Hello Kill_a_watt --
There's been some good advice given here, but I'm going to offer a little from my own experience. I did a degree in zoology, then a PhD at the Natural History Museum in London, and since that time I've done a variety of biological research, museum, teaching, and writing jobs. My field is ammonites, which are cephalopods, not fish, but my experiences are not unlike those of people in the fish-field.
On the one hand, studying science is fun. It is wonderful to be able to spend time doing something you enjoy, and following your curiosity to a better understanding of the natural world.
However, you also need to be realistic. So rather that tell you all the wonderful things about working in science (like the pride in getting published, and the amazing people you'll meet, and the great places you'll visit) I'm going to describe the down-side.
Not all branches of science are equally well funded. There is lots of money in genetics, biochemistry, and biotech disciplines, but there is very little in things like ecology (studing communities) and taxonomy (discovering and naming new species). Money tends to go where there is practical benefits, so animals useful for biochemistry or agricultural purposes and intensively studied while those that are merely interesting are largely ignored. Jobs only exist where there is money, and universities tend to have lecturers and labs focused on fields of biology where the money goes. In other words, if you want to test new drugs or diets on tilapia that is easy enough, but studying the behaviour of shell-dwelling dwarf cichlids from Tanganyika will be much more difficult.
Scientific jobs are short-term contracts and often poorly paid through your 20s and 30s. After your BSc and PhD, you will be expected to do at least one post-doc, and most likely two. That's about 12 years on 3-year contracts. Forget about living in one city, buying a house, or driving a nice car. Not going to happen. There are exceptions, but most young scientists I know have moved many times in their careers and even in their 30s are still in cheap apartments.
Science is very competitive. There are far more science graduates that there are jobs. In other words, it's a buyer's market. You, the graduate, are the seller. You will be up against dozens of equally qualified applicants for PhD posts and post-doc posts.
Science is grant-driven. Universities and museums no longer have money for research or research students. Your PhD or post-doc will be funded by a professor or researcher who gets a large grant. Eventually, you will be expected to become more-or-less self-funded, that is, you will have to apply for grants or you won't have a job. Grant applications are horribly time consuming and rewards are effectively random. I have known young scientists who have failed to get grants and suddenly found themselves having to move back with their parents.
Science isn't a 9 to 5 job. The scientists who succeed largely do so by working late and working weekends. Forget about having a busy social life, and because you have to move to another city every three years, maintaining a long-term relationship with a girl/boyfriend can be very difficult.
This all sounds gloomy. I guess it is. If you spend time with any scientists in their 30s they will often sound very depressed about the whole thing. Science may be fun, but being a scientist is incredibly hard work. All museums and universities are having their funding cut, which means all staff are expect to do research that brings in money. There just isn't scope for "blue skies" research anymore. (Blue skies is research with no goal, that is done just for curiosity. Nowadays, the focus is on work that solves problems.) Anyway, I would encourage you to visit a museum or perhaps a fish biology lab and talk with the people there. Write to them first, and I certainly remember visiting the Natural History Museum at about age 16 for my first visit.
I'm not saying don't be a scientist, but I do think you need to research things now, while you can, before committing yourself to something that might not work out the way you'd like.
Cheers,
Neale
while you don't really get the same on-the-job experience, the engineering fields use the same basic background in the sciences and maths but entry level positions are a lot more secure and lucrative (at least here in the States).
of 'gods creations' and so maybe not as well placed to comment as some others on this thread ie the zoologists etc.
I (dare I say) 'fell' into a job as a research assistant for the agricultural branch of government here, right after I graduated university. The downside was a lot of physical labour, and being on contract. The upside? The contracts were plentiful, and the pay above average for entry level. From there I went on to run a quality control lab for a fish processing company (also decent pay). I took a brief stint as a chiropractic assistant in there somewhere too 
(ok, not so great $), and then moved into pharmaceutical quality control (ever had Nyquil or Dayquil capsules? 
when you are the first person in the world to discover something completely new about some organism, protein, or genetic mutation etc.
