Importance of the Landlocked Pond Smelt Hypomesus nipponensis as a food resource of the Little Tern Sternula albifrons on inland Andong Lake of Korea: A video image analysis

Abstract

We carried out the diet study of the little tern on the sandy islet in inland Andong Lake, Korea, during the beeding season (April to July 2018). To identify its diet and examine the importance of the main prey species as a food resource, we set two remote-control video cameras with 4K-resolution on the islet. One thousand two hundred seventy-five still images that the tern had prey in its bill were identified at the species level and measured on a monitor. Then, they were classified to five length-categories and compared among months and breeding stages. Freshwater fishes dominated the diet (100%; eleven species overall), where the landlocked pond smelt Hypomesus nipponensis (80.8%) and largemouth bass Micropterus salmoides (13.7%) were the primary and next essential prey species, respectively. The average prey item length was 51.04 {+/-} 20.89 mm and significantly differed among months and breeding stages (P < 0.001, respectively). 50-75 mm prey length category was the most frequent in the diet (42.2%). In April and May, larger fish >50 mm constituted the greater part of their diet (93.1%, 66.3%, respectively), whereas the diet in June and July consisted of smaller fishes <50 mm (56.2%, 68.8% respectively). The occurrence frequency of prey length categories also varied significantly among the breeding stages (P < 0.001): 1-25 mm and 50-75 mm were overrepresented and underrepresented, respectively, at the chicks in the nest stage. On the other hand, 50-75 mm was preferred for the pre-laying and incubation stages. In terms of the survival condition of pond smelts, the before- and after water surface temperatures of the day when terns flew away showed a significant difference (P = 0.004), where a threshold looks like between 29.11{degrees}C and 30.04{degrees}C. These results support the prey abundance hypothesis that, when cold-water pond smelts might wholly swim down into the deeper lake in the hot summer, the terns might also leave their colony for another foraging place with higher prey availability.