Title Patch study on the early life ecology of Pacific bluefin tuna, Thunnus orientalis, in the Nansei Islands, northwestern Pacific Ocean
Authers Keisuke SATOH
Citation Bull. Fish. Res. Agen. No.37, 85-145, 2013
Abstract
Annual catch of Pacific bluefin tuna (PBT), Thunnus orientalis, fluctuates largely from 0.9 to 3.5 ten thousand metric ton. Because the most of catch is occupied by premature (non-adult, juvenile) fish, the annual catch of this species is influenced by the recruitment variability, which varied ten-fold yearly for this species. The recruitment variability of marine fish species is generally considered to be adjusted during the early life stage from fertilization to recruitment. The long history of studies on the mechanism of recruitment variability revealed that many mechanisms and factors work simultaneously on conditioning the year class of fish during the early life stage. The factors and mechanisms are different by each species and by year even for the same species and it might be different for each spawning event. The knowledge for growth, mortality, advection and diffusion are piecemeal, and the mechanisms of recruitment variability are little known. The larvae derived from a spawning event react to biological conditions (e.g. food, predation) and physical conditions (e.g. sea temperature, sea flow) during the early life history. The year class is accumulated number of survivor during early life history. In this study, the patch study was applied, which tracked identical larval population (patch), and collected the larvae, food organisms and made oceanographic observation repeatedly. The study is expected to understand clearer relationship between environmental conditions and larval growth and survival.
 The objective of this study is to provide clearer understanding of the early life ecology of Pacific bluefin tuna, especially for recruitment process in relation to environmental condition using patch tracking method (patch study) and to establish a basis for stock management in the northwestern Pacific Ocean. In the first chapter previous studies on hypotheeis of recruitment mechanism, distribution, growth and mortality of early life stage especially in larval stage were reviewed and made it clear the object of this study. In the second chapter the feasibility of detecting and tracking the larval patch was discussed. In the third chapter larval transport and distribution was studied and appropriate protocol of plankton net observation in the sea was discussed. In the last fourth chapter larval survival and growth were studied for modeling growth and survival during the larval stage.
 Seven high density larval populations (patches) of PBT were tracked with reference buoys, and repeated samplings were carried out in the northwestern Pacific Ocean for 28 to 171 hours in May-June from 2004 to 2008. Before detecting patches, a number of samplings using 2-m ring plankton net, average 52 times (6 to 80 times) to catch an initial patch , were carried out. For this study, the successive collection of larvae from the same population is essential. Two pieces of evidence of the success of the trackings over several days to a week were found: the growth rates (0.2 to 0.9 mm day-1) that were estimated from changes in the mode of the length frequency during each tracking closely coincided with the growth rate (0.25 to 0.85 mm day-1) determined from analysis of otolith daily rings; and buoy trajectories and sea current directions showed good agreement. The PBT larvae of same patch were collected in every patch except for the Patch 8 on 2nd tracking day.
 Patches consisted of a number of cohorts. The larval spatial distribution structure was studied using variograms. The range, which is one of the parameters of variograms and horizontal distance indicating no correlation of larval abundance between two stations, is considered as an indicator of horizontal distribution size of cohort. The ranges did not change significantly during trackings and the age specific ranges did not change significantly. The analysis of variogram revealed that the patch typically spread horizontally 15 km ranges and the range of cohort showed stability at least five days. Although the detailed larval horizontal distributions change in accordance with the oceanographic conditions and predation pressure, the larvae were identified one assemblage typically in 15 km range. We should make plankton net tow every 7.5 km in order to detect larval patch, and at least 30 km range in order to observe entire distribution of larval patch. Larvae were only distributed in the surface mixed layer and diel vertical movement was not clearly observed. Patches were entrained in mesoscale eddies (~100 to 500 km diameter) which propagated westward. Some of such mesoscale eddies in this area are known to coalesce with the Kuroshio Current. The spawning area and the recruitment fishing grounds are thereby linked by the Kuroshio. Results suggest that cohorts have a stable spatial structure after fertilization (i.e. during advection, while entrained in mesoscale eddies). Therefore, the spatial relationship between spawning events and mesoscale eddies is concluded to be important for the recruitment process.
 The significant difference in the length frequency distributions between day and night sampling clearly indicates that net avoidance occurred. There was also a significant difference in the length frequency with different mesh apertures of the bongo net. It is likely that net extrusion occurred in the 2-m ring net. Therefore raw larval density must be corrected considering the net avoidance and net extrusion in order to obtain an estimation of the true larval density.
 It is tested that the hypothesis that large body size and rapid growth rate always affect the larval survival of bluefin tuna positively, and analyzed larval growth in relation to environmental conditions. The otolith radius and its daily growth rate of the survivor (collected on later tracking days of each tracking session) tend to be larger and more rapid, respectively than those of the original (collected on earlier tracking days). A large body size was found to positively affect the survival of larval bluefin tuna, as did a rapid growth rate even at an early larval stage (after DAH 7). The otolith radius is influenced positively by sea temperature, the stratification parameter and food density and the growth rate is influenced positively by sea temperature and food density.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull37/37-03.pdf