Its that time of year again when apparently no one has anything else on their minds other than "what's the story with Costco (Kirkland) Frozen Salmon for Passover". If this product would have a Passover hashgacha, I wonder what I'd do all day. Since it doesn't, the office is inundated with various variations on this question.
OK, here's the story with the "why does it say I should rinse wild salmon" inquiry:
1. Kirkland sells both wild and farmed salmon products in their freezer section, typically 3lb bags with multiple portions (individually wrapped in clear plastic, often mixed with a salt brine to keep them moist). See below:
2. A concern was raised several years ago that some wild salmon fisheries glaze the fish with a solution made of a minute amount of corn syrup (batul MANY times past shishim). Most glazes are plain water, or salt water. The purpose of the glaze is to protect the fish from bruising and dehydration, so the advantage of adding small amounts of corn syrup is minimal and its use (while legal) isn't all that extensive.
3. Corn syrup itself is kitniyos shenishtaneh, meaning that even for Ashkenazim we would normally be lenient (particularly since the amount is VERY batul, and it is used before Passover and otherwise unable to be detected). However, since there was a concern, it was deemed appropriate to recommend consumers rinse the offending glaze off of the wild portions.
4. Why are wild different than farmed? Due to the nature of the processing plant, we have never seen farmed fish glazed prior to their injection with the salt water brine (which effectively does that job).
5. As such, it was recommended to rinse wild, with no need to rinse farmed, and technically there is no need to rinse wild either (just a Passover stringency).