In a large pot over medium heat, melt butter; add the shallots and cook until soft and translucent, not browned, about 5 minutes.
Stir in the tarragon.
Add the potatoes and sprinkle with salt and pepper.
Add the fish stock and bring it to a boil over medium-high heat.
Cover, reduce the heat to medium, and simmer for 20 minutes.
Add the salmon fillets in a single layer; cover with the lemon slices, then add the bay leaf.
Pour in the wine.
Cover again and cook over low heat until the salmon flakes when tested with a fork, and the potatoes are tender (10 to 12 minutes).
Add more wine as needed if the potatoes are sticking.
Meanwhile, in a medium skillet, cook the bacon in its own drippings until lightly browned.
Remove and discard the bay leaf and lemon slices.
Remove the salmon fillets and divide them into chunks.
Add the yogurt and cream to the soup.
Stir occasionally over medium heat for 5 to 10 minutes, without allowing the soup to boil.
Gently mix in the salmon and bacon.
Taste, add additional salt and pepper if needed.
Serve hot.
If any chowder is left over, it may thicken up overnight.
Simply thin it out slightly with a little more yogurt.