The Sea Fish Industry Authority Seafish Technology. Improving Selectivity in Towed Fishing Gears

Transcription

1 The Sea Fish Industry Authority Seafish Technology Improving Selectivity in Towed Fishing Gears Further Investigations into the use of Low Diameter Twines for the Construction of Square Mesh Panels Seafish Report SR544 Ken Arkley Gary Dunlin September 2002 The Sea Fish Industry Authority

2 Sea Fish Industry Authority Seafish Technology Improving Selectivity in Towed Fishing Gears Further Investigations into the use of Low Diameter Twines for the Construction of Square Mesh Panels Seafish Report SR544 Ken Arkley Gary Dunlin September 2002 Summary This report is one of a series describing the potential to reduce discarding by the use of technical conservation measures. It covers the second phase of work undertaken to improve the performance of the square mesh panels that are now mandatory in a number of towed gear fisheries. Legislation defines the main dimensions and positioning of panels. It also requires that they be constructed of non-slip knots and rigged so that the meshes stay fully open whilst fishing but it leaves much of the choice of the construction details to the user. The work here has built on what is known about the influence that twine diameter has on size selection of fish. The development of high performance polyethylene (HPPE) twines raised the possibility of using low diameter twines in square mesh panels and improving their selective performance. Eleven days of sea trials were carried out on a twin rig trawler targeting Nephrops. Panels made from HPPE twines were used in two different mesh construction and their performance was compared to panels made of standard PE netting. The results showed a significant improvement in performance attributed to the use of the low diameter twines. They also show that cheaper, and more widely available constructions can be used to equal effect. Technical and financial support from DSM High Performance Fibers of the Netherlands helped to make this work possible and is gratefully acknowledged.

3 CONTENTS: SUMMARY 1. Introduction Background Aims and Objectives Materials and Methods Approach Vessel details Gear details Panel Construction Trials Procedures Catch Sampling Results Week One Dyneema knotless (UC) Vs. Standard PE knotless (UC) Haddock Whiting Week Two Dyneema knotless (UC) Vs. Standard PE knotless (UC) Haddock Whiting Cod Nephrops General Observations Discussion and Findings Acknowledgements... 35

4 FIGURES: Figure 1 MFV Heather Sprig...8 Figure 2 Schematic drawing of trawls used during trials...11 Figure 3 Standard PE single knot SMP...14 Figure 4 Dyneema low diameter double knot SMP...15 Figure 5 Standard PE UC knotless SMP...16 Figure 6 Dyneema low diameter UC knotless SMP...17 Figure 7 Relative position of SMP within the trawl...18 Figure 8 Length/Numbers distribution for Haddock Week One...22 Figure 9 Length/Numbers distribution for Whiting Week One...23 Figure 10 Length/Numbers distribution for Haddock Week Two...24 Figure 11 Length/Numbers distribution for Whiting Week Two...25 Figure 12 Length/Numbers distributions for Cod Weeks One and Two...26 APPENDICES Appendix I: Catch Data Week 1 Table 1 Table 2 Catch data for haddock...ii Catch data for whiting... iii Appendix II: Catch Data Week 2 Table 3 Table 4 Catch data for haddock...v Catch data for whiting...vi Appendix III: Catch Data Weeks 1 and 2 Table 5 Table 6 Catch data for cod week one... viii Catch data for cod week two...ix Appendix IV Photographs showing details of SMPs...xi

5 1. Introduction This report describes a project, which follows on from work presented in Seafish Report No SR539 (Investigating the use of low diameter twines for the construction of square mesh panels). The previous work found that the materials used for the construction of square mesh panels (SMPs) can influence the selectivity of the gear. Careful selection of netting materials and construction can improve size selection and bycatch reductions for certain roundfish species. The work centered on the use of high performance polyethylene (HPPE) twines *(Dyneema ) and Ultra Cross (UC) knotless netting construction. This combination produces a low diameter netting with high mesh stability, which is ideal for square mesh panel construction. Unfortunately, the cost of this combination of Dyneema and UC construction is very high when compared to standard knotted PE netting. Additionally, availability of this material is currently limited in the UK. Having identified the potential benefits of these alternative constructions, Seafish initiated further investigations to evaluate other practical alternatives. The aim was to achieve consistent mesh shape and stability with the lowest practical twine diameter in a readily available, cost-effective material and construction. The previous work was conducted with the co-operation of the Dutch company DSM High Performance Fibers which manufactures the Dyneema material. This collaboration was continued in a second round of trials. DSM and Seafish identified another Dyneema netting product with a specification that potentially fitted the requirements. Sea trials were carried out to evaluate and compare the performance of a number of different SMP constructions, including the new material. *Dyneema Dyneema is the trademark of DSM High Performance Fibers of the Netherlands. It is claimed to be the strongest fibre in the world. It is classed as a high performance polyethylene fibre (HPPE). The density of Dyneema is less than one; in other words it will float in water. The tenacity of Dyneema twines can be up to 15 times that of good quality steel. Elongation at break point is as low for Dyneema fibres as for other high performance fibres, but due to a very high tenacity, the energy required to break the fibres is high. The combination of low density and high strength makes Dyneema attractive as a material for the construction of netting and ropes. The material is resistant to water, most chemicals, UV light and micro-organisms. The high molecular weight PE used to produce the Dyneema fibre is also well known as an engineering plastic used for its superior wear and abrasion resistance. All of these characteristics have enabled a twine material to be produced with high strength at low diameter and weight, which in combination with the Ultracross netting construction, provides further options for improving square mesh panel selectivity. Seafish Report No SR544 1

6 Seafish Report No SR544 2

7 2. Background Current legislation regarding square mesh panels (SMPs) states that: Any square-meshed panel: shall be constructed of knotless netting or of netting constructed with non-slip knots, and shall be inserted in such a way that the meshes remain fully open at all times while fishing. It can be demonstrated that the selectivity of a given mesh size of netting can be improved by reducing the diameter of the twine used in its construction. It is also generally accepted that knotless netting has better selective properties when compared, size for size, with similar knotted material. The combination of low diameter and knotless construction can therefore improve the effectiveness of bycatch reduction devices such as SMPs. Seafish has demonstrated that the use of low diameter, high performance polyethylene (HPPE) twines such as Dyneema, in the form of Ultra Cross (UC) knotless netting can improve the performance of SMPs in certain applications (Seafish Report No SR539). The combination of Dyneema and UC construction results in an effective but relatively expensive netting material. Weight for weight, Dyneema twine is considerably more expensive than standard PE. However, due to its high tenacity, the same strength characteristics can be achieved at half the twine diameter. This means that the weight of material is significantly reduced, resulting in the overall difference in cost also being reduced. The UC knotless construction further increases the price differential compared to standard knotted netting. However, options are available that could produce more cost-effective alternative constructions that take advantage of the selective properties associated with low twine diameter, HPPE materials. By using the same low diameter HPPE twine, in a double-knotted, but otherwise conventional construction, a somewhat cheaper option may be possible. Seafish Report No SR544 3

8 Seafish Report No SR544 4

9 3. Aims and Objectives The overall objective of the programme within which these trials took place is to reduce discarding by improving the selectivity of demersal towed fishing gears. The principal mechanisms for achieving this are improvement of existing technical conservation measures (TCMs) and the development of new ones. Here the aim is to improve the effectiveness of an established TCM within the scope of the current fisheries regulations. The objective was to evaluate the performance of a SMP constructed from low diameter (2.5mm), Dyneema twine in a double-knotted netting configuration as a cost-effective alternative to the current regulation requirement. This was done by comparisons with other materials. The aims of the exercise were: To test the effect of twine diameter on bycatch reduction for two types of mesh by comparing the catch composition from trawls fitted with SMPs of the following constructions; 2.5mm double-knotted Dyneema netting Vs 4mm single-knotted standard PE netting (Regulation maximum for Nephrops trawls) 2.5mm Dyneema UC knotless netting Vs 5mm standard PE UC knotless netting To determine if any reductions in round fish bycatch could be achieved without adversely affecting the catches of the target species (Nephrops). To determine if the Dyneema panel material is a practical option for panel construction in Nephrops trawls. To assess the suitability of using double-knotted twine netting for the construction of square mesh panels. Seafish Report No SR544 5

10 Seafish Report No SR544 6

11 4. Materials and Methods 4.1 Approach These trials were designed to follow on from and add to the knowledge gained from the work recently completed under the Industry Partnership Programme (Seafish Report No SR539 refers). The promising results obtained from the previous trials encouraged Seafish to further examine the benefits of low diameter twines in order to promote and encourage commercial acceptance of the new twine materials. The offer of both practical and financial assistance from DSM High Performance Fibers to run these trials was gratefully accepted. This demonstrated the Company s commitment to developing its interest in selective fishing gear technology. Collaboration of this type with manufacturing companies is welcomed as it can greatly speed up the development process to the benefit of all those concerned. The format of the trial was the same as the previous one. The approach taken was to compare a SMP of a specification as currently used by the industry, with one constructed from low diameter Dyneema twine in double-knotted construction as a possible improvement and cost effective alternative. At the same time, the opportunity was taken to gather further data/evidence on the direct effect of twine diameter on selection by comparing two identical knotless (UC) constructions, one in 2.5mm and the other in 5.0mm twine diameters. In the interest of consistency and comparability, the same twin-rig vessel was selected to carry out the second set of trials. The trials were conducted as a twin-trawl catch comparison exercise following normal commercial fishing practices in a targeted Nephrops fishery in the northern North Sea. Standard catch handling/sampling routines were followed, consistent with those used in the previous trials. Catch data for the main target and bycatch species were recorded for subsequent analysis and interpretation. Two new standard industry specification SMPs were provided by the vessel for this work. All other SMPs were provided by Seafish/DSM to be compatible with the vessel s net design and existing SMP arrangements. The trials were split into two consecutive trips with the intention of conducting 4 full days of fishing in each trip. Two Seafish representatives were present for the duration of the trials (total of 11 days), to carry out the catch sampling procedures and general observations of panel performance. Seafish Report No SR544 7

12 4.2 Vessel details The Buckie registered twin-rig trawler Heather Sprig (BCK 181) was selected for this work (Figure 1). The vessel operates as a twin-rig trawler using the threewarp system. The vessel normally works out of the port of Peterhead, but will change base to other suitable ports in closest proximity to the fishing grounds as and when fishing conditions dictate. The vessel concentrates its operations in the northern North Sea targeting groundfish species including Nephrops, cod, haddock, whiting, saithe and flatfish. The average trip length is 6 to 8 days. Figure 1: MFV Heather Sprig Vessel details: Registered Length 18.6m Breadth 6.86m Depth: 2.81m Tonnage: 49.96t Main Engine: Deutz developing 413kw Seafish Report No SR544 8

13 4.3 Gear details The two nets used in the twin-trawl set-up were the same as those used for the previous panel tests except for the codends and extension sections. Here the 100mm mesh size sections were replaced with 80mm, which is the minimum legal mesh size allowed when targeting Nephrops. Both trawls were as near identical as is practically possible to arrange. They were those normally used by the skipper when targeting Nephrops and groundfish. The nets were of fairly typical scraper trawl design by Pisces Nets of Peterhead. Ground gears consisted of 200mm (8-inch) rubber discs in the centre section of the footrope and 150mm (6-inch) discs in the wing sections. The basic dimensions of the trawls are shown in Figure 2. The nets were rigged to 2.1m (7-feet) Skagen double-cambered V doors by a combination of 3.7m (2-fathom) bridles and 128m (70-fathom) single sweeps. The sweep line was made up of 2 x 55m (30-fathom) combination wire lengths plus 1 x 18m (10-fathom) length of rubber covered wire. The codends and extensions were constructed in nominal 80mm mesh from 4mm braided PE twine. Seafish Report No SR544 9

14 Seafish Report No SR544 10

15 MESHES DEEP MESH SIZE HEADLINE 59.76m FISHING LINE 67.1m MESH SIZE mm MESHES DEEP 160mm mm 100mm mm mm 100mm mm mm 100mm mm 100mm 108 Figure 2: Schematic drawing of trawls used during the trials 11

16 4.3.1 Panel Construction Current legislation stipulates that square mesh panels shall be constructed of knotless netting or of netting constructed with non-slip knots. The majority of SMPs currently being used are constructed from normally produced diamond mesh netting turned on the square i.e. the run of the netting is turned through 90º. In this configuration the netting has a tendency to try and return to its original diamond shape. This results in distortion of the overall panel shape. This tendency is more pronounced when using heavier and stiffer twine. When using knotted netting, particularly in double twine, the combination of twine stiffness, knot construction and the tendency to revert to the diamond shape, all combines to distort the shape of the meshes in the panel. This distortion reduces the potential escape area of the individual meshes reducing the overall effectiveness of the panel. Three different constructions were used for the netting panels used in this experiment: Standard weave, single-knotted braided twine, which had been heatset to help stabilise the knots to resist knot slippage. UC knotless construction which provides the best guarantee against mesh distortion. Standard weave, double-knotted braided twine, heat-set for additional mesh stability. Because Dyneema material has a low coefficient of friction, there is some risk of knot slippage when used in knotted netting. In order to overcome this potential drawback, the Dyneema knotted netting was produced by double knotting each mesh. Additionally, the netting was treated with a special coating (in the manufacturing process) and heat-set to further stabilise the meshes. The drawback to these processes is that it increases the weight of the panel and also reduces the number of meshes per square metre of SMP when compared to the UC construction. Seafish Report No SR544 12

17 The mesh size of all SMPs was nominally 90mm. The following Table (1) contains the panel details including the actual mesh sizes as measured using an ICES approved wedge gauge. Table 1: Panel Details Panel Type Mesh Size (nominal) Mesh Size (actual)* Twine diameter (mm) Panel weight (g) Meshes/ square metre Standard PE single knotted Dyneema double knotted 90mm 91mm 4.0 ~ mm 90.72mm 2.5 ~1000 (-20%) 400 (+11%) Standard PE UC knotless Dyneema UC knotless 90mm 91.02mm 5.0 ~2250 (+80%) 441(+22%) 90mm 89.9mm 2.5 ~700 (-44%) 529 (+46.5%) *Average over 100 meshes ( ) % increase in number of meshes/square metre compared to Standard PE single knotted reference panel. ( ) % difference in weight of panel compared to Standard PE single knotted reference panel. Figures (3), (4), (5) and (6) give the details of the panel construction and Figure (7) shows the position of the panels within the body of the trawls. Seafish Report No SR544 13

18 Figure 3: Standard PE single knot SMP (361 meshes/square metre) 80mm mesh 80 meshes including selvages 19 meshes 3 mesh selvage 64 meshes 90mm mesh ~3.2m 6 open meshes (80mm) meshes 80mm mesh 6 Seafish Report No SR544 14

19 Figure 4: Dyneema low diameter double knot SMP (400 meshes/square metre) 80mm mesh 80 meshes including selvages 20 meshes 3 mesh selvage 62 meshes 91mm mesh ~3.2m 6 open meshes (80mm) Double knot construction meshes 6 80mm mesh Seafish Report No SR544 15

20 Figure 5: Standard PE UC knotless SMP (441 meshes/square metre) 80mm mesh 80 meshes including selvages 21 meshes 3 mesh selvage 6 open meshes (80mm) 64 meshes 91mm mesh ~3.2m meshes 6 80mm mesh Seafish Report No SR544 16

21 Figure 6: Dyneema low diameter UC knotless SMP (529 meshes/square metre) 80mm mesh 80 meshes including selvages 23 meshes 3 mesh selvage 64 meshes 90mm mesh ~3.2m 6 open meshes (80mm) meshes 6 80mm mesh Seafish Report No SR544 17

22 Figure 7: Relative position of SMP within the trawl 9m from codline Seafish Report No SR544 18

23 4.4 Trials Procedures The trials were run over a period of 10 days during July The exercise was split into two 5-day periods with the aim of achieving 4 fishing days from each. The first half of the exercise compared the two knotless panel configurations, (standard Vs. low diameter) and the second half was used to test the two knotted configurations, (standard Vs. low diameter). The vessel made a half landing between the two trial periods. Operations were conducted on the basis of normal commercial fishing practices using an established three-warp, twintrawl arrangement fishing with two identical nets. General gear performance and geometry was monitored using the vessels own acoustic gear monitoring instrumentation. The skipper dictated the area of operation, following the normal pattern for the time of year but with the brief to target Nephrops grounds with an abundance of round fish bycatch species such as haddock and whiting. In this case, fishing took place in the northern North Sea (ICES IVa FO49 (Block no.35) Two Seafish representatives were onboard for the duration of the trials to make observations and record all relevant catch data. The vessel sailed from Peterhead and commenced fishing on grounds to the east of Shetland close to the area where the vessel had been operating during its previous trip. The species mix encountered satisfied the requirements of the trial, i.e. Nephrops and finfish, however, the quantities of small haddocks and whiting, used as the indicators of panel performance, were relatively low. A total of 12 valid paired hauls were completed. Towing times averaged five hours and fishing took place throughout the 24-hour period Catch Sampling Gear handling arrangements on the Heather Sprig allow for one codend at a time to be hauled onboard, (standard practice). This results in the second codend remaining in the water at the stern of the vessel whilst the first codend is being emptied. In this situation, there are concerns that catch is washed out from the codend by wave and vessel motion, particularly in poor weather. Every precaution was taken to limit this effect. Hauling time was reduced to the minimum whilst maintaining constant tension on the codend remaining in the water. The catches from the two nets were kept separate by the use of a split fish hopper. All discarded fish and assorted debris from each codend were quantified, sampled, measured and recorded. Similarly, the retained catch was quantified; this included finfish and Nephrops. The marketable finfish were sampled by grade, measuring all the retained cod, haddock and whiting when quantities were small, or by measuring representative samples when large quantities were being caught. Seafish Report No SR544 19

24 The discarded elements of the catches were quantified by baskets, they being the remainder of the catch on the sorting conveyor, after the sorting operation. Representative samples of the discards were collected throughout the sorting operation, (beginning, middle and end) for measurement. Fish length data were recorded from the samples and raised to represent the total catches for each codend on each haul. The sampling routine slowed up the normal catch handling operations as this necessitated processing all the catch from one codend from each haul before shooting the gear for the next tow. This added approximately 1 hour on to each tow but was necessary to keep the sampling accurate and consistent. Haul by haul observations were made on the general handling and performance of the square mesh panel arrangements. Catch data were recorded for all the main target species for later analysis. Seafish Report No SR544 20

25 5. Results The main indicator species used to assess the performance of square mesh panels in exercises of this nature are haddock and whiting. This is based on their generally positive escape responses to devices of this type. Additionally, these species normally make up the bulk of the round fish bycatch in Nephrops fisheries. It is the level of discarding of these species that these technical measures aim to address. Although the numbers of haddock encountered during the trials were sufficient to enable a reasonable assessment of performance, whiting numbers were much lower producing a less reliable indication. Data for cod were also collected. This species is of limited value in assessing panel performance, however cod catches do help to give an indication of general net performance and therefore some data have been included. Throughout the exercise, the affects of the panels on the target catch of Nephrops were monitored. No losses of Nephrops were attributable to any of the panel configurations under examination. The overall results gave a good indication of the relative performance of the panel materials under test. Catch data for haddock, whiting and cod were collected from a total of 24 valid paired hauls, 12 from each trip covering all the panels under test. The results have been split into week one and week two describing the catch data for each of the main species caught using the different panel constructions. 5.1 Week One Dyneema knotless (UC) Vs. Standard PE knotless (UC) Haddock The size of haddocks retained during week one ranged from 17 to 34cm, the majority of fish being between 23 and 30cm with two peak size ranges at 25 and 29cm and a smaller peak at around 27cm i.e. below the minimum landing size (MLS) of 30cm. This seemed to indicate the predominance of more than one year class on the grounds at the time. This also meant that most of the haddocks caught (~76%) were discards, (<30cm), (Table 1, Appendix I). In practical commercial terms, this figure was greater than 90% when taking into account the actual minimum size to which the crew save fish to ensure that no sub-legal fish are landed. The marketability of this class of fish must also be considered. At the time of these trials, much of the small haddock landed was only attracting withdrawal price. In reality, the minimum landing size is probably closer to 35cm. Seafish Report No SR544 21

26 Figure 8 shows the size distribution of haddock caught during week one comparing the Dyneema knotless (UC) SMP with the standard PE knotless (UC) panel. Figure 8: Length/Numbers distribution for Haddock Week One 600 HADOCK CATCH COMPARISON-WEEK ONE MLS 30cm Numbers 300 DYNEEMA KNOTLESS (UC) SMP STANDARD PE KNOTLESS (UC) SMP size(cm) From Figure 8 and the catch data contained in Table 1, Appendix I, it can be seen that the net fitted with the low diameter, (2.5mm) Dyneema panel retained fewer haddocks across the full size range of fish encountered (~38%). However, in percentage terms, the proportion of discarded to retained catch was similar at 77% discards for the Dyneema panel and 75% for the standard PE panel net. Similarly, the amount of marketable haddocks (>30cm) was comparable at 23% and 25% for the Dyneema and standard panels respectively. Seafish Report No SR544 22

27 5.1.2 Whiting The number of whiting retained during week one was relatively low and almost all fish were above the minimum landing size (MLS) of 27cm (Figure 9 and Table 2, Appendix I refer). Sizes ranged from 13cm to 49cm for the standard PE panel and from 26cm to 47cm for the low diameter Dyneema panel. There was a slight increase in the size of whiting retained in the Dyneema panel net as indicated by the shift of the length distribution curve to the right. The peak length class for this panel was 35cm as compared to 30cm for the net with the standard panel. The few undersize fish that were retained were taken from the codend fitted with the heavier standard PE knotless panel, (numbers not significant). The overall difference in numbers of whiting retained by the two nets was quite noticeable. The standard panel net retained more than twice the number of fish (733 compared to 351). In percentage terms, the proportion of discards associated with the Dyneema panel net was 7%. The standard net produced 12% discards. This meant the proportion of marketable fish was higher for the Dyneema panel net. The difference in numbers of marketable fish was seen at fish lengths up to 40cm+. Figure 9: Length/Numbers distribution for Whiting Week One WHITING CATCH COMPARISON-WEEK ONE 100 MLS 27cm DYNEEMA KNOTLESS (UC) SMP STANDARD PE KNOTLESS (UC) SMP Numbers size (cm) Seafish Report No SR544 23

28 5.2 Week Two Dyneema knotless (UC) Vs. Standard PE knotless (UC) Haddock It can be seen from the Length/Numbers distributions for week two shown in Figure 11 that the size range of haddocks retained was almost identical for the two nets. The size of haddocks retained ranged from 19cm to 48cm for the net with the standard panel and 20cm to 45cm for the Dyneema panel net. The majority of fish were between 27cm and 32cm peaking at 29cm and 30cm for the Dyneema net and standard net respectively, (Table 2, Appendix II). This meant that the haddocks caught were approximately evenly split either side of the MLS of 30cm, (53% discards: 47% retained for the Dyneema panel net compared to 46% discards: 54% retained for the standard panel net). Carrying out a similar check to week one, the actual proportion of haddocks discarded was 75% for the Dyneema panel net and 80% for the standard panel net. The results were similar to week one in that the two panel types showed similar size selection properties but a considerable difference in the actual numbers of haddock retained. The net fitted with the lower diameter Dyneema panel retained ~61% fewer haddocks compared to the standard panel. This was across the full size range of haddocks including marketable fish. Figure 10: Length/Numbers distribution for Haddock Week Two 1200 HADOCK CATCH COMPARISON - WEEK TWO 1000 MLS 30cms Numbers DYNEEMA DOUBLE KNOT SMP STANDARD PE SINGLE KNOT SMP size (cm) Seafish Report No SR544 24

29 5.2.2 Whiting The number of whiting retained during week two was lower than week one. Almost all fish were above the minimum landing size (MLS) of 27cm (Figure 11 and Table 4, Appendix II). Sizes ranged from 24cm to 44cm for the standard PE panel and from 25cm to 48cm for the lower diameter Dyneema panel. A slight shift of the length distribution curve, similar to that seen in the results from week one indicated a possible increase in the size of whiting retained in the Dyneema panel net. As with the results from week one the overall difference in numbers of whiting retained by the two nets was the more significant factor. The standard panel net retained ~41% more than the net fitted with the lower diameter panel. The proportion of discards was similar to that observed with the panel comparisons during week one. In percentage terms, the proportion of discards associated with the Dyneema panel net was 5%. The standard net produced 13% discards. A greater proportion of the whiting catch caught during the second week was above MLS. Figure 11: Length/Numbers distribution for Whiting Week Two 50 WHITING CATCH COMPARISON - WEEK TWO MLS 27cms Numbers DYNEEMA DOUBLE KNOT SMP STANDARD PE SINGLE KNOT SMP size (cm) Seafish Report No SR544 25

30 5.3 Cod It is generally accepted that SMPs rigged in the topside of net extensions do not contribute much to improving selectivity for this species. The data for Cod are included to compare and contrast with those of haddock and whiting. Relatively small numbers of cod were caught over the two trips and the results are fairly typical of what is expected for SMPs for this species. No discernible pattern emerged from the length distributions obtained (Figure 12). However, whilst bearing in mind the small numbers of cod caught, the proportions of discarded cod were reduced for the Dyneema panels in both halves of the trial. In week one the discards for the knotless Dyneema panel in the low diameter twine were ~60% less than the standard PE knotless panel (Figure 12a & Table 5, Appendix I). In week two, when comparing the Dyneema double knot construction with the standard panel design, the proportion of discards was halved in favour of the Dyneema once again (Figure 12b and Table (6), Appendix II ). Figure 12: Length/Numbers distributions for Cod Weeks One and Two 50 COD CATCH COMPARISON-WEEK ONE MLS 35cms DYNEEMA KNOTLESS (UC) SMP Numbers STANDARD PE KNOTLESS (UC) SMP 0 12a 50 COD CATCH COMPARISON - WEEK TWO DYNEEMA DOUBLE KNOT SMP MLS 35cms STANDARD PE SINGLE KNOT SMP Numbers size (cm) size (cm) 12b Seafish Report No SR544 26

31 5.4 Nephrops Since Nephrops were the principal target species in these trials, it is important to be able to demonstrate that the technical measures under test do not have any adverse affect on the effectiveness of the gear in catching this target species. The catches of Nephrops were closely monitored during the course of the trials and the results for both trips are summarised in Table 2 below. Table 2: Total Nephrops catches for weeks one and two Week One Panel Type Total Nephrops (kg) % Difference Dyneema knotless (UC) SMP 229 6% Standard PE knotless (UC) SMP 215 Week Two Dyneema double-knot (DK) SMP 121 Standard PE single-knot SMP % The differences in Nephrops catches of 5% and 6% between the nets are not considered to be significant. Under normal operating practices with twin rig trawls it is accepted that Nephrops catches will vary between sides depending on a number of factors including fishing conditions. No other indications that differences in Nephrops catches may have been attributable to SMP performance were observed. 5.5 General Observations During both weeks of the trials, relatively large numbers of saithe were observed enmeshed in the Dyneema panels. The average size of these fish was 43cm and although each codend contained saithe, very few of these stickers were seen in the other panel configurations. It appeared that this species reacted more positively from the point of view of escape response to the low diameter, Dyneema panels. When considering the construction of the SMPs, the knotless (UC) netting was easier to rig when trying to achieve consistently square meshes and overall panel shape. Also, over time it showed fewer tendencies to distort when compared to standard knotted diamond mesh turned on the square. The lower diameter Dyneema materials, even in the knotted construction are even more user friendly when used for this application. Despite the relatively short test periods, the indications to date are that the Dyneema materials are superior to the standard PE alternatives currently being used. However, further, longer term testing would be required to establish if the double knot (DK) construction will provide the necessary mesh stability to ensure Seafish Report No SR544 27

32 consistent selection over time. Additionally, it is too early to quantify the Dyneema materials ability to withstand wear and tear. The lower diameter twines being advocated may be more susceptible to damage. Examination of the panels after the experimental fishing trips confirmed that the standard (vessel s own) panels were showing signs of mesh distortion in contrast to the knotless and Dyneema materials, which showed little or no change. There were no signs of knot slippage with the double knot Dyneema material. It was also noticeable that the Dyneema material, in both constructions, remained very pliable and retained the desired square mesh shape irrespective of handling. All the knotted materials used, showed signs of mud picked up from the soft seabed conditions on which the gears were operated. The mud has the tendency to cling to the knots and generally discolour the twine as a result of ingress of particles into the weave of the twine. Some evidence of this can be seen in the photographs showing details of the panels in Appendix IV. The more open construction of the standard PE braid, and to some extent the knotless (UC) PE material appeared to be more vulnerable to this effect. The knotless Dyneema was the least vulnerable to this action. Also, possibly as a result of the twine treatment, the Dyneema double-knot only showed signs of mud on the twine surface. There were no signs of mud penetrating the braid. However, the braid used for the double-knot netting is not as tight as that used for the UC construction. This material may be prone to particle ingress over time as the coating wears off. These are important considerations from the point of view of abrasion resistance and general life expectancy of the materials. Seafish Report No SR544 28

33 6. Discussion and Findings It is accepted that using traditionally constructed knotted netting on the square for the construction of SMPs is far from ideal. Although the desired effect is achieved, the material is being forced to work in a way that it was not designed for and this results in numerous limitations. The advent of new materials and alternative net making processes has provided an opportunity to improve on these designs. The previous trials described in Seafish Report No SR539 demonstrated the benefits of the use of low diameter materials for the construction of SMPs. However, these Hi-Tech materials and improved construction processes significantly add to the cost of the final product. At a time when every effort is being made to encourage fishermen to use more selective measures and sustainable practices, any new developments must be seen to be affordable and cost effective. One of the main concerns/problems associated with knotted netting used on the square is that of knot stability. Knot slippage results in mesh distortion, leading to whole panel distortion and reduced effectiveness of the SMP. Having identified the potential benefits of the Dyneema twine itself, the question remained, could a stable, knotted construction be achieved in the new material? Because of the inherent properties of Dyneema, (namely low coefficient of friction), the subject of knot stability has received considerable attention from the manufacturers. This has resulted in double-knotted, treated netting being produced. When using low twine diameters it is easier to produce very tight knots in the netting weave, (See photographs in Appendix IV). The incorporation of thinner twines with the double knotting process and surface treatment has produced a suitably stable diamond mesh. However, the square mesh configuration places different demands on the netting. The short tests conducted within these trials produced very encouraging results. There were no signs of knot slippage or mesh distortion. The indications are that double-knotted Dyneema material is a more suitable material for the construction of SMPs than standard diamond mesh netting used on the square. However, only extended use of the doubleknotted panel under commercial conditions will determine its longer-term performance and suitability. This latest exercise has supported the findings of the previous trials in further demonstrating that improved bycatch reductions are achievable with the use of low diameter twines. This has also been shown to be possible using cheaper, more readily available knotted constructions. The work conducted in this trial and that reported on in SR539 has compared panel construction and materials without account being taken of the influence of colour/contrast on the performance of the netting used. The Dyneema materials Seafish Report No SR544 29

34 used were white and grey (light contrast) as compared to the standard PE materials, which were green and black (dark contrast). The influence of colour/contrast was largely ignored for these trials as it was assumed that because of the ambient seabed/water conditions, panel colour would be masked. The close proximity of the netting panels to the soft seabed conditions resulted in the netting taking up mud and sand resulting in rapid discoloration of the netting. However, it was noticeable that some of the materials took up the mud more readily than others. The observation of large numbers of relatively large saithe stickers (average 43cms) enmeshed in the lighter coloured low diameter Dyneema panels, but not in the darker standard PE panels suggested a more positive escape reaction towards the Dyneema materials for this species. This response may just be a feature of the low twine diameter or may be related to colour/contrast, (or a combination of both). At this stage this explanation is still speculative and would benefit from further investigation. When comparing the characteristics of the different panel materials under test, there are considerable differences in such things as the number of meshes/square metre and the weight of the panels. With reference to panel area (see Table (1), the knotless materials have significantly more meshes for a given area of panel when compared to the knotted netting. In particular, by using the Dyneema UC netting, the escape area of the panel is increased by ~ 46%. The increase in number of meshes is not as large for the double knotted Dyneema at ~11%, but this is still considered a significant factor in panel performance, especially when considering the overall size of the panel relative to the size of the net. This factor/characteristic will have more significance for SMPs used as size selection measures in fish nets. In this situation the panel is expected to select for the whole catch passing the panel. The overall weight of the panel has particular importance when considering the cost effectiveness of the technical measure. Since netting is sold on the basis of weight (price/kg), the difference in cost of the Hi-Tech materials compared to the standard PE can be offset by the much-reduced weight. The panel details shown in Table 1 include the weights of the completed SMPs. From the figures it can be seen that by using knotless Dyneema and reducing the diameter by almost half, the weight can be reduced significantly (~44%) compared to the standard PE knotted panel material. Against the same material, the Dyneema double-knot shows a weight saving of ~20%. A comparison of the standard PE knotted material with the knotless (UC) PE panel indicates an increase in weight (~80%), when changing from the knotted to the knotless construction. However, this is not a direct comparison because of the difference in twine diameters, (4mm and 4.9mm respectively). Even taking this into account, the weight difference is significant. Seafish Report No SR544 30

35 Comparing the two knotted constructions with each other, the change from standard knotted PE to Dyneema double-knot would show a weight saving of ~20%. This is for a reduction in diameter from 4mm to 2.5mm. From the indications to date it may be possible to reduce the Dyneema twine diameter to 2mm thus increasing the weight saving even further. A similar comparison of the two knotless panels indicates that halving the twine diameter by the use of Dyneema twine reduces the panel weight by ~69%. However, it must be borne in mind that in this case, the PE knotless twine that was used was 4.9mm compared to the 2.5mm Dyneema. The maximum twine diameter allowed by the regulations is 4mm. Taking this factor into account, the difference in weight is still very significant. Compared to a 4mm-twine diameter, the weight saving is estimated to be in the region of 50%. As previously mentioned, netting is sold on the basis of weight, therefore this factor is an important one in determining the overall suitability and benefit of this type of material. At the time of this project the relative costs of the materials considered in this project are listed in the table below: Material Price/Kg ( ) PE Single knot 6 PE Ultracross (UC) 20 Dyneema Double knot (DK) 55 Dyneema Ultracross (UC) 120 The Dyneema UC combination is considered to be the best option but is likely to be considered prohibitively expensive compared to the material currently most commonly used, that is the PE in the single knot construction, (by a factor of 20:1). Bearing in mind the life expectancy of the new high-tech material, the Dyneema DK however, does provide an option giving the benefits associated with low diameter at a more realistic cost, (less than half the cost of the Dyneema UC). The difference in the costs of the actual square mesh panels made from the different materials, to meet current regulations, may not necessarily be of the same magnitude as shown above. Different manufacturers and suppliers have different ways of producing and costing these panels. In achieving the aims, this exercise demonstrated that there is still scope for improving the effectiveness of square mesh panels within the current regulations. The trials highlighted the importance of selecting the best materials for the construction of bycatch reduction devices. The materials used for the construction of these devices do influence performance. The use of low diameter Dyneema twines, in both knotless and double knot netting constructions produced significant reductions in the numbers of haddock and Seafish Report No SR544 31

36 whiting bycatches during these trials when compared to the panels constructed from the standard PE material. This was achieved without affecting the catches of the main target species, namely Nephrops. The bycatch reductions are summarised in the following table (Table 3). Table 3 : Summary of Bycatch reductions Panel configuration Sample Nos. Raised Nos. % difference in numbers Haddock Whiting Haddock Whiting Haddock Whiting Dyneema knotless (UC) Standard PE knotless (UC) Dyneema double knot (DK) Standard PE single knot The results indicated little or no difference in the size selectivity between the panel types tested but suggested an improvement in efficiency in terms of bycatch reduction in favour of the lower diameter panels. This is most likely to be related to the larger number of open meshes per square metre of panel that is achievable with the lower diameter twines i.e. larger selection area. Considering the sorting practices observed onboard to be fairly typical and indicating the actual level of discarding to be in excess of 90% at times, the commercial losses attributable to the loss of those fish above MLS would be negligible. The actual amount of haddock discarded over the duration of the trials ranged from 75 to 93% of the total caught. This situation may vary with market demand and pricing. Whiting are generally accepted as the best indicator species for evaluating the performance of TCMs such as square mesh panels because of their vigorous escape responses to such devices. It was unfortunate that the catch profiles encountered did not include a bigger make-up of whiting. The reduction in numbers of whiting attributable to the low diameter Dyneema panels (41-52%) should only be considered as indicative of panel performance at this stage because of the relatively low numbers retained. The positive indications in favour of the low diameter construction for cod may warrant further investigation. However, as with the whiting data, because of the low numbers retained the results must be viewed with caution. Seafish Report No SR544 32

37 It is recommended that the new double-knot Dyneema panel material be tested under commercial conditions over an extended period of time in order to complete the appraisal of performance in this application. To this end Seafish has approached the skipper of the vessel used during these latest trials to consider replacing one of his own standard panels with the Dyneema DK panel. Seafish Report No SR544 33

38 Seafish Report No SR544 34

39 7. Acknowledgements Seafish gratefully acknowledge the following: Judith Migchels, Application Engineer and all the other staff from DSM High Performance Fibers of The Netherlands for their collaboration and financial support to this project. The skipper and crew of the twin-rig trawler MFV Heather Sprig for their services and co-operation during these trials. Skipper John Smith for his participation in these trials acting as industry observer and being actively involved in the catch sampling and data collection on behalf of Seafish. Seafish Report No SR544 35