Effects of substrate and sedimentation on the abundance of upland bullies (Gobiomorphus breviceps)

Most indigenous New Zealand freshwater fish are benthic, and use substrate elements for shelter, cover, and nesting sites. Sedimentation can reduce th e amount of interstitial space provided by the substrate, thus reducing the amount of available cover and potentially influencing fish abundance. To t est the effects of sedimentation on fish density, upland bullies (Gobiomorp hus breviceps Stokell) were introduced into experimental channels with diff erent substrate treatments. After an acclimatisation period, they were allo wed to emigrate. The maximum fish density and relative habitat suitability was then determined by the number of fish remaining after 6 days. Bully num bers declined by over 50% after 6 days in the trials with no cover, whereas with 20 cobbles providing cover for 40 bullies, numbers reduced by only 10 %. When sufficient fine sediment was added to fill the spaces under the cob bles, bully numbers reduced by more than 60%, regardless of the number of c obbles. However, when cobbles were raised above the fine sediment, the numb er of bullies remaining in the channels was similar to the number remaining with cobbles and no sediment. The overall effect of high levels of sedimen tation on bully density was similar to that of providing no cover, with the effect diminishing as the level of sedimentation decreased.