Most of the Hummingbird Fish Finders have the Fish ID option and Fish ID Alarm which are very useful options to use. A fish finder usually displays fish as arches or fish icons, depending on the view you have selected. That, of course, if your Hummingbird fish finder has both modes. Some of the fish finders in the 100 and 300 series only display fish as fish symbols on the screen.
The size of a symbol displayed on the screen indicates the intensity of the sonar return. Also, as the fish will be displayed as arches, the bait fish appear as clouds.

In case you have a Hummingbird fish finder from the 500+ series, with a full color display, and a Dual BeaPlus sonar, there are a few things that you should understand about the Fish ID option.
Once you activate the Fish ID, the fish caught by the 200 kHz sonar beam will appear in orange, in three different sizes, representing the intensity of the sonar return. The returns categorized as fish which are caught by the 83 kHz sonar will appear in blue, also in three sizes, depending how big they are. Blue or orange, each fish symbol will have its depth displayed above it.
The Fish ID Alarm obviously sounds when the fish finders detects a fish, or when the sonar sends a return which is interpreted as a fish. You must understand that not exactly every arch or fish symbol on your screen is really a fish. Sometimes it can be just a sunken wooden log, or the branch of a sunken tree.

On most Hummingbird fish finders you can set up the alarm to sound for different sizes of fish. For example, on a Hummingbird Fish Finder 798ci HD SI Combo, you can set up the alarm to warn you if you pass above large, medium and large, all fish and, of course, there’s the “OFF” mode.
Some anglers might find the Fish ID Alarm pretty annoying and rely only on the images on the screen.
Another aspect to mention here is that the more advanced fish finders from Humminbird also have a Fish ID Sensitivity option. So, you can adjust the sensitivity to allow the weaker returns of the sonar to be interpreted and displayed as fish.
This option may be useful to locate smaller species of fish, or bait fish.
Lastly, to adjust the sensitivity, to set the alarm for certain fish or to turn on or off the Fish ID option you will have to use the LEFT or Right cursor after you have selected the corresponding menu.