There are two chief ways of assessing grayling population in rivers, catch returns and electro fishing.  As it implies, catch return data is from anglers inputting data.  In the early 2000’s, the EA ran a Log Book scheme.  This stopped in 2007 but since 2020 has been replaced by the Grayling Society catch return database, readily accessible from their website to input data.  Catch returns are not a direct measure of fish numbers, more of a surrogate measure and is dominated by the rivers that are most fished. 

Over the years, the Environment Agency has been monitoring fish populations using electro fishing counting in many rivers over many years.  This is regarded as being the most accurate way to assess fish populations.  This data is available on the EA Ecology & Fish Data Explorer.  The surveys cover a much broader range of rivers than the catch return scheme, not restricted to the majority of well fished grayling rivers.

A comparison of the results for the EA regions in England is shown below.  The EA areas are as defined in the early 2000’s.  The catch returns for both the log book scheme and the GS database are quoted as grayling caught per hour.  The electro fishing data is quoted as population density; the number of fish per 102m.  This is because the fished area of the rivers varied. 

For the Thames region, the both catch returns and electro fishing shows a small decrease.  The inclusion of a number of electro fishing surveys from river Mimram post 2007 dramatically increased the overall grayling numbers, but this was countered by a dramatic reduction in grayling population in other rivers, such as the Kennet.  Similarly, recent increases in catch returns from the river Lambourne masked a drop in catch returns from other rivers in the region. 

The three regions: S, NE, and SW also showed changes in the same direction, a decrease in the S and SW and an increase in the NE.  In a more refined river by river analysis, the rise in the NE was dominated by increases in the river Don, more pronounced in catch returns when more anglers started fishing the Don, but other rivers, such as the Wharfe and Hull (Driffield Beck), suffered from a decrease, with virtually no grayling catch returns from the Hull in recent years. 

There were, however, discrepancies in the comparisons for both the midlands and NW.  On closer examination, this discrepancy was due to different rivers being electro fished from those fished by anglers.  For the NW region, most of the rivers included in the catch returns were from the Ribble and its tributaries.  These rivers were much less well represented in the electro fishing surveys.  Similarly in the Midlands, many of the surveys were from the river Onny and Rea brook, poorly represented in the catch returns.

I believe that both methods are of great value in assessing the grayling population in rivers.  Where differences occur, it is normally down to the electro fishing of rivers, or sections of rivers, not included in the catch returns.  Further analysis is on-going for further refinement.