The Ecology of the Fish Pond Ecosystem

Developments in Hydrobiology 72

Series editor

H. J. Dumont

The Ecology of the Fish Pond Ecosystem

with special reference to Africa

by

Guy Delince Institute of Animal Ecology,

University of Ghent, Belgium

SPRINGER-SCIENCE+BUSINESS MEDIA, BV.

Library of Congress Cataloging-in-Publication Data

Delince. Guy. The ecology of the fish pond ecosystem with special reference to

AfrIca I Guy Del Inee. p. cm. -- tDevelopments in hydrobiology ; 72)

Includes bibl iographical references and index. ISBN 978-90-481-4132-6 ISBN 978-94-017-3292-5 (eBook) DOI 10.1007/978-94-017-3292-5 1. Fish-culture--Africa. 2. Fish ponds--Africa--Fertilization.

3. Fresh water--Ferti lization. 4. Pond ecology--Africa. 1. Title. I!. Ser les. SH121.D45 1992 639.3' l1--dc20

ISBN 978-90-481-4132-6

Printed on acid-free paper

Ali Rights Reserved © 1992 Springer Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1992 Softcover reprint of the hardcover 1 st edition 1992

92-54

No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, inc\uding photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner.

Contents

Preface ............ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii Acknowledgements .................................................... xviii Introduction ........................................................ 1

Chapter 1. Pond soil .................................................. .

1. Soil characteristics ................................................ .

Importance of soil for water productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pond bottom constitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Morphological features of a soil ......................................... . Some clay properties ................................................ . Organic matter in soil ............................................... . Stratification of flooded soil ............................................ . Pore water or interstitial water .......................................... . Interstitial water as a transition compartment Mechanisms for transport across the interface

Importance of soil for the productivity of water

3

3

3 3 4 4 5 5 7 7 8

9

2. Soil properties ................................................... 9 2.1. Colloids and their adsorption properties .................................. 9 2.1.1. Mineral colloids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 10

Electric charges on clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 10 Other types of charges ................................................ 10

2.1.2. Organic colloids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 12 2.1.3. Ions sorbed by colloids ........................................... 13 2.1.4. Ionic exchange capacity ........................................... 13

Cation exchange capacity .............................................. 13 Anion exchange .................................................... 14 Sorption isotherms .................................................. 14

Importance of the colloidal properties of a soil .................................. 15

v

vi

2.2. The acid or alkaline reaction of soil ..................................... 15

Acidifying role of aluminium and iron ...................................... 16 Active and potential acidity ............................................. 16 Acidification and alkalinization factors ...................................... 17 Nutrient availability and pH ............................................. 17

2.3. Redox potential ................................................. 17

Importance of pH and redox potential properties ................................. 18

3. Mineral and organic components of a soil .................................. 19 3.1. Mineral component of soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 19 3.1.1. Calcium .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 19

Importance of calcium in soil ............................................. 20

3.1.2. Nitrogen .................................................... 20

Nitrogen compounds present in pond muds ................................... 20 Origin of nitrogen in sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 21 Transfer of nitrogen across the interface ..................................... 21 Conditions at the interface .............................................. 21 Denitrification in sediments ............................................. 21 Nitrogen fixation ................................................... 23 Nitrification ...................................................... 24 Ammonification .................................................... 24 Modifications occurring during the drying of pond bottoms ......................... 24 Changes occurring after refilling .......................................... 25

Importance of nitrogen in the sediments ....................................... 26

3.1. 3. Phosphorus ................................................... 27 3.1. 3.1. Introduction ................................................. 27

Concentration of phosphorus in sediments .................................... 27 Complexity of phosphorus dynamics in sediments ............................... 27

3.1.3.2. Different forms of phosphorus in sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 28

Sediment-bound and interstitial phosphorus ................................... 28 Importance of interstitial phosphorus ....................................... 28 Particulate phosphorus ................................................ 29 Sorbed or immobilized sediment phosphorus .................................. 30 Precipitation of phosphorus or authigenic mineral formation ......................... 31

3.1. 3.3. Phosphorus loading to sediments .................................... 31

Phosphorus accumulation in sediments ...................................... 31 Origin of phosphorus in sediments ......................................... 31

Vll

Loading models and kinetics of sorption ..................................... 32 Phosphorus sorption in sediments - the Einsele-Mortimer paradigm .................... 33 Factors affecting phosphorus immobilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 34 Influence of mineralization on the availability of phosphate ......................... 36 Effect of nitrate on mineralization ......................................... 37 Changes in phosphorus loading with depth .................................... 37 Buffering mechanisms of sediment phosphorus ................................. 38

3.1.3.4. Phosphorus release from sediments ................................... 38

Phosphorus release and sediment characteristics ................................ 38 Phosphorus release and trophic level of a water body ............................. 39 Factors in sediments promoting desorption of phosphorus .......................... 39 Pore water as a source of phosphorus .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ., 39 Aerobic and anaerobic transfers .......................................... 39 Importance of phosphorus release rates ...................................... 40 Gaseous ebullition ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ., 41 Changes caused by resuspension of soil particles ................................ 41 Effect of benthic organisms on the sediment-water exchange ......................... 41 Removal of phosphorus from sediments by epipelic algae . . . . . . . . . . . . . . . . . . . . . . . . . .. 42 Transfer and phosphorus regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 42 Assessment of bioavailability of phosphorus ................................... 42 Effect of pond fertilization on pond bottoms ............. . . . . . . . . . . . . . . . . . . . . ., 43

Importance of phosphorus in sediments ....................................... 43

3.1.4. Sulphur ..................................................... 44

Importance and forms of sulphur present in sediments ...... . . . . . . . . . . . . . . . . . . . . . ., 44 Sulphate and organic matter decomposition ................................... 44 Sulphate oxidation and alkalinity increase .................................... 44 Microorganisms responsible for decomposition ................................. 45

3.1.5. Iron ....................................................... 45

Importance of sulphur and iron ............................................ 45

3.2. Organic matter and humus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 46

3.2.1. Importance of organic matter ........................................ 46

Origin of organic matter ............................................... 46 The flocculent layer as the centre of organic matter decomposition ..................... 46

3.2.2. Decomposition and nutrient regeneration ................................ , 46

Decomposition ..................................................... 46 Composition of the benthic microbial fauna ................................... 47 Aerobic decomposition and sediment oxygen demand ............................. 48 Anaerobic decomposition .............................................. 49

viii

Decomposition rate and the nutritional value of organic matter ....................... 50 Effects of decomposition processes on sediments ................................ 50 Effects of benthic organisms on decomposition and benthic fluxes ..................... 50 Exchanges with the water column ......................................... 51

Importance of organic matter and its decomposition in the sediment 52

Chapter 2. Water and productivity .......................................... 53

1. Water characteristics

2. Physical characteristics of water

2.1. Temperature, density and viscosity 2.2. Water movements and stratification

..................................... 53

................................... 53

53 54

Effects of wind and waves ............................................. 54 Convection currents .................................................. 54 Different types of stratification of ponds and other water bodies ...................... 55

2.3. Colour, transparency and turbidity of the water .............................. 56

Water colour ...................................................... 56 Light propagation in water and attenuation of light by plankton ....................... 56 Water turbidity .................................................... 57 Movement and maintainability of suspended particles ............................. 57 The role of coagulation in particles sedimentation . . . . . . ............. . . . . . . . . .. 57 Effect of turbidity on production .......................................... 58 Turbidity measurement ............................................ 58 Turbidity control ................................................... 59

Importance of physical characteristics of water .................................. 59

3. Chemical variables of water ........................................... 60

Factors determining water chemistry ....................................... 60

3.1. Dissolved oxygen ................................................ 60

Importance of dissolved oxygen .......................................... 60 Oxygen transfer at the air-water interface and within water ......................... 60 Solubility of oxygen in water ............................................ 61 Oxygen sources and consumption in water .................................... 61 Dynamics of change in dissolved oxygen in ponds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 61 Factors controlling dissolved oxygen ...................... . . . . . . . . . . . . .. 63 Measurement of dissolved oxygen ......................................... 64 Methods for controlling dissolved oxygen .................................... 64

Importance of dissolved oxygen in ponds ...................................... 65

ix

3.2. Conductivity and salinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 66

Definition ........................................................ 66 Factors responsible for mineral load of water ................................... 66 Dominant ions ..................................................... 66 Types of water based on conductivity ....................................... 66 Modes of representation of ionic composition ................................... 67 Salinity and chlorinity: definition ......................................... 67

3.3. Alkaline cations ................................................. 67

(1) Calcium and magnesium ............................................ 67 (2) Sodium and potassium .............................................. 69

ImpOrlance of conductivity and alkaline ions .................................... 69

3.4. Alkalinity, pH, hardness and carbon dioxide ............................... 70

3.4.1. pH ........................................................ 70

Definition of pH and pH regulation ........................................ 70 Optimum pH values ................................................. 70

3.4.2. Alkalinity and hardness ........................................... 70

Definition of alkalinity ................................................ 70 Processes modifying alkalinity ........................................... 70 Definition of hardness ................................................ 72

3.4.3. Carbon dioxide ................................................ 72

Importance of carbon dioxide ............................................ 72 Carbon dioxide and buffer capacity ........................................ 73 Carbon dioxide and photosynthesis ........................................ 74 Influence of alkaline ions on buffer capacity ................................... 76

Importance of alkalinity, hardness, pH and carbon dioxide ........................... 77

3.5. Liming ...................................................... 77

3.5.1. pH control of water by liming ....................................... 77

3.5.2. Effects of liming ............................................... 78

3.5.3. Types of lime used and their equivalences ................................ 80

3.5.4. Liming methods ................................................ 81

3.5.5. Identification of the need for lime ..................................... 81

x

3.5.6. Other methods of neutralization ................ 82

Neutralization of sediments with sodium bicarbonate 82 Use of nitrogen fertilizers .............................................. 82 Other products ..................................................... 82

Importance of liming .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 82

4. Nutrients in water ................................................. 83

Relation between nutrients, water fertility and fertilizers ........................... 83 Recycling and regeneration of nutrients .................... . . . . . . . . . . . . . . . . 83

4.1. Rules for the use of fertilizers 84

Importance of nutrients for the production of natural feed ............................. 84

4.2. Nitrogen and nitrogen compounds ...................................... 85

4.2.1. Nitrogen cycling in water .......................................... 85

Nitrogen compounds in water ............................................ 85 Nitrogen fixation in water .............................................. 85 Factors inhibiting nitrogen fixation ........................................ 86 Ammonification .................................................... 87 Nitrification ...................................................... 87 Denitrification or dissimilation .......................................... . Assimilation of nitrogen by plants and phytoplankton ............................ . Relative importance of the various processes ................................. .

88 88 89

Ammonia toxicity for aquatic organisms ..................................... 89 Amounts of ammonia excreted by fish ...................................... 91

Nitrogen and nitrogen compounds in water: salient points ................. . . . . . . . . . .. 92

4.2.2. Nitrogen fertilizers .............................................. 92

Nitrogen fertilizers available ............................... 92 Acidity released by fertilizers ............................... 92 Destiny of fertilizers in water .................................... 94 Relation between nitrogen needs and needs for other nutrients ........................ 94 Fertilization frequencies ............................................... 94 Controversy about the need for nitrogen fertilizers ............................... 94 Doses of nitrogen fertilizers applied to ponds ......................... 95 Recommendations for use of nitrogen fertilizers ....................... 95

Nitrogen fertilizer application ................................... 96

4.3. Phosphorus and phosphorus compounds .................................. 96

Xl

4.3.1. Phosphorus cycling .............................................. 96

Phosphorus species present in water ........................................ 96 Sources of particulate phosphorus ......................................... 96 Composition of dissolved phosphorus ....................................... 97 Composition of dissolved organic phosphorus .................................. 97 Phosphorus concentration in water ......................................... 98 Transformations of phosphorus in water ..................................... 98 Time needed for various transformations ..................................... 100 Importance of phosphorus mineralization ............... . . . . . . . . . . . . . . . . . . . . . . 100 Phosphate uptake and influences on phosphorus availability ......................... 101 Bacterial mineralization and role of phosphatase ................................ 102 Phosphatase sources ................................................. 102 Differences between alkaline and acid phosphatases .............................. 103 Measurement of enzyme activity .......................................... 103 Relation between phosphorus deficiency and phosphatase concentrations . . . . . . . . . . . . . . . . . . 104 Phosphorus uptake by autotrophs ......................................... 106 Processes of phosphate uptake by algae and bacteria ................... . . . . . . . . . . . 106 Phosphate competition between phytoplankton and bacteria .......................... 107 Bacterial and algal synergy .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Intracellular phosphate stocks in algae ...................................... 108 Importance of phosphate effluxes ......................................... 108 Amounts of phosphorus excreted by fish ..................................... 108 Measurement of phosphorus in water ....................................... 109

Phosphorus and phosphorus compounds in water: salient points ........................ 109

4.3.2. Phosphorus fertilizers 110

Phosphorus fertilizers available ........................................... 110 Destiny of phosphorus fertilizers in ponds .................................... 110 Interaction of fertilizer with other water characteristics ............................ 111 Phosphorus fertilizer doses applied ........................................ 112 Application mode of phosphorus fertilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Recommended application rates for phosphorus fertilizers .......................... 114

Phosphorus fertilizer application 114

4.4. Manure or organic fertilizers ......................................... 115

Differences between inorganic fertilizers and manure ............................. 115 Decomposition processes in water ......................................... 115 Types of fish cultured in manure-fertilized ponds ................................ 116 Nutritional value of manure ............................................. 117 Disadvantages of the use of manure ........................................ 117 Mode of use of fertilizers - precautions for their use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Types of organic manures used and their composition ............................. 118 Comparative value of manures ........................................... 118 Effects of manure on water quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Plankton concentration and primary productivity obtained with manure . . . . . . . . . . . . . . . . . . . 120

xii

Yields obtained with manuring ........................................... 120 Number of livestock heads needed for integrated fish farming ........................ 121 Maximal amounts of manure to be used ..................................... 121

Organic fertilizers, role and use ....................................... . . . . . 122

4.5. Other nutrients involved in productivity 122

4.5.1. Silicon ..................................................... 122

Importance of silicon ................................................. 122 Species and concentrations of silicon in water .................................. 122 Silicon assimilation by algae ............................................ 123 Influence of silicon concentrations on plankton composition ......................... 123 Measurement of silicon ............................................... 123

4.5.2. Sulphur ..................................................... 124

4.5.3. Iron ....................................................... 124

Importance of iron .................................................. 124 Iron species in water ................................................. 124 The importance of chelators ................................ . . . . . . . . . . . . . 125 Iron concentration in water ............................................. 125 Effect of iron on fish and algae .......................................... 126 Iron determination in water ............................................. 126

Importance of silicon, sulphur and iron in water ................................. 126

Chapter 3. Productivity and fish production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

1. Primary productivity of water .......................................... 127

1.1. Natural productivity and fish production .................................. 127

Types of aquatic organisms present in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Spectrum of algae found in ponds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Effects of the phytoplankton on the environment ................................ 129 Relations between phytoplankton and planktivorous fish ........................... 129 Common features for lakes and ponds ...................................... 129 Productions in tropical and temperate regions .................................. 129

Natural productivity: what the concept covers 130

1.2. Determinants of primary productivity 130

Primary productivity characteristics ........................................ 130 The photosynthetic capacity ............................................. 131 Methods to measure photosynthetic rate ..................................... 132

xiii

Factors determining photosynthetic rate ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 Maximal densities of chlorophyll a ........................................ 132 Phytoplankton density and photosynthetic rate ........... . . . . . . . . . . . . . . . . . . . . . . . 132

1.2.1. Influence of light ............................................... 133

Autotrophic and heterotrophic organisms ..................................... 133 Light absorption by water and stratification according to light intensity ................... .133 Light absorption or attenuation and trophic loading .............................. 134 Use of light by photosynthesis ........................................... 134 Effects of limiting or excess light on phytoplankton .............................. 135 Efficiency of the use of solar energy in ponds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

Photosynthesis determinants and influence of light 136

1.2.2. Influence of phytoplankton biomass .................................... 136

Phytoplankton abundance and measurement of phytoplankton density ................... 136 Phytoplankton growth ................................................ 136 Factors affecting specific growth rate ....................................... 137 Specific growth rate and biomass ......................................... 139 Community production and biomass ........................................ 141 Steps in the increase of plankton biomass .................................... 142 Effects of water circulation on phytoplankton production ........................... 142 Oxygen release and algal biomass ......................................... 143 Plankton population composition and trophic state of the water ....................... 143

Phytoplankton biomass and primary productivity 143

1.2.3. Elimination of phytoplankton biomass 144

Types of elimination or biomass decreases .................................... 144 Plankton sedimentation ................................................ 144 Algal consumption .................................................. 144 Phytoplankton defenses against predation ..................................... 145 Controls of the dynamics of phytoplankton density ............................... 145

Importance of elimination of algae .......................................... 145

1.2.4. Phytoplankton behaviour and life strategies ............................... 146

Importance of life strategies ............................................. 146 rand K strategies of phytoplankton species ................................... 146 The plankton paradox ................................................ 146 Vertical distribution and migration of phytoplankton ............................. 146 Algal exudations '" ................................................. 147 Algal blooms ...................................................... 148 Algal crashes ...................................................... 148 Causes of algal crashes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Effect of zooplankton on algal blooms ...................................... 150

xiv

Algal disappearance or physiological death ................................... 150 Phytoplankton management ............................................. 150 Algal abundance and off-flavours ......................................... 150

Phytoplankton life-strategies and behaviour 151

1.2.5. Influence of nutrient concentration 151

Primary productivity and nutrient concentration ................................ 151 Trophic loading of a water body .......................................... 151 Relations between nutrient concentration and requirements ......................... 151 Relations between intracellular and extracellular nutrient concentrations and limiting concentrations . .153 Limiting nutrients, and essential differences between nitrogen, phosphorus and silicon cycling .... .153 Relations between phosphorus and nitrogen requirements ........................... 153 Chronic or temporary and changing limitations ................................. 154 Control model for carbon, nitrogen and phosphorus .............................. 155 Reaction of the environment to nutrient limitation ............................... 156 Ways of determining nutrient limitations ..................................... 157 Reliability of predictions of the appearance of algal groups .•....................... 157 Plankton group-succession or temporal modification of plankton assemblage structure ......... .158 Spatial heterogeneity of nutrient concentrations ................................. 159 Temporal heterogeneity of nutrient concentrations ............................... 160 Control of excess nutrients ............................................. 160

Influence of nutrient concentrations 161

1.2.6. Influence of zooplankton 161

Zooplankton composition .............................................. 161 Zooplankton distribution within ponds ...................................... 162 Influences of zooplankton on the plankton community ............................. 162 Influence of macro zooplankton on microzooplankton ............................. 165 Cqntroversy about the diversity of tropical zooplankton compared to that of temperate zooplankton .. 165 The phytoplankton-zooplankton interface .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 Types of control in plankton popUlations ..................................... 166 Controls and regulations on plankton populations ................................ 167 Interaction and exploitative competition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Control by limitation of resources ......................................... 168 Herbivore tactics to escape predation ....................................... 168 Zooplankton predator.to-prey proportions ..................................... 169

Influence of zooplankton on the plankton community ............................... 169

1.2.7. Contributions to secondary production .................................. 170

Limitation of the concept of primary production ................................ 170 The microbial food loop ............................................... 170 Relations between bacterial populations, living plankton and dead cells .................. 170 Differences in organic material ........................................... 171 Bacterial types and decomposition in water ................................... 172

xv

The microbial food loop : source or sink of energy? .............................. 172 Trophic links between organisms ......................................... 173 The trophic pyramid or trophic food web concept ............................... 173

The bacterial food loop: contribution to production 174

1.2.8. Influence of fish production 174

Effects of fish production on primary production and on water quality ................... 174 Effects of fish on artificial environments ..................................... 174 Effects of fish on ponds ............................................... 176 Specific effects on ponds of some groups of fish ................................ 179 Effects of benthophagous fish, especially common carp ............................ 179 Effects of zooplanktivorous fish on ponds .................................... 180 Impact of phytoplanktivorous fish ......................................... 183 Plankton consumption by fish ............................................ 184 Synergism between aquatic organisms ...................................... 185 Interactions between fish feeding regimes ..................................... 185 Influence of fish on the decomposition cycle .................................. 185 Biomanipulation and relations within the aquatic environment ........................ 186 Control of planktivorous fish by piscivorous fish ................................ 186

Importance of fish on plankton production

2. Fish production in a pond

2.1. Stocking, management practices and production

Types of management operations in a fish pond Importance of growth rate and production rate

186

187

187

187 188

2.1.1. Feeding operations .............................................. 189

Importance of feeding ................................................ 189 Influence of feed on growth rate .......................................... 190 Relations between feeding rate and production rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

2.1.2. Stocking density and growth ........................................ 192

Definition of stocking density ............................................ 192 Relations between density, growth rate and final mean weight ........................ 192 Density and reproduction of tilapia ........................................ 193

2.1.3. Stocking density and production ...................................... 193

Influence of density on the production rate .................................... 193 Appropriate densities ................................................. 195

2.1.4. Length of the culture period ........................................ 196

xvi

2.2. Fish production and production factors 196

2.2.1. Production and production factors ..................................... 196

Definition of fish production ............................................ 196 Factors determining production ........................................... 196 Production factors in fish farming in CAR .................................... 197

2.2.2. Critical factors limiting production 198

Deterioration of production conditions ....................................... 198 Problems with oxygen consumption in ponds .................................. 198 Measures to avoid anoxic conditions ....................................... 199 Biological control of oxygen production and consumption .......................... 199 Means of eliminating deleterious decomposition products ........................... 200 Successive limiting factors in fish farming .................................... 200

2.2.3. Ranges of fish yields ............................................. 201

Yield levels according to production intensity .................................. 201 Maximal yields obtained in Africa ......................................... 203

Fish production in ponds ................................................ 205

References ......................................................... 206

Index .......................................................... 223

Preface

This exhaustive review on the potential of fertilization as a means to increase fish production in ponds under African tropical conditions is much overdue. Pond fish farming has been introduced to Mrica over the past forty years. Although it is generally agreed that this continent has a considerable potential for freshwater aquaculture, past development efforts have met with variable but generally limited success. Several constraints have recently been identified while suggesting future research topics and ranking their respective priority. From the technical point of view, pond fertilization has been recognized as the main limiting factor for the development of tilapia farming.

In fact, even outside Africa, most of the present aquaculture production depends at least partly on the use of fertilizers. Over 70 % of the world production of farmed finfish and shrimp is realized with extensive to semi-intensive pond cultural systems, where natural productivity is enhanced through the controlled additions of fertilizers.

Most of the basic scientific research on pond fertilizations was carried out in temperate latitudes, mainly in Central and Eastern Europe, and more recently in the Southern United States and Israel. Not only should much of this past research be open to criticism on the design of experiments, but its results should only be applied in tropical Africa with the greatest caution, if at all. It is a fact that our understanding - and therefore our possible control - of the intermediate stages through which fertilizers are converted into fish food and fish flesh is still far from complete, particularly in African ponds.

The principles of pond fertilization are similar to those applied to agriculture. Fish production is increasing by the addition of certain essential substances present in limited concentrations and by the stimulating effect of these substances on the production of natural live fish food. Such effects relies on the improved operation of several interlinked food chains based on highly complex chemical and

biological interactions taking place simultaneously in the bottom soil and the overlying water of the pond. This review is unique in bringing together and synthesizing the available knowledge on all factors involved and their multiple interactions. The importance of the mud surface as the "chemical laboratory" of the pond is duly stressed. Water quality, nutrient cycling and natural productivity are fully discussed. Such a complete illustration of the complexity of pond fertilization pathways towards fish production should hopefully promote further research on these particular subjects, in Africa as well as in other tropical regions.

The application of fertilizers to tropical waters, although showing great promise, is still in its infancy. For example no standard procedure has yet been devised for the on-farm determination of fertilizer requirements. Therefore the process is still largely empirical and depends on the fish farmer's judgement. In Far Eastern Asia, the latter is often based on the evolution, by trial and error over several centuries, of productive farming methods well suited to local conditions. In Mrica, however, where the people have no background of practical experience and where both the farming conditions and the fish are different, such judgement can only be based on locally-acquired knowledge. Undoubtedly, this is one of the main reasons, which has prompted the author to prepare this multidisciplinary review. It should be a standard reference text to enable African scientists to become increasingly interested, it is hoped, in removing the numerous questions marks still attached to pond fertilization.

Dr. Andre G. Coche

xvii

Former Senior Fishery Resources Officer (Aquaculture) Food and Aquaculture Organization of the U.N. Rome, Italy

Acknowledgements

Words cannot express my profound gratitude to Prof. Dr. H. J. Dumont for the many ways in which he assisted me in bringing this manuscript to the press. I am grateful to him for his critical reading and commenting of the manuscript, and for the many suggestions for improvements, which helped me to shape the manuscript into its final form. I also thank him for his constant availability, whenever his assistance was required, and his patience with me.

My thanks go to Dr. A. G. Coche, former Senior Fishery Resource Officer (Aquaculture), F AO, for writing the foreword and for his useful comments and advice on the manuscript.

I also wish to gratefully acknowledge the material provided by Prof. Dr. Tilzer on primary productivity.

This handbook could only be completed thanks to the logistical assistance of a number of people, especially Mr. D. Claeys, my brother, Jean-Marie, Mr. K. Roche, Mr. E. Bruyneel, Dr. F. Fiers, Ms.

N. Moortgat. Mr. R. S. Coppola, from the Fishery Information, Data and Statistics Service, F AO, computed the statistical analysis of the pond production data. I gratefully acknowledge his help.

Most of the information was gathered at various public libraries in Brussels, in particular the Library of the Royal Institute for Natural Sciences, the Library of the Institute for Hygiene and Epidemiology, the Centre for Research and Information on Tropics (CERDAT), and the library of the Laboratory for Ecology, University of Ghent. I wish to thank them all for making publications available to me.

I also thank Kluwer Academic Publishers for agreeing to publish this volume in the Developments in Hydrobiology series.

Lastly I wish to thank my wife, Martine, for her cheerful support throughout the work.

This volume is dedicated to my parents.

xviii