Factors affecting the fatty acid composition of mesopelagic fish of the continental slope in the South China Sea

Abstract

Study of the ecology of mesopelagic fish is central to assessing the active biological pump in the ocean, especially in the mesopelagic layer. As a consequence of the small size and pressure change, traditional stomach content analysis is not useful for analyzing the feeding behavior of mesopelagic fish. The use of δ 13 C and fatty acid analyses can help to resolve this problem. The mesopelagic fish examined in this study were collected from the continental slope in the north of the South China Sea (SCS), and are compared with nearshore SCS fish and fish collected from the Southern Ocean. The unusually high lipid content of the mesopelagic fish resulted in △δ 13 C values (i.e., the difference in δ 13 C between unextracted and extracted tissues) exceeding 1‰, which is more than the enrichment factor in the food web. Thus, extraction of lipids was conducted prior to δ 13 C isotope analysis for the study of trophic interactions of mesopelagic fish. Compared with other fish, mesopelagic fish had high C18:1n-9/C18:1n-7 and C20:1n-9/ C18:1n-7 ratios, which confirms that plankton is their main dietary source. Diatoms comprise a higher proportion of phytoplankton in the Southern Ocean and a lower proportion in the SCS, and this is reflected in the C20:5n-3/C22:6n-3 (EPA/DHA) ratio in mesopelagic fish in each region. The low EPA/DHA ratio in SCS fish indicates that diatoms are not the main component in the diet of mesopelagic fish. The SCS mesopelagic fish had higher C20:4n-6/C22:6n-3 (ARA/DHA) and C20:4n-6/C20:5n-3 (ARA/EPA) ratios than fish in the SouthernOcean. This result suggests that physical factors (e.g., temperature) also affect the fatty acid composition of these fish, particularly because certain fatty acids enable the fish to better adapt to extreme environmental conditions. Future studies of the synthesis of fatty acids in particular species should take account of both the dietary sources and physical factors in their environment. Keywords: mesopelagic fishes, lipids, δ 13 C, fatty acids, temperature, South China Sea Abstract Study of the ecology of mesopelagic fish is central to assessing the active biological pump in the ocean, especially in the mesopelagic layer. As a consequence of the small size and pressure change, traditional stomach content analysis is not useful for analyzing the feeding behavior of mesopelagic fish. The use of δ 13 C and fatty acid analyses can help to resolve this problem. The mesopelagic fish examined in this study were collected from the continental slope in the north of the South China Sea (SCS), and are compared with nearshore SCS fish and fish collected from the Southern Ocean. The unusually high lipid content of the mesopelagic fish resulted in ∆δ 13 C values (i.e., the difference in δ 13 C between unextracted and extracted tissues) exceeding 1‰, which is more than the enrichment factor in the food web. Thus, extraction of lipids was conducted prior to δ 13 C isotope analysis for the study of trophic interactions of mesopelagic fish.Compared with other fish, mesopelagic fish...