Whisk together first 8 ingredients; gradually whisk in olive oil until well blended. Remove half of juice mixture, and set aside.
Place catfish in a shallow dish or heavy-duty zip-top plastic bag; pour remaining marinade over catfish. Cover or seal bag, and chill 15 minutes, turning catfish once.
Remove catfish from marinade, discarding marinade.
Coat grill rack with cooking spray; place on grill over medium-high heat (350° to 400°). Place catfish on rack, and grill 5 minutes on each side or until done. Remove to a serving platter; drizzle with reserved marinade. Garnish, if desired.