The first thing Westjet will likely do is offer you a refund on the shipping and/or some kind of credit on future shipping. They will offer to send you a cheque. If you want to press for proper reimbursement, do not accept a cheque. Once you do it can be seen as you having reached settlement. Also, watch your email communication closely as you will probably rely on it if you go to small claims court.
I do not know how small claims court works in Sask, but in BC any claim under $10,000 can be persued in small claims by an individual. You do not need a lawyer, it is not hard to do. If you have a strong case usually the company will just settle with you before going to trial since the cost of them having to pay their lawyer will outweigh the cost of reimbursing you. Sometimes the company will let the claim go to court and then still not send a lawyer. In this case it is uncontested and the judge will side in your favour.
Without knowing the details it is hard to say how strong a case you have. Did the rays die as a result of Westjet mishandling the fish? Was the bag punctured in transit? I think everyone on this forum has a strong interest in helping you any way we can, along with learning as much as we can about what went wrong and why/how.