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Peter Grønkjær

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P. Grønkjær
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Marine ecosystems experience rapid change driven by natural processes and human activities. Some of the changes in marine fish populations are easily observable such as variability in population sizes and distribution, but the underlying processes are difficult to study in wild, free-living marine fishes. My research aim to unravel the processes that lead to change in fish populations and to use this knowledge to improve predictions of future states of the fish populations.

My group study recruitment processes to determine what exogenous and intrinsic factors influence the survival of the early life stages of fish. We have studied how food abundance and temperature interact to shape growth trajectories, and how starvation resistance is related to metabolic rate of individual fish larvae. Recently, we have developed a method that allow us to examine a single fish egg or larva and from its stable isotope signature determine the size of its mother. This will allow us to explore the importance of mother size on offspring survival in the wild.

Another line of my research explores the drivers and consequences of changes in marine food webs. We have developed a technique that allow us to explore historic and contemporary food-webs using stable carbon and nitrogen isotopes from otolith (earstone) protein. Otoliths have been routinely collected during the last 120 years and enable us to map out trophic positions of fish with very high temporal and spatial resolution. This area is now a rapidly growing field in otolith and fish ecology research, and my group is still leading the field with a wide range of international collaborators.

Our newest contribution to otolith and fish research builds on 50-year-old observations, but has the potential to revolutionize future studies of wild fish physiology. In the 1970’ies it was hypothesized that observed variation in otolith aragonite carbon isotope values were due to variation in the amount of carbon from these two sources, and that the proportion of diet carbon was determined by the metabolic rate of the fish. Recent focus on warming oceans and the potential detrimental impact on fish physiological performance and the productivity of fish populations called for a way to measure fish performance in the wild. Inspired by our success with the otolith protein we again setup experiments to determine the relationship between metabolic rate and carbon isotope values of the otolith aragonite. In 2019, we have established this relationship and show that we can accurately determine the metabolic rate of individual Atlantic cod with a temporal resolution of ca 14 days. In principle, this means that it is possible to follow variations in metabolic rates over the entire life of an individual fish, documenting ontogenetic, life history and seasonal effects on fish metabolic performance.

 

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Grønkjær FishEco Lab

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