The fish finder is a tool that sends an electric impulse which is converted into a sound wave. What can be seen depends on the frequency and power of the pulse transmitted.
The speed of sound to the water column depends on salinity, pressure and temperature. That means that the pressure the temperature and the salinity affect the frequency of the signal. The bigger the speed of sound the more data in a certain bandwidth. The speed is proportional to the depth and the salinity and inversely proportional to temperature. In deep saline water in cold temperature the amount of data may be bigger.
The fish finder works with the logic of the sonar and the result is given by calculating the difference of the speed of sound in the return signal because of the presence of an obstacle such as fish.
Deep trawl fishing boats use low frequencies to see at the bottom of the sea at 50-200 kHz. When the frequency of the transducer is high then the result is more detail at the screen. In more recent boats the fish finder see in higher frequencies in details and in screens that can be split in multiple parts for better observation.
The bandwidth of the fish finder plays a role also. The wider bandwidth and advanced digital signal processing in the fish finder are better at defining weaker signals from fish such as tuna.
The bandwidth is the ability to transmit data per second basis. The more thick the bandwidth the better chances to catch a weak signal of a fish.
Frequency is the number of waves that pass a fixed place in a given amount of time.