The biggest fish decline in the Bow happened with the 2013 flood. Numbers of fish certainly dropped from being sent to Sask or stranded in a field somewhere. Since then, the fish in the Bow are growing to record sizes. I have never seen this many rainbows this large in 40 years. While numbers may be lower than pre-flood, there seems to be quite a few young 10" rainbows around. There also seems to be good numbers of 2 year old fat browns in the city section.
A yearly inventory certainly would be beneficial for watching long term trends and provide some quality data beyond angler catch rates.
I grew up fishing the Highwood around Longview in the 70's. The river was stuffed with young trout (rainbows, cutts and Bulls) and lots of whitefish. Through the 80's things started to drop off and there was some major Didymo that blanketed the river in the late 90's. After the river became catch and release, the numbers seem to have rebounded somewhat, but it is no where what it used to be. Being one of the Bow's main nurseries, the health of the Highwood certainly effects the Bow. Low oxygen with high temps below High River have been a problem as well. As the populations in towns along the rivers grows, there will be more pressure on water quantity and quality