OF ESCAPE CRITICAL LOADS AND
DAMAGES FROM FISH BITE Cite this article as: Høy E, Volent Z, Moe H, Dempster T, Arechevala P, Sanchez-Jerez P (2013) Assessment of escape critical loads and damages from fish bite. In: PREVENT ESCAPE Project Compendium. Chapter 6.3. Commission of the European Communities, 7th Research Framework Program. www.preventescape.eu ISBN: 978-82-14-05565-8
Authors: Erik Høy1, Zsolt Volent1, Heidi Moe1, Tim Dempster1, Pablo Arechevala2 & Pablo Sanchez Jerez2 1 2
SINTEF Fisheries & Aquaculture, Norway, University of Alicante, Spain
INTRODUCTION Within Norway, official statistics suggests that after the Norwegian technical standard (NS 9415) for sea-cage farms took full effect in 2006, the total number of escaped Atlantic salmon (Salmo salar) declined substantially, yet no similar decline has occurred for Atlantic cod (Gadus morhua; Jensen et al. 2010). This suggests that some causes of cod escape differ to those of salmonids (Moe et al. 2007a) and that different measures must be taken to reduce the amount of cod lost to the environment. Holes in the net play an important role in most escape episodes and are the cause of the greatest proportion of cod escapes by number (Jensen et al. 2010). Holes may form due to overloading of the sea-cage, abrasive contact with objects, from predator damage or through biting of net materials by the cultured species itself (Pemberton and Shaughnessy 1993, Moe et al. 2007b). Cod display different behaviour than salmon and they may actively escape through small holes in the netting (Hansen et al. 2008); this may lead to a ‘continuous leakage’, which is believed responsible for a considerable part of the total number of escapees (Moe et al. 2007a). In an assessment of commercial net-cage diver inspection logs across the Norwegian cod farming industry, Moe et al. (2008) estimated that small holes formed on average once per month per net cage. The small holes observed in cod cages may develop, or be accentuated as a result of biting of the nets if their initial appearance is due to other causes (Moe et al. 2007a, 2009). Cod spend a lot of time close to the net wall (Rillahan et al. 2010) and they seem to explore structures by biting or nibbling. The incidence of biting of the net wall may depend on factors such as feeding regimes, the distribution of feed, the level of biofouling and other factors (Moe et al. 2008). For species that display biting behaviour or spend a large proportion of their time exploring the net walls and bottom, a greater understanding of how the formation of holes can be prevented, through improved netting materials or operational procedures, is essential to prevent loss of farmed fish into the wild.
A procedure to test and compare the resistance of netting materials to repeated biting has been suggested (Moe et al. 2009). As data on the biting and pulling forces of cod were previously unavailable, this procedure assumed that cod pulled with a force corresponding to its weight in air, as occurs in other fish species (Steinberg 1963). Knowledge of both the frequency and force of the pull related to cod bites will improve the quality of such test results.
OBJECTIVE We aimed to develop a method to measure lateral pulling forces exerted on nets by Atlantic cod during biting events. We then compared lateral pulling forces on two different netting materials from two different sizes of cod.
Experimental setup Trials were conducted in May 2010 over a 10-day experimental period in two tanks (2.5 m diameter, 1 m deep) with a continuous flow-through of seawater of 6-7째C and 70-80% oxygen saturation (Figure 6.3.1). Fish were held in the tanks for one month and fed rations of 1% of total biomass per day, following a schedule of feeding for two days, then non-feeding for two days. Periods of non-feeding were necessary in order to obtain an adequate number of biting events. The fish were farm-raised Atlantic cod obtained from Atlantic Codfarms AS and were of two different size distributions. At the start of the trial, one of the tanks were stocked with a group of about 300 cod of mean weight 178 g (hereafter referred to as 180 g), the other with 150 cod with mean weight of 609 g (hereafter referred to as 610 g). As tank volume was 4 m3, stocking densities were 13 kg m-3 for the 180 g fish and 23 kg m-3 for the 610 g fish. Figure 6.3.1. (A) Bite force sensor design, (B) example data indicating forces measured during a biting event, (C) set-up of the bite force sensors within the experimental tanks.
Bite force measurements A sensor for automatic logging of the lateral pulling force was built to obtain a large set of undisturbed, objectively collected bite events. The lateral pull measurement system consisted of a fixed, circular neodymium magnet (Sura Magnets N35, 5 x 5 mm) and a hall-effect sensor (Allegro A1321LUA-T) attached to the end of a rod of spring steel (3 mm Ø, 190 mm length; Figure 6.3.1). The hall-effect sensor measures magnetic field strength, and thus the position of the steel rod relative to the fixed magnet could be recorded. The sensor signal was recorded by a data acquisition device (LabJack U3-HV) and a logger system was developed and implemented in LabView code. Each sensor was then carefully calibrated with known forces applied with a 0 – 1000 g spring scale (Pesola Medio). A total of four sensors were used, two in each tank (Figure 6.3.1). The system was operating at 10 Hz and when a lateral pull exceeding a certain threshold (0.9 N) was registered, the system entered an “event log” state storing the last 3 s of history from a buffer and the next 3 s after triggering. As well as the high resolution event logs, the system recorded a continuous baseline log at a rate of 1 Hz to keep track of possible drift in the signal at zero loads. On one occasion, a video camera was deployed in the tank with the 610 g fish to monitor one of the nets. The video was later matched to the logged events so that registered biting events could be validated with video of the fish behaviour. Two different nylon netting panels where attached to the sensors to test if net type affected the pulling force from cod bite: a) 200 x 200 mm sheet of 20 mm bar length mesh size (mesh half-width), 1792 tex, black, nylon, hard laid, twisted line, knotted netting (hereafter referred to as twisted twine; TT): b) a 200 x 200 mm sheet of 30 mm bar length mesh size, 1688 tex white, nylon, medium laid, braided twine, knotless netting, (braided twine; BT).
Data processing and statistical analysis The 10 Hz event logs were analysed with respect to the power of each pull or tug at the net. The magnitude of a single pull on the net was defined by the value of the recorded spike. Some bite events were recorded as prolonged pulls which could span over several tenths of a second
and contain several peak values. These where then classified as continuous pull-events and characterized by their maximum peak value (see example in Figure 6.3.1b where 11 pulls are recorded and pulls 8 â€“ 11 represent a continuous pulling event). To test for differences in bite force exhibited by cod of different size, among the two net panels used and between singular and continuous pulls, we used independent Student's t-Tests to compare the mean of groups. As bite forces of the 180 g and 610 g cod differed, we tested whether bite forces of cod differed on the two net panels and between singular and continuous biting events separately for the 180 g and 610 g fish.
RESULTS Observations of biting Video of the biting cod showed a behavioural pattern where cod actively targeted the net. Cod approached from below and first touched the net with the chin barbells before grabbing and biting into the net. To complete a biting event, cod commonly opened their mouth and expelled the net before swimming away. If the net became caught in the teeth, cod made vigorous, lateral head movements to free themselves from the net. Fibres in the twine that were caught in the teeth could break during the pull.
Bite pulling forces Over the 10-day experimental period, the bite force measurement sensors recorded 571 and 486 biting events for 180 g and 610 g fish, respectively. 36% and 42% of the recorded biting incidents had a measured bite force of )1.2 N (Figure 6.3.2a). The strongest 5% of all pulls averaged 3.7 N and 6.6 N for the 180 g and 610 g fish, respectively, with maximum recorded forces exerted of 5.9 N, and 9.4 N, respectively. When the average values for the 5% of the strongest pulls are compared to fish weight in air, this corresponds to 204% of the body weight in air for 180 g fish and 106% for the 610 g fish.
On average, 610 g cod made more forceful bites (mean ± S.E.; 2.6 N ± 0.05) than did 180 g cod (1.95 N ± 0.04; Fig. 6.3.2b). Net type affected bite force for both the 610 g and 180 g cod. The loosely laid, braided netting attracted 1.5 – 1.6 times more forceful bites on average from the 610 g cod (2.88 N ± 0.06) than did the hard laid, twisted twine netting (1.86 N ± 0.09; Figure 6.3.2c). Similarly, significant differences were detected in the bite forces exerted on different net types by 180 g cod (Figure 6.3.2c). Twisted twine netting (1.60 N ± 0.09) was subject to bite events with significantly less force exerted than towards the braided twine netting (2.14 N ± 0.05).
Fig. 6.3.2. (a) Frequency distribution of bite events by force for the 180 g and 610 g cod, (b) bite force by cod weight, (c) bite force exerted by 180 g and 610 g cod on the two different net panel types, and (d) bite force exerted by 180 g and 610 g during singular and continuous biting events. BT = braided twine net panel, TT = twisted twine net panel. S = bite event characterised by a single pull; C = bite event characterised by a continuous pulling at the net over several seconds (see example in Fig. 1). Data points for (b), (c) and (d) are mean±S.E.
Through analysis of the recorded bite sensor data, events were identified as continuous biting events or single bites to the net (see example in Figure 6.3.1b). For 610 g cod, continuous biting events were 1.9 times more forceful (4.54 N ± 0.38) compared to single bites (2.42 N ± 0.06) to the net (Figure 6.3.2d). For 180 g cod, a similar result occurred with continuous bites 1.4 times more forceful (2.64 N ± 0.14) than single bites (1.88 N ± 0.04) to the net (p< 0.001; Figure 6.3.2d).
DISCUSSION Bite pull sensor method and relevance to sea-cage aquaculture The sensor we developed for the trials proved effective and robust by logging approximately 1000 bite events over the 10-day experiment period. While most bites involved relatively low pulling forces (< 2 N), strong bite pulling forces of up to 9.4 N were recorded. The breaking strength of a single fibre of nylon netting material is about 0.5 N (Moe et al. 2009). Thus, all of the recorded bite events were of a force sufficient to break multiple fibres. Sea-cage walls will vary in tightness, from extremely taut to loose in some sections when deformed in currents (Lader et al. 2008). In this trial, the items tested were loose hanging sections of netting. Thus the pulling forces due to biting measured in this study are relevant to actual farming conditions as cod bite damage is normally found in areas of loose netting, where there are loose ends between joins of different net panels or where structural, loadbearing ropes are sewn into the net (Moe et al. 2007a).
Differences in bite pull forces with cod size, net type and type of bite Larger cod (610 g) were responsible for more forceful pulls than the smaller cod (180 g), which corresponds with their larger total mass of muscle and larger fin-area. However, smaller cod pulled with a greater force relative to weight. The mechanism responsible for the greater pulling force relative to body weight of small cod remains unclear. The maximum load exerted towards the net by the fish will not solely be a product of the maximum pulling strength, but depend on the breaking strength of each individual fibre, the number of fibres caught in the teeth, and how easy it is to pull single fibres and undo the knitted or twisted construction of the thread. Different net material properties, such as the hardness of the knitted or twisted twine and different coatings, result in different degrees of damage from code bite events (Moe et al. 2009). Twines exposed to cod bite-events become frayed (Moe et al. 2009). In this trial, different net types influenced the recorded pulling force, depending on how readily the teeth of cod became entangled. The availability of single fibres was lower in the hard-laid twisted twine netting, whereas the braided twine net had a looser, Raschel knitted structure. Thus, it is unlikely the cod bit the braided net with more force per se, but greater entanglement of teeth in the fibres due to the structural properties of the net twine likely led to the greater measured pulling forces. We have not found any data which indicate that the colour of the nets influence on pull force. Colour might be a factor in motivation for interacting with the nets, but as this has been a technical study, the setup was not designed to evaluate behavioural aspects of cod biting. The measured pulling forces were higher for prolonged pulls, which is consistent with the mechanism of teeth becoming highly entangled in the net and the subsequent struggle to free the teeth and pull away.
CONCLUSION We have demonstrated that pulling forces applied to nets by cod during biting incidents can be measured empirically. With nearly 1000 analysed pulls recorded, our results shed new light on the mechanisms by which the biting behaviour of cod damages nets. Bite pulling forces measured for all recorded bites were sufficient to break several single fibres in standard nylon net twines. The structural properties of the net twine are an important factor in the mechanism of cod biting nets as load is applied directly to the fragile, single fibre structures when cod teeth become entangled in the netting. These results are relevant for the development of aquaculture nets and benchmarking their bite-resistance, when combined with additional information on the frequency and position of bites to nets. Several other species under industrial culture are known to bite nets (e.g. seabream; Sparus aurata) and similar methods could be applied to assess bite pulling forces exerted by these species.
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