I too play in Lake Michigan! Salmon should be getting shallower very soon.
See below from a previous post, the text comes from the Lowrance web site but is generic concerning sonar technology;
I found some decent text on the Lowrance web site (http://www.lowrance.com/en/Support/Tips
... -Tutorial/ ), explaining why fish show up as arches when the watercraft is moving, note that when the watercraft is stationary or you're ice fishing, you will NOT typically see fish arches and you would tend to see straight horizontal lines appear and dissappear on the FF display as fish swim into, stay a bit and leave the transducer's cone angle;"The reason fish show as an arch is because of the relationship between the fish and the cone angle of the transducer as the boat passes over the fish. As the leading edge of the cone strikes the fish, a display pixel is turned on. As the boat passes over the fish, the distance to the fish decreases. This turns each pixel on at a shallower depth on the display. When the center of the cone is directly over the fish, the first half of the arch is formed. This is also the shortest distance to the fish. Since the fish is closer to the boat, the signal is stronger and the arch is thicker. As the boat moves away from the fish, the distance increases and the pixels appear at progressively deeper depths until the cone passes the fish.
If the fish doesn't pass directly through the center of the cone, the arch won't be as well defined. Since the fish isn't in the cone very long, there aren't as many echoes to display, and the ones that do show are weaker. This is one of the reasons it's difficult to show fish arches in shallow water. The cone angle is too narrow for the signal to arch.
Remember, there must be movement between the boat and the fish to develop an arch. Usually, this means trolling at a slow speed with the main engine. If you are anchored or stopped, fish signals won't arch. Instead, they'll show as horizontal lines as they swim in and out of the cone."
Jim; The partial arch as you describe may be that fish are simply not directly below your transducer, but I would think that you would occasionally see a decent arch. Do you ever see a full or 3/4 arch? if you never see a full arch it may be that your transducer is tipped a bit. Actually I would think that the t-ducer's face or bottom surface must end up being parallel to the surface of the water (in all directions) and not necessarilly just parallel with the bottom of the yak, although I'm not certain just how fussy one needs to be about being parallel.