Example: Finding Atlantic Mackerel with Low Frequency Sonar
As an example one pelagic species which is difficult to find with low frequency sonar is the Atlantic Mackerel (also known as the Spanish Mackerel) due to its lack of a swim bladder. There are more than 60 types of Mackerel in the world and more than half do not have a swim bladder. Without a swim bladder the only parts of the fish which can reflect sound waves are its bones which are not very large. The best frequency to find Atlantic Mackerel is any frequency higher than 85 KHz. At 90 KHz for example it is possible to see a small school of Mackerel at 300M to 400M in good conditions.