The population dynamics, food and reproductive habits of rudd (Scardinius erythrophthalmus L.) in New Zealand

Abstract

Rudd (Scardinius erythrophthalmus L.) were imported into New Zealand in 1967. Their existence in this country provoked great anxiety regarding possible competition with the established trout populations, and led to the present study of population dynamics, food and reproductive habits of rudd. Growth in total length was found to be exceptionally rapid. Maximum observed total lengths of rudd in their first, second, third and fourth years were 120 mm, 155 mm, 198 mm and 246 mm, respectively. Although a change in the length: weight relationship indicated that there was a short period of slow growth in the winter, this was not reflected in annuli formation on body hard-parts in rudd captured north of Auckland. The onset of sexual maturity was also very rapid. Both sexes matured after one year. Two major spawnings were observed in each year and microscopic examination of ovaries indicated that there were at least some multiple spawnings by females in a single season. Fecundity at each spawning was found to be at least as great as that observed in rudd from the Northern Hemisphere (in New Zealand a 33 gram fish was found to contain 15,550 eggs, while an 89 gram fish held 41,500 eggs). Recruitment of young-of-the-year fish also occurred at discrete times and seemed, from population abundance estimation in the Riverhead pond, just to compensate for the poor (calculated) rudd survival of 11.5% per year. Food habits were very similar to those of rudd in Irish waters. Three size related divisions became evident after analysis of food preferences of rudd in New Zealand. From the onset of feeding to 75 mm, total length, rudd mainly consumed zooplankton. When they reached 75 mm to 135 mm, their preferred food had become terrestrial and aquatic insects. Rudd larger than 135 mm were mostly herbivorous. Information on rudd population biology gained over the course of this study suggested that competition with trout, should it exist, would most probably occur between rudd and rainbow trout (Salmo trutta) sub-adults (40 mm to 100 mm, total length). Subsequent experimentation indicated that trout tended to interfere (physically) with rudd confined within the same environment. In spite of this interference by trout, there was a suggestion that rudd were still able to compete successfully for the food resource.