interesting, what do the walnuts do in that recipe? i'm allergic to all nuts.
I'd guess it's to substitute the richness from the fish, as well as to balance the sunflower seeds to have more depth of flavour. They are also a bit softer and more crumbly than sunflower seeds. I've definitely made a tuna salad with sunflower seeds and no walnuts though, and I remember it being pretty good. I'm partial to shredded and drained jicama though.