[giaour]

I would probably go for natural resources management or population biology.

Masters degrees are (from my experience in the US workforce) generally only professionally useful when there is an explicit requirement for one set by a professional standards body or codified in law. As in, you usually need a masters to get tenure as a teacher in a public school, and some government jobs have specific and inflexible degree requirements. But for private sector employment, masters degrees are mostly just for personal enrichment.