Lowrance Structure Scan (LSS): The Fishfinding Necessity

By Capt. Tim Egenrieder

I have been using Lowrance Structure Scan on my fishing charters since 2010. Side and down imaging started as a novelty that I would turn on when I was looking at rock piles and underwater cliff faces.  I now consider it to be the most useful fishfinding device on the market.

How it works: All fishfinders operate as echo sounders, which are a form of active sonar.  An electrical signal is sent from your fishfinder to your transducer that converts it into an acoustic pulse.  A conventional fishfinder broadcasts this pulse straight downward in the vlf and lf range of the radio wave spectrum at 50khz and/or 200khz. This sound wave (or beam) spreads into a cone shape as it travels away from the source, very much like a flashlight projected onto a distant wall.  Any object with a different density than seawater such as a fish’s swim bladder will reflect back (echo) this signal to your transducer.  The fishfinder then calculates the short amount of time between when the 3d cone shaped pulse was sent and received back then converting it to the 2d image you see on screen.

Lowrance Structure Scan (LSS) works differently in two key ways.  The primary difference is that the LSS transducer broadcasts a very narrow beam both straight down and side-to-side instead of the traditional cone shape.  This means the display is receiving a nearly 2d signal allowing for much more detailed 2d displayed image.  Secondly, the LSS transducer operates at the mf range of the radio wave spectrum at 455khz and 800khz, the same frequency range as on your AM radio dial.  As a rule, the higher the frequency, the greater the resolution.  These two significant differences are what allow for such vivid imagery of both structure and fish.

Setup: The sidescan is displayed full width on a 10” display.  I find that keeping the range at 80 feet to each side allows me to see an individual Striped Bass and a school is easy to spot.  The only time I zoom out further to 120 or 150 feet is when I am looking for a schooling fish like mackerel and then zoom into 80 feet when I find them.

Making Bait: Having sidescan provides a huge advantage when catching bait.  Mackerel are usually found outside of the outer islands of Boston Harbor near underwater structure.  I will get to a likely area and begin a search pattern.  Once I mark a school, I move the curser on my screen over on top of the school and simply hit – Enter / Enter, and a waypoint is set.  The downscan capability allows you to see the individual fish in the school of bait once you reposition the boat.  You can then look at exactly what depth that fish are holding and drop your mackerel rigs to an exact strike zone.

Shallow water: The LSS has a huge advantage is shallow.  The cone of the beam of a traditional transducer is measured as cone angle.  Most current traditional 200khz transducers have a cone angle of much less than 20 degrees.  This means if you’re fishing in less than 10 feet of water, the cone diameter on the bottom is well less than 3 feet.  You’ll be lucky to have a fish come into range at that depth. The sidescan capability of the LSS will give you significantly more range as it projects well beyond the limitations of traditional transducers.

The LSS allows you to view both sides or just left / just right.  If you’re running along a ripline or casting into a shoreline, you can set the device to only look into the shallow.  This allows you to set the range to well over 100 feet to the side and still make out important detail.  You can also look for offshore weed lines with this method.   When you approach the likely area, set the device to one side and look several hundred feet for a steady line.  The weeds and all of the trapped debris will be enough to create a noticeable mark on your screen

Identifying fish: The primary forage fish in Boston Harbor are mackerel, menhaden and river herring.  Mackerel typically appear in schools of 10-50 individuals and look like a loose ball of white dots.  River herring (bluebacks and alewives) are typically found in massive, tightly packed schools that appear on screen as giant wave of dots.  Menhaden (pogies, bunker) look larger and more tightly packed than mackerel.  Striped Bass look like a small white pencil line ¼” long.  A school is far more obvious.  It doesn’t seem like much but becomes easy to spot once you’ve trained your eyes.

Fishing Structure: Every harbor has humps, bumps, channels, edges and other structure that typically hold fish.  Boston Harbor is loaded with them.  The sidescan affords me the ability to approach a structure and scan until I find if the fish are holding in the shallow or the depths and on which side.  I can then set a drift and rig the rods for how the fish are holding in that moment rather than blind casting to a spot.  You can also determine the exact depth by moving the curser over a marked fish and drop your lure to the strike zone.

Everyone loves to see surface feeding stripers.  The fishing is easy and fun until the fish go back down because of the guy next to you – it’s always somebody else’s fault…  With the sidescan I can see which side they moved to and tell my clients, “cast to the port side, the left side, — no your other left.”  There have been many times where we could stay with an active school long after the fleet gave up on the fish returning to the top.

Good things don’t come cheap and new marine electronics are far from the exception.  That is the only downside that comes to mind when describing this functionality.  I can tell you that having this equipment onboard has made my fishing charters more efficient, enjoyable and successful.  I couldn’t be happier with the investment.