Community Structure Of Pelagic Fish And Trophic Level Of Key Species In The Central Area Of The Tropical Pacific Ocean

Researches on oceanic ecosystem are relatively weak compared with those on coastal ecosystem, due to the difficulty in conducing scientific surveys and samplings. In recent years, China began to dispatch scientific observers annually to collect real time fishery data at sea in the Pacific, Atlantic, and Indian Oceans, as an implementation of high sea fishery management measures. This provides an important way for China to conduct scientific researches on oceanic fisheries and the associated ecosystems. Based on a tuna longline fishery observer trip in the tropical Pacific Ocean(03°30′N-14°52′S, 151°02′W-168°52′W) from September 2012 to January 2013, community structure of pelagic fishes and trophic level of key species are preliminarily investigated in this thesis. The results provide important information for improving our understandings of the structure and dynamic of pelagic ecosystem in the tropical Pacific Ocean.(1) According to the index of relative abundance, bigeye tuna(Thunnus obesus) and yellowfin tuna(Thunnus albacares) are dominant species in this area, and sickle pomfret(Taractichthys steindachneri), albacore(Thunnus alalunga), skipjack(Katsuwonus pelamis), swordfish(Xiphias gladius), blue marlin(Akaira nigricans), blue shark(Prionace glauca), bigeye thresher(Alopias superciliosus), silky shark(Carcharhinus falciformis), oceanic whitetip shark(Carcharhinus longimanus), wahoo(Acanthocybium solandri), snake mackerel(Gempylus serpens), escoler(Lepidocybium flavobrunneum), and sting ray(Dasyatis violacea) are common species. Overall, correlation test shows that these species are significantly positively correlated. According to the analysis of species link and correlation, the number of species with positive associations is much greater than that with negative associations. This indicates that these species may share niches and have similar requirements for habitats and resources. However, as the main target species, the relevance of bigeye tuna with other species is weak.(2) The average of Pielou’s evenness index is 0.92 in this area. The species evenness and diversity indices remain relatively stable and high in the survey area, except for some stations with whale inerations, which caused many species to escape from being captured. Shannon-wiener’s diversity index and Margalef’s divesity index show similar patterns. The diversity index in 5°-10°S is higher than that in 0 °-5 ° N, 0 °-5 ° S, and 10 °-15 °S. The diversity index in 150°-155°W is higher than that in 155°-160°W, 160°-165°W, and 165°-170°W.(3) Based on cluster analysis of community structure, the entire survey area can be divided into two sub-communities. In the sub-community I, bigeye tuna and escoler are predominant species in the catches, accounting for 31.31% and 12.97% respectively, followed by yellowfin tuna(9.02%), sickle pomfret(8.83%), snake mackerel(8.76%), albacore(4.20%), swordfish(4.07%), skipjack(3.17%), and blue shark(1.86%). In the sub-community II, yellowfin tuna, bigeye tuna, and skipjack are predominant species in the catches, accounting for 25.62%, 23.61%, and 12.60%, respectively, followed by swordfish(5.33%), snake mackerel(3.53%), escoler(2.56%), sickle pomfret(2.14%), albacore(2.00%), and blue shark(0.76%). Blue shark is the common species in the sub-community I, but is rare in the sub-community II. The average capture depth of bigeye tuna is deep, and almost the same as that of sickle pomfret, crocodile shark, longnose lancetfish, dagger pomfret, and velvet dogfish. Therefore, it is possible to reduce the rates of other bycatch speices by deploying deeper longline hooks.(4) The prey composition and trophic level of bigeye tuna, yellowfin tuna, and swordfish, as key species in this area, are analyzed. Fishes are the dominated preys, followed by cephalopods for all three species. However, the similarity of prey type of bigeye tuna is different from that of yellowfin tuna and swordfish, indicating low competitive pressure for food between these species. The frequency of fishes consumed by bigeye tuna is 68.23%, followed with cephalopod(20%) and crustacean(10.59%), resulting in a trophic level estimate of 3.42 for bigeye tuna. The frequency of fishes consumed by yellowfin tuna is 69.56%, followed with cephalopods(26.08%) and crustaceans(4.34%), resulting in a trophic level estimate of 3.39 for yellowfin tuna. The frequency of fishes and cephalopods consumed by swordfish is 87.17% and 12.82%, respectively, resulting in a trophic level estimate of 3.70 for swordfish. Thus, these three species are all at higher trophic levels, and swordfish takes up a little higher trophic position than the other two species.