There have been multiple reports in social media (or more accurately reposted reports) and mainstream media about the presence of blue-green algae in the Hamble recently, and Eastleigh Borough Council have displayed signs in the Botley area warning people to avoid the water.
What we know (updated June 2025)
- A member of the public reported 3 sightings of potential blue green algae (aka cyanobacteria) in March, April and May 2025 in the area between the A334 road bridge at Botley Mills (what3words ///lush.reactions.responds) and the YMCA Fairthorne Manor (what3words ///crop.delighted.royal).
- The Environment Agency reviewed the photos provided and concluded that blue green algae may be present.
- The Environment Agency was not able to take actual samples because this stretch of the river is saline. Saline conditions cause the algae to break down, specifically causing the cell walls to burst. This makes identification very difficult and taking a sample would not provide reliable results.
- Although saline conditions breakdown blue green algae, evidence suggests that toxins produced by such algae are not neutralised because cell death may lead to a release of microcystin toxins into the water, potentially increasing the concentration of extracellular toxins.
River Hamble Adventures policy
- We’ve been running sessions on this stretch of the Hamble since March 2025 and have not seen anything resembling blue green algae. There have been no further reporting of sightings on any of the active social media groups, nor any reported illnesses.
- Whilst we’ll continue to both monitor the river conditions ourselves and monitor the reports from other river users, our assessment is that the presence of blue green algae is questionable and the risk from blue green algae is currently very low.
- The usual precautions such as keeping your mouth closed when falling in the water and washing hands after the session continue to apply and we will remind clients of these as part of the session briefs.
- As a further precaution we will not be carrying out sessions (such as Paddleboard Rescues) that involve extensive immersion in the Hamble until further notice.
Please note that this is a statement of River Hamble Adventures policy and we expect river users to make their own assessment.
Further reading
Both the links below include useful images of what blue green algae looks like.
- https://canalrivertrust.org.uk/support-us/our-campaigns/safety-on-our-waterways/blue-green-algae-what-you-need-to-know
- https://www.bbc.co.uk/news/articles/cz9x9yrqzqlo