Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Sprinkle fish fillets with salt and white pepper. In an extra-large skillet combine Fish Stock, white wine, mint sprigs, olive oil, and butter. Bring to boiling; reduce heat. Add fish fillets; cover and simmer gently for 8 to 10 minutes or until fish flakes easily when tested with a fork, adding the yellow squash and zucchini the last 2 to 3 minutes of cooking.
With a slotted spatula, transfer fillets, squash, and zucchini to serving plates; keep warm. Discard cooked mint.
Bring juices in skillet to boiling over high heat; cook about 6 minutes or until the sauce is thickened and reduced to 1/2 cup. Pour over the fillets. Squeeze a lemon wedge over each fillet. Sprinkle with the snipped mint.
In a large saucepan combine fish bones with the cold water. Bring to boiling over high heat. Reduce heat and simmer, covered, for 45 minutes. Strain stock; cool. Store leftover stock in a covered container in the refrigerator for 1 day or freeze for up to 3 months.