What YOU see on Screen

Fish Arch Diagram

USING THE 405SX

Thermoclines. Thermoclines are

sharp

differences

water

Thermocline Second Return

temperature.

These

are

easily

identified by

the

continuous

nature of the return.

Second Returns. When a sonar

signal is reflected off the bottom

back to the transducer, there is

often enough energy left in the

signal to be reflected off the

surface of the water back to the

bottom a second time. Second

returns appear as a slightly

weaker

bottom representation

exactly twice the depth of the primary bottom return. The second return is most likely to occur in shallow water and in areas of relatively hard bottom.

Fish Arches. With Fish ID off, schools of bait fish as well as individual fish are clearly visible on the 405SX display. Bait fish appear as "clouds" having different shapes and sizes depending on the number of fish and boat speed. Individual fish appear as smaller black and gray lines often appearing as a "fish arch."

A fish arch forms as the boat move over the fish. Due to the transducer beam angle the distance to the fish decreases as it moves into the beam, and then increases as it moves out. When the window graphs this distance change, an arch appears. The true depth of the fish is the top of the arch when the boat is directly over the fish. Boat speed and movement of the fish greatly affect the shape of the arch. When moving slowly, a fish creates an elongated arch. With the boat moving fast the arch appears shorter. A partial arch forms when the fish does not move through the entire cone angle.

The 405SX displays structure such as submerged grass, brush, trees and wrecks on the bottom. Structure can be distinguished by comparing the area just above and below the main bottom return. Usually structure shows as areas of dark to light gray on top of a dark bottom contour. The appearance of structure is greatly affected by boat speed and direction; to