Echo Sounder Use in Thailand: Pay Lakes to Offshore Deep Water

The echo sounder is the single most informative piece of electronics a Thai angler can carry, yet it is also the most commonly misapplied. A unit purchased for offshore sailfish trolling at 200 metres depth will produce a useless smear at the 4-metre depth of a Bangkok pay lake. Conversely, a shallow-water unit calibrated for freshwater reservoir use will fail to show bottom contact at 60 metres in the Andaman Sea. Getting the transducer specification right for the actual fishing environment is the foundational decision, and everything else — reading arches, interpreting structure, understanding the display — follows from that.

Understanding Transducer Frequencies

The transducer is the physical element that sends and receives the acoustic signal. Its frequency determines the balance between detail at shallow depth and range at deeper depth.

High Frequency (200 kHz)

At 200 kHz, the acoustic beam is narrow (typically 6 to 12 degrees cone angle) and produces fine detail — individual fish can be separated clearly, bottom texture is visible, and the image resolution on screen is high. The limitation is depth penetration: above 100 metres, the signal attenuates significantly, and at 200 metres it is usually inadequate for reliable bottom contact.

In the Thai context, 200 kHz is the correct choice for shallow freshwater venues: pay lakes, reservoirs to 30 metres, tidal creeks, and inshore coastal fishing to depths of 50 metres.

Low Frequency (50 kHz and Lower)

At 50 kHz, the beam is wider (20 to 30 degrees) and penetrates to depths beyond 300 metres with good bottom contact. Individual fish target separation is poorer than at high frequency, but large bait ball formations, bottom structure, and thermocline layers are clearly visible. This is the correct frequency for offshore deep-water jigging in the Andaman at 80 to 200 metres, for deep canyon fishing south of the Similan Islands, and for FAD fishing in the Gulf of Thailand where pelagic fish may be holding at 60 to 150 metres.

CHIRP Technology

CHIRP (Compressed High Intensity Radar Pulse) sounders transmit a sweep of frequencies rather than a single frequency. An 83/200 kHz CHIRP unit transmits through both frequencies simultaneously, providing a composite image that combines the depth range of the lower frequency with the target separation of the higher. For Thai fishing where a single unit must serve multiple environments — freshwater pay-lake trips, coastal reef fishing, and occasional offshore charters — a dual-frequency CHIRP unit is the most versatile investment.

Pay-Lake Sounder Use

Shallow-Water Setup

Most Thai pay lakes range from 3 to 8 metres in depth at their deepest points. In these conditions the sounder works at minimal depth, and its primary value is not finding fish by depth — you cannot cast accurately to a fish shown 40 metres away horizontally, and the platforms are fixed — but in revealing the lake's bottom structure: where the level bottom becomes a depression, where gravel gives way to silt, and how deep the water is at the swim's casting range.

Set the depth range manually to 1.5 times the maximum depth of the lake. Automatic depth ranging at a Bangkok pay lake will cycle between extremely compressed and expanded ranges as the transducer passes over the shallow margins and slightly deeper centre — manual setting gives a stable, consistent picture.

Reading Bottom Composition

Soft silt shows as a thick, fuzzy bottom return on the sounder display. Hard sand or gravel shows as a thin, dense line. Clay or compacted mud shows as a double bottom return — the primary signal and a ghost echo immediately below. At Bungsamran Lake in Bangkok, the areas of soft silt in the deeper basin are where catfish hold during the heat of the day, while the harder substrate near the platform edges is preferred by carp. The sounder confirms what experienced local anglers already know, but for a visiting angler it removes guesswork about the bottom at the specific swim being fished.

Fish Arches versus Clutter

At pay-lake depths, the sounder display often shows considerable clutter — small particles, algae suspended in the water column, and interference from other sounders running on adjacent platforms at busy venues. Learning to distinguish a fish arch from this background requires calibrating the sensitivity setting correctly.

Start with sensitivity at 70 to 75% of maximum. A fish arch should appear as a defined curved mark with sharp edges and significant colour difference from the surrounding water column. Clutter appears as diffuse, irregular patches of colour. If the display shows solid colour through most of the water column, sensitivity is set too high — reduce it until the water column between the surface and bottom shows predominantly blue or black with isolated marks for fish returns.

Cheow Lan and Reservoir Side-Imaging

Why Cheow Lan is Unique

Cheow Lan reservoir in Surat Thani province is unlike any other major Thai fishing venue in its acoustic properties. The reservoir was created in 1987 by flooding the Khlong Saeng valley, submerging a karst limestone landscape of pinnacles, caves, and steep-sided towers. The resulting underwater topography creates acoustic echoes of unusual complexity: the same karst pillar produces returns from multiple angles simultaneously, and the vertical faces of flooded cliffs generate confusing double-return artefacts on a standard sounder.

Side-imaging is significantly more useful than downward-only sonar in this environment, because it illuminates the near-vertical structures laterally rather than relying on a downward-pointing beam to image a sidewall. Side-imaging transducers emit a thin, high-frequency fan-shaped beam to each side of the boat, producing a horizontal image of the bottom and structure at up to 60 metres to either side.

Practical Side-Imaging at Cheow Lan

Set the side-imaging range to 30 to 40 metres — broad enough to locate the karst structures from the boat without creating a cluttered, hard-to-read image. Move the boat slowly (3 to 5 km/h) along the margins of karst towers to build a lateral picture of the structure.

Submerged forest — trees flooded when the reservoir was created — appears as a distinctive irregular canopy structure on the side-image, distinct from the smooth face of karst rock. This drowned forest holds giant snakehead, barramundi, and large freshwater fish that use the structure as ambush and shelter. A clear side-image of the forest canopy's depth range allows the angler to position a lure or bait at the correct depth.

Temperature thermoclines at Cheow Lan — the reservoir is deep enough (over 60 metres at the dam wall) to develop seasonal thermal stratification — appear on a sounder as a colour change in the water column rather than a defined line. Fish concentrate at or slightly above the thermocline where oxygen levels are adequate and baitfish are abundant.

Offshore Deep-Water Use: Andaman Sea

Depth Requirements

Offshore fishing in the Andaman Sea from Phuket, Khao Lak, and the Similan Islands charter bases operates across a range of depths: 30 to 60 metres at inshore reef edges, 80 to 200 metres at offshore pinnacles and the deep canyons south of the Similans, and beyond 200 metres at the continental shelf edge.

A dedicated offshore sounder must achieve reliable bottom contact at 200 metres minimum, with usable fish target returns to 150 metres. This requires a low-frequency element (50 kHz or below) in the transducer and a transmit power of at least 500W RMS. Budget units with 200W transmit power are adequate for depths to 100 metres but begin to lose bottom definition at greater depth.

Reading Bait Balls versus Fish Arches

In offshore Thai conditions, distinguishing bait balls from fish is one of the most practically important sounder skills. A bait ball appears as a dense, cloud-like mass of returns concentrated in a vertical zone, often with a slightly more open structure at the edges where individual fish within the school become resolvable. The mass does not show clear individual arches — the school density prevents this.

Predators — GT, Spanish mackerel, wahoo — appear as discrete arches above or at the edges of the bait ball mass. The arches may be brief, appearing and disappearing rapidly as the predators move. In the Gulf of Thailand near FAD structures, a sounder showing a dense bait ball at 40 to 60 metres depth with discrete arches scattered above it at 20 to 30 metres is a classic picture for a productive long-tail tuna and mackerel patch.

Ice-on-Display Warning in Thai Heat

Sounder display screens are not designed for prolonged direct exposure to tropical sun. A unit left in full sun on a boat console in Thai summer conditions (air temperature 36°C, surface temperature 50°C+) may develop liquid crystal layer damage — the display shows white patches or unresponsive areas that initially resemble the icing effect of over-chilling. This is heat damage, not cold damage, but the visual symptom is similar.

Mount the sounder in a location with shade access, or use a dedicated sounder cover when the unit is not in use. Most quality marine sounder units specify a maximum operating temperature of 55 to 60°C for the display — achievable in direct Thai sun on a metal or fibreglass console. A simple shade bracket fabricated from PVC tubing costs under 200 baht at a local hardware store and extends screen life significantly.

Network Integration: Sounder to Plotter

Modern sounder units with NMEA 2000 or Ethernet networking connect directly to chart plotters, sharing depth data, GPS position, and bottom composition information between screens. This integration allows a sounder mounted transom-side to display its information on the helm chart plotter simultaneously — a useful configuration for a sport fisher running offshore from Khao Lak or Koh Lanta where the skipper needs navigation information at the helm and the sounder information is needed at the fishing cockpit.

In a pay-lake context this networking capability is irrelevant, but for anglers who use the same electronics across multiple vessel types — a portable transducer bracket for pay-lake use and a transom mount on a charter boat — buying into a networked ecosystem allows the same display unit to serve both environments with appropriate transducer swaps.

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