I am advised by Inland Fisheries Ireland (IFI), that the presence of low volumes of algal growth on resistivity strips does not cause a pronounced attenuation of the electrical signal nor does it adversely affect a counters operation. The build-up of such algal material is common on all in-stream structures. The routine maintenance of fish counters usually includes the removal of such material when and where feasible, however this is more for health and safety reasons for the staff involved (weir access/prevention of slips and falls) rather than for operational ones.
The River Maine fish counter suffered a number of episodic flood events in 2011 and it was these events that caused the counter to malfunction. The counter is currently operational and counting salmon. Some further upgrades are planned which should prevent flood events causing counter outages in the future.