prevent_escape_chapter_3.2

56

3.2. FACTORS AFFECTING ESCAPE-RELATED BEHAVIOURS IN ATLANTIC COD (Gadus morhua L)

authors:Chris Noble1, Tor H. Evensen1, Ronny Jakobsen1, Richard D. Hedger2, Ingebrigt Uglem2, Emily Zimmermann3, Ian A. Fleming3, David Izquierdo-Gomez4, Erik Høy5 & Børge Damsgård1

1 Nofima, The Norwegian Institute of Food, Fisheries and Aquaculture Research, Muninbakken 9-13, P.O. Box 6122, NO-9291 Tromsø, Norway2 Norwegian Institute for Nature Research, Sluppen, NO-7485 Trondheim, Norway 3 Ocean Sciences Centre, Memorial University of Newfoundland, St. John's, Newfoundland, A1C 5S7, Canada 4 Department of Marine Science and Applied Biology, University of Alicante,03080 Alicante, Spain5 SINTEF Fisheries and Aquaculture, Brattørkaia 17C, 7010 Trondheim, Norway

The escape of farmed fish is a major challenge to the overall environmental and economic sustainability of the aquaculture industry. Whilst the major causes of escape events in cage aquaculture are associated with structural and operation failures (Jensen et al. 2010), numerous European farmed species including Atlantic cod (Gadus morhua L), gilthead seabream (Sparus aurata L) and European seabass (Dicentrarchus labrax L) can express behaviours that increase the risk of escapes. Major technical or operational failures can lead to large-scale losses, but escapes through small holes have also been frequently reported (Jensen et al. 2010). With regard to Atlantic cod, cage-held fish form unsynchronised shoals (Huse 1991) and display exploratory behaviours that include swimming close to the net wall (Rillahan et al. 2011). They also exhibit a tendency to inspect and bite the wall of aquaculture cages (Moe et al. 2007). Net inspection behaviour may increase the likelihood of fish discovering a hole in a net and escaping (Hansen et al. 2009), whereas net biting behaviour may increase the risk of fish damaging the cage netting materials (Høy et al. 2011).

Although recent data show that the number of Norwegian cod farms and their production tonnage has decreased since 2010, production showed a steady yearly increase prior to this point, doubling from 10,370 to 20,620 tonnes per year between 2007 and 2010 (Norwegian Directorate of Fisheries 2012a). This increase in production tonnage did not equate to an increase in the number of escaped farmed cod. In fact, although the absolute number of

INTRODUCTION

ATLANTIC COD FARMING IN NORWAY

Cite this article as: Noble C, Evensen TH, Jakobsen R, Hedger RD, Uglem I, Zimmermann E, Fleming I, Izquierdo-Gomez D, Høy E, Damsgård B (2013) Factors affecting escape-related behaviours in Atlantic cod (Gadus morhua L). In: PREVENT ESCAPE Project Compendium. Chapter 3.2. Commission of the European Communities, 7th Research Framework Program. www.preventescape.eu

ISBN: 978-82-14-05565-8

www.preventescape.eu 57

escapes from Norwegian cod aquaculture facilities peaked in 2008, it has decreased since then, culminating in a rapid fall in the total number of cod escapes in 2011 to 650 fish (see Figure 3.2.1, based upon data from Norwegian Directorate of Fisheries 2012b).

This reduction in the number of escapes may be related to fewer fish being farmed during the same corresponding period (a reduction from 2.5 million to 1.1 million from 2008 - 2010). Percentage data on cod escapes between 2008 - 2010 supports this, as escapes were a relatively constant 1.1 - 1.5% of the total farmed stock (see Figure 3.2.2, data calculated from Norwegian Directorate of Fisheries 2012a). As of March 2012, there is currently no data available from the Norwegian Directorate of Fisheries on the percentage of farmed stock that escaped during 2011, but as only 650 individuals have been reported as escaping cod farming facilities, the percentage of current cod escapes may be markedly less than 2010 figures.

Even with this marked reduction in escapes, cod farmers and regulatory bodies are still diligently striving to realise their target of zero escapes from Norwegian aquaculture (see for example, the Norwegian Aquaculture Escapes Commission http://www.rommingskommisjonen.no/). The Prevent Escape project aims to provide these stakeholders and industry regulators with robust, quantified data on escape risks and mitigation strategies to help them achieve this goal.

Figure 3.2.1. Number of Atlantic cod (Gadus morhua) escapes from

Norwegian aquaculture facilities from 2007 – 2011. Data from the Norwegian

Directorate of Fisheries, 2012b.

Figure 3.2.2. The percentage of escapes in relation to the number of Atlantic cod

(Gadus morhua) farmed in Norwegian seacages from 2007 – 2010. Calculated

using data from the Norwegian Directorate of Fisheries, 2012b.

OBJECTIVES

The behavioural motivation of individual cod to inspect and bite cage netting materials was examined in four replicate multi-patch experiments carried out at the Aquaculture Research Station (Tromsø, northern Norway). Individual cod (a total of 160 fish, weight 560 ± 171 g., mean ± SD) were used to test how cod interact with either a hole in the net or holes that were mended with different repair techniques. These interactions were evaluated in both the presence and absence of feed outside the net panel.

METHODS – SECTION 1

The factors that encourage the expression of escape-related behaviours in Atlantic cod are poorly understood. Moe et al. (2007) and Hansen et al. (2009) suggested that hunger may increase the incidence of net biting and inspection behaviour, but the effects of other farm management factors on the manifestation of these behaviours is lacking or unpublished.

The main objectives of our experiments were to assess the key husbandry factors that can affect the incidence of escape-related behaviour in Atlantic cod and increase the risk of escape. The work consisted of intensive experimental studies in both laboratory tanks and sea-cage facilities and was broken down into three sections:

Tank experiments that focused on two key risk factors that assessed whether a) the presence/absence of a small hole in a net, and b) different types of net repairs affect escape-related behaviour in relation to feeding motivation. The severity and location of net damage was also examined at the end of the study.

Sea cage experiments that investigated the effect of the following four risk factors: a) different types of net repair, b) short-term feed restriction, c) algal biofilms and macrophytic biofouling, and d) short-term changes in stocking density upon the expression of escape-related behaviour.

A final sea cage experiment that evaluated a possible mitigation strategy of providing cage-held cod with enrichment materials to reduce their net wall inspection and biting behaviour.

58


The experiment was conducted in a large indoor observation tank (diameter 5.0 m, depth 1.2 m) that was supplied with unfiltered seawater at ambient water temperatures for approximately 2 weeks. A hexagonal tank wall was constructed within the experimental tank. This tank wall presented the cod with six separate 50 x 50 cm net panels constructed of white, square meshed (measuring 20 mm by 20 mm bar length), untreated nylon netting commonly used in salmon and cod farming. These nets consisted of either a) a pair of plain control nets with no damage or net repairs, or b) two pairs of treatment panels that had four holes (ca. 10 cm in length), three of which were mended with different repair techniques including short and loose thread ends and contrasting colour repair threads (see Figure 3.2.3). To test the effect of feeding motivation a feed source (a net bag with fish pellets) was placed outside two of the net panels for one hour. All panels were filmed using six underwater cameras that recorded the frequency of inspection and biting behaviour for three hours a day from 09.00 - 12.00. Inspection behaviour was defined as a distinct and directional interest towards the net. Biting behaviour was defined as a clear bite or nibble on the net. The location of these behaviours in relation to the net panel, net hole or specific net repair was also recorded. Fish were not fed for the duration of the two week study.

Figure 3.2.3. Examples of the how Atlantic cod (Gadus morhua) biting behaviour can damage repairrs to

the net panel a) knots nearly undone with a severely frayed loose thread, b) one knot undone but with little visible

damage to the loose black thread, c) open hole with minor fraying of

net twine around the hole, and d) all knots undone and the repair thread completely absent. Net panel twines also have frayed ends. Photo: SINTEF

Each net panel was visually examined for damage at the end of the experiment according to the method described by Moe et al. (2009). Damage to both the netting material and the repair thread were assessed using the following categories: i) no visible wear; ii) partly fluffed ends and abrasion; and iii) severely fluffed ends and abrasion. Any knot damage was also assessed using the categories i) no visible damage, ii) one knot undone, or iii) two knots undone. A total score was then calculated for each repair.

60

The frequency of inspection and biting behaviours were significantly affected by the presence of a hole and motivation to access feed outside the hole; they increased if there was a feed source present outside the net wall (see Figure 3.2.5). In summary, 50.5% (n = 3355) of the total number of inspections and 83.6% (n = 459) of the total number of bites occurred in the treatment where the net panels had a hole, various net repairs and feed present outside the net wall. In the treatment where the net panels had a hole, net repairs and no feed present, this percentage dropped to 28.9% and 13.7% for inspections and bites, respectively. The control treatment only attracted 20.6% and 2.7% of the total number of inspections and bites, respectively. These differences were significant, with the number of inspections and bites being significantly greater in the Hole + Feed treatment > Hole treatment > Control.

To a large extent the cod focused their inspection and biting behaviour on loose thread repairs and the net hole, rather than on the net panels themselves. This meant that the type of net repair significantly affected both inspection behaviour and biting behaviour, and fish primarily directed these behaviours at nets repaired with black thread (see Figure 3.2.4).

RESULTS – SECTION 1

Figure 3.2.4. The frequency of a) inspection and b) biting behaviours directed at different types of net repair in tank-held Atlantic cod (Gadus morhua).


Interestingly, in the feeding motivation treatment, 62.5% of the inspections and 86.8% of the bites occurred during the presence of the feed, in comparison to 12.5 and 3.1% before, and 25.0 and 10.1% after removal of the feedbag, respectively.

Fish were capable of causing severe net damage, which in turn was significantly affected by different types of net repair. Control panels and the area around unrepaired holes exhibited little damage at the end of the experiment. Repairs made with white thread and loose ends were more damaged than all the other repairs and the open hole. Repairs made with the black thread and loose ends were also significantly more damaged than the net hole and the white thread repair that had no loose ends (perfect repair). Holes mended with a perfect repair had little or no visible damage.

Figure 3.2.5. The frequency of a) inspection and b) biting behaviours

around a meal in tank-held Atlantic cod (Gadus morhua). Different treatments

were either i) an undamaged net panel, ii) a panel with repairs and a small hole, or iii) a panel with repairs, a small hole

and a feeding stimulus on the other side of the net.

62

The motivation of groups of cage-held cod to inspect and bite cage netting materials was examined in four further experiments carried out at the Aquaculture Research Station sea farm (Tromsø, northern Norway). Experiments were conducted between October and March, where and fish were held in 5.3 m x 5.3 m x 6 m square cages, and exposed to ambient water temperatures and light levels. Each cage was fed via an automated feeding system, which delivered a fixed ration between 11.00 - 12.00 daily.

To see if the type of net repair affected fish interactions with the net wall, each cage (six cages in total) had a single 50 cm x 50 cm net panel constructed of white, untreated nylon square mesh (measuring 20 mm by 20 mm bar length and of the type commonly used in salmon and cod farming) attached to the existing net wall at a depth of 1 m. These net panels had two holes (ca. 10cm in length) that were repaired with either a black or white thread that had loose thread ends (ca. 3 cm in length). Each panel was filmed for 2 hours daily between 10.30 and 12.30 using underwater video cameras connected to a mobile video recording system. From this footage, the frequency of inspection and biting behaviour for 10 mins before, during and after a meal were investigated. Inspection behaviour was defined as a distinct and directional body orientation towards the net. Biting behaviour was defined as a clear bite or nibble on the net. The location of these behaviours in relation to the net panel, specific net repair or net frame (where the panel was attached to the existing net) was also recorded.

To test the effect of short-term feed restriction upon the frequency of interactions with the net, six cages of cod (ca. 700 g average weight, 550 fish per cage) were split into two treatments for a total of 42 days. The first three cages were fed a 100% ration according to commercial feed tables, whilst a further three treatment cages were fed a 100% ration for the first 14 days, 50% daily rations for the next 14 days, and 100% for the final 14 days of the study.

To test the effect of net cleanliness and biofouling upon the frequency of interactions with the net, two cages of cod (ca. 1040 g average weight, 600 fish per cage) were subject to a 10 day period with “clean” net panels that had been submerged in seawater for 30 days during winter, but exhibited no macro-biofouling and only 30 days exposure to algal

METHODS – SECTIONS 2 & 3

biofilm. After 10 days, these net panels were replaced with “biofouled” panels of the same size and type, but which had been submerged in seawater for 140 days and exhibited extensive fouling with algal biofilm. In addition, a 20 cm x 20 cm section of this panel was artificially fouled with a mix of two different live seaweeds: knotted wrack (Ascophyllum nodosum L.) and kelp (Laminaria spp.). These seaweeds are found on and around the sea farm, and can often break off and foul the cage nets. The daily frequency of net inspection and biting behaviour around meal times was studied before and after the change in the level of biofouling, for a total of 20 days.

To investigate the effect of short-term changes in stocking density upon the frequency of interactions with the net, an additional cage of cod was stocked with 600 fish (ca. 1030 g average weight). After a 10 day period, an additional 1800 fish were transferred to the cage, increasing stocking density from a low (3.7 kg m-3) to a medium (14.7 kg m-3) density (current stocking density thresholds are 25 kg m-3 for Atlantic cod in Norway). The daily frequency of net inspection and biting behaviour around meal times was studied before and after this change in stocking density for a total of 20 days.

To investigate whether environmental enrichment could be used as a mitigation strategy to reduce the frequency of cod interactions with the net wall, six cages were stocked with 600 cod (ca. 960 g. average weight) for 1.5 months. The first three cages received no environmental enrichment for the duration of the experiment and acted as control cages. A further three treatment cages were subject to a) 10 days without environmental enrichment, b) 30 days of enrichment, followed by c) no enrichment for the remainder of the study. Enrichment material consisted of one piece of commercial artificial kelp, ca. 3.5 m in length whose strands (ca. 50 cm in length) are manufactured from reinforced PE fabric. A section of this seaweed measuring 2.5 m was submerged 1 m from the net wall opposite the net panels. Additional enrichment was provided by a manufactured PE tube rack (32 tubes in an 8 x 4 tube rack, tube diameter 10 cm and tube length 50 cm), that was also submerged 1m from the net wall opposite the net panels (see Figure 3.2.6). The daily frequency of net inspection and biting behaviour around meal times in relation to whether fish were subject to environmental enrichment was studied in addition to any interaction with cage enrichment materials.


64

With regard to the components of the net panels that attracted the attention of group-held cod, results from the sea cage trials somewhat differed from the findings of the tank studies. For example, in the control cages of the feed restriction experiment, inspection behaviour was primarily directed at the net panel > net frame > net repair (see Figure 3.2.7). In the sea cage studies there was no clear relationship between the components of the net panel and the frequency of biting behaviour.

Data from the short-term feed restriction experiment suggests that a 2-week period of 50% reduced rations during autumn/winter had little effect upon net panel inspection behaviour, aside from inspection behaviour remaining relatively stable during feeding when 100% rations were restored, compared to a decrease in inspection behaviour in corresponding control cages during the same period (see Figure 3.2.8). In fact, the only relationship that short-term feed restriction had upon the expression of escape-related behaviour was an increase in the frequency of net biting during feeding, but this biting frequency decreased when 100% rations were restored.

RESULTS – SECTIONS 2 & 3

Figure 3.2.6. Photograph showing how environmental enrichment materials (an 8 x 4 tube rack or artificial kelp) were deployed in seacage containing Atlantic cod (Gadus morhua). Picture also shows cod inspecting the tube rack. Photo: Nofima

The frequency of net inspection and biting behaviour decreased during a meal regardless of treatment. In addition, there was a long-term decline in net biting and inspection frequency as winter approached, irrespective of whether fish were subject to reduced rations or not.

There was a clear relationship between biofouling and the number of interactions with the net. The frequency of inspection and biting behaviours increased when fish were exposed to a dirty net (see Figure 3.2.9). This increase was specifically directed at biofouling with seaweed, and long-term biofouling with algal biofilms appears to have had little or no effect upon net inspection frequency (see Figure 3.2.10) which also applied to net biting behaviour. The frequency of net inspection and biting behaviour also decreased during a meal.

There was no clear relationship between stocking density and the expression of net inspection and biting behaviours. Although the total number of net inspections increased when stocking density was quadrupled, this appeared to be primarily a function of the increase in the number of fish, rather than stocking density per se. When data were adjusted to account for this increase in fish number, no clear relationship between stocking density and cod inspection behaviour could be found, aside from after a meal when the frequency of inspections was marginally higher in fish held at low densities (see Figure 3.2.11).

Environmental enrichment had no clear effect upon the frequency of net inspection or biting behaviour in sea cages and there was high variability between cage replicates within each treatment. Exposure to enrichment materials did not significantly reduce the number of net wall inspections in relation to control cages, (see Figure 3.2.12). When cod did interact with the enrichment materials, their interactions were primarily directed at the artificial seaweed, rather than the tubes that were provided to offer potential shelter for the cod (see Figure 3.2.13). No cod were observed either swimming through the tubes or resting in them.

65www.preventescape.eu

66

Figure 3.2.8. The frequency of a) inspection and b) biting behaviours around a meal in cage-held Atlantic cod (Gadus morhua) subjected to a short-term period of feed restriction. Different coloured lines represent the different 2 week periods prior-, during and post- feed restriction. Dashed lines show data from corresponding time periods in the control cages that received 100% rations for the duration of the study. SD bars are omitted to aid clarity.

Figure 3.2.7. The frequency of a) inspection and b) biting behaviours directed at different types of net repair in cage-held Atlantic cod (Gadus morhua) fed full satiation rations during autumn and winter (data represents mean + SD).


Figure 3.2.9. The frequency of a) inspection and b) biting behaviours

around a meal in cage-held Atlantic cod (Gadus morhua) exposed to clean

or dirty nets (mean ± SEM).

Figure 3.2.10. The frequency of inspection behaviours (mean + SD) directed

at different types of net repair and biofouling materials with a) clean nets and b) dirty nets in cage-held Atlantic cod (Gadus morhua). NB there was no

seaweed on the nets during part a).

68

Figure 3.2.11. The frequency of inspection and biting behaviours around a meal in cage-held Atlantic cod (Gadus morhua) in relation to short-term changes in stocking density. SD bars are omitted to aid clarity.

Figure 3.2.12. The frequency of net inspection behaviours around a meal in cage-held Atlantic cod (Gadus morhua) in relation to whether the cages had environmental enrichment. Different coloured lines represent the different periods pre-, during and post- enrichment. Dashed lines show data from corresponding time periods in the control cages that received no enrichment for the duration of the study. SD bars are omitted to aid clarity.

Figure 3.2.13. The frequency of a) inspections and b) bites with different types of environmental enrichment in cage-held Atlantic cod (Gadus morhua) exposed to

either artificial kelp or a tube rack (mean ± SEM).


70

We have demonstrated that a number of common aquacultural factors can increase the expression of escape-related behaviours in Atlantic cod and have also demonstrated the efficacy of measures to reduce and mitigate these risks. In controlled studies carried out in tanks, feeding motivation emerged as a strong risk factor for inspecting and biting a net when fish were deprived of feed (Damsgård et al. in press), and this data supports previous studies that suggested complete feed deprivation increases net inspection and biting in cod (Moe et al. 2007, Hansen et al. 2009). These behaviours were primarily directed at poorly repaired areas of the net wall, specifically where a contrasting coloured thread was used to repair net damage.

In cages, the overall frequency of net inspection and biting was relatively low, and decreased further as winter approached. Results from the sea cage trials differed somewhat from the tank studies in relation to what aspects of the net attracted cod interactions. Specifically, when fish were fed a full satiation ration during autumn/winter in sea cages, their inspection behaviour was primarily directed at the net panel and net frame, rather than net repairs. There was no clear relationship between the components of the net panel and the frequency of biting behaviour. The majority of the sea cage studies showed that the number of inspections and bites decreased during feeding which suggests that the presence of feed in the water column distracts the fish from browsing the net wall. These results were markedly different from the earlier tank studies, where the presence of feed outside the net wall affected the frequency of interactions with the net. In the sea cage studies, fish could access feed when it was presented to them, thus reducing the frequency of their interactions with the net.

Appetite varies within and between days for many farmed fish species (see for example, Noble et al. 2008). However, numerous farmers feed their fish a fixed daily ration according to commercial feed tables that do not account for short-term changes in appetite levels, meaning there is a risk of farmers under- or over-feeding their fish. Our experiments show that a short-term period of feed restriction (not complete deprivation of feed) does not emerge as a robust risk factor for stimulating the expression of escape-related behaviour in Atlantic cod during autumn and winter. The only risk is an increase in the already low frequency of biting behaviour during meals under the feed-restricted regime. Biofouling in the form of algal biofilms does not emerge as a risk factor for net inspection or net biting in cage-held Atlantic cod during spring. However, macro biofouling with seaweeds does increase the frequency of net biting and inspection behaviour, and farmers should attempt to reduce this if high volumes of seaweeds get snagged on the net wall. There is no clear relationship between short-term changes in stocking density and the frequency of net inspection behaviours. This means that when farmers transfer fish to holding cages during transport and grading procedures, the increase in density per se does not increase the expression of escape-related behaviours.

Providing environmental enrichment with artificial seaweed and tubing materials for potential shelter was not a robust mitigation strategy for reducing the expression of net inspection behaviours in sea cages during winter/spring. In fact, there was no significant effect of

DISCUSSION

Complete feed deprivation increases the risk of cod inspecting and biting the net panel. Farmers should follow their existing strategies of limiting the time they starve fish during common husbandry practices e.g. during size grading or fish transports.

Short-term feed restriction during winter does not appear to be a robust risk factor for stimulating the expression of inspection behaviours in cage-held Atlantic cod, but does increase net biting frequency during feeding. This means that even if farmers miscalculate the daily feed requirements of their fish for short periods, it may not be as severe as first thought.

Short-term changes in stocking density, such as when fish are transferred to a central holding cage, do not appear to affect net inspection frequency before or during feeding.

Biofouling with long stranded seaweeds does increase the risk of cod biting and inspecting the net. Farmers should attempt to remove these fouling objects as and when they build up.

Environmental enrichment by suspending artificial kelp and tubes within a cage during winter/spring is not a robust mitigation strategy for reducing the expression of inspection and biting behaviours at the net wall.

enrichment upon either inspection or biting behaviour, although there was high variability between cage replicates in each treatment. When cod did interact with enrichment materials they did so in different ways: the majority of interactions with the enrichment materials were directed at the artificial kelp rather than the tube rack and no cod were observed swimming through or resting in the tubes.

RECOMMENDATIONS


72

Damsgård B, Høy E, Uglem I, Hedger RD, Izquierdo-Gomez D, Bjørn PA (in press) Net-biting and escape behaviour in farmed Atlantic cod, Gadus morhua: Effects of feed motivation and net traits. Aquaculture Environment Interactions

Hansen LA, Dale T, Damsgård B, Uglem I, Aas K, Bjørn PA (2009) Escape-related behaviour of Atlantic cod, Gadus morhua L., in a simulated farm situation. Aquaculture Research 40:26–34

Huse I (1991) Culturing of cod (Gadus morhua L.). In: Finfish Aquaculture. Ed: McVey JP. Handbook of Mariculture, Vol 2/CRC Press

Høy E, Volent Z, Moe-Føre H, Dempster T (2011) Loads applied to aquaculture nets by the biting behaviour of Atlantic cod (Gadus morhua). Aquacultural Engineering 47:60-63

Jensen Ø, Dempster T, Thorstad EB, Uglem I, Fredheim A (2010) Escapes of fish from Norwegian sea-cage aquaculture: causes, consequences and prevention. Aquaculture Environment Interactions 1:71–83

Moe H, Dempster T, Sunde LM, Winther U, Fredheim A (2007) Technological solutions and operational measures to prevent escapes of Atlantic Cod (Gadus morhua) from sea-cages. Aquaculture Research 38:90–99

Moe H, Gaarder RH, Olsen A, Hopperstad SO (2009). Resistance of aquaculture net cage materials to biting by Atlantic cod (Gadus morhua). Aquacultural Engineering 40:126–134

Noble C, Kadri S, Mitchell DF, Huntingford FA (2008). Growth, production and fin damage in cage-held 0+ Atlantic salmon pre-smolts (Salmo salar L.) fed either a) on-demand, or b) to a fixed satiation-restriction regime: data from a commercial farm. Aquaculture 275:163–168

Norwegian Directorate of Fisheries (2012a) Escape statistics of farmed cod. http://www.fiskeridir.no/english/statistics/norwegian-aquaculture/aquaculture-statistics/cod

Norwegian Directorate of Fisheries (2012a) Escape statistics of farmed cod. http://www.fiskeridir.no/statistikk/akvakultur/oppdaterte-roemmingstall

Rillahan C, Chambers MD, Howell HW, Watson WH (2010) The behavior of cod (Gadus morhua) in an offshore aquaculture net pen. Aquaculture 310:361–368

REFERENCES CITED

Seabass (Dicentrarchus labrax) and seabream (Sparus aurata) aquaculture is a major industry in the Mediterranean (130 000 and 170 000 tonnes, respectively; FAO 2010), and involves growing juvenile fish to harvest-sized adults in sea cages. Fish escapes from sea cage facilities can be a serious problem, and threatens the sustainability of the aquaculture industry and viability of wild fish populations. The main causes of escapes are severe environmental conditions, such as strong winds and storms, and fish predators attacking the farmed population (Jensen et al. 2010). Fish farmers also regularly report escape incidents of the main farmed species (seabream and seabass) through holes in the net of sea cages, and fish escape has been associated with species-specific interactions with sea cage nets (Jensen et al. 2010, Papadakis et al. 2012). Such fish interactions include general net inspection behaviour, such as overt net biting that can lead to structural weakness and holes in the net, and exploratory or risk taking behaviour exhibited when fish pass though holes in the net. An understanding of the conditions that increase the propensity for aquaculture fish species to swim through holes in sea cage nets is of key importance for helping both researchers and farmers develop mitigation measures to reduce escape risk.

INTRODUCTION

authors:Ioannis E. Papadakis, Vassilis M. Papadakis, Alexios Glaropoulos, Maroudio Kentouri*

* University of Crete, Greece

3.3. THE ESCAPE-RELATED BEHAVIOUR OF EUROPEAN SEABASS (Dicentrarchus labrax) AND GILTHEAD SEABREAM (Sparus aurata)Cite this article as: Papadakis IE, Papadakis VM, Glaropoulos A, Kentouri M (2013) The escape-related behaviour of European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata). In: PREVENT ESCAPE Project Compendium. Chapter 3.3. Commission of the European Communities, 7th Research Framework Program. www.preventescape.eu


ISBN: 978-82-14-05565-8

prevent_escape_chapter_3.2

Documents

Transcript of prevent_escape_chapter_3.2