Resident cod population benefits from the seasonal feast on fat herring

Monday 15 Aug 16
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by Line Reeh

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Mikael van Deurs
Associate Professor
DTU Aqua
+4535 88 34 28

Study finds substantial cross-system lipid transportation in the transition zone between the Baltic Sea and the North Sea (Øresund) where a resident cod population interact with a migrating herring population on a seasonal basis.

A new study led by Researcher Mikael Van Deurs, DTU Aqua, describes how immense quantities of lipids arrive in Øresund every fall, a short distance from Copenhagen. The lipid are conveyed by more than one hundred thousand tons of fat herring that return from productive feeding grounds to the North, and presents the local cod population with an opportunity to dwell into a veritable feast. However, since these migratory herring are relatively large, only relatively large cod have access to the lipid feast.

"Sometimes we don´t have to look in the distance to discover ecological spectacles worth studying, communicating, and preserving," says lead author Mikael van Deurs, DTU Aqua with a reference to the location of the strait of Øresund right next to the capital of Denmark, Copenhagen..

It has previously been suggested that prey subsidies, by movement of either prey or predators, often enhance predator production beyond what local resources can support. The present study supports this notion, and thereby highlights the importance of addressing migrant-resident interaction in ecosystem models, conservation initiatives, infrastructure planning, and fisheries management.

From feeding ground to cod liver

The study trace the lipids, using fatty acid trophic markers (FATM), from the herring feeding grounds in the North Sea to the cod livers in the Western Baltic Sea. Time series analysis of population dynamics indicate that population-level production of cod is positively affected by the herring subsidies. However, the underlying mechanisms were more complicated than anticipated.

During the herring season, large cod received most of its dietary lipids from the herring, whereas smaller cod were prevented from accessing the lipid pool due to a mismatch in predator–prey size ratio. Furthermore, while the herring were extremely rich in bulk energy, they were surprisingly poor in a specific functional fatty acid. Hence, the study was the first to illustrate how the magnitude cross-system fluxes of subsidies in migrant–resident systems are potentially constrained by the size structure of the resident predator population and the nutritional quality of the migrants

The research was funded by Danish council of independent research DFF-4002-00114. The study Marine ecosystem connectivity mediated by migrant–resident interactions and the concomitant cross-system flux of lipids was published in Ecology and Evolution.

Read the paper:

Mikael van Deurs, Anders Persson, Martin Lindegren, Charlotte Jacobsen, Stefan Neuenfeldt, Christian Jørgensen, P. Anders Nilsson: Marine ecosystem connectivity mediated by migrant–resident interactions and the concomitant cross-system flux of lipids
Ecology and Evolution, Volume 6, Issue 12, June 2016, Pages 4076–4087
10.1002/ece3.2167