Large-scale Distribution Of Micronekton In The Tropical Western Pacific

Micronekton generally denotes a specific group of nekton with body length between 2 and 20 cm,including small fishes,cephalopods,crustaceans and some gelatinous organisms.They are thought to be one of the largest unexplored marine biological resources.Micronekton plays a key role in marine ecological systems.It is not only a crucial link of the ocean food web but also an important portion of biological carbon pump in mesopelagic zone.The tropical ocean is one of the most active areas of marine creatures,and is also an important fishing ground of high economic value fishes.The deep scattering layers have been widely explored in the large-scale investigation of nekton communities using acoustic methods featuring rapid sampling,as well as noninvasive and nonextractive advantages.Nowadays,many studies have been conducted to simulate and analyze the changes of sound propagation in the ocean,and a convenient and universal method of raw data processing has been obtained.To explore the spatial patterns of deep scattering layers(DSLs)and their possible influencing factors,we investigated the daytime(10:00-14:00)DSLs in the tropical western Pacific Ocean(12–26°N,150–165°E)using active acoustic data collected with a shipboard acoustic Doppler current profiler(SADCP,OS38K)at 38 k Hz.The distributions of nautical area scattering coefficient(NASC,a relative proxy of mesopelagic biomass),center mass of mesopelagic zone(CM),boundaries of DSL,and gathering level of mesopelagic community(MGL)have shown a strong latitudinal gradient in the study area.Three spatial zones were detected:the northern part(NP,22-24°N),the central part(CP,17-22°N),and the southern part(SP,12-17°N)by k-means cluster analysis on the physiochemical environment features.The variables of DSLs were significantly different in the three parts.The NP showed the deepest core DSLs(average in the range of 460-600 m),the largest NASC(130.8±41.0 m² nmi^-2),and the most concentrated MGL(6.7±0.7%).The vertical migration strength of mesopelagic micronekton was also stronger in NP(39.7%)than in other areas(18.6%in CP and 21.5%in SP).The results of GAM showed that MAO(mesopelagic average oxygen)was the most important factor on the depth of DSLs.The light was another important factor to the depths of UBD(upper boundary of DSL)and CM.Moreover,there was about a 4-month delay in the correlations between NASC and net primary productivity.Our study provide base data for environmental management on the deep-sea mining and a optimized approach to exploring the properties of DSLs with ADCP data in the future.