Dietary analysis of the herbivorous hemiramphid <i>Hyporhamphus regularis ardelio</i>: an isotopic approach

Abstract

The stable isotope values for a range of size classes of Hyporhamphus regularis ardelio from Moreton Bay, south‐east Australia were determined. There was a positive linear relationship between δ 13 C and standard length ( L S )(δ 13 C = 0·034 L S − 16·23; r 2 = 0·78). δ 13 C ranged from −8·48 to −17·29‰ with the smallest size class (50 mm L S ) being on average 1·04‰ enriched with respect to that of zooplankton ( Temora turbinata ) and 7·97‰ depleted compared to Zostera capricorni . δ 13 C was positively correlated with L S ( P &lt; 0·01)(more enriched with increasing L S ) with those fish of the largest size class (225 mm L S ) being 9·86 and 0·84‰ enriched than T. turbinata and Z. capricorni , respectively. There was no detectable trend in δ 15 N values with L S ( P &gt; 0·01) with δ 15 N, ranging from 9·18 to 11·00‰. Fish of all size classes were on average 2·32 and 7·63‰ more enriched than zooplankton and seagrass, respectively. Carbon isotope data indicate that H. r. ardelio commence life as carnivores and change to a diet in which seagrass is the primary carbon source. The dependence on animal matter, however, is always present. Due to the low percentage of nitrogen in Z. capricorni (2·5%) compared to zooplankton (9·1%) it appears that nitrogen from zooplankton is necessary throughout their life history with the carbon requirements for these fish coming chiefly from Z. capricorni .