Jimbo wrote on May 4^th, 2013 at 11:56am:

Ditto to what Westwood has said. While I HAVE actually witnessed fish taking the hook & being reeled in on a friend's larger, more powerful, higher resolution "fish" finder, my small unit packs more easily & answers my main need: to establish depth.  Am I in lake trout depth or walleye depth?  Tells me which lures to put on the end of the line or what water I need to move to depending on what I'd like shore lunch to  look like. Having said that, in the interests of minimizing weight, I COULD be persuaded to leave my depth finder at home if I KNEW there were no lake trout present in the waters I would be fishing.  If there was a lot of portaging or bushwhacking involved, I might just leave it at home if no lakers were to be had [this remark comes from someone who has finally learned his lesson from chronic "overpacking" on bad-ass trips]. Jimbo   8-)

I'm hardly what you'd call a seasoned veteran when it comes to fishing but based on the parameters that you've set for what you need to know, Jimbo, I wonder if a depth finder is the best thing considering the bulk, weight etc.  Consider a Fish Hawk
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This is a tool that fits in the palm of your hand and weights a couple of ounces.  If you're concerned about trout (or Walleye,) it seems to me that depth isn't really what you want to know anyway.  What you want to know is temperature.  Trout tend to be where the water temp. is between 48 and 53 degrees.  The Fishhawk will tell you the temp every 5 feet, the depth and whether your bait is in the zone.  That's a lot of useful info.  It won't tell you anything about structure.  So, unless somebody tells me about it or I happen to paddle over it or get some clue based on outcroppings, sand bars etc. I won't be able to know that there is an underwater reef, for example.  However, in terms of likely fishing spots, people have been fishing with great success without depth finders for years.  I can generally get a pretty good idea of what is going on under the water by paying attention to what is going on above it.  A hard drop off like a a cliff face above the water is probably not going to be a good place to fish whereas a gentle drop off or point with large rocks and small boulders ought to be a spot worth looking for Walleye.  And if there happens to be blow down and such like then there's a good chance for small mouth and pike etc., although I'm sure you get all that already.  I don't know if this Fish Hawk will be the ticket for me.  I bought one and I'll be trying it this summer when I stalk the wily trout.  I'll let you know if it's blaster or disaster come September.  I have high hopes.  I did pretty well last summer based mostly on guess work so I'm hoping a little tech will just add to my success.  But really, I don't quite buy the notion that without a fish finder, you're wishing, not fishing.