Jellyfish: flexible filter feeders to keep an eye on

Published: 3 February 2017

Jellyfish are an increasing problem for fishing industry, tourism and inlet stations. With rising sea temperatures, the numbers of some species are increasing and the times they amass at the coast are changing. And that has an impact on the food chain.

Jellyfish studies in North Sea and Wadden Sea

Comb Jellyfish

Expert in Ecosystems Victor Langenberg: ‘Until now, we have known little about jellyfish in the North Sea and the Wadden Sea. We were concerned about invasive species and their impact but we actually didn’t know exactly which species, or how many species, were present in our seas. So the Netherlands Institute for Marine Research (NIOZ), the University of Groningen and Deltares conducted a study. It led to unexpected discoveries.’

This study showed that the comb jellyfish (Mnemiopsis Leidyi) is also present in large numbers now. Lodewijk van Walraven completed his doctorate thesis on the consequences for the food chain. Victor Langenberg and Henk van der Veer (NIOZ) were his supervisors.

Predicting problems and changes

Jellyfish polyps

The researchers also found that jellyfish appear earlier in the year following warm winters. In addition, they looked for polyps (bottom-living stages of jellyfish) in many places in Dutch coastal waters: on the seabed, on wrecks and in ports. The polyps tend to settle on hard surfaces on the coast but it has now been found that they also do this a long way offshore.

The invasive comb jellyfish, for example, is now thriving and it is already commonplace in the Netherlands. Victor Langenberg: ‘This animal is  tough: it adapts well to the circumstances. It can not only survive, but also reproduce, in brackish waters. The jellyfish eat zooplankton, which fish also eat. The scale of this study was too small to arrive at conclusions about the consequences but if the Dutch coastal waters get warmer, the comb jellyfish could reproduce more and earlier, and eat up the zooplankton. That will affect food availability for other gelatinous species, and fish such as herring, sprat and anchovies. I would advise establishing a monitoring system here in the Netherlands for both zooplankton and jellyfish polyps so that we can predict problems and changes.’

Problem now on Europe’s radar

The first results of the study resulted directly in a number of European jellyfish studies. Deltares has been working with institutes in Belgium (ILVO), the United Kingdom (CEFAS) and France (IFREMER and ULCO-LOG). Computer simulations, using Delft3D-PART, the particle tracking module of the Delft3D 4 Suite, show that the Netherlands could be a source of invasive comb jellyfish for other European waters. These findings, and others, have been discussed with organisations including the ICES (International Council for the Exploration of the Sea) and the FAO-GFCM (General Fisheries Commission for the Mediterranean).

Modeling Jellyfish at Dutch coast

Dr. Lodewijk van Walraven is currently working at NIOZ