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Why fish run.
Thu 5th May, 2016
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 I mentioned last time that there had been an improvement in the fishing which has continued this week. Anglers reported fish moving through the lower river on Tuesday and now that we're into May these runs should gradually build in frequency until numbers peak around October.
News travels fast in Turangi and there were several anglers in and around the Bridge pool early yesterday morning. I watched them for a few minutes before I met up with Bevis and one of them landed a nice trout as I was about to leave.
It will take a little while before you get an even spread of fish throughout the river so it still pays to keep moving. Up to now they seem to be in small pods here and there.
 We started around town a bit farther up and had a couple of rainbows and an unsuccessful attempt at a brown. But with a few anglers around and more showing up we decided to relocate.
We had a quick flick in Big Bend but this place tends to produce consistently later in the year. Last year it was a "banker" pool from around the end of June on ... often fishing well all day long.
It was a different story just down river and we spotted half a dozen as soon as we arrived.
Bevis took two on a small gold-head before they spooked and disappeared into the deeper water.
 
So after a quiet few weeks it looks as if things may have kicked off on the Tongariro at last.
But why do fish run?
Rain or impending rain is often cited as one of the key triggers ... but there's obviously a lot more to it than that. So is it coincidence that all of a sudden the fishing improves and then a day or two later the forecast rain arrives. I have a feeling that in the unlikely event we had no rain from now until Christmas the fish would probably still run ... but what are the facts?
Michele Dedual explains the science behind it:
Hi Mike
This is a really good but big question and I’ll try to provide the most salient points.
1) Fish need to reach maturity before spawning and maturity is triggered by several factors. Growth is an obvious one, when fish grow quickly they will spawn earlier. If they don’t grow quickly they will “wait” longer in the lake before running trying to maximise the output of eggs. However, at some stage there is a trade-off between growing further with the risk of mortality and running a bit smaller. This is also when the sexual hormones cycles controlled by temperature, day length and also fat content (like in humans fish need some fat to metabolize hormones and again if the growing conditions are not right they will affect the maturity) become the dominant trigger.
2) There are also environmental triggers as you indicate. In the Tongariro our radio-tracking experiments indicate that the early spawning fish are entering the river in an early state of maturation with eggs not in the advance stage of development (vitellogenesis). These fish will react strongly to freshes to move upstream but they will stop when conditions returned to normal milling in the same pool for long periods until the next fresh when they’ll push further. During this early phase of the run some fish can take as much as 89 days to move from the Delta to the Waipa Stream...This is probably why at the moment fish in the lower T-T are concentrated in such densities that I haven’t seen for many years, they are waiting for the next fresh. Interestingly these fish are really nervous and hard to catch having something else on their mind than just grabbing a nymph!
  3) Later in the season (Sep-Nov) fish are running in a more advance stage of maturity and are less reactive to freshes with some fish rushing through the river in 9 days.
This indicates that the early running fish are very important for the fishery as they sustain the typical winter fishery. This is why we looked at the genetic make-up of the early v late running fish and more importantly if fishing (in the lake) was more likely to remove early running fish. We didn’t find any evidence of this but this is also where we met the limit of science.
4) Water temperature and water flow are other triggers but how fish use them is not fully understood. There are some theories that the flow in the gravel which can go up or down is important and even used differently by brown and rainbows but again the science on this is not that strong.
5) The presence of fish spawning is another strong trigger as it provides a “smell” to the water that will attract others, in that sense it is the first run that is the hardest to get, after things are more predictable.
Tight lines
Michel
The rain hasn't stopped all day in Turangi but it has made very little difference to the flows. Tomorrow ... overcast with patchy drizzle to start. Then a brief let up as we head towards the weekend before a change early next week with more persistent rain on the cards.
It could be an interesting weekend .. .keep an eye on the lower river and town pools.
See you out there.
Mike |
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