Article - The River Blog

Riverfly Monitoring End of Season Update 

Author: Susan Mitzi
Categories: Nature, Wildlife

We have reached the end of another successful riverfly monitoring season in the Holme Valley. Our fantastic team of volunteers have been out across the catchment monitoring 12 different sites. This year, it has been our busiest year yet with a total of 58 submissions and 9953 invertebrates recorded! 

Our riverfly monitoring programme is part of the nationwide Anglers Riverfly Monitoring Initiative (ARMI) run by the Riverfly Partnership. Volunteers collect samples of river invertebrates once a month from April to October and then record what they find. There are eight groups of invertebrates to look out for: mayflies, olives, blue-winged olives, flat-bodied mayflies, cased caddisflies, caseless caddisflies, stoneflies and freshwater shrimp. These are chosen for how easy they are to identify and their different sensitivities to pollution.  

Each sample is given a score based on the invertebrate abundance and the data is added to the Calder Rivers Trust dashboard, a central database which includes all the samples taken in the Calder catchment since 2010. These results collected by volunteers build up a picture of the water quality throughout the river over time.  

What I have found most interesting from joining some of the volunteers on their samples is the number of invertebrates in each sample. Just a 3-minute kick sample can bring up hundreds of invertebrates from the sediment and it can be mesmerising to watch them all swimming around in the tray. The swimming patterns of the invertebrates can also be an easy method of identifying them. For example, olives dart rapidly around the tray whilst stoneflies wiggle from side to side and caseless caddisflies look like they are doing the worm. 

Cased caddisfly larvae have definitely been a favourite invertebrate among volunteers and can be difficult to spot until you see a twig moving in the tray and it turns out to have antennae and legs. Mayfly larvae are the rarest by far due to their extreme sensitivity to pollution with only 6 being sampled this year.  

The invertebrate abundance also shows huge variation between sites. Some sites contain hundreds of freshwater shrimp and not much else, and others contain more cased caddisflies and stoneflies. This represents the variation in water quality throughout the river and the importance of monitoring to detect any significant changes which could indicate a pollution event. 

Next season, we will be training up more volunteers and adding new monitoring sites to get an even better picture of the water quality in the River Holme. We will also be handing out water monitoring kits to volunteers to test for different nutrients in the water.   

Thank you again to our amazing volunteers for all their hard work. 

If you would like to get involved with riverfly monitoring, please email 


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