reduce bycatches from prawn trawling is to install physical devices (collectively termed bycatch reduction devices—BRDs) in the trawl, which are designed to improve selectivity (for a review see Broadhurst, 2000). These devices can be classified under two categories, according to the methods used to facilitate the escape of bycatch. The first category includes designs that mainly function by mechanically partitioning the catch according to size. These BRDs include some sort of separating panel (usually a rigid grid) terminating in an escape exit anterior to the codend, and are designed to exclude individuals that are larger than the openings in the separating panel. The most well-known example of these sorts of BRDs is the Nordmøre-grid, which has been adopted in several prawn-trawl fisheries throughout the world (e.g. Isaksen et al., 1992; Hickey et al., 1993; Broadhurst and Kennelly, 1996a; Pettovello, 1999). The second category of BRDs includes those that are designed to operate by exploiting behavioural differences between prawns and fish using strategically placed panels and escape exits (Broadhurst, 2000). These devices may be used on their own in a trawl, or as secondary BRDs behind designs that mechanically partition the catch according to size (in this case, the latter are termed primary BRDs). The presence of sea turtles in areas of Shark Bay and concerns over the potential for their capture and mortality by prawn trawlers, resulted in recent drafting of bycatch action plans. These specify the implementation of rigid mechanical-type separating BRDs (with bar spacings no larger than 100 mm) and secondary BRDs to reduce bycatch. Numerous existing BRDs, including the Nordmøre-grid, have already been tested and modified by prawn trawlers working in Shark Bay. Anecdotal information suggests that many of these designs maintain catches of prawns, although the presence of very large quantities of sea grass (Amphibolis antarctica) on the trawl grounds at certain times and locations can negatively affect performances of these devices (i.e. block bars and escape exits and result in loss of prawns).
Continual refinements of grids by industry to address the problems associated with sea grass are likely to result in suitable mechanical-type separating BRDs that also exclude sea turtles and other large animals from trawls. However, such designs do not address the capture and mortality of smaller fish which are unlikely to be excluded in any great quantities from mechanical-type separating BRDs. Because of this, modifications that exclude small fish via differences in behaviour need to be developed and tested as secondary BRDs in this fishery. Further, to facilitate the acceptance and implementation of these sorts of BRDs, they should have no adverse effects on efficiency of the trawl and operate over the range of commercial conditions experienced. More specifically, the problems associated with sea grass in Shark Bay means that any secondary BRDs should be simple and free of components or panels that might become blocked. One such BRD (termed the composite square-mesh panel) has been shown to be very effective in other Australian prawn-trawl fisheries. This device involves different-sized square-shaped meshes located in the top of the posterior section of the codend (Broadhurst and Kennelly, 1996b; Broadhurst et al., 1999a). Previous studies have suggested that anteriorly displaced water in front of the catch can assist fish to maintain position in this area of the codend and facilitate their escape through the strategically located composite square-mesh panel (see Broadhurst et al., 1999b). The composite square-mesh panel has no rigid components or complicated guiding panels, is easy to construct and has been proven to maintain catches of prawns and reduce bycatch from several different designs of trawls over a range of commercial conditions in Australia (Broadhurst and Kennelly, 1996b; Broadhurst et al., 1999a). Our specific objectives in the present study were to: (i) quantify the effectiveness of an industry-modified Nordmøre-grid in maintaining catches of prawns in Shark Bay; (ii) examine the bycatch-reduction capability of the composite square-mesh panel as a secondary BRD. As part of this second objective, we aimed to examine some of the possible effects on fish escape due to displaced water in front of the catch. This was achieved by quantifying the performance of the composite square-mesh panel at two positions (forward and aft) in the codend.
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2. Materials and methods
2.1. BRDs examined
The experiment was done on established prawntrawl grounds in Shark Bay (Fig. 1), Western Australia, in August 2000 using a chartered commercial prawn trawler (FV Cape Conway), rigged to tow two trawls (each with a headline length of 14.6 m) in a conventional twin-rig system. The trawls were identical, made from polyethylene twine and had a stretched mesh size of 52 mm in the body and 47 mm in the codend (NB: unless stated otherwise, all stretched mesh sizes were measured as centre knot to centre knot). All tows were done over a combination of sandy and light mud bottoms in depths ranging from 13.7 to 18.5 m and at speeds between 3.5 and 4.6 kn (mean S:E: of 4:2 0:04 kn per tow).
One primary BRD and two secondary BRDs were used in the experiment. The primary BRD, termed the Nordmøre-grid, consisted of an aluminium grid with the upper third offset at 458 and vertical bars spaced at 100 mm (Fig. 2A). The grid was located at an angle of 458 in a 30-mesh extension piece, measuring 120 meshes in circumference and made from 47 mm, 60 ply, UV-stabilised, high-density polyethylene twine. The anterior end of this extension was laced to the trawl body, and a zipper (Burashi S-146R, pinlock side) attached to the posterior end to facilitate the changing of codends. Unlike original designs of the Nordmøre-grid (e.g. Isaksen et al., 1992; Hickey et al., 1993), the design tested in the present study
Fig. 1. Location of Shark Bay and area fished.
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Fig. 2. (A) The Nordmøre-grid and (B) its location in the trawl.
had no funnel or guiding panel, but included two flexible panels of 47 mm diamond-shaped mesh (60 ply, UV-stabilised, high-density polyethylene twine) that were hung loosely above and below the escape exit and extended to the grid (Fig. 2B). These panels were attached to the net only anteriorly, and were free to lift through and above the escape exit (see also Broadhurst and Kennelly, 1996a). The theory behind these panels was that they would limit the escape of prawns, but still allow organisms larger than the bar spacings to be released from the trawl. To maintain stability, two plastic floats (16 cm in diameter) were attached to either side of the upper edge of the grid (Fig. 2B). Three codends were constructed and rigged with zippers (Burashi S-146R, pinlock side) so that they could be attached posterior to the extension containing the Nordmøre-grid. The first codend was a normal
design (termed the conventional codend) and comprised 47 mm diamond-shaped mesh (60 ply, UVstabilised, high-density polyethylene twine) throughout with a circumference of 120 meshes and a length of 70 meshes (Figs. 3A and 4A). The second and third designs, termed the aft composite square-mesh panel (ACSMP—Figs. 3B and 4B) and forward composite square-mesh panel (FCSMP—Figs. 3C and 4C) codends had the same circumference and length as the codend described above, but included secondary BRDs. These BRDs comprised composite panels made of 47, 94 and 155 mm mesh cut on the bar (Fig. 3D—see also Broadhurst and Kennelly, 1996b) and inserted into the top sections of the codends at distances anterior to the draw strings of 24 and 43 meshes, respectively (Fig. 3B and C). The location of the panel in the ACSMP codend was based on designs developed for use in New South Wales (NSW)
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Fig. 3. Schematic diagram of the (A) conventional, (B) aft composite square-mesh panel, (C) forward composite square-mesh panel codends and (D) composite square-mesh panel. T, transversals; N, normals; and B, bars.
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Fig. 4. The locations of the Nordmøre-grid and the (A) conventional, (B) aft composite square-mesh panel and (C) forward composite squaremesh panel codends in the prawn trawl.
and South Australian prawn-trawl fisheries (e.g. Broadhurst and Kennelly, 1996b; Broadhurst et al., 1999a). Broadhurst et al. (1999b) showed that at this location there is substantial displacement of water anterior to the catch in the codend, that might assist small fish to maintain position under the composite square-mesh panel, increasing their probability of randomly encountering the open meshes and escaping (see discussion below). In the FCSMP codend, the square-mesh panel was located further forward and away from any flow-related effects due to catch (Broadhurst et al., 1999b). Instead, a restricting cord (4 mm diameter polypropylene rope, 1.3 m in circumference—Figs. 3C and 4C) was located at a distance of 10 meshes posterior to the composite square-mesh panel. This cord reduced the circumference of the codend by approx. 35% (calculated assuming a fractional mesh opening of 0:35 the stretched mesh length between the knots the mesh circumference;
Broadhurst et al., 1999a). We hypothesised that reducing codend circumference immediately behind the composite square-mesh panel would create a short but sharp taper in the codend that would displace water forward and thereby assist fish to maintain position beneath the composite square-mesh panel (as is believed for the ACSMP). The control codend represented those normally used commercially and was made entirely of 47 mm diamond-shaped meshes (60 ply, UV-stabilised, high-density polyethylene twine), measuring 120 meshes in circumference and 100 meshes in length. This codend was the same length as each of the codends described above attached to the extension containing the Nordmøre-grid and was laced to the end of the trawl body in a similar manner. Using zippers, the conventional design (i.e. with no secondary BRD), FCSMP and ACSMP codends described above were alternatively attached posterior
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to the Nordmøre-grid (Fig. 4) and the entire assembly tested against the control codend, on each side of the twin-rigged gear (i.e. three separate paired comparisons: Nordmøre-grid only, i.e. with conventional codend vs. control; Nordmøre-grid and FCSMP codend vs. control; and Nordmøre-grid and ACSMP codend vs. control). Two replicate 40 min tows of each paired comparison were made on each night, providing a total of 10 replicate comparisons of each configuration over five nights. The position and order of the three codends attached posterior to the Nordmøre-grid were randomly assigned. However, because the Nordmøre-grid could not be easily removed from the net (i.e. it was not possible to attach a zipper between the trawl body and this BRD), this device was alternated between nets on different nights so that equal numbers of paired comparisons of each treatment against the control were made on port and starboard trawls. Trawls were tested prior to the trials to ensure no biases between the sides of the vessel. 2.2. Data collected and statistical analyses After each tow, the two codends were emptied onto a partitioned tray. Prawns, all individuals of commercially and/or recreationally important species and individuals of the most abundant non-commercial species were separated. The following categories of data were determined for each tow: the total weight of prawns; the weights of individual species of prawns; the numbers of western king prawns and brown tiger prawns and a subsample (3 kg from each codend) of their lengths (to the nearest 1 mm carapace length); the weight of bycatch; the weights and numbers of commercially and/or recreationally important species and the most abundant non-commercial species; and the sizes (to the nearest 0.5 cm) of the most abundant and commercially and/or recreationally important fish. Several species (commercial and non-commercial) were caught in sufficient numbers (i.e. 1 individual in at least 7 replicates) to enable meaningful comparisons (see Table 1). Catch data for all replicates that had sufficient number of each variable were analysed with one-tailed paired t-tests ðP 0:05Þ testing the hypothesis that the BRDs caught less than the control. To examine the relative effectiveness of BRDs, the differences in
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Table 1 List of species caught in sufficient quantities to permit analyses Scientific name
Common name
P. latisulcatus P. esculentus Metapenaeopsis crassissima Sillago maculata Platycephalus endrachtensis Pseudorhombus jenynsii Lethrinus genivittatus Portunus pelagicus Leiognathus leuciscus Pentapodus vitta Paramonacanthus choirocephalus Upeneus asymmetrius Pelates quadrilneatus Sardinella gibbosa Sorsogona tuberculata
Western king prawns Brown tiger prawns Coral prawns Trumpeter whiting Bar-tailed flathead Small-toothed flounder Threadfin Blue swimmer crab Ponyfisha Butterfisha Leatherjacketa Goatfisha Trumpetera Sardinea Heart-headed flatheada
a
Denotes non-commercial/non-recreational species.
catches (between each treatment and their respective controls) for those variables with numbers present in all tows were analysed using a balanced two-factor orthogonal ANOVA (BRDs and nights were treated as fixed and random factors, respectively) after preliminary tests for heteroscedasticity using Cochran’s test. Data with heterogeneous variances were transformed, but if heteroscedasticity persisted, ANOVA was done at a more conservative probability level (0.01) to reduce the likelihood of Type I errors (Underwood, 1981). Means were compared following significant F-tests using Student–Newman–Keuls (SNK) multiple comparisons. With the exception of data for catches of prawns, where analyses provided similar results for weights and numbers of variables, only data concerning numbers were included in Fig. 5 to conserve space. Size-frequencies of prawns and fish retained were combined across all tows and where there were sufficient data (>30 individuals in each codend), they were compared with two-sample Kolmogorov–Smirnov tests ðP 0:05Þ.
3. Results Compared to the control, the Nordmøre-grid only and the Nordmøre-grid with the FCSMP and ACSMP codends significantly reduced the total weight of prawns by similar amounts (e.g. 12.5, 14
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Fig. 5. Differences in mean catch (S.E.) between the BRDs and the control for (A) weight of total prawns, (B) number and (C) weight of western king prawns, (D) number and (E) weight of brown tiger prawns, (F) weight of coral prawns, (G) weight of bycatch and numbers of (H) trumpeter whiting, (I) butterfish, (J) leatherjacket, (K) goatfish, (L) trumpeter, (M) threadfin, (N) sardine, (O) heart-headed flathead, (P) bartailed flathead, (Q) small-toothed flounder and (R) blue swimmer crabs. Histograms in black represent significant reductions. Ng, Nordmøregrid; FCSMP, forward composite square-mesh panel codend; ACSMP, aft composite square-mesh panel codend.
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Fig. 5. (Continued ).
and 12%, respectively) and the numbers (19.5, 19.3 and 16.1%, respectively) and weights (by 14.8, 18.8 and 13.5%, respectively) of western king prawns (Fig. 5A–C and Table 2). The Nordmøre-grid in
combination with the ACSMP codend also significantly reduced the weight of bycatch (by 49%) and the numbers and weights of leatherjacket (by 55.6 and 58.6%), sardine (by 57.4 and 70.2%),
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Fig. 5. (Continued ). Table 2 Summaries of one-tailed paired t-tests comparing three BRDs against the controla Ng only vs. control
Total weight of prawns No. of western king prawns Weight of western king prawns No. of brown tiger prawns Weight of brown tiger prawns Weight of coral prawns Weight of bycatch No. of trumpeter whiting Weight of trumpeter whiting No. of butterfish Weight of butterfish No. of leatherjacket Weight of leatherjacket No. of goatfish Weight of goatfish No. of trumpeter Weight of trumpeter No. of threadfin Weight of threadfin No. of sardine Weight of sardine No. of heart-headed flathead Weight of heart-headed flathead No. of bar-tailed flathead Weight of bar-tailed flathead No. of small-toothed flounder Weight of small-toothed flounder No. of blue swimmer crab Weight of blue swimmer crab
Ng and FCSMP vs. control
Ng and ACSMP vs. control
pt-v
P
n
pt-v
P
n
pt-v
P
n
2.616 3.657 3.801 0.489 0.582 0.785 0.675 – – 1.013 0.725 0.079 0.410 4.088 3.616 1.670 1.582 – – 1.620 1.792 0.838 0.637 – – 1.414 0.367 2.375 2.691
0.014* 0.003** 0.002** 0.318 0.287 0.771 0.258 – – 0.170 0.245 0.469 0.654 0.999 0.997 0.065 0.074 – – 0.925 0.942 0.217 0.274 – – 0.097 0.362 0.972 0.982
10 10 10 10 10 8 10 – – 10 10 10 10 10 10 10 10 – – 8 8 7 7 – – 9 9 7 7
2.263 3.446 2.682 0.303 0.689 0.558 1.666 0.338 0.334 0.634 1.240 1.680 0.921 3.524 1.847 1.284 0.922 1.251 0.539 – – 0.403 1.096 0.584 0.420 1.021 1.321 0.432 0.379
0.025* 0.004** 0.013* 0.616 0.746 0.297 0.065 0.372 0.374 0.273 0.127 0.064 0.191 0.997 0.951 0.123 0.196 0.128 0.305 – – 0.349 0.152 0.712 0.345 0.167 0.109 0.338 0.357
10 10 10 10 10 8 10 8 8 8 8 10 10 10 10 7 7 7 7 – – 9 9 9 9 10 10 10 10
3.132 3.314 3.390 0.996 0.896 0.987 2.431 1.163 1.236 0.393 1.257 3.558 3.515 0.105 0.162 1.668 1.621 1.678 1.833 2.076 2.032 1.318 1.010 3.103 3.500 2.511 2.548 0.043 0.041
0.006** 0.004** 0.004** 0.173 0.196 0.173 0.019* 0.144 0.131 0.352 0.122 0.003** 0.003** 0.459 0.438 0.073 0.078 0.072 0.058 0.038* 0.041* 0.114 0.173 0.007** 0.004** 0.018* 0.017* 0.516 0.484
10 10 10 10 10 10 10 7 7 9 9 10 10 10 10 7 7 7 7 8 8 8 8 9 9 9 9 8 8
a Ng, Nordmøre-grid; FCSMP, forward composite square-mesh panel codend; ACSMP, aft composite square-mesh panel codend; pt-v, paired t-value; n, number of replicates; (–) insufficient data. * Significant at P < 0:05. ** Significant at P < 0:01.
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Table 3 Summaries of F-ratios from two-factor analyses of variance to determine effects on variables due to fishing with three different BRDs on five different nights. The transforms used to stabilise the variances where required are listeda Treatment
Nights BRDs Interaction Residual
d.f.
4 2 8 15
Total weight of prawns
2.17 0.14 0.67
Western king prawns
Brown tiger prawns
No.
Weight
No.
Weight (1=ðx þ 1Þ)
8.57** 0.21 1.32
3.08 1.26 0.44
2.29 0.18 1.74
0.66 0.98 2.16
Weight of bycatch
14.35** 2.83 5.97**
Leatherjacket
Goatfish
No.
Weight
No.
Weight
6.03** 6.16* 1.15
2.68 3.79 1.50
0.93 6.84* 0.76
1.94 1.74 1.85
a
Weight of king prawns was tested using a Type I error rate of a ¼ 0:01 because Cochran’s test of the transformed data was still significant at P ¼ 0:05. * Significant at P < 0:05. ** Significant at P < 0:01.
bar-tailed flathead (by 75.7 and 73.4%) and smalltoothed flounder (by 50 and 46.5%) (Fig. 5G, J, N, P, Q and Table 2). While not significant, this combination of BRDs also reduced the numbers and weights of trumpeter whiting, butterfish, trumpeter, threadfin and heart-headed flathead (differences between 9.4 and 68.2%) (Fig. 5H, I, L, M, O and Table 2). No other significantdifferencesweredetected(Fig.5andTable2), although the Nordmøre-grid only and the Nordmøregrid in combination with the FCSMP codend showed some reduction in the weight of bycatch (by 4.8 and 15.5%, respectively) and the numbers and weights of butterfish, trumpeter, heart-headed flathead and smalltoothed flounder (by between 7.1 and 44.4%) (Fig. 5G, I, L, O, Q and Table 2). Further, the numbers and weights of trumpeter whiting, leatherjacket, threadfin and blue swimmer crab were also reduced by the Nordmøre-grid and FCSMP codend (by between 17.7 and 73.6%), but these catches were not significantly different from the control codend (Fig. 5H, J, M, R and Table 2). ANOVA of the differences in catches between the BRDs and their controls detected significant differences in the numbers of leatherjacket and goatfish for
the main effect of BRDs and an interaction between nights and BRDs for the weight of bycatch (Table 3). Subsequent SNK tests of these means showed that the Nordmøre-grid with the ACSMP codend retained significantly fewer goatfish that the other BRDs, less bycatch on the third night and fewer leatherjackets than the Nordmøre-grid only (Table 4). Two sample Kolmogorov–Smirnov tests comparing size-frequency distributions of western king prawns, brown tiger prawns, butterfish, and heartheaded flathead retained by the codends showed no significant differences in relative size compositions (Figs. 6A, B and 7A, F). No significant differences were detected in size compositions of leatherjacket and trumpeter between the Nordmøre-grid only and control codends or between the Nordmøre-grid with the ACSMP and control codends, but the Nordmøregrid with the FCSMP codend caught proportionally fewer medium and large-sized leatherjacket and trumpeter than did the control (Fig. 7B). Similarly, the Nordmøre-grid with the ACSMP and the Nordmøre-grid only caught fewer large-sized sardine and goatfish, respectively (Figs. 7E, C).
Table 4 Summaries of SNK multiple comparisons of means for the significant interaction between nights and BRDs for weight of bycatch and the main effect of BRDs for number of leatherjacket and goatfisha Weight of bycatch No. of leatherjacket No. of goatfish
Night 3: Ng and ACSMP < Ng and FCSMP ¼ Ng only; other nights: no differences between BRDs Ng and ACSMP ¼ Ng and FCSMP < Ng only Ng and ACSMP < Ng and FCSMP ¼ Ng only
a Ng, Nordmøre-grid; FCSMP, forward composite square-mesh panel codend; ACSMP, aft composite square-mesh panel codend; (<) caught less.
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Fig. 6. Size-frequency distributions of (A) western king prawns and (B) brown tiger prawns retained in the BRD and control codends.
4. Discussion All three modified codends containing the BRDs significantly reduced the catches of western king prawns, contributing to a significant reduction in total weight of prawns. However, because the amounts of these reductions were similar (compared to their controls) and ANOVA failed to detect any significant
differences among the various designs (Table 3), it is likely that these prawns escaped at the Nordmøre-grid and not through the composite square-mesh panels. That is, no additional prawns were lost when using a composite-square-mesh panel behind the Nordmøregrid. This result may be explained by the partial blocking of the Nordmøre-grid due to sea grass encountered during many tows and/or the movement
M.K. Broadhurst et al. / Fisheries Research 58 (2002) 349–365 Fig. 7. Size-frequency distributions of (A) butterfish, (B) leatherjacket, (C) goatfish, (D) trumpeter, (E) sardine and (F) heart-headed flathead retained in the BRD and control codends. * Significant at P < 0:05; ** significant at P < 0:01.
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Fig. 7. (Continued ).
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of this sea grass out through the escape exit. For example, some prawns may have been unable to pass through blocked sections of the grid, or were mixed in with the sea grass and escaped as it was released from the trawl. These results are comparable to those detected by Broadhurst and Kennelly (1996a) during an experiment with a similar design of the Nordmøregrid in the Clarence River, NSW, Australia. In this work, large quantities of debris and weed contributed to a reduction in catches of school prawns, Metapenaeus macleayi by approx. 10%. The large quantities of sea grass encountered in Shark Bay means that, at times, some loss of prawns out through the escape exits of mechanical-type separating BRDs will probably be unavoidable, although the potential for this could be minimised via subtle modifications to the existing design. One possible improvement may be to extend the outer flap of the existing Nordmøre-grid a greater distance past the escape opening. Another option may be to remove the inner and outer panels at the escape exit and insert a long, shallow-angled, guiding panel, starting approx. 2 or 3 m in front of the upper edge of the grid and terminating immediately in front of its base. Broadhurst and Kennelly (1996a) suggested that similar problems of seaweed blocking grids in the Clarence River, NSW, were alleviated using this type of modification, primarily because the geometric angle and contours of the panel rolled the seaweed into a ball before reaching the grid, facilitating its movement out through the large exit. An additional modification that could facilitate the removal of sea grass may be to reduce the bar spacings in the grid (e.g. to approx. 60 mm), thereby increasing the surface area of the grid and possibly the flow of water out through the escape exit. The influences of sea grass on prawn loss also may have contributed to the escape of some larger fish and may partially explain some of the significant reductions in various individuals from the Nordmøre-grid and ACSMP codend and non-significant reductions in number of individuals from the other BRDs (Table 2). For example, significant differences were detected between size-frequency distributions of sardine caught with proportionally fewer large individuals retained in the codend containing the Nordmøre-grid and ACSMP (Fig. 7E). It is possible that some of these larger individuals passed through the escape exit of the grid during the movement of sea grass and other debris. In
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support of this, proportionally fewer large-sized goatfish (13–17 cm) were retained in the Nordmøre-grid only than in the control codend (Fig. 7C). The sizes of these individuals means that they could easily have passed through the 100 mm bar spacings in the grid and into the codend. Similar results were observed for leatherjacket and trumpeter caught in the codend containing the Nordmøre-grid and FCSMP (Fig. 7B, D). Regardless of the potential for this effect, the results showed that most fish escaped through the composite square-mesh panel when it was located closer to the end of the codend (i.e. the ACSMP design). This may be explained in terms of: (i) the swimming ability and behaviour of fish in the trawl; (ii) the influences of water flow anterior to the catch in the codend; (iii) the location of the composite square-mesh panel. Previous studies have shown that fish, unlike benthic invertebrates, have certain characteristic responses to towed trawls (see Chapman, 1964; Wardle, 1975, 1983; Watson, 1989). Small fish that enter the trawl attempt to maintain station, but are quickly fatigued and fall back into the codend. An area of possible escape for fish occurs in the codend because, as they are herded together, the balance of the school is upset which can initiate an escape response upwards and/or to the sides. Further, Broadhurst et al. (1999b) determined that there is substantial displacement of water forwards, owing to twine area and the build-up of catch, that can help fish maintain position up to approx. 1 m in front of the catch in the codend. For example, at a towing speed of 2.5 kn, the displacement of water immediately in front of the catch can reduce relative flow (i.e. for fish attempting to maintain position with the trawl) by up to 40% (Broadhurst et al., 1999b). The inclusion of strategically positioned openings (e.g. a composite-square-mesh panel) above this area can be used to direct some individuals out of the trawl. The critical importance of positioning the composite square-mesh panel correctly is illustrated by the non-significant reductions in bycatch from the FCSMP codend (Table 2). In this codend, we located the panel forward (e.g. 43 meshes or approx. 2.2 m anterior to the draw string) and attempted to induce some flow-related effects by restricting codend circumference immediately behind the panel. Despite the slightly narrower codend, there were no significant reductions of fish using this design (Table 2). These results conflict with those from previous studies that
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have examined different locations of square-mesh panels in crustacean trawls (Hillis et al., 1991; Armstrong et al., 1998). For example, in the Irish Sea, Armstrong et al. (1998) showed that a square-mesh panel located 7 m anterior to the codend extension of a Nephrops trawl reduced more small (<27 cm) whiting, Merlangius merlangus, than an identical panel located 1 m anterior to the codend extension. These results were attributed to a greater width of the square-mesh panel, and therefore more surface area, when located forward in the taper of the trawl (i.e. the trawl stretched the panel laterally). It is unlikely that trawl geometry had any great influence on the results observed in the present study, since both panels were located in the codend (i.e. an area with no taper). A more plausible explanation for the anomaly may involve the relative towing speeds of the trawls. Armstrong et al.’s (1998) experiment involved trawls towed at speeds between 2.2 and 2.8 kn (Mike Armstrong, Pers. Comm.), while in the present study, we towed trawls at up to 4.8 kn. At such speeds, small fish (<20 cm) would not have been able to maintain position in the vicinity of the composite square-mesh panel located in the forward position and probably passed very quickly to the back of the codend. The results presented here have illustrated the utility of simple panels of strategically located square mesh as secondary BRDs for reducing the bycatch of unwanted fish. The composite square-mesh panel is a modification that has no effect on the geometry of the trawl, but provides openings in that area of the codend where fish are able to maintain position (owing to influence of anteriorly displaced water) and either randomly or actively attempt escape. Since the influences of water flow anterior to the catch appear to be the main contributing factor in the escape of fish through this BRD, it may be possible to insert similar other modifications in the same area, provided they do not negatively affect codend geometry. Alternatively, if behavioural-type BRDs are to be located in other parts of the trawl or codend, they need to: (i) be positioned in an area where there are similar reductions in relative water flow; or (ii) contain components that reduce relative water flow to a velocity where small fish are able to maintain position close to the escape exit. Future research in the Shark Bay prawn-trawl fishery should include an assessment of some of these types of modifications, although any designs will have
to be constructed with regard to the possible influences of sea grass on their performance. A concern by fishers in this fishery is that the potential exists for sea grass, that passes through the Nordmøre-grid and into the codend, to build up past an aft composite square-mesh panel and effectively block the square meshes. It may be possible to use the composite square-mesh panel in a more forward position and initiate some displacement of water forward by restricting posterior codend circumference further than that attempted in the present study (i.e. to 70% instead of 35%). Alternatively, it may be appropriate to attach some form of semiporous panel (e.g. fine mesh) on the bottom of the codend, behind the panel and at an angle to the direction of tow (see Broadhurst et al., 1999b for details) to force the required changes in water flow. The willing involvement of Shark Bay prawn-trawl fishers in the design, modification and day-to-day testing of the sorts of modifications discussed above, should result in improvements to the basic designs tested in this study. Further experiments by scientists in this fishery will then be required to validate the performance of any new BRDs.
Acknowledgements Funding for this work was provided by the Fisheries Research and Development Corporation (Grant No. FRDC 2000/189). Thanks are extended to Errol Sporer for technical support and to Mike Moran, Peter Stephenson and Nick Caputi for constructive comments to the manuscript. This work would not have been possible without the advice and assistance of Norm Stevens, netmaster at Nor-west Seafoods. We would also like to thank the staff of Nor-west Seafoods Pty. Ltd., and the Shark Bay prawn-trawl fleet for their ongoing cooperation and support. The Departamento de Pesca, Universidade Federal Rural de Pernambuco in Recife, Brazil is acknowledged for contributing towards the involvement of Matt Broadhurst. References Andrew, N.L., Pepperell, J.G., 1992. The by-catch of shrimp trawl fisheries. Oceanogr. Mar. Biol. Annu. Rev. 30, 527–565. Armstrong, M.J., Briggs, R.P., Rihan, D., 1998. A study of optimum positioning of square-mesh escape panels in Irish Sea Nephrops trawls. Fish. Res. 34, 179–189.
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