Diss. ETH No. 13706 Population dynamics of whitefish ( Coregonus suidteri Fatio) in artificially oxygenated Lake Hallwil, with special emphasis on larval mortality and sustainable management Dissertation submitted to the SWISS FEDERAL INSTITUTE OF TECHNOLOGY, ZURICH for the degree of Doctor of Natural Sciences presented by Carole Andrea Enz Dipl. Natw. ETH Zurich bon1 August 3, 1972 Citizen of Sch(1nholzerswilen (TG), Switzerland accepted on the recommendation of Prof. Dr. J. V. Ward, examiner Prof. Dr. H. Lehtonen, co-examiner Dr. R. Muller, co-examiner Kastanienbaum, 2000 Meinen Eltern und Max Copyright ~;i 2000 by Carole A. Enz, EA WAG Kastanienbaurn All rights reserved. No part of this book rnay be reproduced, stored in a retrieval systen1 or transmitted, in any fonn or by any ineans, electronic, rnechanical, pho- tocopying, recording or otherwise, without the prior written permission of the copyright holder. First Edition 2000 PUBLICATIONS CHAPTER 3 OF THE THESIS HAS BEEN ACCEPTED FOR PUBLICATION: ENZ, C. A., SCHAFFER E. & MOLLER R. Growth and survival of Lake Hallwil whitefish (Co reg onus suidteri) larvae reared on dry and live food. - Archiv fUr Hyclrobiologie. CHAPTERS 4, 5, 6 Ai~D 7 OF THESIS HA VE BEEN SUBMITTED FOR PUBLICATION: ENZ, C. A., MBWENEMO BIA, M. & MULLER. R. Fish species diversity of Lake Hallwil (Switzerland) in the course of eutrophication, with special reference to whitefish ( Coregonus suidteri). Submitted to Conservation Biology. ENZ, C. A .. SCHAFFER, E. & MULLER, R. Importance of prey movement, food particle and tank circulation for rearing Lake Ha11wil whitefish (Coregonus suidteri) larvae. Submitted to North Alnerican Journal of Aquaculture. ENZ, C. A., BtTRGl, H. R., STOSSEL, F. & M(LLER, R. Food preference of whitefish (Coregonus suidteri) in eutrophic Lake Hallwil (Switzerland), and the question of cannibalism. Submitted to Freshwater Biology. ENZ, C.A., MOLLER, R., MBWENEMO BIA, M. & HEBB, J. A population dynamics 1nodel for evaluating mortality factors in Lake Hallwil whitefish (Coregonus suidteri) larvae. Submitted to Arch. Hydrobiol. Spec. Issues Advanc. Lirnnol. ENZ, C. A., BORG!, H. R., MBWENEMO BIA, M. & MOLLER, R. Factors controlling growth and maturity of whitefish (Coregonus suidteri) in eutrophic Lake Hallwil (Switzerland). Submitted to Journal of Fish Biology. CONTENTS Publications v Contents vii Summary xi Zusammenfassung xiv 1 Introduction 1 ,.., 1.1 Description of Lake Hallwil ;_, 1.2 Population dynamics of Lake Hall wil whitefish 7 1.3 Objectives 15 1.4 References 17 2 Fish species diversity 35 2.1 Abstract 35 2.2 Introduction 35 2.3 Methods 38 2.4 Results 39 2.5 Discussion 42 2.6 Acknowledgements 49 2.7 References 49 3 Growth and survival of whitefish larvae reared on dry and live food. 53 3.1 Abstract 53 3.2 Introduction 54 3.3 Materials & Methods 55 3.4 Results 59 3.5 Discussion 62 Vll 3.6 Acknowledgements 69 3.7 References 70 4 Importance of prey nioven1ent, food particle size and tank circulation 79 4.1 Abstract 79 4 •"-'') Introduction 80 4.3 Methods 81 4.4 Results 83 4.5 Discussion 87 4.6 Acknowledge1nents 89 4.7 References 90 5 Food preference of adult whitefish, and the question of cannibalism 95 5.1 Abstract 95 5.2 Introduction 96 5.3 Methods 97 5.4 Results 99 5.5 Discussion 105 5.6 Acknowledgements 109 5.7 References 109 6 A model for evaluating mortality factors in whitefish larvae 117 6.1 Abstract 117 6.2 Introduction 117 6.3 Methods 119 6.4 Results 126 6.5 Discussion 131 6.6 Acknowledgements 135 6.7 References 135 Vlll 7 Factors controlling growth and maturity 141 7.1 Abstract 141 7.2 Introduction 141 7.3 Methods 144 7.4 Results 146 7.5 Discussion 150 7.6 Acknowledgements 156 7.7 References 157 8 Synopsis 163 8.l Introduction 163 8.2 Population dynan1ics of L. Hallwil whitefish 163 8.3 Implications for artificial reproduction 165 8.4 Implications for stocking practice 166 8.5 Implications for the fisheries 167 8.6 Effects of the restoration measures 168 8.7 References 169 Dank 173 Curriculum vitae 177 lX x SUMMARY This work basically describes the fish fauna and the dynamics of a whitefish pop- ulation in a typical eutrophic lake. Because whitefish are cmnmercially important, and because their natural reproduction in eutrophic lakes is ineffective, the life history, management and conservation of whitefish are investigated, taking the example of eutrophic Lake Hallwil in the Swiss Midland. The structure of today's fish assemblages in Swiss lakes is the result, first of all, of colonization after the last Ice Age some 15,000 years ago. After the retreat of the glaciers, fish immigrated fron1 the previously ice-free regions into the newly- formed lakes. Lake Hallwil is such a post-glacial lake. Today, it is inhabited by twenty fish species. In 196L a rapid decrease in oxygen concentration due to cul- tural eutrophication led to a devastating fish kill. In the following years, most of the fish populations successfully recovered on their own (chapter 2). However, in cu trophic lakes. sahnonid fish like whitefish are greatly endangered due to unsuc- cessful natural propagation and the threat of fish kills. In Lake Hallwil, the so called Lake Hallwil "Ballen" (Coregonus suidteri, Fatio), a :fast-growing autoch- thonous coregonid, was commercially important already during the Middle Ages. In 1898, the first algal bloom heralded the beginning of eutrophication. Stocking whitefish larvae to sustain the decreasing whitefish population was conducted already at the beginning of the 20th Century. Nevertheless, whitefish yield contin- ued to decrease due to the negative effects of progressing eutrophication. Follow- ing the fish kill of 1961, Lake Hallwil whitefish probably disappeared from the lake. Thanks to stocking Lake Sen1pach "Ballen", a conspecific whitefish form in a near-by lake, a whitefish population was successfully restored in Lake HallwiL Since 1970, an increasing munber of whitefish larvae have been pre-fed for at least six weeks in hatcheries prior to stocking in the lake. These pre-fed larvae are expected to survive ten times better in the lake than newly hatched larvae. Larvae are usually reared on live zooplankton or on dry diets. In order to further improve the rearing of whitefish larvae, smne feeding experhnents were carried out (chap- ters 3 and 4 ). Thereby it was found that live zooplankton is still the best diet to rear xi whitefish larvae. However, low zooplankton density in the lake in spring often leads to difficulties in supplying sufficient numbers of zooplankton in the hatch- eries. Nauplii of Artemia salina, various dry diets (chapter 3) or frozen zooplank- ton (chapter 4) may serve as alternative food. The results of the experiments showed that larvae reared on dry diet need an adaptation phase to switch from dry food to live zooplankton (chapter 3). As an important detail when setting up rear- ing tanks, the water inflow should be placed below the water surface of the tank in order to increase floating time and thus availability of the dry diet (chapter 4 ). Thanks to stocking pre-fed larvae, whitefish has been the dmninant fish in Lake Hallwil since 1977. Nevertheless, annual yield fluctuates strongly. This is a result of variation in year class strength caused by a number of mortality factors acting primarily during the early life stage of whitefish. Among the inortality factors investigated, intraspecific food cmnpetition due to high stock size and cannibal- ism were found to be unimportant. Larvae and adults showed different food pref- erences, leading to niche segregation during the most critical time in spring (chap- ter 5). However, strong evidence for early life mortality due to eutrophication- related processes was found using computer modelling: Sunny weather in May, together with high nutrient concentration, leads to strongly increased algal bimn- ass and photosynthesis. This may result in extreme oxygen supersaturation which causes lethal gas bubble syndrome in the whitefish larvae (chapter 6). The large whitefish stock in Lake Hallwil is certainly of great con11nercial interest. It also has to be seen as the positive result of fish species conservation. Growth retarda- tion, however, occurred in parallel with increasing whitefish density (chapter 7). Trophic state in tenns of total phosphorus concentration of Lake Hallwil decreased since 1977. External and internal restoration measures (chapter 1), in particular sewage diversion and artificial oxygenation since 1986, have had a pos- itive effect on the lake ecosystem. Hypolinmetic oxygenation has enlarged the living space of fish and benthic organisms (chapters 5 and 8). Within the context of these water protection ineasures, this thesis provides new knowledge on the biology and ecology of Lake Hallwil whitefish in a eutrophic lake undergoing res- toration. The results presented herein may set the basis for better conservation and rnanagen1ent through improved rearing, stocking and protecting this valuable fish (chapter 8). The new findings may also help to assess, on a more general basis, the Xll usefulness of external and internal lake restoration measures for conservation and management of lake whitefish. Xlll ZUSAMMENFASSUNG Die vorliegende Arbeit befasst sich im wesentlichen nut den Then1enbereich der Fischfauna und der Populationsdynamik von Coregonen in einc1n cutrophen See. Weil Felchen fischereiwirtschaftlich von Bedeutung sind und weil die natiirliche Fortpflanzung dieser Fische in eutrophen Seen nicht mehT funktioniert, standen in dieser Arbeit Lebenszyklus, Fischerei1nanagement und Arterhaltung der Felchen im V ordergrund. Das Untersuchungsgewasser, der Hallwilersee, ist ein Beispiel ftir einen eutrophen Coregonensee im Schweizer Mittelland. Die heutige Zusammensetzung der Fischgesellschaften in Schweizer Seen ist primar das Ergebnis der Neubesiedlung nacheiszeitlicher Gewasser vor ungefahr 15'000 Jahren. Nach dem Rilckzug der Gletscher wanderten Fische aus eisfreien Gebieten in die neuentstandenen Seen ein. Der Hall wilersee ist ein solcher nach- eiszeitlicher See.