The Development of Multi-Rig Trawling Systems in the UK Past, Present and Future? Technical Information Sheet No: 2000/04/FG Background Multi-rig trawling developments started in the Gulf of Mexico shrimp fisheries in the early 1950's. Fig. 1. Adoption elsewhere in the world was slower; for example, most of the Australian shrimp and prawn fisheries did not take up the multi-rig option until the early 1970's. Fig. 1 Gulf of Mexico style multi-rig prawn trawl arrangement 1
In Europe, beam trawling has been well established for many decades, particularly in its twin-trawl mode. Fig. 2. However, interest in multi-rig otter trawl systems did not really come about until the early 1980's. Fig. 2 Twin beam trawl arrangement Around 1983/4, Danish fishermen, with assistance from the Danish Fisheries Institute at the North Sea Centre in Hirtshals, developed a three-warp, twin-rig system for Nephrops. Uptake of the system at this stage was limited by the requirement for three barrelled main winches. Interest in twin rigging however continued at a pace. With the continued involvement of the Danish Fisheries Institute, the three-warp system was perfected and further developments took place to enable other sectors of the fleet to take advantage of the new ideas. This involved the conversion of single net operators to twin rigging using the two-warp arrangement. Experience in these fisheries soon produced excellent results. Twin rig catches regularly doubled those of comparable single net operators. By early 1987 the Danes had made considerable advances in multi rig trawling technology. By this stage vessels were being specifically designed and purpose built for twin rigging. These vessels were targeting predominantly shrimps and Nephrops with later developments designed to target flatfish species such as plaice. The Danish experience created a considerable amount of interest amongst other European fishermen. In the UK, this eventually led to what has probably been one of the most significant developments in demersal trawl fisheries in the last 20 years. UK interest in multi-rig systems The Sea Fish Industry Authority (Seafish) started to get interested in multi-rig systems around the mid 1980 s. 2
At around the same time as the Danish developments were taking place, interest in twin and triple trawling was increasing in the flatfish fisheries around the east and south east coasts of England. These were much smaller scale operators successfully targeting mainly Dover sole with very small trawls. Seafish Gear Technologists examined the multi-rig systems that were operating in the English sole, Danish shrimp and Nephrops fisheries with a view to assessing the feasibility of introduction to other fisheries. This initiated the ideas for developing systems for UK Nephrops fisheries. The work that followed resulted in the development of a successful two-warp system for small vessels in the lower horsepower categories (up to ~300hp). The idea being that this class of vessel, being constrained to a greater extent by weather conditions, could become more economically efficient by adopting this system. The technology was rapidly adopted by a large number of vessels in the UK Nephrops fleet. This uptake was spread across the whole range of vessel sizes and horsepower. The systems were designed such that conversion from single to multi-rig could be accomplished with minimal alteration to existing deck layouts and/or operating procedures. This was obviously attractive to those fishermen that still had some reservations about the method, but wanted to try the systems before committing themselves to major investment or change. As the benefits of this new technology were realised and more and more interest developed, there was a steady move towards the more flexible three-warp systems. This option involved more investment and change to existing vessel layouts. It was not long before UK, and in particular, Scottish fishermen got wind of the benefits of these systems. Scottish and Danish fishermen often found themselves operating alongside each other on the fishing grounds, or landing to the same markets. This lead to close, regular contact and introduction to the new technology. At this stage a number of Scottish skippers started to experiment with these new ideas. It was only a matter of time before the UK industry followed the Danish path and started to invest in purpose-built multi-rig vessels. By the early 1990 s nearly every new vessel built for the Scottish fleet was built with multi-rig capability. The three-warp system became the preferred option for most of the fleet. By this time, multi-rig techniques were no longer confined to Nephrops, shrimp or flatfish fisheries. After the initial development of a two-warp system, Seafish involvement in the development process was less prominent. The wheels had been set in motion and with so many fishermen adopting the technique the technical development was rapid. Why use multiple trawls? Before looking further at the development process, it is worth briefly describing why two nets are better than one and why multi-rig trawling has attracted so much attention.
For a given horsepower, it is possible to tow two or more small trawls providing a greater potential catching power than that of one larger trawl. Fig. 3. Fig. 3 This principle works most effectively for ground fish species such as flatfish, monkfish, shrimps and prawns (Nephrops). Trawls designed to catch these species generally have relatively wide bosom sections. It is this design feature that makes them so effective. Two or more small trawls can provide a much greater bosom area than that of one single trawl, for the same overall net drag. This improved efficiency was the main driving force behind multi-rig development. Fig. 4. Fig. 4 Two warp sole rig
Two warps or three? Multiple trawls can be rigged in a variety of arrangements to match the gear design to the target species. Two basic systems have developed in the UK, each distinguished by the number of warps being used in the system: The two-warp system for towing twin or triple trawls. This is basically the conventional single trawl warp arrangement with the addition of four warp bridles, two attached to the ends of each of the main warps. (Each main warp shares the loads of the system equally when the system is in balance). Fig. 5. (a) Twin trawls (b)_ Triple trawls Fig. 5 Two warp, arrangement The three-warp system for towing two trawls. This requires a minimum of three barrels on the main winch. The centre warp carries a greater load than the outside warps (typically between 1.3 and 1.7 times greater). This system is the more flexible of the two as it allows independent adjustment of the three warps to bring the system into balance. With the two-wire system, warp lengths are fixed. This has developed as the preferred option, particularly with the larger classes of vessel. Fig. 6 overleaf.
Fig. 6 Three warp, twin trawl arrangement Technical problems Although technical development was rapid, this is not to say that the industry did not come up against problems. From the onset, there were fundamental problems inherent with this technology that caused fishermen considerable headaches. Seafish Gear Technologists have been kept extremely busy over recent years providing advice and answering technical queries on the setting up and operation of multi-rig systems. The commonest problems still stem from a lack of understanding of the basic geometry of the systems. Common to all the variations of multi-rig set-ups is the problem of gear alignment. In other words ensuring that all the trawls in the system are towing evenly or all square. These problems and the means of solving them can be best described by the use of diagrams. Figs. 7&8. Fig. 7 represents a twin trawl, three warp system in which all three trawl warps are set at the same length. It can be seen from this diagram that, as the doors spread, they move out of alignment with the centre weight. They are effectively being towed ahead of the centre weight with the result that the trawls are distorted. As the door spread increases, the distance (a) on the diagram increases.
Fig. 7 All three warps of equal length In order to restore alignment, (Fig. 8), the centre warp has to be shortened relative to the two outer warps. The problem lies in determining this difference in length. By using scale model trawls in the Seafish Flume Tank in Hull, in combination with theoretical mathematical procedures, Seafish technologists have been providing valuable guidelines to fishermen, assisting them in the process of determining these warp length differences and setting up their rigs correctly. Fig. 8 Centre warp shortened to bring doors and trawls into alignment
As an additional aid to fishermen involved with multi-rig systems, Seafish have produced a video describing the basic principles involved and giving guidance on the setting up and adjustment of the gear. Multi-rig systems are demonstrated by the use of scale models operating in the Authority s Flume Tank. Recent developments As the catching sector increases investment in multi-rig operations, we are seeing similar trends from some of the larger electronic and engineering equipment manufacturers. New systems incorporating gear mounted monitoring equipment and automatic winch controls, designed to ensure that multi-rigged trawls are towing evenly, are now on the market. At present this technology is being targeted at the larger vessel sector and as such is inappropriate for the smaller, inshore multi-riggers. These operators will still have to rely on their knowledge and understanding of the geometry and forces involved in order to set up and optimise the performance of their multi-rig systems. As multi-rig developments have progressed, the trend has been towards larger and more powerful vessels operating larger and heavier gear. Four warp systems capable of towing four trawls are currently being developed for use by large, factory, stern trawlers, specifically designed for this multitrawl role. These techniques are now being applied to a wider range of fisheries targeting a wider range of species. As a consequence, there is a danger that fishermen may loose sight of the basic concepts from which the whole development took place. In which case, benefits may be lost. Inappropriate application of multi-rig systems can sometimes lead to reduced net returns. In other words the catch rates produced can not justify the costs involved in using these systems, (increased gear, fuel and vessel maintenance costs). In such circumstances the question should be asked: could similar, or better results be produced by a down sizing of gear, or even reverting to a more suitable single net arrangement? In recent years progress within the multi-rig sector has been so rapid that it has been extremely difficult to keep pace. Fisheries R&TD in the UK and in Europe has been focussed on conservation and improving selectivity rather than on improving efficiency. As a consequence, the influence of this sector on the development of multi-rig techniques has been limited, probably to the detriment of the whole process. One of the main concerns of fisheries management has been that of limiting effort. As such, multi-rig trawling has had its critics. It is fair to say, that in some respects, the development of multi-rig trawling has added to the problems of discarding. These systems are designed to increase catch rates and as such have proven themselves in numerous fisheries. Unfortunately, in mixed species fisheries, bycatches of non-target species and undersize target species are a fact of life (to a greater or lesser degree depending on the particular fishery). As a consequence of their greater catching efficiency these systems also have the potential for taking increased quantities of unwanted catch, which often ends up being discarded. Very little attention has been paid to comparing the selectivity of single trawls with that of multi-rigs. However, from the point of view of research and development, twin trawl systems have provided researchers with an excellent tool for conducting comparative fishing experiments. If there is to be a sustainable future for these systems, then it could be argued that there is an urgent need to determine if, or how, the selectivity of multiple trawl systems vary from single trawls. There may be a greater need for the trawls used in them to be more size and/or species selective.
The net gains from a multi-rig system should be greater than that from a comparable single rig operator, all other factors being equal. There is anecdotal evidence to suggest that this is not always the case. If this observation is correct, it suggests that some multi-rig systems could bare closer scrutiny with regard to their suitability for the fisheries at which they are directed. Attentions in the future should perhaps be focussed on developing better applications of this technology so as to avoid wastage in all respects. Further information can be obtained from Seafish at the address below: The Librarian, Sea Fish Industry Authority, St. Andrews Dock, HULL HU3 4QE Tel. No. 01482 327837 Fax. No. 01482 223310 E-mail: d_dalton@seafish.co.uk Website: www.seafish.co.uk