Population structure, distribution, reproduction, diet, and relative abundance of koaro (Galaxias brevipinnis) in a New Zealand lake

Abstract

The koaro population of Lake Rotoaira was investigated during September 1998 and April 1999. Abundance (mean CPUE) in fyke nets was lower on the eastern side of the lake than on the western side where underground springs occurred and rocky substrate predominated. Adults were 54–135 mm long and the sex ratio varied with size, with 68% of the smaller fish (60–7) mm) being male and 79% of the larger fish (70–135 mm) being female. All fish over (6) mm were sexually mature with 60% having either well developed or ripe gonads by September. Few females (12.5%) had spawned by September whereas most (83%) were spent by April, so spawning occurred over summer months. Abundance in the lake dropped by over 70% between September and April and this seasonal decline is consistent with a summer emigration of many adult koaro into inlet springs and streams to spawn in lake tributaries during summer and autumn. Overall, the diet of lake‐dwelling koaro was dominated by purse caddis larvae (Paroxythira sp.), but small fish (<70 mm) also fed extensively on Daphnia, whereas larger ones (>9 0 mm) fed on Odonata larvae, snails, and common bullies. Although the introduction of rainbow trout in 1906 reduced the koaro population of Lake Rotoaira, they were still relatively abundant in the lake in the early 1970s. However, by 1999, no koaro were found in the sublittoral zone of Lake Rotoaira, and trapping in the littoral revealed a relatively low abundance relative to other comparable lakes. These data, plus anecdotal information on abundance, suggest that they are now much less abundant than in the 1970s.