Otolith retrieval from faeces and reconstruction of prey-fish size for Great Cormorant (Phalacrocorax carbo) wintering at the East Dongting Lake National Nature Reserve, China

Abstract

The food composition of Great Cormorant (Phalacrocorax carbo) was studied by analyzing 360 faecal samples collected from their roosting sites at the East Dongting Lake National Nature Reserve in south-central China during November 2008 and January 2009. A total of 223 fish otoliths were retrieved. Only sagittal otoliths (N = 74) with intact margins, good medial relief, and well-defined sulcus were used to identify the prey-fish species and to reconstruct prey-fish size. Through shape indices and elliptical Fourier analyses, these otoliths were identified as belonging to two cyprinid fish species, crucian carp Carassius auratus (72%) and white amur bream Parabramis pekinensis (28%). The otolith size (maximum length, Lo)—prey-fish size (standard length, Ls) relationships determined for the two fish species were Ls = −19. 5 + 37.1 Lo (N = 74; R2 = 0.95; P < 0.05) for crucian carp and Ls = −31.9 + 36.9 Lo (N = 70; R2 = 0.97; P < 0.05) for white amur bream. The back-calculated Ls from the otoliths retrieved from the faeces were 96.2 ± 10.2 mm (range 72.7–110.4 mm) for crucian carp and 86.3 ± 10.4 mm (range 74.1–104.6 mm) for white amur bream. Based on setnet sampling of the fish community near roosting sites, crucian carp and white amur bream were found as the first and second most abundant species. Great Cormorants were very selective for the size range of prey. These results demonstrate that analysis of otoliths retrieved from faeces can be an effective, inexpensive, and non-invasive method to identify prey-fish species and to estimate prey-fish size in studies of diet composition for piscivorous birds.