Seiji Ohshimo

Age validation, growth and annual reproductive cycle of chub mackerel Scomber japonicus off the waters of northern Kyushu and in the East China Sea

The age and growth of chub mackerel Scomber japonicus collected from the East China Sea and the northern waters off Kyushu between June 2000 and June 2001 were determined by observing the otolith surface after dipping it in xylene. The translucent and opaque zones on the otolith surface were identified, and the number of translucent zones was counted. Monthly changes in the frequency of fish with translucent zones on the otolith margin, and in the marginal increments, indicated that the translucent zones were formed between April and June. The seasonal pattern of annulus formation on the otolith became clear by observing the otoliths of fish with known ages, and the otolith formation in wild fish was consistent with that of fish with known ages. The mean gonadosomatic index of male and female fish was high from March to May, and spawning females were observed from mid-March to mid-May. The estimated ages were 1–5 years for males and 1–6 years for females. The von Bertalanffy growth parameters did not significantly differ between male and female. The model was obtained as FLt=406×{1−exp[−0.372×(t+1.68)]

Classification of fish schools based on evaluation of acoustic descriptor characteristics

Acoustic surveys were conducted from 2002 to 2006 in the East China Sea off the Japanese coast in order to develop a quantitative classification typology of a pelagic fish community and other co-occurring fishes based on acoustic descriptors. Acoustic data were postprocessed to detect and extract fish aggregations from echograms. Based on the expert visual examination of the echograms, detected schools were divided into three broad fish groups according to their schooling characteristics and ethological properties. Each fish school was described by a set of associated descriptors in order to objectively allocate each echo trace to its fish group. Two methods of supervised classification were employed, the discriminant function analysis (DFA) and the artificial neural network technique (ANN). We evaluated and compared the performance of both methods, which showed encouraging and about equally highly correct classification rates (ANN 87.6%; DFA 85.1%). In both techniques, positional and then morphological parameters were most important in discriminating among fish schools. Fish catch composition from midwater trawling validated the fish group classification through one representative example of each grouping. Both methods provided the essential information required for assessing fish stocks. Similar techniques of fish classification might be applicable to marine ecosystems with high pelagic fish diversity.

Growth and spatiotemporal distribution of juvenile shortfin mako (Isurus oxyrinchus) in the western and central North Pacific

This paper presents an estimation of growth curves and spatiotemporal distributions of juvenile shortfin mako shark (Isurus oxyrinchus) in the western and central North Pacific Ocean using port sampling data collected from 2005 to 2013. The monthly length compositions show a clear transition of three modes in the size range of smaller than 150-cm precaudal length (PCL), which were believed to represent the growth of age-0 to age-2 classes, and they were then decomposed into age groups by fitting a Gaussian mixture distribution. Simulation data of lengths at monthly ages were generated from the mean and standard deviation of each distribution, and fit with a von Bertalanffy growth function. Parameters of the estimated growth curves for males and females were 274.4 and 239.4cm PCL for the asymptotic length and 0.19 and 0.25 year–1 for the growth coefficient indicating apparently faster growth than previously reported. Generalised linear models were applied to age-0 to explore the seasonal changes of PCL by area. They were born during late autumn and winter off the coast of north-eastern Japan, an area known to have relatively high productivity compared with other pelagic areas, and gradually expanded their habitat eastward and northward with the seasons as they grew.

Age, growth and reproductive characteristics of round scad Decapterus maruadsi in the waters off west Kyushu, the east China Sea

The purpose of this study was to determine the growth and reproductive characteristics of round scad Decapterus maruadsi in the East China Sea. The characteristics regarding growth and reproduction of round scad based on otolith reading and gonad histology, respectively, were estimated. The von Bertalanffy’s growth model for round scad was estimated as follows: FLt=342[1−exp{−0.55(t+0.58)}], (1<t<7)where FL, is fork length (mm) at age in years t. The maturation stage of the ovary was observed by histological techniques. June was the main spawning period; the maturation and migratory nucleus stage of the oocyte was observed when the gonad somatic index value was approximately greater than 4.

Relative abundance trend of the blue shark Prionace glauca based on Japanese distant-water and offshore longliner activity in the North Pacific

The Kuroshio–Oyashio transition zone is one of the most important fishing grounds for pelagic fish, including blue shark and swordfish, which are primarily targeted by shallower Japanese longliners. We evaluated the fishing behavior of Japanese longliners based at the Kesennuma fishing port to estimate the standardized catch per unit of effort (CPUE) of blue shark. In our analysis, we first used data that had been filtered for blue shark sets and found that annual target shifts occurred seasonally and geographically; the greatest change in target species, from swordfish to blue shark, occurred in spring (April–June). Beginning in the early 1990s, the fishing grounds shifted from the southwest to the northeast of the North Pacific, and the number of sets targeting blue shark gradually increased. To incorporate this variable target behavior into the abundance index, the 10th percentile of the swordfish CPUE values was applied as the target indicator in the CPUE standardization model. The results indicate an upward population trend that has been caused by a decrease in fishing pressure from the early 1990s due to the prohibition of drift nets in 1992 and the subsequent decrease in longline vessels.

Biomass fluctuation of two dominant lanternfish Diaphus garmani and D. chrysorhynchus with environmental changes in the East China Sea

Acoustic surveys have been conducted for estimating the biomass of commercially important fish (e.g., anchovy, jack mackerel), lanternfish (Diaphusgarmani and D. chrysorhynchus), and pearlside (Maurolicus japonicus) in summer in the East China Sea (ECS) since 1997. The biomass of lanternfish and pearlside was 2.26–19.16 times that of commercially important fish, and these species represented substantial biomass in the ECS. Though there were no correlations between biomass of pearlside and environmental indices, significant correlations between biomass of lanternfish and southern oscillation index (SOI) in March (positive correlation), arctic oscillation (AO) in March (negative) and October (positive), monsoon index (MOI) in February (positive), and Kuroshio flow mass in winter (positive) were observed. Weak AO and strong MOI would cool down the sea temperature and would lead to increased primary and secondary production in the ECS, thereby enhancing larval survival of lanternfish. The SOI would affect the Kuroshio meander in the ECS, and strong SOI and Kuroshio flow mass would transport larvae of lanternfish to the present survey area. This is the first report on the lanternfish standing stock and its fluctuation in the ECS.

Reproductive biology of the blue shark (Prionace glauca) in the western North Pacific Ocean

The reproductive biology of the blue shark (Prionace glauca) in the western North Pacific Ocean was investigated to contribute to future stock assessments because of limitations of recent studies and the lack of information about the reproductive cycle. Reproductive data were obtained from 490 males (precaudal length (PCL), 33.4–252.0cm) and 432 females (PCL, 33.4–243.3cm). Size at 50% maturity was estimated to be 160.9cm for males and 156.6cm PCL for females. Litter size varied from 15 to 112 (mean 35.5) and was positively correlated with maternal PCL. Parturition, ovulation and mating occurred sequentially from spring to summer. The gestation period was estimated to be 11 months. The ovarian follicles of pregnant females developed synchronously throughout the gestation period along with embryonic growth, indicating that females reproduce annually. Our results showed that the productivity of North Pacific blue sharks is higher than previously thought, based on larger fecundity and a shorter reproductive cycle. These new findings will improve future stock assessments and provide management advice.

Evaluation of growth-dependent survival during early stages of Pacific bluefin tuna using otolith microstructure analysis

The present study used otolith analysis-based body size back-calculation to: (1) evaluate the hypothesis of growth-dependent survival in young Pacific bluefin tuna (PBF; Thunnus orientalis) from the north-western Pacific Ocean; (2) identify critical developmental stages for survival; and (3) compare interannual differences in early growth. To this end, we compared the daily growth trajectories of a large number of larvae (standard length (SL) 150mm) collected between 2011 and 2015. Otolith radius and SL were highly correlated and yielded a single relationship applicable for the five year-classes. Body size back-calculation showed that only larvae with fast, steady growth successfully transitioned to the juvenile stage. The ontogenetic analysis of daily growth rates revealed interannual differences only in larvae, and not in the larval stage of juveniles and YOY. Neither sudden decreases nor increases in growth rates were observed during the larval stage of any of the stages, suggesting that the observed variability in larval body size may be the result of individual differences in growth rates rather than of drastic, one-time events. Overall, the results of the present study indicate that growth-dependent survival of larvae may be the most critical for PBF recruitment.

Age, growth, and reproductive characteristics of dolphinfish Coryphaena hippurus in the waters off west Kyushu, northern East China Sea

The growth and reproductive characteristics of dolphinfish Coryphaena hippurus collected in the waters off western Kyushu from May 2008 to April 2011 were determined based on scale and otolith readings and gonad histological examinations, respectively. Based on annual increments in scales and daily increments in sagittal otoliths, the von Bertalanffy growth curves in male and females were determined as