Have a look at the chart below, which shows the 33 salmon I've taken during our regular week on the Upper Findhorn over the past 10 years, subdivided by sex and time of day. In this post I will focus on the group of cock fish caught before breakfast.
The change in air temperature in the hour is no more than 2-3 degrees, which would not impact either the water's temperature or its oxgen content (see RV Righyni's 'Salmon Taking Times'). Barometric pressure does not obligingly change with the time of day or month, and in any event rarely changes significantly in a 20 minute period when the weather is settled. The imminence or otherwise of rain did not appear to change the behaviour. So I don't think it's the weather.
Within limits it doesn't seem to matter whether the water is rising, falling or steady, provided that it's high enough for fish to run. The colour of the water did not have any observable effect. The exceptions are bottom low water and spate conditions.
What about light? In the interests of brevity I shall spare you the calculation of the proportions of the light that is reflected or absorbed by the water at different times of day,
and go straight to the graph below:
The significant time is circled. Provided that there is a clear sky the underwater light intensity more than doubles in a 30 minute period. However, at Tomatin sunrise occurs when the sun clears the mountains at an elevation of about 4 degrees. Consequently, when the sky is clear, the sub-surface light level can go from near-zero to 25% of its full midday value in as little as 10 minutes. This change is so abrupt and intense that it probably has an impact on fish behaviour. The issue is why it affects cock fish so markedly, and not hens.
The picture from September 2011 shows one of the bed-loving young rods in the party with the fruits of an early start (outside my sample). He and his host each caught near-identical cock fish that had been displaying competitively for the previous 20 minutes, in the same area, within the space of 10 minutes. Despite the relatively high water level (12” above summer low, starting to rise further) and rapid flow in the head of the pool, both fish were caught on floating lines, conventional leaders and large shrimp doubles fished near the surface. The 3 hen fish caught in the same area of the pool later in the day by other rods were all taken with weighted tubes and sink tips.
When I started fishing the Upper Findhorn in 2002 the received wisdom was that it was not worth fishing before breakfast. That's probably true earlier in the season before the cock fish get charged up on testosterone, but it isn't so in September.
I'll be looking at why the hens are at their best later in the morning in another post.