Doctoral defence in Biology - Gotje Katharina Gisela von Leesen

Live stream: https://livestream.com/hi/doktorsvorngotjekatharinagvonleesen

Ph.D. student: Gotje Katharina Gisela von Leesen     

Dissertation title: Temperature selectivity in Icelandic and Northeast-Arctic cod

Opponents: Dr. Karin Hüssy, Senior Researcher at the Technical University of Denmark
Dr. Arild Folkvord, Professor at the University of Bergen, Norway

Advisor: Dr. Steven E. Campana, Professor at the Faculty of Life and Environmental Sciences at the University of Iceland

Doctoral committee: 
Dr. Audrey J. Geffen, Professor at the University of Bergen, Norway
Dr. Steffen Mischke, Professor at the Faculty of Earth Sciences at the University of Iceland
Dr. Sigurður Sveinn Snorrason, Professor at the Faculty of Life and Environmental Sciences at the University of Iceland

Chair of Ceremony: Dr. Arnar Pálsson, Professor and the Vice Head of the Faculty of Life and Environmental Sciences at the University of Iceland

Abstract:
Increasing water temperatures are predicted worldwide, with high amplitudes in the Arctic and sub-Arctic regions exceeding predictions for other regions. An understanding how Atlantic cod (Gadus morhua) reacted to changing environmental conditions in the past is essential for predicting re-distribution under climate change. In this thesis, I examined the temperature selectivity of Icelandic and Northeast-Arctic (NEA) cod in response to fluctuating temperature conditions and changes in the stock dynamics. Multiple century-long data time series and linear mixed-effect models were used to investigate the effect of fluctuating water temperatures and changes in stock dynamics on the temperature selectivity of cod, using stable oxygen isotope composition in otoliths as a proxy of ambient temperature. Icelandic cod δ18Ootolith values were significantly correlated with water temperature time series, indicating that they were exposed to fluctuating water temperatures during the past 100 years and did not move appreciably in response to increasing ocean temperatures. Furthermore, abundance changes have affected the temperature selectivity of Icelandic and NEA cod as a density-driven response; however, the response of the two populations was different. Increasing abundance resulted in increasing intraspecific competition and decreasing individual fitness levels, which expanded the distribution area of both cod stocks into previously unfavorable thermal habitats. To validate the accuracy of high-resolution otolith isotopic records as a temperature proxy, stable oxygen isotope records of wild, free-swimming Icelandic cod tracked with data-storage tags (DST) were analyzed with high-resolution secondary-ion mass spectrometry (SIMS). Results demonstrated that the method is well-suited as a relative index of temperature exposure, although maximum ambient temperatures were often overestimated. Using century-long data time series and oxygen isotope chronologies of cod otoliths have markedly improved our understanding of the Icelandic and NEA cod temperature selectivity. Thus, this thesis provides important information that helps to predict how two commercially important cod stocks might respond to global warming and fluctuating stock dynamics.

About the doctoral candidate:
Gotje von Leesen was born in Germany and graduated with a Bachelor of Science in Biology from the University of Bremen in 2014. Afterwards, she did an International Master of Science in Marine Biodiversity and Conservation (EMBC+), jointly organized by the University of Ghent (Belgium). After stays in Bremen (Germany), Split (Croatia) and Ovideo (Spain), she graduated in 2016. Before she started her PhD at the University of Iceland in August 2017, she worked as a research fellow at the Alfred-Wegener-Institute in Bremerhaven (Germany) for several months. She has already started a Postdoc at Aarhus University (Denmark), where she continues her work with stable oxygen and carbon isotopes in cod otoliths.