Larval release and attachment modes of the ... - Wiley Online Library

Aquaculture Research, 2011, 42, 1056^1060

doi:10.1111/j.1365-2109.2010.02659.x

SHORT COMMUNICATION Larval release and attachment modes of the hydroid

Ectopleura larynx on aquaculture nets in Norway Christina Carl1,2, Jana Guenther1 & Leif Magne Sunde1 1

Centre for Research-Based Innovation in Aquaculture Technology, SINTEF Fisheries and Aquaculture,Trondheim, Norway

2

Institute for Chemistry & Biology of the Marine Environment, Carl von Ossietzky University Oldenburg, Oldenburg, Germany

Correspondence: C Carl, School of Marine and Tropical Biology, James Cook University, Angus Smith Drive, Douglas, Qld 4811, Australia. Email: [email protected] Present address: C Carl, School of Marine and Tropical Biology, James Cook University, Townsville, Qld, Australia.

Introduction In the ¢n¢sh industry, clean cage nets are essential for the health of the stock (Braithwaite & McEvoy 2005). The accumulation of biofouling, the unwanted attachment of marine organisms on submerged surfaces, decreases the water £ow through nets. Subsequently, the water quality within sea cages is reduced, which may a¡ect ¢sh health negatively (Cronin, Cheshire, Clarke & Melville 1999; de Nys & Guenther 2009). Over the last decade, the hydroid Ectopleura larynx (syn. Tubularia larynx) has become one of the most common fouling organisms in the Norwegian ¢sh farming industry, causing increasing problems for farmers (Guenther, Carl & Sunde 2009). The rapid growth of E. larynx on aquaculture nets requires ¢sh farmers to clean their nets regularly, often on a fortnightly basis during the peak of the biofouling season between July and November (Guenther et al. 2009). To reduce biofouling, the majority of Norwegian salmon farmers use copperbased coatings on nets, combined with regular underwater high-pressure washing (200^300 bar) with rotating discs (Olafsen 2006). Anecdotal observations suggest that hydroids grow back faster once the ¢rst washing has taken place (B. Jensen, salmon farm manager, pers. comm.). Despite the dominance of E. larynx and its associated problems for the ¢sh farming industry, only a few studies have focused on hydroids on aquaculture nets. These have examined the settlement and

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successional development of hydroids on aquaculture nets (Greene & Grizzle 2007; Guenther et al. 2009) and the increase in drag force on the netting due to hydroids and other fouling organisms (Swift, Fredriksson, Unrein, Fullerton, Patursson & Baldwin 2006). Given the increasing occurrence of hydroiddominated biofouling in the Norwegian aquaculture industry (Guenther et al. 2009), there is a need to identify the factors in£uencing the fouling community, particularly hydroids, on sea cages. Understanding these factors and the link between E. larynx and its response to the underwater washing of nets could allow control over this problematic fouling organism in an aquaculture environment. Therefore, the speci¢c aims of this study were to identify the impact of the underwater washing on this fouling organism by determining the number of released actinulae, juveniles and polyps during the underwater washing. Furthermore, the ability of polyps to release actinulae, after polyps were cut from the hydrorhiza (see description in Fig. 1), was determined. Finally, strategies of E. larynx to maintain their attachment to the net were classi¢ed to determine whether the net material facilitated the colonization and attachment.

Materials and methods To investigate the response of E. larynx to the underwater washing of cage nets and quantify the number of actinulae, juveniles and polyps of E. larynx in the

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Larval release and attachment modes of Ectopleura larynx C Carl et al.

to the water temperature at Sunde. The number of released actinulae was counted after 24 h. To qualitatively determine the strategies of E. larynx to maintain their attachment to the net, eight samples of fouled ¢sh cage netting (8 5 cm, 15 mm halfmesh) from 10 to 12 m depth at a commercial salmon farm in Reitholmen (63136.95 0 N; 09107.48 0 E), Mid-Norway, were cut out in December 2007 and preserved in 1% formalin in seawater. Hydroids were examined under a compound microscope and photographed using a Nikon DS camera.

Figure. 1 Diagram of Ectopleura larynx (modi¢ed by Hayward & Ryland 1990).

water column, plankton samples were collected 2 m downstream from a sea cage at a commercial salmon farm at Edya (63139.31 0N; 08141.05 0 E), Mid-Norway, in December 2007. Plankton samples were collected 40 min before (n 5 3) and during (n 5 3) a ¢rst washing cycle in the morning and again 40 min before (n 5 3) and during (n 5 3) a second washing cycle at noon, when a di¡erent part of the cage net was washed. The samples collected 40 min before the washing cycles provided information on the natural release of larvae and polyps by E. larynx. The water column was vertically sampled from 0 to10 m water depth using a 100 mm mesh sized plankton net (KPT Naturfag) with an opening diameter of 30 cm. The sampled volume was estimated by assuming that a water column of10 m height was sampled. All plankton samples were preserved in1% formalin in seawater, and the actinulae, juveniles and polyps of E. larynx in each plankton sample were identi¢ed and counted using a compound microscope. Given the occurrence of several 0 values in the data, the mean sums of all E. larynx propagules (actinulae, juveniles and polyps) before and during each washing cycle were statistically compared using paired t-tests (SPSS version 16). The signi¢cance level of the t-test was adjusted accordingly for each of the two analyses (a 50.025). Data were log-transformed to meet the assumptions of normality and homogeneity of variances (Levene’s test). To quantify the number of released actinulae of cut polyps as a potential for colonization, adult E. larynx were collected from a commercial salmon farm at Sunde (63130.21 0N; 09111.59 0 E), Mid-Norway, in October 2008. Thirty polyps were cut o¡ using scissors and placed individually in Petri dishes with 10 mL of 0.45 mm ¢ltered seawater at 12 1C, which was similar

Results All the mean numbers of propagules (actinulae, juveniles and polyps) of E. larynx increased several-fold during the washing cycles (Fig. 2). Polyps increased the most in comparison with the initial mean numbers before the underwater washing. More than 300 polyps were found in the water column during the ¢rst washing cycle, whereas none were found before the ¢rst washing cycle in the morning (Fig. 2a). In general, the mean numbers of all propagules (actinulae, juveniles and polyps) were higher during the ¢rst washing cycle than during the second one (Fig. 2b). The amount of E. larynx propagules in the water column increased signi¢cantly during the ¢rst underwater washing (t 5 5.81, d.f. 54, P 5 0.004) and also during the second washing cycle (t 5 6.25, d.f. 54, P 5 0.003). Furthermore, cut polyps of E. larynx released a mean number of 3.6 0.7 actinulae within 24 h under laboratory conditions. The growth of E. larynx on salmon cage nets was examined, and three strategies were determined to maintain their attachment to the nets (Fig. 3). First, the hydrophytons grew around threads and often intertwined, creating compact tufts of hydrophytons around the thread. The chitinous perisarc of the hydrophytons made the tuft in£exible, and the only way to detach the hydroids from the netting was to break it. Second, the hydrophytons grew between loose nylon ¢laments and threads. The hydrophytons were strongly attached to the net, because the loose nylon ¢lament functioned as a strap that secured the hydroid to the netting. Third, some hydrophytons also incorporated nylon ¢laments into their chitinous perisarc (Fig. 3).

Discussion This study demonstrates that the underwater washing of sea cage nets resulted in higher numbers of

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Figure. 2 Numbers of actinulae, juveniles and polyps of the hydroid Ectopleura larynx collected from 0 to 10 m depth at a commercial salmon farm at Edya, Norway, before and during (a) the ¢rst washing cycle in the morning, and (b) the second washing cycle at noon, when a di¡erent part of the net was washed. Data are mean SE.

Figure. 3 Overview of the three strategies of the hydroid Ectopleura larynx (H) to maintain their attachment to the net (N). (a) Winding of the hydrophyton around threads; (b) growth of the hydrophyton between loose nylon ¢laments and threads; and (c) incorporation of nylon ¢laments into the chitinous perisarc. Matching schematic representations are shown below the photographs. Arrows indicate loose nylon ¢laments of the thread. (a^c) have the same magni¢cation. Scale bar 5 500 mm.

E. larynx actinulae, juveniles and polyps in the water column. Strong currents of approximately 235 m s 1 created by underwater high-pressure washers (AKVAgroup ASA, Bryne, Rogaland, Norway) not only exceed the critical value of 8 cm s1 (Pye¢nch & Downing 1949) to dislodge settled actinulae and

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juveniles from the substratum but also cause damage to the hydroids. The strong currents induce the gonophores to burst and release actinulae. These actinulae then encounter available space for settlement and growth on the cleaned nets and other underwater structures as soon as the current speed returns




to the ambient velocity. Pye¢nch and Downing (1949) found that 440% of actinulae settled within 6 h after their release under laboratory conditions. Furthermore, detached polyps of E. crocea (syn. Tubularia crocea) continued to release actinulae (Rungger 1969), and this study showed that cut polyps of E. larynx were also capable of releasing an average of 3.6 actinulae within 24 h. Because the spread of hydroids within and between farms may be enhanced by the use of underwater high-pressure washers, this ¢nding could be of concern for other aquaculture sites in the vicinity of farms cleaning their nets with this method. After settlement, E. larynx ¢rst establishes an extensive system of hydrophytons (Pye¢nch & Downing 1949) to spread the colony and reduce the risk of mortality (Gili & Hughes 1995). This study showed that current net constructions with multi-¢lament nylon threads provided a structure for maintaining the attachment of E. larynx on the nets, such as the growth under loose ¢laments. The hydroid is fastened to the thread and their removal may be hindered. Therefore, fragments of the hydroids may remain on the washed netting and a fast and extensive regrowth of E. larynx may be facilitated. The loose nylon ¢laments may be created during the cleaning of the fouled net with high-pressure washers. This procedure strains the netting and possibly decreases the strength of ¢laments. The results of this study contribute to an improved understanding of the factors in£uencing the abundance of hydroids on aquaculture nets, and to possible solutions to control and reduce their growth more e⁄ciently. The current underwater washing is only a temporary measure to control E. larynx as a fouling organism and e¡ective long-term solutions are needed (Guenther, Misimi & Sunde 2010). Future underwater washers may be modi¢ed to collect the cut polyps of hydroids to reduce the number of actinulae released in the water column. Furthermore, to hinder the growth of hydroids between loose ¢laments and threads, which maintains their attachment on the nets, netting threads could be made of a single ¢lament or have a coating enhancing the cohesion of the nylon ¢laments, which can also withstand the high pressure used during the underwater washing. This might also increase the e⁄ciency of the cleaning by reducing the numbers of fragments of hydroids on the nets after the washing procedure. Thus, the possibility of fast and extensive re-growth is minimized. The removal and onshore cleaning of nets is not popular with Norwegian salmon farmers due to operational lo-

gistics (Guenther et al. 2010). Further studies are also needed to determine powerful strategies to kill remaining fragments on the nets after the washing and to develop strategies for the complete removal of hydroids from the nets.

Acknowledgements We thank T. Dempster, K. Tangen and three anonymous reviewers for comments on earlier versions of this manuscript, employees at the commercial salmon farms for their support during sample collection and J.-M. Gili for the identi¢cation of the hydroid species. Financial support was provided by the Norwegian Research Council and the Fishery and Aquaculture Research Fund (Project number 164719) and the Centre for Research-Based Innovation in Aquaculture Technology (CREATE).

References Braithwaite R.A. & McEvoy L.A. (2005) Marine biofouling on ¢sh farms and its remediation. Advances in Marine Biology 47, 215^252. Cronin E.R., Cheshire A.C., Clarke S.M. & Melville A.J. (1999) An investigation into the composition, biomass and oxygen budget of the fouling community on tuna aquaculture farm. Biofouling 13, 279^299. de Nys R. & Guenther J. (2009) The impact and control of biofouling in marine ¢n¢sh aquaculture. In: Advances in MarineAntifouling Coatings andTechnologies (ed. by C. Hellio & D.Yebra), pp.177^221.Woodhead Publishing Limited, Cambridge, UK. Gili J.M. & Hughes R.G. (1995) The ecology of marine benthic hydroids. Oceanography ^ Marine Biology Annual Review 33, 351^426. Greene J.K. & Grizzle R.E. (2007) Successional development of fouling communities on open ocean aquaculture ¢sh cages in the western Gulf of Maine, USA. Aquaculture 262, 289^301. Guenther J., Carl C. & Sunde L.M. (2009) The e¡ects of colour and copper on the settlement of the hydroid Ectopleura larynx on aquaculture nets in Norway. Aquaculture 292, 252^255. Guenther J., Misimi E. & Sunde L.M. (2010) The development of biofouling, particularly the hydroid Ectopleura larynx, on commercial cage nets in Mid-Norway. Aquaculture 300,120^127. Hayward P.J. & Ryland J.S. (1990) The marine fauna of the British Isles and North-West Europe.Volume I. Introduction and Protozoans toArthopods. Clarendon Press, Oxford.


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Olafsen T. (2006). Cost analysis of di¡erent antifouling strategies. SINTEF Fiskeri og Havbruk report, SFH80 A066041, ISBN: 82-14-03947-9 (in Norwegian). Pye¢nch K.A. & Downing F.S. (1949) Notes on the general biology of Tubularia larynx Ellis & Solander. Journal of the Marine Biological Association of the United Kingdom 28, 21^43. Rungger D. (1969) Autotomy in Tubularia crocea and its ecological and physiological signi¢cance. Pubblicazioni della Stazione Zoologica di Napoli 37, 95^139.

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Swift M.R., Fredriksson D.W., Unrein A., Fullerton B., Patursson O. & Baldwin K. (2006) Drag force acting on biofouled net panels. Aquacultural Engineering 35, 292^299.

Keywords: biofouling, ¢sh farming, antifouling technologies, hydroids
