While the unit has many positive features, anglers should also be aware of its limitations. The SI unit's transducer must be located externally on the stern of a boat. The system works best at speeds under 10 mph and on straight-line courses, as turns distort the imagery.

How the stern is shaped and outboard motor is mounted influence whether the unit can "see" well to either side. Nothing may be placed in the line of sight to the sides of the transducer, and external mounting exposes the transducer to possible damage. Yet, I fish mine in Lake Fork, a water known for its hard-to-navigate stump fields, without damage.

I'm a beginner with the SI system and learn to better identify echoes and use other features on every outing. If you're a non-techie like me, you may find the complexity of menus and some options non-intuitive, but learning the details is rewarding. Users need to buy a map chip and may want another chip to record images for review with a personal computer.

Beyond conventional "fish-finding" sonars, the SI is worth the price if a user wants an advanced search tool.

Fishery scientist Ralph Manns, Rockwall, Texas, has contributed to In-Fisherman for over two decades, on topics ranging from conservation to features on how best to catch fish. He is a principal author of the Bits & Pieces column.

(Left screen shot) Mike Bucca, a bass guide (tritonmike.com) at Lake Allatoona, Georgia, located this school of baitfish surrounded by suspended spotted bass with his Humminbird 987c SI in SI mode. Bucca reported catching five of these large spots by vertically working a ballhead jig with plastic stickbait attached. The shadows of the bass indicate these are fish echoes.

(Right screen shot) One of several images of an underwater bridge and surrounding area that Ralph Manns screen-captured. Referring to several pictures of the same structure or cover objects from several angles greatly aids interpretation of what is shown in the imagery.

(Left screen shot) The ability of the SI to reveal details of underwater cover is shown here. This stump with a cluster of laydowns at its base produced two 6-pound largemouths at Lake Fork this spring. The shadow reveals the large stump of the main trunk that protrudes from the surface. The SI revealed a second laydown resting diagonally on the bottom.

(Right screen shot) A bed of milfoil, coontail, and filamentous algae at Lake Fork. Not all weedbeds are this distinct or provide as sharp a return. Note that the SI sees right into clumps of vegetation so the front edges of beds merge with the vegetation behind them. In some cases beds merely looked like bubbles or bumps on the bottom.