Summary
Laboratory studies were conducted on 15 schools of blackchin shiners, Notropis heterodon, to determine if they altered their structure in response to changing environmental demands. The hypothesis tested was that fish schools should sacrifice a flat, hydrodynamically efficient structure in favour of an unobstructed visual field in the presence of a predator by staggering in the vertical plane. Ten schools were exposed for two weeks to a simple environment with only a current. For the next two-week period a predator was added. Five control schools were exposed to the simple environment for both two-week periods. Six of the ten treated schools increased their staggering in the vertical dimension as predicted while none of the control schools changed. This result was suggestive that hydrodynamic advantages were sacrificed. Respirometer experiments indicated these fish were capable of achieving some hydrodynamic benefits from schooling but these benefits may be a function of fish size.
Methodology
Blackchin shiners, Notropis heterodon, were obtained in a series of seines during late August and September of 1981 from Lake Opinicon (40 km northeast of Kingston, Ontario). They were maintained at the Queen's Biology Department in well oxygenated flow tanks (86 cm by 47 cm by 31 cm) in groups of thirty with a 12 h photoperiod and fed a diet of fish flakes (Nutra Fin).
A circular channel was used to simulate different environments. The channel was created by placing a steel ring (102 cm by 76 cm) in the center of a wading pool (183 cm by 76 cm) and a 7 cm sec -1 current was generated by a submersible pump (Fig . 2). The test area was restricted to a third of the circumference of the channel by pieces of fiberglass screening and coloured white allowing the fish and their shadows to be clearly visible on film. The apparatus now simulated an environment where the only function required by the school was to swim against a current.