ASFB Home > 2003 > Biological Invasions: Consequences for Parasites, Pathogens, Emerging Diseases, and Fisheries in the Marine Environment.

Diel changes in density and size structure of giant kokopu within individual stream pools: Daytime tolerance vs nightime agro.

Bruno David¹, Gerard Closs², Eric Hansen², Shannon Crow²

¹ Northland Conservancy, Department of Conservation, P.O. Box 842, Whangarei, New Zealand.
² University of Otago, P.O. Box 56, Dunedin, New Zealand.
Email: bdavid@doc.govt.nz

Giant kokopu within eight individual stream pools were observed during the day and night for a two month period. The number and size of diurnally active fish within each pool was recorded on each day and compared through time. In addition, all fish within pools were individually identified by their unique markings. Recorded observations indicated that the population of individuals within each pool was remarkably stable during the study period. During the day, pools were consistently dominated by high numbers of fish representing a variety of size classes. In contrast, at night, each pool was consistently dominated by low numbers of large fish (typically the same one or two individuals) while smaller individuals were located in either riffle and run habitats or unseen. A removal experiment was conducted to determine whether the large resident individuals excluded smaller resident individuals from pools at night. Following the removal of large individuals, a significant increase in the number of smaller resident fish was observed in pools at night. These findings indicate that large dominant giant kokopu can influence distributions of subordinate fish at fine spatial scales within streams, especially at night.

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