HTI (Hydroacoustic Technologies, Inc.) – HTI is a nickname for the scientific instrument consisting of an HTI echosounder mounted onto a towed body. The echosounder emits sound waves into the water column and listens for sound which has reflected (echo) off particles in the water. The strength of the returning signal can be used to identify the size and abundance of particles (usually plankton or fish). The size of the particle that can be detected is dependent upon the frequency of the sound emitted by the echosounder. The higher the frequency, the smaller the particle that can be detected. For krill, a frequency of 150 – 200 KHz is often used.