Vertical segregation and seasonal changes in fish depth distributions between lakes of contrasting trophic status

Abstract

High frequency (200kHz) echosounding was used to determine vertical structuring and seasonal changes in the depth distributions of limnetic fish in three New Zealand lakes. Juvenile (f.l. 30–50 mm) and large‐sized (f.l. 50–80 mm) smelt Retropinna retropinna were segregated into discrete layers by depth in all three lakes throughout the year. Larval and post‐larval bullies Gobiomorphus cotidianus (f.l. 8–20 mm) formed a further layer between the layers of small and iarge‐sized smelt. Although the depths of the large‐sized smelt and bully layers changed on a seasonal basis, vertical segregation between the three layers of fish persisted throughout the year, even when the lakes were homothermal. Seasonal movements of fish were disrupted by the autumnal deoxygenation of the hypolimnion in Lake Rotoiti. The schools of large‐sized smelt were forced to ascend towards the layer of bullies, but segregation between these smelt and the bullies was maintained as the smelt occupied a much reduced depth range above the hypolimnion, and just below the bullies. The winter descent of bullies, which precedes their transition to a benthic existence failed to occur in Lake Rotoiti, even though the lake was fully mixed at this time.