Identification of fish responsible for five layers of echoes recorded by high‐frequency (200 kHz) echosounding in Lake Rotoiti, North Island, New Zealand

Abstract

Abstract Layers of echoes, segregated by depth, were recorded by high‐frequency (200 kHz) echosounding in several lakes near Rotorua, New Zealand. The fish species responsible for the echo layers in Lake Rotoiti are identified. Schools of 1+ ‐year‐old smelt (Retropinna retropinna) produced a band of spike‐like echoes between 2 and 10 m depth. Below this layer, larval bullies (Gobiomorphus cotidianus) produced a second, scattering‐type layer. A third layer of echoes, below the scattering layer, was produced by 2+ ‐year‐old smelt. Smaller fish were responsible for a deeper, fourth layer of echoes, apparent only in Lake Rotoiti, but the actual species or life history stage was not identified. Echoes within 5 m of the lake bottom at depths over 40 m comprised a fifth layer and were caused mainly by adult koaro (Galaxias brevipinnis). Echoes from trout (Oncorhynchus mykiss), from schools of smelt in Layers 1 and 3, and from adult koaro were of a similar amplitude. As acoustic surveys of trout density will depe...