In a small bowl, stir together the pineapple juice, soy sauce, garlic, and cayenne pepper.
Place the salmon fillets in a small shallow baking dish then pour the marinade over the fish, cover tightly with plastic, then refrigerate for 2 hours or overnight.
While the salmon marinades, prepare the salsa: Combine the pineapple tidbits, avocado, jalapeno, red onion, cilantro, and salt in a small mixing bowl. Pour lime juice over the top, then toss gently to coat. Refrigerate until ready to serve.
When ready to bake, preheat the oven to 400 degrees F. While the oven preheats, remove the fish from the refrigerator. Line a baking sheet with aluminum foil, then coat with non-stick cooking spray. Remove the salmon from the marinade, shake off any excess, then place the salmon flat-side down on the prepared baking sheet.
Transfer the marinade to a medium saucepan. Simmer over medium-high, stirring often, until the marinade is reduced by half, about 6 minutes. Brush liberally all over the top of the salmon.
Bake the salmon for 5 minutes, brush again, then bake an additional 5 to 10 minutes, until the fish flakes easily with a fork. Serve topped with pineapple salsa.
TO STORE: Leftover salmon and salsa may be kept in a refrigerated airtight container for up to 3 days.
TO REHEAT: Warm the salmon in a 350 degree F oven until heated through. Serve with leftover chilled pineapple salsa.