I can say first hand that it salmon identification is harder than it looks, especially when you're rusty or new to it. If you have four salmon species sitting beside each other and can compare it would be easy. But that's not the way it works out.
Let's say in the heat of battle, you get a keeper size fish, maybe 6 or 7 pounds. Has been a slow day -- sure would be nice to have something for the BBQ, you want to keep this fish if it is legal.
You get out the DFO book:
Spots on tail: spring, pink, or coho. Big spots pink, small spots spring, some spots coho. But what is a "big" spot when you haven't caught a fish this year? And what about that spring that only has some spots? Spring and pink ok, coho closed if not hatchery.
No spots on tail: sockeye or chum. Neither have big teeth, neither teeth in tongue. Chum has white anal fin, but so does sockeye in the picture. Chum always open, sockeye rarely.
Black mouth: spring. But picture of pink has a lot of black in mouth too (teeth on tongue being the key, but better read close).
Big scales: again, first fish in a while, how big is a big scale?
Slimy: yup, pinks are slimier, but again, all fish are slimy when it's been a while.
Meanwhile the fish is flopping, other guys on board are still trolling with the other rod, the boat is bouncing...got to decide quick. Benefit of the doubt on your side or the law's? By now, the fish is effectively dead anyway -- been netted, flopping, bleeding, etc.
Not saying it's right, just saying that's what happens.
For this guy (next time it happens), I'd suggest a friendly explanation of the difference so that he'll know for next time. Learning from the DFO book is hard -- having someone point it to you firsthand would be very helpful for most novices. As you say, if got snippy about it at that point, then you could go to the next level if you wanted.
That's my 2 cents.
