I've encountered a tough surface when I've done salmon, and have never quite figured out how to avoid this. Your suggestion to reduce the temperature makes sense. Another possibility that I've toyed with is that it's taking too long to get the fish up to cooked temperature (I'm obviously hot smoking). I've got a brother-in-law in AK who sends down salmon, and those fillets are usually an inch or more thick. So I'm spending about 7 hours to get them up to temp.
Kummock has excellent advice about smoking times and temperatures for salmon in his Smoked Wild Alaskan Salmon thread at the beginning of this fishing sub-forum. This is what he says:
Step 5: SMOKING
Smoke using the following Bradley Smoking guideline:
100°-120°F for 1-2 hours, then increase to
140° for 2-4 hours, then increase to
175° for 1-2 hours to finish
Use the longer times given for thicker/higher oil content fish. As a general rule, the higher temp you use or the longer you hot smoke, the more the meat cooks the oils out, HOWEVER, the meat becomes dryer/tougher in the process. I've "accidently" left meat (silver salmon) at the 140-150°F range for up to 8 hours and it still turned out great. I personally believe that you'd have to try REAL hard to make a batch of smoke salmon unpalatable by over smoking/cooking. If you get white “boogers” on the meat, you’re cooking too high/too fast.I suspect that the reason the surface of your smoked salmon is getting tough is that you're leaving it in the smoker too long at a temperature which dries it out, but which is too low to produce the internal temperature necessary for the fish to be safe to eat. The objectives of smoking fish are to get smoke flavor into the fish, to produce a desirable amount of dryness and flakiness, and to get the internal temperature of the fish high enough so that the fish will be safe to eat. For some thoughts about the internal temperature required to produce safe smoked fish, see
http://forum.bradleysmoker.com/index.php?topic=30863.msg364765#msg364765 . The trick is to get the fish to the necessary internal temperature before it gets too dry overall and/or before it gets too hard on the outside. The seven hours you are smoking your salmon is drying it out before it reaches a sufficient internal temperature.
When cooking fish, you aren't trying to dry it out. In fact, you usually want it to be nice and moist when it reaches the internal temperature of 145 degrees which is required to produce safe cooked fish. So fish is cooked at higher temperatures for shorter periods of time. Salmon is typically baked at 350-425 degrees, and broiled at higher temperatures for shorter periods of time. And those temperatures don't make the outside of the fish get hard, because the fish isn't being cooked for long enough to dry it out.
So here's my suggestion, which is a little different from Kummok's recommendations: take the fish out of the brine and put it on your smoker racks in front of a fan for an hour to dry the surface a bit, which is called forming a pedicle. Then put it in the smoker for half an hour at 100 degrees with no smoke to finish drying off its surface, and to start raising the temperature of the fish. Then smoke it for an hour at 130 and an hour at 150, which I usually find is enough to produce a nice smoked fish flavor. Then quit smoking, and raise the temperature in the smoker to a moderate temperature, say 170-200 degrees, and leave it there until it reaches what you consider to be a desirable level of dryness and flakiness. How long this takes will depend on the kind of fish you are smoking, the thickness of the fish, and whether there is skin on the fish. If you find that when the fish has reached a nice level of dryness and flakiness, it still hasn't reached the necessary internal temperature, you didn't have the temperature high enough towards the end of your schedule. So next time you smoke that kind of fish, crank up the temperature higher towards the end of the schedule.
One thing to keep in mind is that different kinds of fish respond differently to being smoked. If I use the same brine and smoking schedule on kokanee salmon, rainbow trout, and mountain whitefish, they come out differently. And different kinds of salmon also smoke up differently. So another possibility is that the salmon you liked so much was a different kind of salmon from the salmon which you are currently trying to smoke.
Here's some kokanee, rainbow trout, and mountain whitefish which I smoked up recently, using Kummok's brine recipe and other recipes. These thin fillets can be smoked quite a bit faster than the thick salmon fillets which you are smoking, but the objective of my smoking schedule is the same: to get the fish to an internal temperature of at least 145 degrees before it gets too dry.
