The Yurappu River, which flows through Yakumo Town in southern Hokkaido, is a river where salmon run up and spawn and we can see natural spawning. Let's look at the spawning environment. Let's start with "water temperature".
2. Flow velocity measurement - white salmon eggs
This video explains the flow velocity, water depth, and riverbed sediment (medium gravel) in the spawning environment and fertilized eggs that have been dug up by other spawning pairs and have died and turned white. As explained in this video with dead eggs, live eggs become also hard.
3. Biofilm - Aquatic Insects
Biofilms and aquatic insects are described as being involved in the supply of substances from post-spawning carcasses.
4. Electric shocker
Electric shock fishing gear (electric shocker) is used when collecting salmon and other fish in relatively shallow rivers. A special harvest permit from Hokkaido is required for use.
5. Secondary sex characteristics
In the training, the external morphology of males and females of post-spawning carcasses, especially secondary sexual characteristic-related traits is measured. We compare with the numbers of immature salmon caught in the open ocean.
Training at the laboratory
1. morphological characteristics of fish during spawning season
Reproduction-related morphological changes called "secondary sexual characteristics" are observed in salmon that return from the sea to the river for spawning. Males and females, which were difficult to distinguish in the open ocean, are also easier to distinguish due to secondary sexual characteristics.
2. Measurement of Body Length and Weight
Students measure the length and weight of males and females while anticipating them.
3. Observation of organs and removal of the brain Caution: this video contains dissection images.
First, the abdomen of the male during the spawning season is opened, and the gonads (testes), liver, pyloric caeca, stomach, etc. are observed. Next, the brain is removed to explain each part, and the pituitary gland, which is the center of hormone secretion, is taken out.
4. Otolith sampling Caution: this video contains dissection images.
The otolith in the head is taken out. The salmon at the Hatchery of the Fisheries Resources Research Institute are labeled with temperature on the otoliths so that their origin can be identified even in the open ocean.
5. Scale sampling Caution: this video contains dissection images.
Scales are collected from males during the spawning season. You can see the age and growth history from the scale pattern. However, it is difficult to remove the scales of fish during the spawning season, and it seems that they are having a hard time.
6. Observation of ovulated eggs Caution: this video contains dissection images.
The abdomen of the female during the spawning season is opened, and the eggs that have been ovulated and just before spawning are observed. Explains egg growth and maturation, egg membrane hardening and digesting enzymes.