Fish pots trials in the bay of Douarnenez

Transcription

1 Not to be cited without prior reference to the authors ICES CM 2010 / I:01 Fish pots trials in the bay of Douarnenez Méhault, S., Morandeau, F., Larnaud, P., Mouchel-Drillot, M., Meillat, M., Bavouzet, G. Abstract The fish pot technique is of growing interest along the French coast line. Ifremer, together with the fishing industry, conducted a project to develop new prototypes of fish pots suitable to the local fisheries. After development in flume tanks, 5 different conceptions of pots have been tested on board of the RV Thalia in the bay of Douarnenez (Brittany, France). These sea trials aimed to observe the behaviour of the gears and test the effect of pots height and conception on catches. Specific objectives were limiting the catches of conger eel (low commercial interest) while maintaining high catches of other commercial species. Each model of pot has been used at 3 different heights daily (0, 1 and 2m). Catch data have been obtained for 3*5 pots during 13 days. The main catch was common pout ant conger eel. The analysis of catch data showed that the gear conception and mesh size have an effect on the pot catch performances and selectivity. One of the pot models was efficient in limiting the conger eel catch with acceptable catch of common pout (cylindrical pot with a bottom side entrance). The pot height affected the catch of both species. Keywords: Fish pot, common pout, conger eel Contact author: Sonia Méhault, 8 rue François Toullec, Lorient, France, tel: , soniamehault@ifremer.fr 1

2 1- INTRODUCTION The awareness of the fishing industry to preserve the marine resources grown over the past years. The fishermen showed an increasing interest to diversify their fishing methods with more potentially sustainable gears. The work presented in this document focused on fish pots. The fish pot technique is used all around the world but more scarcely in Europe. The main species targeted in Northern Europe are cod (Gadus morhua) and torsk (Brosme brosme) (Furevik and Lokkeborg, 1994) and common pout (Trisopterus luscus) in Spain. Presently, the fish pot technology is not developed along the French coast, but more and more small scale fishermen are demanding for more information on this technique. This work specifically aimed to develop new pot designs adapted to the artisanal local fishery and test their potential of capture in coastal French waters. A better understanding of this technique is indispensable to support the fishermen in their orientation towards new or alternative fishing gears. 2- MATERIAL and METHODS 2.1- Fish pot development This project was initiated within the pole of competitiveness of Brittany (France). It was set up and carried out in partnership with a marine research institute, a net maker, a marine security institute and a plastic material company. Fishermen have been consulted to determine the specifications a fish pot need to comply according to the species they target, the type of zone they exploit, the type and size of boat they work with. The main specifications drawn for a new gear from this consultation can be listed as follow: (1) foldable gear (2) double chamber gear (3) floating gear (ie. at a controlled distance above the bottom) (4) large volume of the upper chamber (5) entrance in the water flow axis, opposite to the anchor (6) biodegradable side, and (7) designed to cope with strong marine currents. Together with the partners of the project, foldable fish pots have been designed and tested step by step in flume tank. According to the specifications above, four models of fish pots have been designed and tested together with the standard Norwegian cod pot, named here as model 1. With 25mm square mesh size, it is used as a control gear (Figure 1). The concept of model 2 (Figure 2) aimed to increase the pot volume by lengthening the upper chamber. The inner volume was also increased at the front part with a deflector which weak the current inside the pot. The whole gear is made of a 50mm square mesh. The difference between model 2 and model 3 (Figure 3) remains in the mesh size of the upper chamber of 25mm square. The smaller black mesh size aimed to create a shadow effect, such as a Fish Aggregating Device, which may attract the fish as a refuge. Model 4 (Figure 4) was inspired from the ending part of a fyke net used in Northern Brittany. This cylindrical pot was made to be deployed by the water current, making the fish entering upstream. Finally, model 5 (Figure 5) was conceived to be deployed by its float in calm water. The entrance of this last gear is on its lower face. For all the gears developed and tested, the mesh sheets were made of black braided polyamid. All of them had only one entrance made of nylon monofilament. Annular floats of 0.45kg have been used during the sea trials. 2

3 Entrance Figure 1 : Model 1 - Standard Norwegian cod pot with one entrance. 25mm square mesh. Figure 2 : Model 2 -Modified Norwegian cod pot with longer upper chamber and deflector. One entrance. 50mm square mesh. Figure 3 : Model 3 - Modified Norwegian cod pot with longer upper chamber and deflector. One entrance. 50mm square mesh for lower chamber and 25mm square mesh for upper chamber. Figure 4 : Model 4 - Cylindrical and horizontal pot. One entrance. 50mm square mesh. Entrance Figure 5 : Model 5 - Cylindrical and vertical pot. Entrance on the lower face. 50mm square mesh. The behaviour of each pot has been observed under water currents from 0 to 3 knots in the flume tank. The opening of the pot entrance and the volume of the pot under various levels 3

4 of currents were observed. These parameters allowed defining the suitability of the pot models according to the type of fishing areas (ie. current strength). The floatability of the pot was adjusted in the flume tank and the height of the pot above the bottom was defined by the length of the rope between a weighted braid and the fish pot (Figure 6). There was a specific interest in keeping the pot above the bottom, first to let them orientate in the current, and second to test the effect of the pot height on the catch and more specifically on conger-eel which is (most of the time) not targeted by fishermen. Figure 6 : Setting of individual fish pot. (1) annular floats (2) foldable fish pot (3) adjustable rope (4) weighted braid (5) surface floats (6) buoy rope (7) weight (35kg) Three units of each of the 5 fish pot models have been tested in May 2009 during 15 days. Each model was immerged at three distinct heights: on the bottom, 1 and 2m. The model 5 set on the bottom was actually about 50cm above the sea floor due to its conception with the entrance on the lower side Sea trials Sea trials to test the 5 fish pot models were carried out in the Bay of Douarnenez (Brittany, France, Figure ) on board of the RV Thalia. It is a sheltered area, with low current strength and some wrecks. The mean depth is 30m. These fishing grounds are harvested by artisanal netter and long liners targeting sea bass and Pollock mainly. The area explored with the pots changed along the trials, but all fish pots were always immerged individually in the same area (Figure ) in order to avoid bias due to a variation of fish abundance. 4

5 Figure 7: Sea trials carried out in the bay of Douarnenez (West of Brittany, France). The squares of the right side figure represent the areas where the set of 15 pots was immerged during the survey The pots were always baited the same way with two bait bags of 0.5kg of frozen mackerel and 0.5kg of frozen pilchard each. One bait bag was placed in each chamber (ie. 2 kg of bait per pot). The quantity of bait was voluntarily over abundant in order to not be a limiting factor to the catch. The bait was integrally replaced before each immersion. The fish and crustacean caught were identified, counted and measured for each individual pot (except conger ell which were only counted and weighed). In total, 196 immersions (all 5 models together) have been carried out during the sea trials. The mean time immersion was 21 hours. 15 immersions lasted between 6 and 9 hours (pots were immerged and hauled back on board the same day) and for organisational reasons one pot of model 2 was immerged for 2.5 hours only. 15 immersions were longer than 40 hours (pots remained immerged while the boat was stopped in the harbour). For the 165 other immersions, the gears were sent in the afternoon and hauled back during the morning of the following day. Each set of 15 fish pots were immerged and hauled at the same period of the day. Model number 1 was immerged 42 times, number 2 was immerged 43 times, number 3 was immerged 41 times, number 4 was immerged 28 times and number 5 was immerged 42 times (model number 4 was immerged 28 times because the prototypes used broke due to conception problems and divers observations showed it did not deployed under the water current conditions in the area of study) Data analysis Hypothesis on the effect of pot height on the catch, as well as hypothesis on the difference of catch abundance in the lower and upper chamber of the fish pot were tested with a non parametric test (Wilcoxon) after having tested the distribution of the data (Shapiro test). Considering the model 1 with 25mm mesh size sampled the common pout population (control gear), the selectivity analysis was done using a General Linear Model with a binomial distribution based on the proportions retained at length with model 5 (test gear): Test/(Test+Control) ~ Length. 5

6 2.4- Underwater video observation at sea Two complementary underwater video observation systems have been used to observe both the pot and fish behaviour. Divers collected information on the diversity and abundance of fish in the fishing area. They also made observation of the shape of the pots during the fishing operation. The second source of visual observations was based on black and white video camera attached to one single fish pot and a metallic structure (Figure 8). Figure 8 : Metallic structure for fish pot underwater video observation (A) camera (B) fish pot fixation and (C) container support (battery, video recorder, electronic supplies) 3- RESULTS 3.1- Pot and fish behaviour by underwater observations The observations made during the development of the fish pots in flume tank have been confirmed by the observations made by the divers: models 2 and 4 do not deployed well in area with low current strength, especially model 4 for which the inner volume was extremely reduced due to the closed random position of the hoops in the absence of current (Figure 9). 6

7 Figure 9. Left: Model 4 on the sea bed with no current. Right: Model 4 tested in flume tank with 2knots of current Regarding model 2 (and 3), the extension of the upper chamber seemed to be useless since the current was not strong enough to deploy it. Models 1 and 5 appeared to be well adapted to the studied area, they deployed themselves perfectly. The observations made by the divers indicated that the main species present in the surrounding of the gears were common pout (Trisopterus luscus) and conger eel (Conger conger). Only very few sea bass (Dicentrarchus labrax) and black sea bream (Spondyliosoma cantharus) have been observed in the study area. Divers observed higher fish abundance (common pout) around the wrecks than on sandy grounds. The spatial variation of abundance was also observed in the catches. The observations made with the passive underwater video recording system have been carried out on the 11 and 13 of June On the 11 of June, the gear was immerged at 16:50. The fish approaching and entering inside the pot have been recorded between 18:40 and 19:50. That day, the tide coefficient was 67 with high tide at 20:10. On the 13 of June, the gear was immerged at 17:00. The fish approaching and entering inside the pot have been recorded between 20:20 and 21:10. That day, the tide coefficient was 57 with high tide at 21: The catch The main species captured during the sea trials were common pout (81% in numbers), conger eel (13%) and dogfish (3%). Because of its mesh size (25mm square), the fish pot model 1 realised the highest catches. Among the 4 models with similar mesh size (50mm square), the pot number 5 realised the best capture of common pout, and the lowest capture of conger eel. Some of the common pout have been caught entangled in the pot meshes (ie. captured by the net but not in nay of the pot chambers). The catches of black sea bream (3 individuals) and lobster (2 individuals) have been realised with models 1 and 2 (Figure 10). 7

8 Number of individuals caught / number of immersions Fish pot model Lobster Norway Pout Spinous spider crab Black sea bream Edible crab Dogfish Conger eel Common pout Figure 10 : Mean number of individuals caught by immersion for each species and fish pot models. The catches realised with model one is higher than the other due to the difference of mesh size (model 1 : 25mm square mesh, models 2-5: 50mm square mesh). All models of fish pots tested had 2 chambers, but the catches of common pout made in the lower and upper compartments were not significantly different for none of the model (p > 0.05, Wilcoxon test). However, the catches of conger were significantly higher in the upper compartment (53 individuals) than the lower compartment (8 individuals) for model 1 (p < 0.05, Wilcoxon test). During the first part of the trial each model has been immerged 10 times at 3 distinct heights (10 times on the bottom, 10 times at 1m and 10 times at 2m above the bottom, except model 4 which was immerged only 8 times at 1m height). The following comparison of catches is based on the pot model with the same mesh size (50mm, ie. excluding the data obtain with model 1). The best catch of common pout which were realised with model 5 and decreased as the height of the gear above the bottom increased, though not significantly (p>0.05, Wilcoxon). To a lower extend, the same phenomenon was observed with model 2 for common pout (Figure 11), and with all models for the conger eel (Figure ). Number of individuals Model 2 Model 3 Model 4 Model 5 0 Bottom 1m 2m Figure 11 : Common pout for leg 1 and 2 8

9 Number of individuals Bottom 1m 2m Model 2 Model 3 Model 4 Model 5 Figure 12 : Conger eel for leg 1 and 2 The comparison of the catches realised with model 1 (25mm square mesh) and model 5 (50mm square mesh) showed an escapement of smaller individuals. The test gear (model 5) has an equivalent catch as the control gear for fish above 35.5cm (Figure 13). Figure 13 : catches and proportion retained of common pout realised with model 1 (control: 25mm square mesh) and model 5 (test: 50mm square mesh) The immersion of 2.5 hours took place on the 4 th of June 2009 between 13:45 and 16:15. That day the tide coefficient was 64 and the high tide was at 15:41. 7 common pouts have been caught during this short immersion of pot model 2. 9

10 4- DISCUSSION Four new fish pot models have been developed and tested together with the standard Norwegian cod pot. The experiment done in flume tank and in situ at sea clearly showed that the physical features of the place of use is important, and more specifically the current conditions : foldable gears for which the underwater deployment relies essentially on their buoyancy are suitable for areas with weak currents (eg. model 5). Gears relying on current strength to deploy themselves are not suitable in sheltered areas such as the bay of Douarnenez. To increase the chance of capture with fish pots, a number of parameters should be taken into account. Sonce the gear is static, the catches seem to be highly dependant on the surrounding fish abundance. During that experiment, up to 120 individuals of common pout have been caught in one single pot (model 1) when immerged at proximity of a wreck. From a management point of view, pot technique should be considered with care when it is used widely in productive areas less accessible to other fishing gears. Adverse effects of fish pot fishery have been observed on reef community for example (Hawkins et al., 2007). Underwater observations showed that even if the fish density is high inside the gear, the fish are not stressed while the gear is immerged and static. Fish were well alive when they were hauled back on board, though Cole et al. (2003) showed for other species (blue cod) that flapping as the pot leaves the water is sufficient to compromise the flesh quality. With fish pot technique, most of the fish can be discarded alive if necessary, though some species such as common pout are sensible to decompression. The way of avoiding discarding from the deck is to adapt the mesh size to the targeted species. The comparison of 25 and 50mm square mesh size showed the high potential of selectivity of fish pots. No specific experiment was conducted on the duration of immersion during this study. However, for technical and logistical reasons on board, one fish pot (model 2) has been immerged once for 2.5 hours. This immersion happened during the high tide time and captured 7 common pouts. This observation coincides with the two observations made by passive underwater system, showing that the approach and entrance of the fish occurred just before the high tide time. Further experiment would be necessary to investigate the hypothesis that the tide time has an effect on the pout catch. Though no fish escapement could be observed on the underwater video, some pots were hauled back with bait consumed and null catch. This suggests that fish were able to enter the gear, consume the bait, and get out during the immersion period. Cole et a.l (2004) showed the set duration of pots has an effect on the catch of blue cod (Parapercis colias), therefore, further investigation would be necessary to define the optimum duration of immersion to maximise the catch and minimise the escapement of local fish species. To cope with the escapement issue, pots have been conceived with two chambers in order to reduce the escapement phenomenon. No significant difference was observed between the catch in the upper and lower chamber, and common pout were found almost equally in the upper and lower compartment. We believe the upper part compartment plays an important role in the retention phase of the capture process by increasing the chance of retention. During this experiment, the upper chamber was also baited, which under commercial condition could be viewed as an additional cost. The size of the pot is also a parameter to consider to limit the risk of escapement since the bigger the pot, the lower the risk of 10

11 escapement (Munro, 1974). However, the protocol of this experiment was not designed to demonstrate the pot size effect on the catch. The model 3 had its upper chamber with a smaller mesh size aiming to create a shadow and refuge effect to attract the fish. This model was also baited but no difference between model 2 and 3 was observed in the catch. This could be due to the fact that the 25mm square mesh of model 4 was not small enough to create a refuge effect. However, further work should be continued on the potential of fish pot to attract and capture fish since authors demonstrated it is possible with other species Treasurer (2000). Among the four models tested and developed with 50mm square meshes, model 5 got the best catch performance for common pout and the lowest for conger eel. The concept of this model, with its entrance on the lower face which does not comply with the assumption that the bait diffusion and the pot entrance should be in the same axis, did not prevent the catch of the common pout. However, some of the common pout were caught entangled in the meshes: they probably intended to reach the bait but did not find the entrance. The catch of conger eel were considerably reduced with that pot concept compared to the others. Two hypotheses can be proposed to explain this result: (1) the conger eel is not willing to swim vertically to enter the pot, or (2) it did not found the entrance since it was not in the bait diffusion axis. This result is relevant since only very few fishermen are interested in conger eel (very low market price) and also because this species is a predator of the other potential targeted species (several common pout caught have been killed and half eaten inside the pot). Three distinct heights above the sea bed have been tested in order to observe the effect of this parameter on the catches. The results show that the higher the pot above the sea floor the lower the capture of both, common pout and conger eel. However, having the pot higher above the bottom may be beneficial if the catch of crustaceans and benthic organisms need to be avoided (Furevik et al., 2008). From this experiment it seems that having the pot on, or slightly above the bottom is enough to ensure the best catch of common pout and let the gear orientate itself in the current. The catch of conger eel was reduced as the height increased, but was not avoided totally. The conger eel has a low market value, and most of the artisanal fishermen wish to avoid it, also because this species is difficult to handle when hauling the pot on board. However, some fishermen found a special market to sell conger eel, and for them, fish pot is a very suitable gear if their boat is equipped with appropriate hauling devices. Based on the results obtained during this experiment in the bay of Douarnenez, fish pots have been produced and lent to fishermen volunteer to use them on board of their boat under professional conditions. A wide field of investigation remains on bait, duration of immersion and pot shape according to species targeted on the French coast. This project is currently running with the participation of fishermen, involved in collaborative research to contribute to sustainable fisheries. 11

12 BIBLIOGRAPHY Cole, R. ; Alcock, N. ; Handley, S. ; Grange, K. ; Black, S. ; Cairney, D. ; Day, J. ; Ford, S. ; Jerrett, A. (2003) - ; Selective capture of blue cod Paraperci colias by potting : behaviour observations and effects of capture method on peri-mortem fatigue Fisheries Research 60 PP Cole, R. ; Alock, N. ; Tovey, A. ; Handley, S. (2004) - Measuring efficiency and predicting optimal set durations of pots for blue cod parapercis colias Fisheries Research 67 PP Furevik, D. ; Lokkeborg, S. (1994) - Fishing trials in Norway for torsk and cod using baited commercial pots Fisheries Research, 19 PP Hawkins, J.; Roberts, C.; Gell, F. ; Dytham, C. (2007) - Effects of trap fishing on reef fish communities Aquatic Conserv : Mar. Freshw. Ecosyst 17 PP Furevik, D.; Humborstad, O.; Jorgensen, T.; Lokkebord, S. (2008) Floated fish pot eliminates bycatch of red king crab and maintains target catch of cod Fisheries Research 92 PP Muro, J.L. (1974) The mode of operation of Antillean fish traps and the relationships between ingress, escapement, catch and soak. J Cons. Int. Explor. Mer, 35(3), Treasurer, J.W. (2000) Evaluation of the relative catching of pots for north European wrasse (Labridae) ACKNOWLEDGEMENT Thanks to the net maker Le Drezen, the society ResiConcept, the Institut Marin de Prévention, the Parc Naturel Marin d Iroise and fishermen of the Bay of Douarnenez for their contribution and involvement in this fish pot experiment project. 12