Governing factors for the size-at-age: the influence of sex, food availability, temperature and fishery on the growth performance of North Sea whiting Merlangius merlangus and cod Gadus morhua Dissertation zur Erlangung des Doktorgrades an der Fakultät für Mathematik, Informatik und Naturwissenschaften Fachbereich Biologie der Universität Hamburg KORRIGIERTE FASSUNG vorgelegt von Rebecca Anna Maria Lauerburg aus Trier Hamburg, 2016 Gutachter der Dissertation: Prof. Dr. A. Temming Jun. Prof. Dr. Marc Hufnagl Tag der Disputation: 29.04.2016 Mitglieder der Prüfungskomission: Prof. Dr. K. Dausmann - Vorsitz Prof. Dr. C. Möllmann Prof. Dr. A. Temming Jun. Prof. Dr. Marc Hufnagl CONTENTS SUMMARY ............................................................................................................................................ 1 ZUSAMMENFASSUNG ....................................................................................................................... 5 GENERAL INTRODUCTION ............................................................................................................ 9 WHITING AND COD IN A GLOBAL CONTEXT .................................................................... 9 THE NORTH SEA ECOSYSTEM ............................................................................................. 10 WHITING AND COD IN THE NORTH SEA ECOSYSTEM .................................................. 13 PHYSIOLOGICAL ECOLOGY AND FACTORS INFLUENCING THE SIZE-AT-AGE ...... 15 GENERAL HYPOTHESES—SCOPE OF THE THESIS .......................................................... 18 REFERENCES ............................................................................................................................ 19 Chapter I .............................................................................................................................................. 25 Sex-specific food intake in whiting Merlangius merlangus .............................................................. 25 ABSTRACT ..................................................................................................................................... 25 INTRODUCTION ............................................................................................................................ 26 MATERIALS AND METHODS ..................................................................................................... 29 DATA SOURCES AND SAMPLING ........................................................................................ 29 DATA ANALYSIS ..................................................................................................................... 30 RESULTS ......................................................................................................................................... 34 GROWTH DIFFERENCES ........................................................................................................ 34 DIFFERENCES IN THE EMPTY STOMACH MASS ............................................................. 36 DIFFERENCES IN THE BODY MASS-LENGTH RELATIONSHIPS ................................... 37 DIFFERENCES IN THE LIVER MASS .................................................................................... 38 DIFFERENCES IN THE DIET COMPOSITION ...................................................................... 39 DIFFERENCES IN THE QUANTITY OF FOOD ..................................................................... 40 DISCUSSION .................................................................................................................................. 43 GROWTH DIFFERENCES ........................................................................................................ 43 DIFFERENCES IN METABOLIC COSTS ............................................................................... 43 DIFFERENCES IN ENERGY ALLOCATION PATTERNS .................................................... 44 i CONTENTS DIFFERENCES IN ENERGY UPTAKE ................................................................................... 45 DIFFERENCES BETWEEN SAMPLES FROM 2007 AND 2012 ........................................... 46 REFERENCES ................................................................................................................................. 48 Chapter II ............................................................................................................................................. 53 Forage fish controlling population dynamics of North Sea whiting—evidence from field data and bioenergetics modelling ....................................................................................................................... 53 ABSTRACT ..................................................................................................................................... 53 INTRODUCTION ............................................................................................................................ 54 MATERIALS AND METHODS ..................................................................................................... 56 DATA SOURCES AND SAMPLING ........................................................................................ 56 LENGTH-AT-AGE OF WHITING ............................................................................................ 57 PREY ABUNDANCE ................................................................................................................ 57 STOMACH CONTENTS & INDIVIDUAL FISH DATA ......................................................... 57 CONDITION OF WHITING ...................................................................................................... 58 THE INFLUENCE OF THE FOOD CONSUMPTION ON THE GROWTH OF WHITING ... 59 RELATIONSHIP BETWEEN FORAGE FISH ABUNDANCE AND LENGTH-AT-AGE OF WHITING ................................................................................................................................... 60 RESULTS ......................................................................................................................................... 61 LENGTH-AT-AGE..................................................................................................................... 61 PREY ABUNDANCE ................................................................................................................ 65 STOMACH CONTENT MASSES ............................................................................................. 66 CONDITION OF WHITING ...................................................................................................... 69 ESTIMATED CONSUMPTION AND CORRESPONDING STOMACH CONTENTS .......... 71 RELATIONSHIP BETWEEN FORAGE FISH ABUNDANCE AND LENGTH-AT-AGE OF WHITING ................................................................................................................................... 73 DISCUSSION .................................................................................................................................. 74 LENGTH-AT-AGE OF ADULT WHITING ............................................................................. 74 THE INFLUENCE OF SIZE-SELECTIVE MORTALITY ....................................................... 74 THE INFLUENCE OF TEMPERATURE .................................................................................. 74 WHITING LENGTH-AT-AGE 1 YEAR ................................................................................... 75 PREY ABUNDANCE IN THE NORTH SEA ........................................................................... 76 STOMACH CONTENTS OF WHITING ................................................................................... 77 CONDITION OF WHITING ...................................................................................................... 77 ESTIMATED CONSUMPTION AND CORRESPONDING FIELD DATA ............................ 77 RELATIONSHIP BETWEEN FORAGE FISH ABUNDANCE AND LENGTH-AT-AGE OF WHITING ................................................................................................................................... 78 REFERENCES ................................................................................................................................. 80 Chapter III ........................................................................................................................................... 85 Influence of fishing mortality, temperature and prey abundance on the length-at-age and individual growth of North Sea cod Gadus morhua ........................................................................ 85 ABSTRACT ..................................................................................................................................... 85 INTRODUCTION ............................................................................................................................ 86 ii CONTENTS MATERIALS AND METHODS ..................................................................................................... 88 DATA SOURCES AND SAMPLING ........................................................................................ 88 MEAN LENGTH-AT-AGE ........................................................................................................ 89 SPATIALLY RESOLVED MEAN LENGTH-AT-AGE AND ESTIMATION OF GROWTH PARAMETERS .......................................................................................................................... 90 SPATIALLY RESOLVED