- Water temperature
- Location in the river
- The available nutriment, its abundance and quality
- Population density
The grayling is a fish that is part of the salmon family. Though, unlike salmon (Salmo salar), they spawn in the spring.
The spawning usually takes place between May and June when the water temperature is above +4 centigrades. This normally occurs in the afternoon when the water temperature is highest.
The fish spawn in gravel bottom areas that lie 0,5-1 meter underwater and where stones offer hiding places in the stream. Then the newly-hatched fries stay near the spawning site for months along the riverside.
The grayling has got its latin name Thymallus thymallus after the common hausehold spice, thyme Thymus vulgaris. The fish are said to emit the thyme scent when raised directly out of water.
The Growth of Grayling in The River Iijoki
The Growth speed of grayling varies in different stream and rapid areas. The following outlines the growth factors of the Grayling in River Iijoki. Iijoki is a river in the Northern Finland and it flows through my home village Taivalkoski.
Main growth-affecting factors:
What is actually controlling the growth for the Graylings to mature earlier in certain areas?
What I am about to propose now is not a scientific report. It is rather conclusions of a fishing master drawn from grayling scale (gif-image, 61 kb) dating I made during many years (1995-97) and statistics, which I will present you as average values.
After following the growth of graylings in different parts of the river Iijoki, I have noticed that the graylings growing downstream lake Jokijärvi (a lake the river Iijoki flows through) grow faster. Why is this? Does the water temperature affect favourably on water insects and small fishes and in the end impact the graylings?
There is a hypothesis proposing that downstream Jokijärvi's in an area called "Luusua" live a lot of Hydropsychidae, or, caddis fly larvae. The Graylings eat this type of larvae which is theorized to trigger their accelerated growth. I think that small fish are also an important part of Graylings gourmet menu (I've studied their eatings).
Slower growth downstream from the damp of Taivalkoski, compared to other areas, would result from remarkably smaller population of the water insects (Hydropsychidae) and other nutrients.
I have sometimes measured legal size (30 cm in Finland) graylings living downstream Jokijärvi that aged no more than 2+, meaning that they've been wagging their tails in the river for two years and one summer. Usually, reaching the legal size takes about three years.
About 6-8 kilometers downstream Jokijärvi is a popular fishing site called Sauvonjuurikka. This river area is rich with small islands, and is a beautiful place with a nice amount of fish.
But there is more. Downstream the damp of the river Iijoki (jpg-images, 41 and 42 kb) which bas built in Taivalkoski village the graylings need an average of 4,4 years to achieve 31,8 cm in length. It seems that under the damp the growth factors are out of order, but farther downstream the growth is again faster.
Graylings from Pirinkoski need about 3,9 years to grow 31,9 cm.
As a summary from three years of time I can tell that my fly graylings from the river Iijoki have been 3,7 years old and 32 cm long (gif-image, 30 kb) in an average. So we are not talking about any great fishes and those few wealthier ones don't much affect on the averages.
The main reason why there is not large graylings in the river Iijoki is because of the high
fishing pressure the fish have to face.
My article that was published in the Perhokalastus (Finnish Fly Fishing) magazine issue 3/1999 is about dating, age database and growing factors of graylings. The fine drawing by Simo Yli-Lonttinen (see below) and a couple of other images bring the article to life.
Catch statistics compilation and charts
One third (29%) of my graylings in year 2000 were caught in June. Like the year before, September was the second best fishing month (20%). Third place goes surprisingly to October (17%) because the weather was good for fly-fishing. At that time the temperatures in the upper streams of the river Iijoki differed from one random measuring to another between 8.8 and 6.2 degrees centigrade. The water levels were perfect for the whole October, for there were no long-term rains.
The average size of the graylings in year 2000 was 33,7 cm. Comparing to the year before this means 0,8 cm addition. From year 1998 to 2000 the average size of my fly graylings have grown 1,6 cm. In the past season the biggest graylings measured 39 - 42 cm. I caught these at the end of the season, one in August, one in September and two in October.
My fishing falls varyingly into three categories based on the fly type: the larva imitations presented for the river bottom, pupa imitations swimming more or less near the surface, and mini streamers and dry flies. I made a chart of catch distribution in how different flies caught fish in year 2000. The chart proves that dry flies lured a half and larva imitations a good third of graylings.
The translator wishes to thank
As I have examined the average values of my three-year summaries, they told me that already the growth of graylings dropped. Graylings from Sauvonjuurikka came behind the other graylings living downstream Jokijärvi. The four-year-olds from Sauvonjuurikka measured 31,8 cm. Downstream Jokijärvi the graylings were 3,6 years old at that time and they were already 32,6 cm long!
Catina Tanner (USA) for her assistance
in the translation of this article.
