I will be upfront. I am not a marine-electronics expert. I do not claim to understand every sounder setting or interpret every mark perfectly.

What I can do is set my sounder up effectively, find productive ground and use the information on the screen to decide where to fish. This lesson shares the practical approach that works for me. It is not a technician’s guide to sonar theory.

You do not need to understand every setting in every menu. You need to recognise fishable ground, understand what your sounder is showing you and decide whether an area deserves a drop.

What You Will Learn

• How to research potential ground before leaving home
• Why contour lines alone can be misleading
• How to turn a search mark into an accurate fishing mark
• What useful bait and fish returns can look like in deep water
• Why quiet-looking ground can still be worth a drop
• How manual gain, frequency and bottom-focused views improve the image
• How to turn sounder information into a planned drift

The Search Begins at Home

When you are travelling a long way offshore to fish the continental shelf, you cannot afford to arrive without a plan. Fuel is expensive, fishing time is valuable and an enormous amount of ocean can look completely featureless from the surface.

Before a trip, I identify several areas of potential ground and save some initial search marks. These are not guaranteed fishing spots. They are simply places that look promising and give me somewhere worthwhile to begin sounding when I arrive.

I generally look for:

• Ledges and drop-offs
• Ridges and isolated rises
• Canyons and depressions
• Steep changes in depth
• Broken or irregular ground
• Small features sitting within a larger slope

Rather than putting all my faith in one mark, I save several prospects and group them into a practical route. Some will be worthwhile. Some will look completely different when I arrive. That is normal.

The purpose of planning is not to guarantee a catch. It is to narrow an enormous area of ocean into a manageable list of places worth investigating.

Contour Lines Can Be Misleading

Traditional contour lines are useful for understanding the broad depth and shape of an area, but they do not always tell the full story.

I have wasted plenty of time and fuel travelling to impressive-looking contours, only to arrive and find virtually nothing there. These phantom structures can look like perfect fishing ground on a chart, but the live depth and sounder image may bear little resemblance to what was mapped.

Contours can also mislead you in the opposite way. They can make genuinely interesting ground look completely unremarkable.

Look at the contour-only view above. It appears to show an ordinary area with little to make it stand out. If I relied on those lines alone, I would not give it a second look.

Once Relief Shading is switched on, smaller rises, channels, depressions and irregular sections become visible. When the area is enlarged, there is a substantial amount of broken ground that was almost invisible on the standard chart.

Two of the most useful tools in my planning are Navionics Relief Shading and BathyMaps. In my experience, both are generally more reliable than standard contours alone. They are not infallible, but when they show a clearly defined feature, there is a much better chance that the ground will actually be there.

Navionics Relief Shading presents the seabed as a three-dimensional surface, making larger ridges, canyons, ledges and changes in gradient much easier to identify than with contour lines alone.

If a feature appears promising across more than one source, it moves higher on my list.

BathyMaps provides detailed bathymetric imagery that can reveal isolated rises, channels and broken ground across large areas. I use it to build a shortlist of search marks before travelling offshore.

From Search Mark to Fishing Mark

A coordinate saved at home is an initial search mark. It is not necessarily the exact place where you should drop a jig.

When I arrive at a new mark, I approach while watching the depth and sounder. I cross the feature from different directions and use the boat track to avoid searching the same water repeatedly.

I want to establish:

• The feature’s exact position and depth
• The location of its top, face, base and edges
• Where the steepest or hardest sections appear
• Whether bait or fish are present
• Which part deserves a fishing drift

One pass might show where the depth begins to change. The next may reveal a sharper face, a harder section or a concentration of bait. With each pass, I build a clearer picture and save more accurate marks.

This is how an approximate search mark becomes a genuine fishing mark.

What Am I Looking for on the Sounder?

One of the questions I am asked most often is:

My normal response is:

Deep-water fishing is no different from the way you normally use a sounder. If you pass over promising structure and see a strong show of bait, fish or concentrated activity near the bottom, set up for a drift.

Returns will look different between sounders, colour palettes, transducers and settings. They may appear as arches, lines, streaks, blobs or colours connected to the bottom. Bait might look like a dense cloud, a loose patch or a layer rising from the seabed.

You do not need to identify every return or know exactly which species is there. Ask yourself:

You will know when you see it. Strong returns rise 10-15 metres above the bottom in approximately 300 metres. I may not know exactly what species are there, but I would be setting up for a drift.

Kingfish showing clearly in approximately 220 metres. We set up for a drift and the accompanying video shows the resulting hook-up.

This is the kingfish hook-up that followed the sounder show above.

Strong activity close to the bottom in more than 400 metres. Perfect individual arches are uncommon at this depth, but there is clearly enough happening to justify a drop.

This is the Blue-eye Trevalla hook-up that followed the sounder show above.

A thick layer of bait rising approximately 5-10 metres off the bottom in 415 metres. Individual fish may be difficult to separate, but the concentration of life is unmistakable.

When the Screen Says No One Is Home

Strong shows make the decision easy, but deep-water fishing does not always provide that confirmation.

I have caught plenty of fish, including some very big fish, when there was almost nothing obvious showing. The ground looked right, but according to the sounder, nobody was home.

Fish may be holding so tightly against steep or broken structure that their returns merge into the bottom. They may be sitting within cracks or depressions. The depth, terrain and capability of the electronics may also make them difficult to separate. I cannot say for certain.

What experience has taught me is not to dismiss good ground simply because I cannot see obvious fish marks.

Dropping a heavy jig through 250 or 300 metres takes time and effort, especially when the screen is not giving you much encouragement. At some point, however, you have to stop looking and start fishing.

• Good ground with strong bait or fish shows: definitely set up for a drift.
• Good ground with very little showing: still worth checking.
• Poor ground with no activity: keep searching.

Sometimes the only way to find out whether anyone is home is to send a jig down and knock on the door.

Be Realistic About Deep-Water Returns

Beyond 300 metres, most recreational sounders will not consistently display perfect individual fish arches, particularly when fish are holding close to the bottom.

The full depth range compresses the bottom portion of the display. Fish may merge with the seabed, while movement, beam coverage, frequency, gain and filtering all affect how returns appear.

That is why finding good ground is often more important than waiting for a perfect fish arch.

More powerful electronics can improve your chances of seeing useful returns, but power alone does not determine performance. The display, sounder module, transducer, frequency, installation and settings must work together.

My system uses Garmin displays, a GSD26 sounder module and separate 1kW Airmar B175L and B175M transducers. The low-frequency B175L is my main option for serious depth. The B175M provides a narrower medium-frequency beam and more detail where conditions and depth allow.

A 2kW or 3kW system may provide stronger returns in extreme depth, but most recreational anglers are not running electronics of that specification. Good installation and correct setup remain essential at any power level.

In simple terms:

• Lower frequencies generally provide better deep-water penetration and a wider beam.
• Medium frequencies generally provide more detail and a narrower beam.
• CHIRP transmits across a range of frequencies and can improve target separation.
• If you are using a Medium or High CHIRP transducer, take it out of CHIRP and try a fixed frequency at the lowest frequency available within that band. For example, if the Medium range is 85 to 135 kHz, set it to 85 kHz. In my experience, this can provide a clearer and more stable picture in deep water and while travelling at speed.

Do not judge your sounder against perfect marketing images. Learn what your own system can show and get the most from it.

Take Your Sounder Out of Auto

One of the most important changes you can make is to take your sounder out of Auto.

Auto can provide a starting point, but too many anglers set their sounder once and forget about it. The settings that worked at one depth or over one type of bottom may not provide the best image somewhere else.

Keep Adjusting the Gain

Gain controls how much returning information appears on the screen. Too little can create a beautifully clean display while hiding weaker returns. Too much can fill the screen with clutter.

Experiment with it. Increase the gain and see what appears. If the screen becomes too cluttered, reduce it slightly. Do not assume the setting from the previous mark is still correct.

Increase the Scroll Speed

In deep water, I generally run the scroll speed at the fastest setting. Scroll speed controls how quickly the sounder history moves across the display. It does not make the sounder penetrate deeper, but a faster setting spreads the returns across more of the screen and can make changes in the bottom, bait and fish activity easier to recognise.

Slower scroll speeds compress more history into the display. That can make deep-water returns look crowded or harder to separate. Start fast, watch how the image is being drawn and adjust if your particular unit produces a picture that is too stretched or difficult to read.

Experiment With Colour Filter or Colour Erase

This setting has different names on different sounders. It may be called Colour Filter, Colour Erase, Colour Threshold or something similar.

The basic idea is to remove part of the weaker colour spectrum from the display. In very deep water, this can reduce low-strength clutter and leave the stronger returns easier to see.

Make small adjustments and watch what disappears. If you remove too much colour, you may also erase weaker bait or fish returns that are worth seeing. The aim is not to create an empty screen. It is to reduce enough clutter that the strongest targets and bottom detail become easier to interpret.

A Real Example

Before this screenshot was taken, I had been fishing in more than 400 metres and had increased the gain to display weaker returns. When we moved back into approximately 254 metres, I had not yet reduced it. The screen was heavily cluttered and over-gained for the shallower depth.

Even through that clutter, a very strong concentration of organised returns appeared above the bottom. It was enough to make me stop.

We set up for a drift and sent down an 800g Slobseeker. The result was a 28kg Bar Cod.

The 28kg Bar Cod caught after stopping on the strong sounder show above.

The lesson is not to run excessive gain. It is to recognise useful information through the clutter, then adjust the settings to refine the picture.

The setup:

• Jig: 800g Slobseeker
• Braid: SeriolaPro DeepJigger PE3
• Rod: Fishlogica Deep Parabol
• Reel: Ocea Jigger 2501LD
• Result: 28kg Bar Cod

Focus on the Bottom

Displaying the entire water column in 300 or 400 metres compresses the most important information into a small part of the screen.

Most of our target species hold close to the seabed. Use zoom, bottom lock or depth shift to concentrate on the bottom 30 or 40 metres. This makes bait, fish, ledges and subtle changes near the bottom much easier to see.

Jim’s Tip

Turn the Information Into a Drift

The image moving across the display represents information that has already passed beneath the transducer. By the time a feature appears, the boat may already be moving away from it.

Finding ground on the sounder and putting your jig in front of it are different skills. Save an accurate mark, determine the direction and speed of the drift, then allow for the time your jig will take to reach the bottom.

A short dummy drift can help establish the correct starting point. Dropping directly on top of the mark may be too late. In deep water, you often need to begin well ahead of the target.

Detailed drift planning and boat handling will be covered in Lesson 7.

Build Your Own Knowledge Bank

Mark captures, bites and promising shows. Review your drift tracks and compare successful passes with unsuccessful ones.

One fish can be luck. Several catches from the same small section reveal a pattern.

Repeated trips allow you to refine broad search marks into precise pieces of productive ground. Over time, this knowledge can become more valuable than simply owning a more expensive sounder.

FIND THE GROUND. FIND THE FISH.

FROM SEARCH MARK TO FISHING MARK

1

🗺

Research at homeStudy charts and contours

2

☰

Cross-checkUse Relief Shading and BathyMaps

3

📍

Save search marksPlan several places to investigate

4

)))

Verify the groundSound it from more than one direction

5

◎

Mark the exact spotFind the top, face, base and best activity

6

⛵

Plan the driftAllow for drift direction and sink time

🗺 THE CHART GETS YOU INTO THE AREA ➜ ))) THE SOUNDER FINDS THE EXACT GROUND.

The process is simple:

1. Research potential ground at home.
2. Cross-check contours with Relief Shading and BathyMaps.
3. Save several search marks.
4. Verify the ground with your sounder.
5. Mark the exact feature and establish the drift.
6. Send down a jig.

Final Takeaway

Good electronics are valuable, but they cannot replace preparation and time spent exploring.

Use mapping to identify potential ground before leaving home. Treat coordinates saved at home as search marks, then use your sounder to confirm the structure and identify the exact section worth fishing.

Do not wait for perfect fish arches. Learn to recognise concentrated life, but do not dismiss good ground simply because the screen looks quiet. Take your sounder out of Auto, focus on the bottom and keep adjusting the gain as the depth changes.

Then make the decision the electronics cannot make for you. Set up the drift and send down a jig.

Coming Up in Lesson 6

Offshore Conditions: Weather, Currents, Tides and Moon

Finding promising ground is only one part of planning an offshore trip. Weather, swell, wind, current, tides and moon phase can determine whether an area is safe to reach and practical to fish.

In Lesson 6, we will look at how these factors influence trip planning, drift speed, jig selection and potential bite windows.

Technical References

• Airmar B175L specifications: https://www.airmar.com/Product/B175L
• Airmar B175M specifications: https://www.airmar.com/Product/B175M
• Garmin GSD26 product information: https://www.garmin.com.hk/products/onthewater/gsd-26-asia/