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Reproductive Ecology of Baltic Sprat and Its Application in Stock Assessment

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Reproductive Ecology of Baltic Sprat and Its Application in Stock Assessment Reproductive ecology of Baltic sprat and its application in stock assessment Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Holger Haslob Kiel, Februar 2011 Referent: Prof. Dietrich Schnack Ko-Referent: Prof. Myron A. Peck Datum der mündlichen Prüfung: 24.03.2011 Zum Druck genehmigt: 24.03.2011 Table of contents _____________________________________________________________________________ Zusammenfassung ..................................................................................................... 1 Summary ................................................................................................................... 2 Introduction ............................................................................................................... 3 Baltic Sprat .................................................................................................................................. 3 The Assessment of the Baltic Sprat Stock ................................................................................... 6 Spawning traits of Baltic Sprat .................................................................................................... 6 The Daily Egg Production Method .............................................................................................. 7 Aim of the present study .............................................................................................................. 8 Chapter I: Spatial and interannual variability in Baltic sprat batch fecundity ........ 11 Chapter II: Seasonal variability of sprat spawning traits ........................................ 25 Chapter III: Observations on sex ratio and maturity ogives of Baltic sprat ........... 45 Chapter IV: The dynamics of postovulatory follicle degeneration and oocyte growth in Baltic sprat .............................................................................................. 63 Chapter V: Temperature-dependent egg development of Baltic sprat .................... 75 Chapter VI: Application of the Daily Egg Production Method to Baltic Sprat ...... 81 Conclusion and Outlook .......................................................................................... 99 A stage-based matrix model to resolve critical life stages of Baltic sprat population in relation to temperature .......................................................................................................................... 101 References ............................................................................................................. 105 Annex I: Maturity Keys ......................................................................................... 115 Annex II: Histological features of sprat ovaries ................................................... 119 Annex III: Histology protocols ............................................................................. 126 Contributions of authors ........................................................................................ 128 Acknowledgements ............................................................................................... 129 Curriculum Vitae ................................................................................................... 130 Eidesstattliche Erklärung ....................................................................................... 133 i ii Zusammenfassung _____________________________________________________________________________ Zusammenfassung Die Europäische Sprotte (Sprattus sprattus L.) ist ein kleiner pelagischer Schwarmfisch, der im Schelfgebiet des Nordostatlantiks weit verbreitet ist. Eine besondere Stellung nimmt diese Art im pelagischen Ökosystem der Ostsee ein, da sie einerseits dem Ostseedorsch als Hauptnahrung dient und andererseits als abundanteste planktivore Fischart in diesem System einen erheblichen Prädationsdruck auf Zooplankton und Ichthyoplankton ausüben kann. In der vorliegenden Arbeit wurden einige wichtige Reproduktionsmerkmale der Sprotte untersucht, die nicht nur für die Bestandskunde, sondern auch für zukünftige Studien zur Populationsdynamik und Ökologie dieser Art wichtig sind. Bei der Sprotte handelt es sich um einen Portionslaicher, der mehrere Eiportionen über eine längere Laichsaison abgibt. Dabei ist der Gesamtumfang der saisonalen Eiproduktion nicht vor der Laichzeit determiniert. Aus einem Reservepool werden ständig neue Oozyten rekrutiert, die heranreifen und schließlich abgelaicht werden. Aus dieser Eigenschaft heraus ergeben sich einige Besonderheiten in Bezug auf Untersuchungen zur Reproduktionsbiologie dieser Art. In der vorliegenden Arbeit konnte sowohl die beobachtete jährliche als auch die saisonale Variabilität in den untersuchten Reproduktionsmerkmalen mit der vorherrschenden Hydrographie und der Größe des Laicherbestandes in Zusammenhang gebracht werden. Die Fruchtbarkeit, d.h. die Anzahl abgegebener Eier pro Laichportion, zeigte sowohl Schwankungen zwischen den untersuchten Jahren als auch zwischen verschiedenen Gebieten in der Ostsee, und ein großer Teil dieser Variabilität konnte mit der vorangegangenen Wintertemperatur und der Gesamtgröße des Laicherbestandes erklärt werden. Das Geschlechterverhältnis und der Anteil an reifen Individuen im Gesamtbestand wurden für verschiedene Gebiete in der südlichen und zentralen Ostsee anhand von logistischen Modellen berechnet. Der Anteil an kleinen Sprotten, die bereits zum Laichgeschehen in einer Laichsaison beitragen, zeigte ebenfalls einen Zusammenhang mit der Wintertemperatur und der Bestandsgröße. Saisonale Schwankungen in der Fruchtbarkeit, dem Anteil laichender Weibchen am Laicherbestand, der Follikelatresie im Ovar sowie dem Eidurchmesser und Eitrockengewicht wurden untersucht. Eine weitere wichtige Beobachtung konnte im Zusammenhang mit der Ovarentwicklung und der Follikelzersetzung nach der Ovulation (postovulatory follicles, POFs) gemacht werden. Zum ersten Mal werden in der vorliegenden Studie histologische Details von POFs im Ovar der Sprotte dargestellt. Dabei konnte gezeigt werden, dass die komplette Zersetzung dieser Strukturen nach der Ovulation etwa solange dauert wie das Intervall zwischen zwei Laichzeitpunkten. Dies sind wichtige Ergebnisse für zukünftige Studien, um die Laichfrequenz der Sprotte abschätzen zu können. Desweiteren wurde anhand experimenteller Daten ein Modell zur Sprotteientwicklung in Abhängigkeit der umgebenden Wassertemperatur entwickelt. Alle Ergebnisse wurden abschließend dazu benutzt, um die Sprottbestandsgröße im Bornholm Becken mittels einer Eiproduktionsmethode, der „Daily Egg Production Method (DEPM)“, abzuschätzen. Die Ergebnisse zeigen, dass dies ein vielversprechender Ansatz zur Bestandsabschätzung dieser Art ist. Die Vorteile dieser Methode liegen in der Unabhängigkeit von Fischereidaten und darin, dass alle wichtigen Eingangsdaten direkt aus Feldbeobachtungen gewonnen werden können. Außerdem bezieht diese Methode die natürliche Variabilität der Reproduktionsmerkmale mit ein, was im Standardassessment nicht der Fall ist. Allerdings wurden in Bezug auf die Laichfrequenz und die tägliche Eiproduktion, zwei essentielle Eingangsparameter der DEPM, verbliebene Unsicherheiten aufgezeigt, die in zukünftigen Studien beseitigt werden sollten. Die in der vorliegenden Arbeit gesammelten Erkenntnisse und Daten können eine Grundlage für weitere Studien zur Verbesserung der Bestandsabschätzung sein und dazu dienen, alternative Indizes für die Bewertung des Reproduktionspotentials des Sprottbestandes in der Ostsee zu entwickeln (z.B. Laicherbestand der Weibchen, oder gesamte Eiproduktion des Bestandes einer Laichsaison), die DEPM weiterzuentwickeln und zu implementieren, und die Populationsdynamik und Ökologie dieser Art weiter zu erforschen. 1 Summary _____________________________________________________________________________ Summary The European sprat (Sprattus sprattus L.) is a small planktivorous marine pelagic clupeoid species with a wide distribution in shelf areas of the Northeast Atlantic. Sprat is an ecological key species in the Baltic pelagic ecosystem. On the one hand, sprat serves as main prey for the Baltic cod stock and on the other hand, it is the most abundant planktivorous fish species in the Baltic Sea. Thus, sprat has the potential to exert predation pressure on both ichthyoplankton and zooplankton. Sprat is an indeterminate batch spawner, releasing several egg batches over a protracted spawning season. Oocytes recruit from a reserve pool throughout the spawning season. Due to this feature, some peculiarities challenge the investigation of the reproductive biology of this species. In the present study, a number of reproductive traits of Baltic sprat were investigated, all of which are essential with respect to the assessment and further studies of the population dynamics of this species of the Baltic Sea. Interannual, seasonal and spatial variability in the investigated reproductive traits of Baltic sprat was revealed and could partly be attributed to hydrographic conditions and sprat stock size. Absolute and relative batch fecundity was found to differ among areas and between years in the southern-central Baltic. The seasonality of some important spawning traits, i.e. batch fecundity, spawning fraction, atresia, oocyte dry weight and oocyte diameter were analysed combining histology and modern image analysis methods. Models of sex ratio and maturity at length were established for
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