Wednesday 16 January 2013

Morning Glory - Sex and Flies

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 cocks were all hooked in the faster water in the top third of the pools.  It did not matter what fly I used, if it was a good size and fished hard near the surface.  In every case the take was aggressive: there were no near misses or losses to add to the figures.   So what was the stimulus? 

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.

Once the Tomatin water is more than about 10 inches above average summer levels, fish can run with relative freedom by day and night.  They tend to be more tentative at night, especially when faced with shallow exits from pools.  Consequently running fish accumulate during darkness in the 'short halt' lies towards the heads of pools, especially if the water level is falling.  When the light level suddenly increases, cock fish in the now more crowded area can see the other fish, and most importantly, their male peers.  Until that time a cock fish would have been aware from the smell of expelled testosterone of the presence of any other male directly upstream, albeit without knowledge of proximity, but not of those to its flanks.  By late September cock fish are increasingly charged for their reproductive role, so sex becomes a major factor in their behaviour.  What we see here is a burst of typical testosterone-laden alpha male dominance behaviour, and what happens supports that contention.  Generally it involves larger multi-sea-winter (MSW) fish – mostly in the 8-14 lbs range – jumping, rolling and rushing about being generally aggressive.  Put a large fly into their field of view and they attack it.

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.



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