I would argue that the mood of this first introductory chapter is one of immense sadness and relief as Pi tells us how, on the one hand he is overjoyed that he has survived and made it to Canada, but on the other hand how his experiences still haunt him and leave his joy tinged with sadness that he finds difficult to deal with. For example, note the following example of this:
Richard Parker has stayed with me. I've never forgoten him. Dare I say I miss him? I do. I miss him. I still see him in my dreams. They are nightmares mostly, but nightmares tinged with love. Such is the strangeness of the human heart. I still cannot understand how he could abandon me so unceremoniously, without any sort of goodbye, without looking back even once. That pain is like an axe that chops at my heart.
Clearly, the pain that Pi feels at his "abandonment" from Richard Parker introduces a sombre note to the first chapter, that tempers the joy of his release with the pain of his experience at sea and how that impacted him and even now continues to impact him in his life today, so long after his traumatic experience.