The tiny jellyfish suspected of causing a big problem

The jellyfish, known as hydrozoans, are being closely monitored by salmon farmer Mowi Scotland to better understand how they affect gill health.

Lucy Fry, regional health manager at Mowi, said that daily monitoring of plankton and hydrozoans pointed towards a range of jellyfish species as a possible cause for some previously unexplained gill health challenges.

10% of sites affected

Due to their size, hydrozoans cannot be detected by the naked eye, but microscopic analysis is helping Mowi to map out their presence on its sites. Around 10% were found to have experienced significant blooms of different hydrozoan species since monitoring began 12 months ago.

Hydrozoans can be more problematic than larger jellyfish species, with stinging cells potentially causing damage to gill tissue. They can lead to significant gill damage and potentially complicate other concomitant gill infections such as amoebic gill disease (AGD). Because of their size there is also a risk of them causing significant internal damage to fish by getting into their digestive systems.

Tracking patterns

Fry said: “We are constantly looking for new ways to enhance fish health and, with our improved system for monitoring micro jellyfish, we are now able to begin to track patterns and trends that will indicate the need for further preventative measures to protect fish stocks against hydrozoans.

“By continuing this research and sharing knowledge, we hope to develop a better cross-sector understanding of how fish are affected by micro jellyfish species and identify whether they do contribute to AGD and other unexplained gill health issues, as we currently suspect.”

An international concern

Hydrozoans are known to be an international concern, with producers in Canada, Norway, Chile and Ireland having previously reported their presence, but increased monitoring could be the key to managing their impact.

General mitigation measures for problems caused by other plankton, algae and zooplankton such as jellyfish include cessation of feeding to keep the fish deeper, aeration, bubble curtains and early harvesting.

The issue of hydrozoans is among many gill health influences being discussed at the International Gill Heath Conference, which is being delivered by Scotland’s Sustainable Aquaculture Innovation Centre (SAIC) with support from the Norwegian Seafood Research Fund (FHF).

Free conference

More than 400 delegates have registered for the free conference, which is aimed at professionals, researchers and students from the aquaculture sector. The conference aims to improve understanding of gill health, share knowledge, and hear about best practice from international experts. Registration can be made here.

The conference begins at 9am BST tomorrow and includes an international sector update, and talks on integrated health management (interventions), nutrition and host pathogen interactions.

Wednesday’s programme includes presentations on integrated health management (monitoring and prevention), model systems, and epidemiology and monitoring, as well as an early career researcher session.

The full programme can be accessed here.

Priority area

SAIC chief executive Heather Jones, said: “Enhancing gill health is an international priority area for aquaculture, and innovation and collaboration have the power to make a big difference to fish wellbeing.

“Through conferences like this we can share knowledge to further the understanding and awareness of different factors that impact the sector. It’s incredibly important that we work together, both domestically and internationally, to communicate results and share cutting-edge innovation that can help solve challenges for the benefit of the entire sector.”