Stable isotopic and stomach content analyses reveal changes in the trophic level and feeding habit of large-head hairtail (Trichiurus lepturus) in the northern South China Sea

The feeding habit of large-head hairtail (Trichiurus lepturus) in the northern South China Sea was investigated through isotopic and stomach content analyses. The isotopic features of the hairtail at the same body size differed among regions, with the fish in coastal waters presenting higher delta N-15 and delta C-13 values compared to those in the open sea, indicating different trophic levels (TL), food habits, and isotopic baselines. According to the partial correlation of water depth with delta N-15 values, the sampling stations were divided into three regions based on the depth of water: coastal (20-40 m), near coastal (60-80 m), and open sea (100-200 m) regions. In the coastal region, the hairtail from stations affected by the Pearl River plume exhibited lower delta N-15 and delta C-13 values. The stomach content analysis indicated different feeding habits of the hairtail from different regions. The hairtail in the coastal and near coastal waters fed more on fish and less on crustaceans compared to the hairtail in the open sea. The relationship between delta N-15 and fish size exhibited two contrary patterns. First, the delta N-15 values increased with increasing preanal length in the hairtail sampled from the water depth of 30-40min section F (in fish with preanal length < 200 mm) and those samples from the water depth of 100-200 m. This finding reflected an ontogenetic shift in diet and TL. However, the delta N-15 values tended to decrease with the increasing preanal length of the hairtail samples collected from the water depth of 30-40min section F (fish with a preanal length of similar to 200-300mm). These findings suggested that under the conditions of insufficient availability of high-quality prey, the larger hairtail fed more on low-TL prey to compensate for the increase energy demand, arising due to growth, which led to the observed decrease in delta N-15 values.

Automated summary

The feeding habit of large-head hairtail (Trichiurus lepturus) in the northern South China Sea was studied using isotopic and stomach content analyses. Different isotopic features, trophic levels, and food habits were found among the hairtail in coastal waters and open sea. The depth of water influenced the hairtail's isotopic values, with three regions (coastal, near coastal, and open sea) identified. Stomach content analysis revealed that the hairtail in coastal and near coastal waters fed more on fish and less on crustaceans compared to those in the open sea.