What is a bulging pouch under baby salmon?

You're likely referring to the yolk sac, a large, prominent sac located on the belly of a baby salmon (also known as an alevin).

Here's why it's important:

* Nourishment: The yolk sac is filled with nutrients that the salmon embryo used during development inside the egg.

* Survival: After hatching, the alevin relies on the yolk sac as its primary food source until it develops the ability to swim and hunt.

* Gradually Absorbed: The yolk sac gradually shrinks as the alevin grows and feeds. Eventually, it will be completely absorbed.

So, the bulging pouch isn't a cause for concern, it's a natural part of the salmon's development.