Master’s thesis Ecological impact of the invasive European flounder (Platichthys flesus) on the native European plaice (Pleuronectes platessa) on nursery grounds in Iceland Theresa Henke Advisor: Guðbjörg Ásta Ólafsdóttir, Ph.D. University of Akureyri Faculty of Business and Science University Centre of the Westfjords Master of Resource Management: Coastal and Marine Management Ísafjörður, May 2018 Supervisory Committee Advisor: Guðbjörg Ásta Ólafsdóttir, Ph.D. Reader: Camille Leblanc, Ph.D. Program Director: Catherine Chambers, Ph.D. Theresa Henke Ecological impact of the invasive European flounder (Platichthys flesus) on the native European plaice (Pleuronectes platessa) on nursery grounds in Iceland 45 ECTS thesis submitted in partial fulfilment of a Master of Resource Management degree in Coastal and Marine Management at the University Centre of the Westfjords, Suðurgata 12, 400 Ísafjörður, Iceland Degree accredited by the University of Akureyri, Faculty of Business and Science, Borgir, 600 Akureyri, Iceland Copyright © 2018 Theresa Henke All rights reserved Printing: Háskólaprent, Reykjavík, May 2018 ii Declaration I hereby confirm that I am the sole author of this thesis and it is a product of my own academic research. __________________________________________ Theresa Henke iii iv Abstract In recent years the number of invasive species that have been introduced to new environments has strongly increased, and their further distribution may be favored by both anthropogenic influence and climate change. In 1999, the European flounder (Platichthys flesus), a flatfish species native to central European coasts, was first identified in the southwest of Iceland at the mouth of the Ölfusa river and has since then spread around the whole country. This project investigated the ecological impact of juvenile flounder on juveniles of the native European plaice (Pleuronectes platessa) on nursery grounds. The work focused on two sampling sites representing early and more recent flounder establisment; the fjords Borgarfjörður and Önundarfjörður, respectively. Fish were caught with a beach seine between July and September 2017. Species composition and juvenile size were measured, and stomach content analysis was used to determine niche widhts of each species and overlap in feeding patterns between species. Flounder was present in all collected samples, but both species composition and length distribution of flounder and plaice varied between sites and sampling time. Significant diet overlap between species was observed for some samples, particularly between 1+ flounder and 0+ plaice, which indicates potential for competition between species. Together with observed direct predation of 1+ flounder on 0+ plaice, the presence of the invasive species is likely to have a negative impact on the native plaice population. Differences between the sites of early and more recent invasion could not clearly be linked to the time since invasion. This project highlights that the management of invasive species needs to be improved, especially in locating and restricting pathways of introduction as well as in early detection and rapid response strategies. Útdráttur Á síðustu árum hefur flutningu tegunda út fyrir náttúleg heimkynni aukist mjög. Margvíslegt rask á búsvæðum svo og loftslagsbreytingar auka líkur á útbreiðslu þessarra framandi tegunda í nýjum heimkynnum. Árið 1999, fannst flundra (Platichthys flesus), flatfiskur með náttúrulega dreifingu við strendur mið Evrópu, við Ölfusárósa. Flundran hefur síðan dreifst víða um land. Í þessu verkefni eru rannsökuð möguleg áhrif flundruseiða á seiði skarkola (Pleuronectes platessa) á uppeldisstöðvum. Sýnataka fór að mestu fram á tveimur svæðum sem endurspegla eldra og nýlegra landnám flundrunar; í Borgarfirði og í Önundarfirði. Seiði voru veidd með landnót frá júlí og fram í september 2017. Tegundasamsetning á hverjum stað var metin, lengd seiðanna mæld og þau vigtuð og magainnihald greint til að ákvarða vistbreidd tegundanna og skörun í fæðuvist. Flundra veiddist við alla sýnatöku en bæði tegundasamsetning og stærð seiða beggja tegunda var breytileg eftir svæðum og tíma. Marktæk skörun í fæðuvist var á milli flundru og skarkola seiða, mest áberandi á milli 1+ flundru og 0+ skarkola. Þá kom í ljós að 1+ flundruseiði eru afræningjar á 0+ skarkolaseiði. Það er því talið líklegt að tilkoma flundru hafi neikvæð vistfræðileg áhrif á skarkola á uppeldisstöðvum. Það komu hinsvegar ekki fram sterkar vísbendingar um mun á milli svæða eftir tíma frá landnámi flundrunnar. Niðurstöður þessa verkefnis sýna að það er mikilvægt að og hefja virka stýringu ágengra tegunda við Ísland m.a. með því að greina helstu flutningsleiðir, bæta snemmtæka greiningu framandi tegunda og þróa viðbragðsáætlun. v vi Table of Contents Abstract .............................................................................................................................. v Table of contents ................................................................................................................ vii List of figures ..................................................................................................................... x List of tables ....................................................................................................................... xi Acknowledgments .............................................................................................................. xii 1. Introduction ................................................................................................................... 1 1.1 Goal of this study ........................................................................................... 3 2. Theoretical background ............................................................................................... 5 2.1 Invasive species ............................................................................................. 5 2.1.1 Definition and trends around the world ........................................ 5 2.1.2 The pathways of introduction of invasive species ......................... 6 2.1.3 The influence of Climate Change on invasive species .................. 7 2.1.4 Impacts of invasive species.............................................................. 8 2.1.5 Invasive species in Iceland .............................................................. 12 2.1.6 Management of invasive species ..................................................... 13 2.2 Pleuronectiformes ......................................................................................... 15 2.3 European flounder (Platichthys flesus) ....................................................... 16 2.3.1 Distribution of flounder .................................................................. 16 2.3.2 Life-history traits of flounder ......................................................... 16 2.3.3 Invasion of flounder in Iceland ...................................................... 19 2.3.4 Economic value of flounder ............................................................ 20 2.4 European plaice (Pleuronectes platessa) ..................................................... 21 2.4.1 Distribution of plaice ....................................................................... 21 2.4.2 Life-history traits of plaice.............................................................. 21 vii 2.4.3 Economic value of plaice ................................................................. 24 3. Methods .......................................................................................................................... 25 3.1 The study system .......................................................................................... 25 3.2 Study sites...................................................................................................... 26 3.3 Sampling ........................................................................................................ 28 3.4 Laboratory analysis ...................................................................................... 28 3.5 Stomach content analysis ............................................................................. 29 3.6 Age determination ........................................................................................ 29 3.7 Statistical analysis ........................................................................................ 30 3.7.1 Shannon-Wiener Index to determine niche width ...................... 30 3.7.2 Simplified Morisits Index to determine niche overlap ................ 30 3.7.3 Principal Component Analysis (PCA) .......................................... 31 3.7.4 Generalized Linear Mixed Model (GLMM) ................................ 31 4. Results ............................................................................................................................ 33 4.1 Species identification ................................................................................... 33 4.2 Length distribution ...................................................................................... 34 4.3 Determination of age classes for both species ............................................ 35 4.4 Sympatric occurrence of plaice and flounder ............................................ 36 4.5 Stomach content analysis ............................................................................. 38 4.5.1 Diet items ....................................................................................... 38 4.5.2 Principal