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We analyzed stable isotope ratios, fatty acid, and lipid components of chub mackerel, Scomber japonicus, to evaluate their applicability as chemical biomarkers. Chub mackerel were collected in the East China Sea, the Sea of Japan, and the Pacific Ocean, with body sizes ranging from 51 to 388 mm. The total fatty acid and lipid contents (mg g–1) increased with growth, and the percent composition of fatty acids and lipids changed drastically. The percentages of docosahexaenoic acid (%DHA), stearic acid (%Ste), and arachidonic acid (%ARA) decreased and the percentage of oleic acid (%Ole) increased when the body length (fork length, FL) was about 250 mm. The amounts of triacylglycerols (TAG) in fish in the East China Sea were lower than in the Sea of Japan and Pacific Ocean. The percentage of TAG drastically increased at around 250 mm FL, however, the phosphatidylcholines (PC) drastically decreased. The ontogenetic differences in the fatty acid and lipid composition might be influenced by metabolic events such as growth and reproduction. The geographical differences in chub mackerel can be distinguished by using stable isotope ratios, the ω3:ω6 fatty acid and DHA:EPA (eicosapentaenoic acid) ratios, and six main fatty acids. These chemical biomarkers could be available for understanding habitat and migration of chub mackerel.

The density dependence of growth and body condition have important impacts on fish population dynamics and fisheries management. Although population density is known to affect the temperature of the habitat selected, how this affects the density dependence of growth and body condition remains unclear. Here, we investigated annual changes in body condition, habitat temperature, and cohort-specific growth of chub mackerel (Scomber japonicus) in the western North Pacific and examined quarterly changes in the density dependence of body condition. We hypothesized that chub mackerel body condition is affected both directly (e.g., through competition for food) and indirectly (through changes in habitat temperature) by the abundance of both conspecifics (i.e., chub mackerel) and heterospecifics (the Japanese sardine Sardinops melanostictus). Indeed, chub mackerel body condition, habitat temperature, and growth all decreased with increasing conspecific and heterospecific abundance. Mean annual growth rates in chub mackerel were positively corelated with body condition. The best model showed that conspecific and/or heterospecific abundance had strong negative effects on chub mackerel body condition in all seasons, and influenced habitat temperature in some seasons. By contrast, temperature effects on body condition were weak. Therefore, direct effects likely have more impact than indirect effects on density-dependent body condition and growth.

Length–length and length–weight relationships in various length measurement standards were estimated for dominant small pelagic fishes in the Kuroshio–Oyashio current system (Japanese sardine Sardinops melanostictus, Japanese anchovy Engraulis japonicus, chub mackerel Scomber japonicus, and spotted mackerel Scomber australasicus). Fish samples were collected from surface–midwater trawl surveys and commercial purse-seine fisheries between September and October of 2020 in the western North Pacific. Total length (TL), fork length (FL), and standard length (SL) were measured to 0.01 cm, and total body weight (W) was measured to the nearest 0.01 g for each individual. All length–length relationships (TL–FL, TL–SL, and FL–SL) were highly significant (p 0.98 in all species. Length–weight relationships (W–TL, W–FL, and W–SL) were also highly significant (p 0.98 in all species. This study provides new comprehensive information on length–length and length–weight relationships in various length measurement standards for the four small pelagic fish species. These data will be useful references for biological studies and stock assessments of these small pelagic fishes.

Many small pelagic fishes obligately form schools; some of these schools attain a remarkable size. Although the school is a fundamental and important ecological unit and is the site of biological interactions such as competition and predation, information on schooling processes in the field remains scarce. Here, we examined the quantitative relationships between population density and school size, the number of schools, and other school characteristics (i.e., packing density, volume, and cross-sectional area) in three species of small pelagic fishes: Japanese anchovy Engraulis japonicus, Japanese sardine Sardinops melanostictus, and chub mackerel Scomber japonicus. We found that school size increased almost linearly with population density, whereas the number of schools and other characteristics increased non-linearly with population density, whereby the rate of increase slowed radically as population density increased. These results indicate that, at low population densities, an increase in density causes an increase in both school size and the number of schools, whereas at higher population densities, an increase in density triggers the formation of larger schools rather than more schools. Furthermore, we found that the shapes of the relationships of all school characteristics with population density differed among species. Our results indicate that the schooling behaviour of small pelagic fishes is density-dependent, and responses to changes in density are species-specific. Our results provide insight into how biological interactions such as intra- and inter-specific competition and predator-prey interactions mediate the density-dependent processes that underlie the population dynamics and community structure of small pelagic fishes in marine ecosystems.

Density dependence is a fundamental concept for fish population dynamics. Although density-dependent growth and maturity among older juveniles and adults is important for regulating fish population size and for fisheries management, the mechanism of density dependence for marine fishes remains unclear. Here, we examined changes in Japanese sardine growth with increasing abundance beginning in the 2010s and how the current pattern of density-dependent growth differs from that of a previous stock-increase period from the 1970s to early 1980s. During the current period of increasing abundance, mean standard length has already dropped to the lowest level yet observed and growth has declined more sharply with increased abundance than in the 1970s and 1980s. Mesozooplankton biomass in July in the summer feeding grounds was also lower during the current period. Therefore, our results suggest that summer food availability in the western North Pacific controls the strength of density-dependent growth. A lower carrying capacity for Japanese sardine could account for the stronger density dependence of growth observed in the 2010s; this indicates that future Japanese sardine abundance might not increase as much as in the 1980s unless food availability improves.

SummaryLength–length relationships and length–weight relationships were estimated for four small pelagic fishes (Japanese sardine Sardinops melanostictus, Japanese anchovy Engraulis japonicus, chub mackerel Scomber japonicus, and spotted mackerel Scomber australasicus) in the Kuroshio–Oyashio current system. Fish samples were collected from surface–midwater trawl surveys and commercial purse-seine fisheries between September and October of 2020 in the western North Pacific. Total length (TL), fork length (FL), and standard length (SL) were measured to 0.01 cm, and whole body weight (W) was measured to the nearest 0.01 g for each individual. All length–length relationships (TL–FL, TL–SL, and FL–SL) were highly significant (p < 0.0001), with r2 > 0.98 in all species. Length–weight relationships (W–TL, W–FL, and W–SL) were also highly significant (p < 0.0001), with r2> 0.98 in all species. This study provides a useful reference for biological studies and stock assessments of these small pelagic fishes.

Spatio-temporal patterns in spawning influence growth and survival by affecting the environment experienced by offspring during their early life stages. Therefore, identifying changes in spawning patterns can help researchers understand population dynamics and recruitment fluctuations. The Japanese sardineSardinops melanostictusis a small pelagic fish that undergoes large stock fluctuations. Although spawning patterns are known to change spatially and temporally with stock abundance, little information is available on the processes underlying stock increases. This study aimed to describe changes in spawning pattern and early growth of Japanese sardine during the recent period of stock increase, and to clarify the effects of different spawning periods on offspring growth. We examined trends in egg abundance in the western North Pacific in 2004–2018 and analyzed hatch dates and growth trajectories by otolith microstructure analysis of juveniles from the Kuroshio–Oyashio transitional region (the species’ main nursery area). During the study period, the main spawning area shifted from the western to the eastern part of this region off Japan, and spawning in the eastern part roughly coincided with juvenile hatch dates. Hatch dates also shifted from mid-March at the earliest to February and early March from 2013 onwards. Although early-hatched cohorts (which originate offshore of eastern Japan) experienced slower initial growth, they likely played an important role in the recent stock increase. Successful recruitment of these cohorts may have been facilitated by factors such as early hatching and transport to the nursery, which reduces the frequency of predator encounters.

Species identification based on morphological characteristics includes species misidentification, leading to estimation bias in stock assessment and posing challenges difficult to be resolved. The spawning eggs of spotted mackerel Scomber australicus and chub mackerel S. japonicus in the western North Pacific are used for stock assessment as an index of spawning biomass and are classified based on egg diameter by past evidence. However, the difference in the distribution of egg diameters between the two species has become so ambiguous that the spawning eggs of chub mackerel may be classified as spotted mackerel. This can be explained by the larger distribution of egg diameters in chub mackerel with increasing stock abundance, resulting in overlap with the distribution of egg diameters in spotted mackerel. This leads to species misidentification and biased estimates of spotted mackerel abundance. To overcome this bias, it is necessary to develop a standardization method to remove the effect of species misidentification. Here, we demonstrate that a recently-developed spatio-temporal model can easily and efficiently reduce estimation bias for egg density and stock abundance in the spotted mackerel, using 15 years data for spawning eggs. We incorporated species identification error as the effect of the egg density of chub mackerel on the catchability of spotted mackerel in the spatio-temporal model. The index estimated from the model decreased temporal fluctuation substantially. When using the index accounting for species misidentification, the retrospective bias of abundance estimates for spotted mackerel decreased by about half compared with using the indices that ignored species misidentification. These results suggest that incorporating species misidentification bias is an essential process for improving stock assessment.

Growth is an important factor that regulates survival during early life stages, and growth rates and recruitment are often thought to be positively related. However, environmental conditions generally fluctuate over time, which could lead to temporal changes in growth–recruitment relationships. Here, we investigate whether growth–recruitment relationships are time-varying by applying dynamic linear models, which allow coefficients to vary over time, to a 23-yr data set of recruitment and early life-stage growth rate in Japanese sardine Sardinops melanostictus. We found that the impact of growth rate on recruitment (i.e., the slope coefficient) was relatively high during the late 1990s, then gradually weakened, and has remained low in recent years. The baseline recruitment level (i.e., the intercept) also shifted over time, being relatively low in the late 1990s, gradually increasing, and remaining high in recent years. Furthermore, a model with time-varying intercept and slope was more statistically plausible than models with constant coefficients. These results indicate that the growth–recruitment relationship varied dynamically over time. Our findings highlight the need to consider the possibility of time-varying growth–recruitment relationships when investigating such relationships and predicting recruitment.

Standardization of catch per unit effort is an effective way to not only extract temporal abundance trends but also understand distribution shifts of fish resources. Chub mackerel in the Northwest Pacific has recently been of great concern to fishermen and fisheries managers because the total catch has been increasing, and fishing grounds have expanded from the Japanese exclusive economic zone eastward to high-sea areas. Understanding the spatiotemporal dynamics of chub mackerel is urgently needed for evaluating the current status (i.e., abundance and distribution) and considering future exploitation. Here we constructed two recruitment indices from pelagic trawl survey data of the Pacific chub mackerel. For recruitment indices of age-0 fish, we integrated an algorithm for automatic area stratification into the delta-GLM that combines two separate generalized linear models (for encounter probability and positive catch rates) to simultaneously investigate changes in relative abundance and distribution. Recruitment indices were high in the recent years of 2013, 2016, and 2017, especially at around 160 °E longitude within the high-sea areas, suggesting simultaneous changes in abundance and distribution. Furthermore, we found that our approach had better predictive ability, especially for positive catch rates, than that of a generalized additive model by cross validation, and the obtained standardized indices mitigated potential bias in stock assessment estimates. Our integrated approach will be useful for understanding the spatiotemporal dynamics of fishes and standardizing abundance indices for stock assessment and management.

Growth and reproductive characteristics of jack mackerel Trachurus japonicus collected in the East China Sea were determined based on otolith readings and gonad histology, respectively. Translucent and opaque zones on sectioned otoliths were identified and opaque rings counted. Von Bertalanffy growth parameters did not significantly differ between males and females, and the combined growth curve was: Lt = 401{1 − exp[−0.275 (t + 1.149)]} (0.8 < t < 6.9), where Lt is fork length (mm) at age t. The calculated lengths at age 1 in our study were larger than those reported 50 years ago from the East China Sea. The spawning period was evaluated to be from December to June, but primarily from February to May, based on the monthly changes in the gonadosomatic index and histological observations. The minimum size and age at first maturity were smaller and younger, respectively, than those reported in previous studies.

Growth and reproductive characteristics of gold striped amberjack Seriola lalandi collected in the waters off western Kyushu from April 2003 to March 2008 were determined based on vertebral centrum readings and gonad histology, respectively. Translucent and opaque zones on vertebral centrum were identified, and the number of opaque rings counted. Von Bertalanffy growth parameters did not significantly differ between males and females (F=0.28, P>0.05), and transformed value for the von Bertalanffy growth parameter FL ∞ and K were 1108 and 0.3091, respectively. The spawning period was evaluated to be from April to June for S. lalandi, based on the results from monthly changes in the gonadosomatic index and histological observations. Based on the relationships between the fork length and the developmental stage of testes and ovaries, the smallest fork length of male and female with mature gonads was 624 and 662 mm, respectively. The first maturation age was estimated to be 2 years old for both sexes.

The spawning grounds of the chub mackerel (Scomber japonicus) and spotted mackerel (Scomber australasicus) in the East China Sea were estimated based on catch statistics of the Japanese large- and medium-type purse seine fishery from 1992 to 2006. Biometric data were obtained from specimens caught by purse seiners in the East China Sea from 1998 to 2006. Gonadosomatic index (GSI) at 50% sexual maturity of chub mackerel and spotted mackerel females was 2.5 and 2.6, respectively. Using this criterion for GSI, chub mackerel larger than 275 mm and spotted mackerel larger than 310 mm in fork length were considered to be mature. Mature chub mackerel was observed in the area of 15–22°C sea surface temperature (SST), and mature spotted mackerel was observed in the area of 17–25°C SST. The spawning period of chub mackerel ranged from February to June, and that of spotted mackerel ranged from February to May in the East China Sea. The spawning grounds were estimated from the distributions of catch per unit effort (CPUE) of spawners and SST. As a result, the spawning ground of chub mackerel was estimated to be in the central and southern part of the East China Sea and the area west of Kyushu in February, March, and April, and in the central part of the East China Sea, the area west of Kyushu and Tsushima Straight in May, and in Tsushima Straight and western part of the Sea of Japan in June. The spawning ground of spotted mackerel was estimated to be in the central and southern part of the East China Sea and southern coastal area of Kyushu in February, March, and April, and the central and southern part of the East China Sea and the area west of Kyushu in May.

Proper identification of Anisakis species infecting host fishes is very important to both human health and fish disease diagnosis. The foremost problem in the identification of Anisakis larvae in fishes is that L3 larvae cannot be easily differentiated morphologically, especially between A. simplex (sensu stricto) (s.s.) (Rudolphi, 1809) and A. pegreffii Campana-Rouget et Biocca, 1955. Instead, molecular means such as allozyme, mitochondrial DNA (mtDNA) cox2 region and polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analyses had been successfully used. In this study, morphological differences of L3 larvae collected from fishes and in vitro-cultured L4 larvae and adult A. simplex (s.s.) and A. pegreffii were evaluated. Anisakis larvae were collected from 7 different host fishes within Japan. Undamaged A. simplex (s.s.) and A. pegreffii collected from Oncorhynchus keta (Walbaum) and Scomber japonicus Houttuyn, respectively, were used for in vitro-culture in order to obtain L4 and adult stages. Species identification was confirmed by PCR-RFLP analysis of the ITS region (ITS1-5.8S-ITS2) of ribosomal DNA and by mtDNA cox2 gene sequencing. Results revealed that L3, L4 and adult stages of A. simplex (s.s.) and A. pegreffii are morphologically distinguishable based on ventriculus length, wherein the former has longer ventriculus (0.90-1.50 mm) than the latter (0.50-0.78 mm). For oesophagus/ventriculus ratio, these two species are distinguishable only during L4 and adult stages. Also, adult male A. simplex (s.s.) and A. pegreffii were found to be distinguishable by differences in the distribution pattern of the caudal papillae, particularly the 3rd pair of distal papillae.

耳石にみられる日周輪と着底マークを用いて佐島と金田湾に生息するメバル稚魚の初期生活史を調べ比較した。主産出時期は佐島では2月, 金田湾では1月であった。浮遊期間は, 佐島で28～68日, 金田湾で51～109日と推定され, 浮遊期間は金田湾で長かった。底生生活に移行する推定体長は, 佐島で9.1～19.5mm, 金田湾で12.0～26.1mmであり, 後者で大きかった。浮遊期および底生期の成長速度は佐島の方が有意に高かった。浮遊期の成長速度と浮遊期間の間には地点内でも地点間でも負の相関関係が認められた。