3.4. ESCAPE BEHAVIOUR OF GILTHEAD SEABREAM EUROPEAN SEABASS IN COMMERCIAL SEA CAGES
Cite this article as: Smith C (2013) Escape behaviour of gilthead seabream and European seabass in commercial sea cages. In: PREVENT ESCAPE Project Compendium. Chapter 3.4. Commission of the European Communities, 7th Research Framework Program. www.preventescape.eu ISBN: 978-82-14-05565-8
authors: Chris Smith1 1
Hellenic Centre for Marine Research, Greece
INTRODUCTION European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) are two of the primary marine aquaculture fish species in the Mediterranean region (Rana 2005). Qualitative evidence from researchers and aquaculturists suggests seabass are not prone to net biting, but they are thought to be much more opportunistic about escaping through any potential net openings; with the majority of escapes coming primarily from technical and operational failures (Dempster et al. 2007). It is quite well known that caged seabream have a net biting behaviour that focuses on visual imperfections, whether it is fouling on the net, attachment areas, repairs, or worn and loose threads (ICES, 2006). The fish will also repeatedly re-visit an area of interest and gradually open up a hole in the net material or weaken the net structure. At some point when the opening is large enough they may start to escape.
OBJECTIVES The aim of this cage farm experimental work was to take some of the important issues from the laboratory experiments on seabass and seabream carried out at the University of Crete and investigate escape-related behaviour on a Mediterranean commercial sea cage farm. With regard to seabass, three major investigations were undertaken, including an inspection of cage nets to look at possible damage from biting behaviour, and response experiments with respect to loose threads on the cage wall and a hole in the cage wall. With regard to seabream, two major investigations were undertaken: an inspection of cage nets to look at possible damage from biting behaviour and response experiments with respect to a hole in the cage wall.
CAGE FARM EXPERIMENTS - SEABASS The cage farm experimental work was carried out on a mixed species farm on the north west coast of the island of Crete. To observe evidence of potential biting damage on the inside of cage nets, a Remotely Operated Vehicle (ROV) was used for visual observation in cages. Multiple vertical transects were carried out on the inside of the cages with observation stops at various depths from the surface to the bottom of the net. This was carried out in separate cages of seabass of 170 g and 400 g size. Biting would have been evident from holes, unbraided or loose thread or simply visible white patches from the removal of dark red antifouling coating on the net. However for all the observed transects, there was no evidence of any biting on the inside of the cages for this species. In the second experiment, a divided cage system was used to observe the behaviour of 170 g seabass to loose threads on the net (Figure 3.4.1 and 3.4.2). A divider net was introduced daily to a 12 m x 12 m cage with a camera on the net lined up to observe a loose thread tied to the net (5 cm thread of the same material as the net). The net was deployed down the side of the cage and then gently pulled away from the side of the cage to a position approximately 3 m away, to divide the cage and confine the fish to approximately 60% of the cage (leaving a smaller area of the cage remaining with no fish). Video examination noted the incidents of a number of specific behaviours, including: fish inspecting the thread, contacting (touching the thread with their snout), biting or tearing at the thread. From the first to the second day there was a large increase in the level of interaction with the net. Generally speaking the major part of the seabass shoal kept away from the net with only a few individuals cruising along the net wall. On the first day the number of individuals inspecting the thread was constant but the number of thread contacts decreased as the day progressed. On the second day the pattern was very similar, although there was generally a higher level of interaction. The number of fish inspecting the thread doubled and was again at a constant level through the day. The number of contacts with the thread declined towards midday, whilst contact with the net, apart from the thread, was low and did not change through the day.
The breakdown of contacts was very similar for the two observation periods with approximately 28% of behaviours involving a simple contact with the snout on the thread, 43% involving a small bite with the jaws closing on the thread and 28% a tear, where the individuals grabbed the thread and used the body to turn away and try and tear the thread away from the net.
Figure 3.4.1. Schematic of the split net system.
Figure 3.4.2. Photograph of the split net system.
In conclusion: s Seabass were shown to bite on loose thread on the cage wall s Seabass showed a constant level of interest in the thread, but contact with the thread declined through the morning. s Contact with the thread was primarily through simple biting, but also with more physical tearing at the thread. s There were differences in activities between days.
In the third and final experiment the same experimental procedure as described above was used to investigate the behaviour of 150 - 200 g seabass to a hole in the net over 3 days. In place of the thread, a hole was made in the net by cutting mesh in a vertical line, 10 cm long and representing roughly 2 body heights of the seabass. Video examination noted the incidents of a number of specific behaviours, including: shoaling distance from the net, proximity of individual fish to the hole, inspection of the hole, contact inspection, exiting the confined area, entering the confined area. Seabass exhibited a relatively tight shoaling behaviour in close proximity to the net, and the shoal seemed to be quite sensitive to disturbance, quickly moving away if disturbed, but just as quickly moving back to the net. A few independent fish were noted along the edge of the cage. From day to day, shoaling patterns showed some differences, with the shoal gradually getting closer to the net towards midday for two of the days, allowing greater interaction with the net. Overall there were higher response levels on the third day, which may be due to the fish acclimatising to the presence of the net. The pattern of proximity and hole inspection was relatively constant but largely increasing on Day 3. This was perhaps reflected by the shoaling behaviour, where the shoal would often turn to point at the cage wall and consequently higher numbers of individuals seemed to be inspecting the hole. Seabass exhibited very little contact with the hole (snout contact). In terms of passing through the hole, activity was high, but unfortunately the number of entrances was much higher than the number of exits/escapes. A significant number of individuals had passed around the divider net (an escape response in itself), although this group was extremely small in size compared to the â€œconfinedâ€? group. The number of exits increased into the afternoon, whilst the number of entrances peaked at midday. From general observations, entering the hole seemed to be involve fish swimming in a straight line from the edge of the hole into the cage (the approach could not be seen), but escapes/exits were more likely to be associated with the fish approaching the hole from a distance, followed by a pause where the seabass inspected at the hole, followed by the fish swimming directly through. Figure 3.4.3 shows some of the seabass and net hole interactions.
Figure 3.4.3. Cage with seabass, shoal away from the cage wall, shoal up at the cage wall with an individual escaping and entering the net.
In conclusion: s Seabass are willing to escape through a hole in the net, and the incidence of this escape behaviour is both high and direct. s Exit of fish through the hole seemed to be dependent on proximity of the shoal
GENERAL DISCUSSION - SEABASS Prior to the start of the experiments, the perceived wisdom on the behaviour of seabass was that they were not known as net biters, but did exhibit a propensity to escape through holes in the net. In terms of net biting, the experiments undertaken in this study had mixed results. Whilst the cage nets used on the farm for seabass showed no indication of damage that could be related to biting, the loose thread experiment showed that there could be a high level of biting with this species. The reason for seabass biting on the thread may be that the thread was an attractant on an otherwise homogenous net surface. The loose thread was quite obvious (5 cm long). The danger of the loose thread was that firstly it attracted biting and secondly tearing that transferred strain away from the immediate bite. Seabass were also seen to inspect and contact the net independently of the thread. It is thought that for seabass, interaction with the net was highly dependent on shoaling behaviour. When the fish densely shoaled away from the net there was little chance of interaction with the net. If the shoal was in close vicinity to the net, then the number of interactions increased. Movement of the shoal was mostly parallel to the cage walls especially when the shoal was away form the cage wall, but was also directly toward and away from the net in response, for example, to the presence of person on the walkway (movement away from the person, returning to the net when the person moved away). Seabass seemed to be very reactive to external stimuli, with the shoal often seen to suddenly change direction. The responses of seabass to the thread and hole were different. They would make contact with the thread, but would only pass cleanly through a net and were not observed to come into contact with the periphery of the hole (either inspecting or biting the edge of the hole). Passing through the hole was a straight swimming action with an approach from a distance.
CAGE FARM EXPERIMENTS - SEABREAM The cage farm experimental work was carried out on a mixed species farm on the northwest coast of the island of Crete. To observe evidence of biting on the inside of cage nets, a Remotely Operated Vehicle (ROV) was used for visual observation in cages. Multiple vertical transects were carried out on the inside of the cages with observation stops at various depths from the surface to the bottom of the net. This was carried out in separate cages of fish of 200 g and 400 g size. Biting would have been evident from holes, unbraided or loose thread or simply visible white patches from the removal of dark red antifouling coating on the net. For the smaller sized seabream there was no evidence of biting, but for the large seabream biting evidence was seen on the inside of the cages. Biting was easily evident through removal of the dark red antifouling coating, leaving exposed white patches and areas. There were three types of bite interactions: a) On the inside side wall of the net in contrast to normal net sections (Figure 3.4.4) there were small patches of white net consisting of a few strands to small patches of several centimetres diameter (Figure 3.4.5) â€“ it was assumed that these cleaned areas were caused by large bream biting the netting material, leading to a removal of antifouling agent. This removal of red antifouling may have provided the seabream with a colour contrast stimulus that could increase biting on that particular area. b) There were some previously repaired tears in the net, and the tears had been repaired by drawing together the edges from the outside of the net then using tie-wraps (cable-ties) to close the opening, i.e. the seam was on the outside of the net (Figure 3.4.6). Where some threads or loose ends were visible on the inside these ends showed white frayed thread or small white patches where potential biting had removed the antifouling (Figure 3.4.7). c) Toward the bottom of the net there were several cases of small raised white patches (one to two mesh diameters in size), that appeared to be indentations on the inside of the net. It was assumed that these were caused by hooking the net onto the cage stanchions during maintenance and general husbandry procedures. The weight of the net on this small focal point may have led to stretching that produced this â€œplugâ€?. When the net was dropped back into the water, the stretch plug was retained and may have attracted seabream biting/inspection behaviour as evident by the removal of antifouling and viewed as raised white areas (Figure 3.4.8a,b), which could be quite dense in number (Figure 3.4.9a,b).
Figure 3.4.4. Normal net
Figure 3.4.5. Cleaned patch of net
Figure 3.4.7. Repair with bitten seam
Figure 3.4.6. Normal Repair
Figure 3.4.8b. Bitten net ‘plug’
Figure 3.4.8a. Bitten net ‘plug’
Figure 3.4.9a. Frequent net ‘plugs’ in white circles
Figure 3.4.9b. Frequent net ‘plugs’ in white circles
Clear areas and bite marked repairs were distributed generally around the cage, whilst cleaned â€˜plugâ€™ marks were found generally below 9 m near the cage bottom (see Figure 3.4.10).
Figure 3.4.10. Concentration of bite areas around the cages.
In conclusion: s Markings on the cage wall of large seabream may be evidence of net biting on loose threads and removal of the antifouling coating. s Targeted areas included unblemished parts of the net, loose threads from repairs and raised portions of the net. s There were no overt indications of potential biting damage from juvenile seabream. In the second experiment, a divided cage system was used to observe the behaviour of 150 - 200 g seabream over 3 days in relation to a hole the net. A divider net was introduced daily to the 12 m x 12 m cage with a camera lined up to observe a hole cut into the net. The hole was approximately 10 cm long and represented roughly 1.5 body heights of fish â€“ under tension the hole opened out into an oval shape. The net was deployed down the side of the cage, then gently pulled away from the side of the cage to a position approximately 3 m away, to divide the cage in two, where fish were confined to an area consisting of 60% of the cage and a smaller area with no fish. Video examination noted the incidents of a number of specific behaviours, including: shoaling distance from the net, proximity of individual fish to the hole, inspection of the hole, contact inspection, escaping from the confined area, re-entering the confined area.
Generally speaking, the number of interactions (inspection, contact, exiting) were highest by day 3, probably related to the shoal moving closer to the net wall and the fish consequently having a greater possibility of interacting with the net. Shoaling behaviour of seabream was generally seen as a loose aggregation of fish that did not approach the cage wall very often. The shoal was mostly distant in the early hours of the day getting slightly closer to the net wall towards the afternoon. There were a number of independent fish cruising along the net, but these fish did not approach the hole, and most of them took avoiding action. Individual proximity to the hole increased each day, particularly towards midday. The number of fish inspecting the hole, which was independent of proximity, followed a similar pattern. This is also reflected in the number of contacts with the hole where the fish came up to the hole and touched it with their snout or bit the edge of the hole but did not go through it (the most common practice). In terms of escapes, there were very few escape incidents and these were mostly on the final day and mostly after midday. The fish that passed through the hole approached it in a straight line from a distance and went cleanly through the hole. Fish gave the impression that passing through the hole was accidental, as they just swam through a clear line/space in front of them. Figure 3.4.11 shows some seabream interactions with the net. Incidental observations made for seabream indicated that they kept up a constant level of nibbling/biting/tearing contact with a loose thread from another experiment in the background of the camera view during the hole experiment.
Figure 3.4.11. Cage with seabream, shoal away from the cage wall with a fish nibbling on the opening, shoal up at the cage wall with an individual escaping and re-entering the net.
In conclusion: s Seabream do escape through a hole in the net, but the incidence of escapes were low. s Interactions with the hole were not as dependent on the position of the shoal in relation to the net as in seabass, as there were a number of independently moving fish along the wall of the cage. s Most close interactions with the net would not lead to escape.
GENERAL DISCUSSION - SEABREAM Prior to the start of these experiments, the perceived wisdom on the behaviour of seabream was that they were net biters but were reluctant escapers. In terms of net biting, the experiments undertaken in this task have confirmed that seabream are indeed biters, both from evidence of bite marks on the commercial cage nets and also in the background of the hole experiment. In the previous section of this report it is suggested that seabass interactions with the net are related to their shoaling behaviour, which could be complex. Seabream also had a complex shoaling with similar tight and loose formation, but in contrast would in some cases move towards a person on the walkway (perhaps linking the presence of people around the nets with feeding). Seabream were less reactive than seabass and also exhibited more independent behaviour, with individual fish sometimes swimming along the cage wall separate from the shoal. They exhibited an interest in the hole with a high incidence of contact or biting at the hole edge, but had few escapes.
Net biting behaviour may lead to net damage and a reduction in net strength, which in turn may lead to the formation of holes from strain on weakened net parts. Holes will lead to escapes. In light of the seabass experiments, some recommendations can be made concerning farming practices that can reduce the chances of hole forming, which include: s Ensuring that there are no loose threads inside the net s Frequent inspection of nets for seabass, as they can be highly responsive to a hole in the net. s If a hole in the net is suspected, personnel should stay above the site until the net can be inspected and repaired, as seabass seemed to move away from the net when personnel were present.
In light of the seabream experiments, some recommendations can be made concerning farming practices that can reduce the chances of hole formation which include: s If the net is repaired out of the water, ensure all seams, knots and loose threads are on the outside of the net s All repaired seams should have smooth finishes inside the net s In general there should be no loose threads inside the net s Tie-wraps/cable-ties are very useful for quick repairs until the net can be removed during scheduled maintenance. s If a hole in the net is suspected, personnel should stay away from the site until the net can be quickly inspected and repaired, as seabream seem to move towards the net if personnel were present. s If the same colour antifouling is frequently used, it may be better to have the initial nets in the same colour so there is no colour contrast if the anti-fouling is removed.
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