A few weeks ago, we discussed the first life stage of a salmon – eggs. Today, we continue to follow the life of a salmon into the next stage – alevins.
Inside of the egg, the salmon embryo begins to develop and growing slowly:
- After 7 to 10 days the head and body begin to form
- After about 1 month eyes begin to appear
- After about 2 months the embryo begins to move inside the egg
- And after about 3 months the embryo hatches from the shell inside the egg!
As development progresses, the embryo begins to wiggle around. Finally, when the embryo can no longer get enough oxygen through the egg wall, it releases an enzyme that weakens the membrane. This allows the embryo to breaks through, emerging as an alevin.
Recently, the eggs in our fish tank hatched into alevins. Check it out:
Alevins are mobile embryos meaning that the yolk sac from the embryo remains attached to the alevin’s belly. The yolk sac provides the food it needs as it continues to develop, while hiding in the gravel. They can move through the gravel, but silt can still smother them. The sac shrinks as the alevin develops teeth, eyes and a digestive system. The alevin begins to eat some external food that floats through the water in the gravel. The alevin’s respiration system also develops, allowing it to breathe through its gills.
Since alevin cannot swim and their yolk sac makes movement slow, they are an easy target for predators. To hide from predators, they need clean gravel where they can hide. Aside from predators, threats to alevins include siltation, pollution and floods or other activities that can disturb the gravel.
Organizations like Splitrock Environmental are striving to build and protect ecosystems that foster the development salmon, with special consideration of their most vulnerable life stages. Restoration works include cleaning gravel free of silt and adding vegetation to the surrounding riparian areas. At home you can help protect alevins by not pouring poisons into drains. When out walking with your dog, try to keep them from running in the water. For these vulnerable alevins, they need any help we can offer them.
What happens next in the life of a salmon? Stay tuned to find out.