04/06/2026
Last week, we received a report from one of our local angling clubs regarding a carp only mortality within one of their lakes. We attended the site to investigate, undertook water quality monitoring, and collected a water sample for algal analysis. Initial observations - colour of the lake and dissolved oxygen readings indicating supersaturation - suggested the presence of an algal bloom however, the extent and diversity of algal species present even came as a surprise to us.
Analysis of the algal sample by our biology colleagues confirmed a significant bloom of blue-green algae, identified as Oscillatoria. At least three to four species within this genus were present, with each of the visually distinct types individually exceeding the threshold for a bloom. Estimated cell counts indicated an overall abundance of approximately 905,000 cells/ml, far surpassing the recognised bloom threshold of 20,000 cells/ml. For those that don’t know, blue-green algae can potentially pose a health risk to not only fish, but also humans and other animals.
The sample also contained a moderate to high abundance of spiky diatoms, predominantly Synedra, alongside colonial green algae, non-spiky diatoms, motile unicellular green algae, and non-motile unicellular green algae all of which were recorded in low abundance. We’ve included a photo which shows a couple of the Oscillatoria ‘types’ along with some spiky diatoms and colonial green algae.
The abundance of blue-green algae, spiky diatoms, and overall algal biomass was sufficient that, either individually or in combination, factors such as cyanobacterial toxins, gill irritation and or damage from spiky diatoms, and diurnal fluctuations in dissolved oxygen levels would have stressed the fish and may have contributed to this mortality.
The sample also contained a substantial quantity of organic material, along with some inorganic debris. Within this, there was a moderate to high abundance of diatom frustules (the silica ‘shells’), predominantly from spiky Synedra species. These particles may have further contributed to gill irritation or damage as carp feeding can disturb bottom sediments, temporarily suspending such material in the water column and increasing the likelihood of contact with the fish’s gills for a second time.
We have since provided the angling club with advice and guidance and algae control options which the club will now implement going forward whilst monitoring the situation closely and prepared to call us again if they require any further assistance.
This service is all funded through your rod licence income, with a dedicated Fisheries Officer on standby to respond to fisheries incidents 24/7 365 days a year.
