Single factor flyfishing

In my finance line of business, one would never rely on a ‘one factor model’ for decision-making, regardless of its elegance and appeal. Similarly, in flyfishing, the complexity of what makes a good day usually well exceeds a simple one factor explanation. You know, those single factors such as heat, wind, rain, water level, barometric pressure… Over the years, we combine all these variables into our own mental ‘model’ of how we understand them to affect the region we’re fishing, and then we head out with the best plan to catch some fish. I think that’s called experience?

However, on Saturday 6 January, I had one of those days when a single factor, a very large drop in barometric pressure, coincided with the only opportunity I had to fish in Tasmania on the fantastic Tyenna River. And you know what happens when you mark a special day months ahead on your calendar, to fish somewhere remote? The weather demons must have access to that agenda too, and always seem to prepare poor conditions that will arise just by coincidence on that very day!

Anyway, I went ahead and fished around the Mount Field National Park area, alternating between dries, dry-droppers and Czech nymphing. I averaged one fish per hour of fishing. In addition there were a number of half-committed takes on the dry and a series of ‘followers’ on the nymphs. All in all, I finished that day feeling I hadn’t done very well. But when I looked up into a grey sky, where the wind was whipping the trees to a frenzy, spraying clouds of dry leaves like confetti through the hot air, I realized this was no ordinary day. Indeed, the Bureau of Meteorology data suggested the pressure had fallen from a peak of 1014.0 hPa the night before, to a low of 999.1 hPa. There was my excuse!

The Tyenna River under much friendlier conditions on day two.

My wife knows my fishing passion too well, and the next day, when the weather had completely settled and we were planning to visit the amazing MONA again before heading home, she gave me a leave pass to go back to the Tyenna once more. So after the Sunday market, I dropped her off at MONA and drove up to the Tyenna for an unexpected encore. According to BOM data, the barometer was rising steadily from its big dip the day before and was already back up to 1008.0 hPha. Was this one factor going to explain most of the variation?

I rigged up the Czech nymph system again, as I had seen most of the fish following from the depths the day before, and there was no evidence of surface activity. And sure enough, in the first run I had three trout in rapid succession. All committed to the fly without hesitation; no ‘followers’ today! The pattern continued until I peered over a log at the back of a pool, only to notice the log gently moving from left to right! A very large trout, and feeding hard. I won’t try to explain all the variables going through my mind that kept me from upgrading my 7x tippet – the most obvious being the simple fear the fish might vanish while I changed up. In any case, I made the cast and the big brown just swung over and ate the fly. Fish on! Luckily, I managed to keep the fish in the pool as any run into the rapids up or downstream would have spelled disaster. Twice I tried netting it but the net was too small. What a nice new problem! Third time lucky, and I had a terrific 5 pounder to weigh, photograph and release. It took a size 14 pearl-beaded nymph with a pink hot spot. There you go, even browns will go for a colourful meal every so often.

I need to buy a bigger net!

I finished that session with six fish per hour, or in other words, six times better than the previous day (my skill did not change overnight!). I’ve never before considered my results that way, but I did this time for comparison. Coming home, I looked up Andrew Bett’s book The Barometric Breakthrough. It describes the effect of barometric pressure on salmon and he concludes that the difference in catch rate for him is about 3.5 times better for rising versus falling barometric pressure. This seemed to match my experience on the Tyenna.

The ‘log’ released to surprise another angler.

The only problem of course is that we go fishing despite the trend in the barometric pressure, and that as a ‘one factor model’, the barometer is hopelessly inadequate in explaining failure or success. Most of the time anyway. But once in a while, collecting your own data and developing your own ‘study’ to test a certain hypothesis, can be a lot of fun. If nothing else, my two sessions on the Tyenna also showed me that sometimes, we shouldn’t beat ourselves up too much after a bad day – the cards are simply stacked against us.