The Australian Dietary Guidelines advise eating one or two meals with fish every week for good health. Pregnant women can safely eat one or two meals of fish each week but they should choose the type of fish carefully.

Avoid fish with high levels of mercury - these are billfish (swordfish, broadbill and marlin), shark (flake), orange roughy (sea perch), gemfish, southern blue fin tuna and catfish.
Limit other fish, such as tuna steaks, to one portion per week or two 140g cans of tuna per week (smaller tuna contain less mercury).
There is no restriction needed on the amount of salmon, including canned salmon, which is eaten.
Anchovies and sardines should be OK. Im not sure about muscles and oysters though.
Most shellfish are filter feeders and can absorb all sorts of yucky stuff. I think I would give them a miss or only have a small amount. Also I would not advise eating any raw fish or shellfish.