Cage Aquaculture, 3rd Edition

Cage Aquaculture Third Edition

Malcolm C. M. Beveridge

Cage Aquaculture

© 1996, 2004 by Blackwell Publishing Ltd Editorial Offices: Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK Tel: +44 (0)1865 776868 Blackwell Publishing Professional, 2121 State Avenue, Ames, Iowa 50014-8300, USA Tel: +1 515 292 0140 Blackwell Publishing Asia Pty Ltd, 550 Swanston Street, Carlton, Victoria 3053, Australia Tel: +61 (0)3 8359 1011 The right of the Author to be identified as the Author of this Work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. First edition published 1987 Second edition published 1996 by Fishing News Books, a division of Blackwell Science Third edition published 2004 by Blackwell Publishing Library of Congress Cataloging-in-Publication Data Beveridge, Malcolm C. M. Cage aquaculture/Malcolm C. M. Beveridge. – 3rd ed. p. cm. Includes bibliographical references (p. ). ISBN 1-4051-0842-8 (pbk. : alk. paper) 1. Cage aquaculture. 2. Cage aquaculture – Environmental aspects.

I. Title.

SH137.3.B48 2004 639.8 – dc22 2004000842 ISBN 1-4051-0842-8 A catalogue record for this title is available from the British Library Set in 10/12 pt Sabon by SNP Best-set Typesetter Ltd., Hong Kong Printed and bound in India by Replika Press Pvt Ltd, Kundli The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp processed using acid-free and elementary chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. For further information on Blackwell Publishing, visit our website: www.blackwellpublishing.com

Contents

Preface Acknowledgements 1 Cage Aquaculture – Origins and Principles 1.1 1.2 1.3

Principles of aquaculture Rearing facilities The origins of cage culture

2 Cage Aquaculture – An Overview 2.1 2.2 2.3 2.4

Diversity of cage types Cages and cage aquaculture Cage culture and aquaculture Advantages and disadvantages of cage culture

3 Cage Design and Construction 3.1 Shape, size and materials 3.2 Traditional designs 3.3 Modern designs Appendix 3.1 Current force on a single panel of a net cage (from Løland 1993a) Appendix 3.2 Example of cage flotation computation Appendix 3.3 Calculation of the buoyancy of a 3 ¥ 3 ¥ 3 m bamboo cage (see section 3.3.2) 4 Site Selection 4.1 4.2 4.3 4.4

Environmental criteria for farmed aquatic species Environmental criteria for cages Site facilities and management Concluding remarks

5 Environmental Impacts and Environmental Capacity 5.1 5.2 5.3 5.4

Resource consumption The cage aquaculture process Wastes Modelling environmental capacity

vii vii 1 2 4 6 9 9 14 22 24 32 33 37 40 107 109 110 111 111 134 151 155 159 159 163 164 183 v

vi

Contents

Appendix 5.1 Example of intensive cage rainbow trout production assessment for a temperate natural lake (see section 5.4.1) (modified from Beveridge 1984a) Appendix 5.2 Example of extensive cage tilapia production for a tropical reservoir (see section 5.4.1) (modified from Beveridge 1984a) Appendix 5.3 Example of semi-intensive cage tilapia production assessment for a tropical lake (see section 5.4.1) (modified from Beveridge 1984a) 6 Management 6.1 6.2 6.3

Transport and stocking Feeds and feeding Routine management

7 Problems 7.1 Currents 7.2 Disease 7.3 Drifting objects 7.4 Fouling 7.5 Oxygen 7.6 Security 7.7 Predators and scavengers 7.8 Wastes 7.9 Weather and climate Appendix 7.1 Example of calculation for a aeration system design for a freshwater rainbow trout cage, assuming airlift pumps are employed References Index

198

199

199 201 201 209 226 240 241 243 250 251 256 265 265 275 281

307 308 361

Preface

Since the first edition of this book seventeen years ago, aquaculture has consolidated its position as an important means of producing food and a contributor to global food security. Cage aquaculture too has continued to expand. While undoubtedly there is more caged fish production in fresh waters than in marine environments, there has been much expansion in the intensive rearing of species such as Atlantic salmon – a fifteen-fold increase in as many years – sea bass and sea bream in coastal environments. The third edition tries to maintain the original aim of producing a synthesis of information on cages and cage aquaculture practices. The past ten years have seen tremendous advances in the body of knowledge pertaining to aquaculture. For example, studies of the behaviour of farmed aquatic animals have resulted in improved welfare, growth and survival of stock and reductions in wastes. However, if cage aquaculture is to continue to develop and contribute to global food supplies, its reliance on environmental goods and services must be fully considered. Context is important and judgements on resource use, economic and social impacts must be made in the widest possible context, including alternative means of food production. With expansion and intensification of production methods, integration with other users of coastal and freshwater environments, too long ignored, is now crucial. As in previous editions, this book is intended as a source or reference book rather than as a practical manual and the new edition contains many new references. Its format is little altered, although the balance between sections has been changed to accommodate new information and to reflect redundancy in certain practices. I have included little information on cage or equipment suppliers but refer readers to the internet, to trade papers such as Fish Farmer, Fish Farming International, Northern Aquaculture, and to the trade directories published by the European Aquaculture Society and Aquaculture Magazine.

ACKNOWLEDGEMENTS While I have recently moved on to pastures new, this book is very much the product of my long association with the Institute of Aquaculture, University of Stirling – I could not have written it if I had not been a member of staff there for more than twenty years. I am particularly indebted to my former colleagues, especially Donald Baird, Paul Bulcock, Arturo Chacon Torres, Yrong Song Chen, Roy Clarke, Sylvain Huchette, Kim Jauncey, Sunil Kadri, Liam Kelly, Dave Little, James Muir, Kenny McAndrew, Anne Nimmo, Oscar Pérez, Mike Phillips, Lindsay Ross, Fernando Starling, Alan Stewart, Billy Struthers, Trevor Telfer and Md. Abdul Wahab. vii

viii

Preface

I thank the organizations that have supported me over the years in my work on cages, especially the Overseas Development Administration of the UK Government (now the Department for International Development, DFID) and the Highlands and Islands Development Board (now Highlands and Islands Enterprise, HIE). The Food and Agriculture Organization of the United Nations awarded me an Andre Mayer Fellowship to work at the College of Fisheries, University of the Philippines, and the short period spent in that wonderful country greatly influenced my views of aquatic environments, their conservation and management. Many people have been involved in the evolution and development of this edition of the book and I would particularly like to thank the individuals and organizations who provided information and photographs: Mr Ismael Awang Kechik, Mr Håkan Berg, Dr Asbjorn Bergheim, Mr Alastair Blair, Dr Peter Blyth, Dr Giles Boeuf, Dr Alastair Bullock, Dr Andre Coche, Mr Richard Collins, Dr James Deverill, Dr David Edwards, Dr Magnus Enell, Fusion Marine (One-steel and Mr Coulsen), Dr John Hambrey, Dr John Hargreaves, Dr Steve Hodson, Professor John Huguenin, Dr Kim Jauncey, Professor J. Katoh, Professor Nils Kautsky, Dr Liam Kelly, W&J Knox, Dr M. Kuwa, Dr Geir Løland, Mr Ian Macrae, Mr Ken McAndrew, Professor James Muir, Dr Mike Phillips, Dr Roger Pullin, Professor Ron Roberts, Dr Derek Robertson, Sadco-shelf, Professor T. Sano, Professor Christina Sommerville, Dr Alan Stewart, Stirling Environmental Services, Dr Trevor Telfer, Dr Max Troell, Mr Barney Whelan and Dr F Willumsen. Many people also helped with the production of illustrations and graphs: Graham Brown, Rachel Delaney, Brian Howie, Liam Kelly and Denise Macrae at the University of Stirling and David Hay at Fisheries Research Services, Scotland. To all, I owe a debt of thanks. Last, but not least, I would like to thank my family – Maggie, Sandy and Charlotte – for their patience and constant support.

Cage Aquaculture, Third Edition Malcolm C. M. Beveridge Copyright © 1996, 2004 by Blackwell Publishing Ltd

Chapter 1

Cage Aquaculture – Origins and Principles

Aquaculture is the aquatic counterpart of agriculture and its origins extend back some 4000 years (Beveridge & Little 2002). However, unlike agriculture, which has been the most important way of obtaining food on land for several thousand years, aquaculture has until recently contributed little in real terms to world fish or shellfish production. Instead of evolving towards cultivation, hunter–gatherer methods of procuring food from the aquatic environment developed along a different path: by improvements in finding prey and by increases in killing power. There are several reasons why agriculture and aquaculture did not develop in the same way. First, food in the lakes and seas has, until recently, been abundant. Increases in fishing pressure and development of fisheries technology were sufficient to meet growing demands and there was, therefore, little need to learn to farm. Moreover, the aquatic environment was hostile and something to be feared. It must have seemed impossible that a structure that could hold fish securely and withstand the forces of the tides and currents, waves and storms, could be built in the sea. There were other technical problems, too, to overcome. While the breeding and husbandry of animals and the harvesting and planting of seeds was readily achieved on land, it has proved difficult to breed many aquatic species, to hatch the eggs and to successfully rear the offspring. The problems in part stemmed from the fact that people were dealing with organisms that were very different from themselves and with an environment about which they were largely ignorant. It was not until the rise of the biological sciences in the 19th century that the mysteries surrounding the physiology and reproduction of aquatic animals, and the role the environment played in controlling these processes, began to be solved. World demand for fish, both as a source of food for human consumption and for reduction to fishmeal, has grown at a steady pace since the end of World War II. Until recently demands were met by the expansion of capture fisheries. Growth was around 5% during the 1950s and 1960s, increasing to 8% during the 1980s, and to 10% per annum during the past decade, production peaking at 95 million tonnes in 2000 (FAO 2003) (Fig. 1.1). However, when production from China is excluded, the supplies of fish for human food have changed little since the mid-1980s. There is a dwindling number of conventional stocks that can sustain further increases in exploitation and the situation has been exacerbated by steep increases in fuel oil prices, the development of economic exclusion zones (EEZs), the over-capitalization of many fishing fleets and profound anthropogenic changes to the very ecosystems upon which fisheries depend. 1

2

Chapter 1

Fig. 1.1 Growth, real and projected, in world capture fisheries production, world fish culture and population (data from various sources).

Over the next 25 years or so capture fisheries landings might remain stable, providing appropriate management of stocks and development of new fisheries can be achieved and providing that novel fish products can be successfully marketed. All the indications suggest that by the end of the first quarter of the 21st century farmed fish production will approximate that from capture fisheries production and be the most important means of providing fish for food. This scenario, however, takes no account of likely future shortages in some of the raw materials required for intensive aquaculture and ignores growing constraints on land and water availability (Beveridge et al. 1994b, 1997b; Pauly et al. 1998; Naylor et al. 1998, 2000), assuming instead that human ingenuity will rise to the challenges.

1.1

PRINCIPLES OF AQUACULTURE

Fisheries and aquaculture share the same aim: to maximize the yield of useful organisms from the aquatic environment. The classical theories of Russell (1931) and Beverton & Holt (1957) have determined that the size of exploitable stocks is determined by four factors: recruitment rate, growth rate, natural mortality rate and fishing mortality rate (Fig. 1.2). Capture fisheries try to maximize yields by increasing fishing mortality rate, partly at the expense of natural mortality, although if too many fish are killed recruitment and growth are unable to compensate and stocks dwindle. Aquaculture, on the other hand, seeks to increase yields by manipulation of all four population-regulating factors: growth, reproduction, recruitment and natural mortality rates.

Cage Aquaculture – Origins and Principles

3

Fig. 1.2 Factors governing exploitable stock biomass (redrawn from Pitcher & Hart 1982).

Aquaculture began independently in different societies, both agriculture- and fishing-based, and followed a pattern of development in many respects similar to that of agriculture. The control of natural mortality through the capture and holding of fish and shellfish while they increased in biomass or value was probably an early achievement (Beveridge & Little 2002). The simplest facilities to construct would have been earth ponds, possibly little more than mud walls built to temporarily hold water and fish following the seasonal flooding of a river. Manipulation of growth through feeding with household scraps or agricultural wastes would have been a logical next step. However, with one or two notable exceptions, such as that of carp in China (Li 1994), control of spawning and recruitment is comparatively recent as it is difficult to induce many species to breed in captivity. There are also many technical problems involved in the hatching of eggs and the maintenance and feeding of larval and juvenile stages (Bardach et al. 1972). Aquaculture has gradually gained control over all four of these population-determining processes. Recent decades, in particular, have seen great advances in the fields of nutrition, genetics, engineering, physiology and biochemistry, resulting in hugely improved yields. In summary, aquaculture, or the farming of aquatic organisms, is achieved through the manipulation of an organism’s life cycle and control of the environmental variables that influence it. Three main factors are involved: control of reproduction, control of growth and elimination of natural mortality agents. Control of reproduction is essential otherwise farmers must rely on naturally spawning stocks. The supply of fry from the wild may be restricted to a particular season and a particular area, and there may also be shortages due to over-exploitation of wild stocks. This step remains to be realized in the culture of many, particularly marine, species. Growth can be increased through selection of broodstock (control of breeding is, therefore, a prerequisite), and through

4

Chapter 1

feeding. While the culture of carnivorous species is dependent upon the supply of largely fishmeal-based diets, there is considerable scope for minimizing feed costs providing that the appropriate omnivorous/detrivorous/planktivorous species and systems are used. Finally, rearing systems are essential to all types of aquaculture. They are designed to hold organisms securely while they increase in biomass by minimizing losses through predation and disease and by excluding competitors (Reay 1979). Rearing systems must also facilitate management.

1.2

REARING FACILITIES

Rearing facilities for fish can either be land-based or water-based, the former including ponds, raceways, tanks and silos and the latter comprising enclosures, pens and cages. In dictionaries, the terms ‘enclosure’, ‘pen’ and ‘cage’ appear synonymous and may be used interchangeably. However, in aquaculture this has given rise to a degree of confusion, the term ‘enclosure’ often being used to describe something which could either be a cage or a pen and the word ‘pen’ being used in North America to denote a large sea cage. The terms are used here in a more restricted sense. ‘Enclosure’ is used to denote an enclosed natural bay, where the shoreline forms all but one side, which is typically closed off by a solid, net or mesh barrier (Fig. 1.3a). In pen culture, all sides of the structure, except for the bottom, are man-made, often being constructed from wooden

(a) Fig. 1.3 Water-based aquaculture systems. (a) Enclosure (Lake Buhi, Philippines); (b) fishpen (Laguna de Bay, Philippines); (c) cages (Ireland – courtesy M. J. Phillips).

Cage Aquaculture – Origins and Principles

5

(b)

(c) Fig. 1.3 Continued.

poles and netting (Fig. 1.3b). The bottom of the pen, however, is formed by the sea bed. Cages, by contrast, are enclosed on the bottom as well as the sides by wooden, mesh or net screens (Fig. 1.3c). There are other differences among water-based rearing facilities. Pens and enclosures tend to be larger, ranging in size from around 0.1 ha to some that exceed 1000 ha in area. Cages, however, typically have a surface area somewhere between 1 m2 and 1000 m2. Moreover, because of their small size, cages are better suited to intensive culture methods than pens.

6

1.3

Chapter 1

THE ORIGINS OF CAGE CULTURE

Cages were probably first used by fishermen as a convenient holding facility for fish until ready for sale (Beveridge & Little 2002). The earliest cages may have been little more than modified fish traps or baskets, and such traditional types of holding facility have been in use in many parts of the world for generations. True cage culture, in which fish or other organisms were held for long periods of time while they increased in weight, was until recently thought to be a comparatively modern development. According to Li (1994), however, cage culture was established in China during the Han Dynasty 2200–2100 years ago. In the first apparent written account Hu (1994) relates how Zhou Mi described fry sales in the ancient Jiujiang River in an appendix (‘Beiji’) to a book entitled Kuixinzhashi written in 1243 during the Sung Dynasty (AD 960–1280). The fry ‘. . . reach home and are placed in cloth cages in open water with bamboo sticks supporting the four corners. The fry actively move about in the cages with the waves as if they enjoy playing. One or half a month later, the fry grow bigger for marketing. The cloth cages are fine-meshed cages. The bamboo sticks serve to frame the cages in which the caught fry are nurtured.’ The culture of fry in what were probably small fixed cages must have been for grow-out purposes in ponds. Hu (1994) gives a further account of fry catching in the Jiujiang River in the late 1840s and describes how in 1876 foreign visitors to Jiangxi described fry catching and temporary holding in cages. In the Great Lake region of Cambodia floating cages have been used since the end of the 19th century (Lafont & Savoeun 1951; Hickling 1962; Ling 1977; Pantulu 1979). Snakeheads (Channa spp.), catfish (Pangasius spp., Clarias spp.) and marble-headed gobies (Oxyeleotris marmorata) were held in wood or bamboo cages, fed on a mixture of kitchen scraps and trash fish and transported by river to the markets of Phnom Penh. Cages were either towed behind the boats or occasionally incorporated into the vessel to form a well-boat (Fig. 1.4a, b). During the 20th century, this type of cage culture spread to most parts of the lower Mekong delta and into Vietnam (Pantulu 1979; Tuan et al. 2000). In Mungdung Lake, Sulawesi, Indonesia, floating bamboo cages have been in use since the early 1920s (Reksalegora 1979) to rear Leptobarbus hoeveni fry captured from the lake. A different form of cage culture appeared in Bandung, Indonesia, around 1940. Small bamboo and ‘bulian’ wood cages were anchored to the bottom of organically polluted rivers and canals and stocked with common carp, Cyprinus carpio, which fed on wastes and invertebrates carried in the current (Vass & Sachlan 1957; Costa-Pierce & Effendi 1988). Traditional cage culture, distinguished by its reliance on natural construction materials and on natural or waste feeds, is still practised in parts of Indonesia and Indo-China (Fig. 1.5). However, these traditional cage-rearing practices had a localized influence and did not directly give rise to modern cage fish farming methods (see also Hu 1994). Modern cages utilize synthetic mesh or netting materials and have collars usually fabricated from synthetic polymers and metals, although wood is still widely used in many designs. It is difficult to be precise about the origins of modern cage fish farming although Japan was undoubtedly a key influence. According to Milne (1974), Professor Harada, Director of the

Cage Aquaculture – Origins and Principles

7

(a)

(b)

(c) Fig. 1.4 Traditional fish cage designs, Indo-China. (a) Southern Vietnam; (b) boat-shaped cage from Cambodia; (c) battery of small cages, Cambodia (from Pantulu 1979).

Fisheries Laboratory at Kinki University, first started experimenting with cage fish culture in 1954 and commercial culture of yellowtail Seriola quinqueradiata followed three years later. In Norway, cages were being used to culture Atlantic salmon (Salmo salar) in the early 1960s and in Scotland the White Fish Authority commenced salmon cage rearing trials around 1965. Surprisingly, tilapia (Oreochromis spp.) culture in cages is of even more recent origin and

8

Chapter 1

Fig. 1.5 Cage aquaculture of cobia (Rachycentron canadum) in household cages, Ha Long Bay, Vietnam (courtesy M. J. Phillips).

owes its beginnings to work carried out at Auburn University in the late 1960s (Schmittou 1969). Modern cage farming is thus very much a phenomenon of recent decades. In the following chapter the relative importance of aquaculture in cages and the factors determining its current status are explored.

Cage Aquaculture, Third Edition Malcolm C. M. Beveridge Copyright © 1996, 2004 by Blackwell Publishing Ltd

Chapter 2

Cage Aquaculture – An Overview

Cages are highly versatile and lend themselves to being used in many different ways. This section is intended as an overview of the subject and many of the themes touched on will be explored in detail in later chapters.

2.1 DIVERSITY OF CAGE TYPES Cages have developed a great deal from their humble origins and today there is an enormous diversity of types and designs. They may be classified as shown in Fig. 2.1 (see also Huguenin 1997). There are four basic types: fixed, floating, submersible and submerged. Fixed cages consist of a net bag supported by posts driven into the bottom of a lake or river (Fig. 2.2a). They are in common use in some tropical countries such as the Philippines. Fixed cages are comparatively inexpensive and simple to build, although they are limited in size and shape and their use is restricted to sheltered shallow sites with suitable substrates. The bag of a floating cage is supported by a buoyant collar or, in some cases, a frame. This type is by far the most widely used and can be designed in an enormous variety of shapes and sizes to suit the purposes of the farmer. Floating cages are also less limited than most other designs in terms of site specifications. Some floating types are designed to rotate in order to control fouling (see Chapter 7). The design in Fig. 2.2b rotates about a central axis incorporated into the collar while other designs are rotated by means of moving the flotation elements or by adjusting the buoyancy of the frame members (Fig. 2.2c). The much more widely used non-rotating floating types can be constructed with wide or narrow collars. The former are common on larger cages and serve as work platforms, facilitating many of the routine farm tasks (Fig. 2.2d). Most wide collars are designed to be rigid although some are flexible so that they may be used at more exposed sites (Fig. 2.2e; see also Chapters 3 and 7). Simple and inexpensive flexible-collar narrow cages can be fabricated using rope and buoys (Fig. 2.2f), but in practice are difficult to manage. Rigid narrow collars, constructed from glass fibre or steel section and buoys (Fig. 2.2g), are popular in Western Europe despite the fact that routine operations must be performed from a boat or pontoon. Rigid mesh designs must, of course, utilize a rigid collar. Some floating net bag designs, including early designs for flatfish culture, have a solid bottom (Hull & Edwards 1979). Neither net nor rigid mesh bag submersible cages have a collar, but instead rely on a frame or rigging to maintain shape. The advantage over other designs 9

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Chapter 2

FIXED

FLOATING

Nonrotating

Rotating

Wide collar

Narrow collar

Rigid collar

Flexible collar

Rigid bag

Flexible bag Net floor

SUBMERSIBLE

With central axle

Suspended from surface

Without central axle

Rotation by adjustment of float buoyancy

Rigid

SUBMERGED

Adjustable buoyancy

Flexible

Rotation by adjustment of float position

Solid floor

Fig. 2.1 A classification system for cages (developed from Kerr et al. 1980).

(a) Fig. 2.2 Different types of cages. (a) Fixed cage (SEAFDEC, Laguna de Bay, Philippines); (b) rotating cage with central axle (Kiel, Germany); (c) submersible, rotating cage without central axle (Dunstaffnage, Scotland – courtesy A. Blair); (d) floating, wide collar milkfish broodstock cage (SEAFDEC, Philippines – courtesy R. S. V. Pullin); (e) flexible wide collar design (Ireland – courtesy B. Whelan); (f) flexible, narrow collar cage (Lake Titicaca, Bolivia); (g) rigid, narrow collar frame (Scotland – courtesy I. H. Macrae); (h) submersible cage (Germany); (i) submerged cage (Java, Indonesia – courtesy C. Sommerville).

Cage Aquaculture – An Overview

11

(b)

(c) Fig. 2.2 Continued.

is that the position in the water column can be changed to exploit prevailing environmental conditions. The cages are typically kept at the surface during calm weather and are submerged during adverse weather or during a harmful algal event. Some submersible designs rely on the bag being suspended from buoys or

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Chapter 2

(d)

(e) Fig. 2.2 Continued.

a floating frame on the water surface (Fig. 2.2h) while others have variable buoyancy (see later in Fig. 7.31). While a number of submerged cage designs have been proposed (see Huguenin & Rothwell 1979; Huguenin 1997), far fewer have gone beyond the design

Cage Aquaculture – An Overview

(f)

(g) Fig. 2.2 Continued.

13

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Chapter 2

(h)

(i)

Fig. 2.2 Continued.

concept stage or indeed have been built or widely used. Simple submerged cages, which are little more than wooden boxes with gaps between the slats to facilitate water flow and are anchored to the substrate by stones or posts (Fig. 2.2i), have been used to culture common carp in flowing waters in Indonesia (Vass & Sachlan 1957; Costa-Pierce & Effendi 1988) and to culture lobster in coastal waters in Vietnam (Tuan et al. 2000). Submerged net mesh bag designs have been used in lakes and reservoirs in the former USSR and in China (Martyshev 1983; Li 1994). However, it remains a moot point whether all species readily adapt to rearing in submersible or submerged cages (see sections 7.9.1, 7.9.2). Despite the fact that cage designers and manufacturers have produced all sorts of designs in the past half-century or so, the range of cage types today is, if anything, smaller than it was a decade ago. Cost, always important, has now become the overriding design criterion, particularly in the industrial-scale farming industries (e.g. salmon farming) and this has led to uniformity in terms of shape, size and materials (see section 3.1).

2.2

CAGES AND CAGE AQUACULTURE

Because they are a relatively inexpensive and convenient way to keep captive aquatic organisms, cages have been used for a variety of purposes, some unre-

Cage Aquaculture – An Overview

15

lated to aquaculture. For centuries, cages were used to hold and transport bait fish for tuna pole and line fishing (Ben Yami 1978; Takashima & Arimoto 2000) although today their use has been largely superseded by live-bait holds in boats. They have also been used in Norway and Canada, on a trial basis at least, to move fish such as herring (Clupea harengus harengus) and pollack (Pollarchius pollarchius) to impoundments (see Kreiberg & Solmie 1987 for a review) and in Japan for keeping fish caught in traps until ready for market (Takashima & Arimoto 2000). Caged fish have been used to monitor the water quality of power station effluents (Holt 1977; Chamberlain 1978) and to monitor environmental quality (Grizzle et al. 1988; Meng et al. 2000). Cage aquaculture has been used to treat the symptoms of eutrophication (Yang 1982; Little & Muir 1987; CostaPierce & Effendi 1988; Chang 1989; Starling et al. 1998; Costa-Pierce 2002) and has also been used in conservation initiatives for species as disparate as seahorses (Hippocampus spp.) (Vincent & Pajaro 1997), frogs (Vines et al. 1996) and giant clams (Tridacna spp.) (Bell et al. 1997), and to produce fish for putand-take recreational fisheries. Cages may be useful in experimental work where it is important to exclude environmental effects (see Kulikovsky et al. 1994) or as an alternative to replicate ponds (Struve & Bayne 1991), although particular care must be given to experimental design to ensure sufficient replication and to avoid pseudoreplication issues (Hurlbert 1984). But cage culture is primarily targeted at producing food. Cages can be used at different stages of the life cycle: for breeding, fry and fingerling rearing, and/or production of fish for the table. Cage-based tilapia hatcheries (‘hapas’) were developed in the Philippines in the 1970s (Beveridge 1984b; Smith et al. 1985) and a decade or so later the technology had become widespread in Southeast Asia. Little & Hulata (2000) cite low capital costs and intensive management of broodstock and seed as their primary advantage over other systems. Cage production of fry and fingerlings of freshwater fish species, such as tilapia and carp, is now widespread (Smith et al. 1985; Li 1994; Jayaraj et al. 1998; Anh & Son 2001; Ariyaratne 2001; see also section 2.2.1), as is the use of cages for production of Atlantic salmon smolts in Scotland (Williamson & Beveridge 1994) and elsewhere. The use of cages in growing fish and crustaceans for the table is also widely discussed in this book. Like other types of aquaculture, cage fish farming may be classified on the basis of feed use as extensive, semi-intensive and intensive. In extensive culture fish rely solely on available natural foods such as plankton, detritus and seston. Semi-intensive culture involves the use of low protein (20%) food, usually based on fishmeal (see also Coche 1983).

2.2.1

Extensive cage aquaculture

Various authors have observed caged fish grazing on the fouling community on cage nets (see Norberg 1999; Huchette et al. 2000). In a cage-based trial in which

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Chapter 2

Fig. 2.3 Primary production of fresh waters (from Beveridge 1984a).

the surface area available for colonization by fouling organisms was greatly increased, Huchette & Beveridge (2003) attempted to grow tilapias with no additional food. Although production of up to 0.94 g fish m-2 per day was recorded in the upper 0.5 m of the water column, the system proved not to be economically viable. Further discussion is thus limited to the culture of fish in cages that are reliant on external supplies of food. Extensive cage culture is restricted to fresh waters and may be practised in two types of environment: highly productive lakes and reservoirs (see Shenoda & Naguib 2000) and water bodies that receive sewage or domestic wastes (see Kibria et al. 1999). Primary production, which fuels all successive energy transactions in aquatic food webs other than in waste-fed systems and systems with high allochthonous inputs (i.e. externally-produced materials, such as leaves), is dependent upon the availability of essential nutrients (for example, phosphorus and nitrogen compounds) as well as light and temperature (Le Cren & LoweMcConnell 1980; OECD 1982). Systems with high nutrient loadings are likely to be highly productive. However, productivity is also strongly correlated with latitude (Brylinsky 1980), and between temperate zones (23–67°N and S) and tropical zones (23°N–23°S) there is a considerable increase in the range of annual primary production values (Fig. 2.3). Tropical water bodies with high nutrient loadings offer the best opportunities for extensive cage culture (Table 2.1). Extensive cage culture on any scale seems to have only been practised in the Philippines and China (Beveridge 1984a; Li 1994; Beveridge & Stewart 1998). Caged bighead carp (Aristichthys nobilis) were used at Selatar Reservoir, Singapore, during the late 1970s and early 1980s to help control eutrophication problems in municipal water supplies (Yang 1982), and similar approaches were employed in reservoirs in Brazil in the 1990s using silver carp

Cage Aquaculture – An Overview

17

Table 2.1 Correlation between the plankton biomass and fingerling production of silver carp and bighead carp in reservoirs (from Li 1994). Phytoplankton

Eutrophic Mesotrophic Oligotrophic a

Zooplankton

¥103 cells l-1

mg l-1

ind.a l-1

mg l-1

Stocking density ind.a m-2

>100 30–100 5 2–5 2000 1000–2000 3 1–3