Not sure about the rest of Canada but in Ontario you can buy aquacultured fish from registered aquaculture facilities to keep in artificial waters and aquariums. Below is the relevant regulation that prohibits the purchase/sale of native fish without a license:
O. Reg. 664/98: FISH LICENSING
under Fish and Wildlife Conservation Act, 1997, S.O. 1997, c. 41
Buying or selling fish
51 (1) A person shall not buy or sell fish that belong to a species that exists in Ontario waters or fish prescribed by the regulations, except under the authority of a licence and in accordance with the regulations.
Interpretation
(2) For the purposes of subsection (1), fish cultured in Ontario shall be deemed to belong to a species that exists in Ontario waters.
And here are the amendments that allows you to purchase and transport aquacultured fish without a license:
Buying or Selling Fish
32. (1) A person who buys fish directly from the holder of an aquaculture licence, commercial fishing licence, commercial bait licence or document described in subsection (2) or (3) is exempt from the requirement in subsection 51 (1) of the Act to have a licence to buy fish. O. Reg. 196/00, s. 4 (2); O. Reg. 261/05, s. 7 (1).
Transportation of Fish
29. (1) For the purposes of paragraph 3 (1) (b) of the Ontario Fishery Regulations, each of the following documents shall be deemed to be a licence to transport live fish if the fish have been cultured, caught, bought or sold under the authority of the document:
1. An aquaculture licence.
2. A commercial fishing licence.
3. A document deemed to be a licence to sell fish under section 32.
4. A receipt, invoice or bill of lading issued by the holder of a licence referred to in paragraph 1 or 3.
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The best time to get is when they replenish genetic stock through wild fish. This generally happens more often with salmonids than others as out gov't stocks our rivers and lakes. Make sure you hold onto the receipts for verification purposes for 3 years.
Additional reading re Ontario aquaculture laws:
The policy governing how aquaculture licences, renewals, transfers, amendments, refusals and cancellations are issued. This policy’s goal is to minimize the risk of ecological damage from aquaculture activities.
www.ontario.ca
This policy/procedure identifies the requirements for stocking fish. It also identifies the procedure to issue a Licence to Stock Fish in Ontario Waters.
www.ontario.ca
Welcome to the new e-Laws. It’s now easier than ever to find Ontario laws. We welcome your feedback.
www.ontario.ca
A fishing license allows you to possess and transport up to 120 "baitfish" as well, another workaround for some species. Our province is divided into multiple BMZs (bait management zones) and regulations exist for transport of baitfish between different BMZs - if you buy baitfish from outside your home BMZ (where your primary residence is located) you are required to carry a receipt of the purchase and they must be disposed of within 2 weeks, meaning if you want to be by the book legally you must purchase/harvest baitfish from within your home BMZ. Do note that a fishing license does not allow you to "culture" baitfish (produce in any capacity for a commercial or stocking operation).
More Ontarians should look into keeping aquacultured natives, some of the coolest fish ever. Upside is that aquacultured trout seem to have better temperature tolerances than wilds - here is one of my 3 browns, temp hovers between 68 and 72 depending on season and they're doing great. The aquaculture biologist that runs the facility I got these from said that his brook trout have no problems with temps in this range as long as oxygenation is good:
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