Biology subjects

Yukami, R.

Publications and source records attributed to Yukami, R..

7 recordsLinked to original sources

bioRxiv2024

Group composition and its ontogenetic changes in marine pelagic fishes

Group living is a widespread phenomenon among animals and an important trait in fishes. Understanding group composition and its dynamics is fundamental for elucidating ecological and evolutionary processes in natural populations of gregarious animal species, including fishes, but our knowledge of group composition in marine fishes is limited. Here, we examined group composition and its ontogenetic change in two marine fishes, Japanese sardine (Sardinops melanostictus) and chub mackerel (Scomber japonicus). We found that, at the juvenile stage, groups were composed of individuals with similar body length, age, and growth history. At the sub-adult stage, although body length within a group remained similar, the within-group similarity in age and growth history decreased, approaching the level expected when groups were randomly composed. Therefore, during the early life history, individuals form groups with those spawned at approximately the same time and place (i.e., those from the same batch). As they develop, however, individuals of various origins intermingle and selectively form groups with others exhibiting similar phenotypes. We further found that environmental and geographical factors were also related to the body size and age of individuals distributed. This indicates that Japanese sardine and chub mackerel exhibit phenotype-specific and growth-stage-specific habitat use, which also contributes to the formation and maintenance of structured groups. Our findings provide valuable information on group composition, its ontogenetic change, and its underlying mechanisms in marine fishes, for which the information is limited. These insights may help to clarify ecological and evolutionary processes such as population dynamics, density-dependent phenomena, and the evolution of grouping behaviour in small pelagic fishes.

ecology↗

bioRxiv2022

Density-dependent attributes of schooling in 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.

zoology↗

bioRxiv2021

Stronger density-dependent growth of Japanese sardine with lower food availability: Comparison of growth and zooplankton biomass between a historical and current stock-increase period in the western North Pacific

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.

ecology↗

bioRxiv2021

Intra- and inter-specific density dependence of body condition, growth, and habitat temperature in chub mackerel (Scomber japonicus)

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.

ecology↗

bioRxiv2020

Length-length and length-weight relationships for four small pelagic fishes in the Kuroshio-Oyashio current system

Length-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.

ecology↗

bioRxiv2020

Spatio-temporal spawning patterns and early growth of Japanese sardine in the western North Pacific during the recent stock increase

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 sardine Sardinops melanostictus is 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.

ecology↗

bioRxiv2020

Spatio-temporal model reduces species misidentification bias of spawning eggs in stock assessment of spotted mackerel in the western North Pacific

Species identification based on morphological characteristics includes species misidentification, leading to estimation bias of population size. The eggs of spotted mackerel Scomber australicus and chub mackerel S. japonicus in the western North Pacific has been identified based on egg diameter. Recent density of spotted mackerel was considerably high despite its low stock biomass. A possibility of this phenomenon is due to overestimation because the difference in egg diameter has become ambiguous between two species. However, we cannot test this possibility using DNA analysis because the eggs are fixed with formalin. Here, we estimated the index of egg density of spotted mackerel using a spatio-temporal model that incorporates the effect of egg density of chub mackerel on the catchability of spotted mackerel, using 15 years data of spawning eggs. We then examined how retrospective biases in estimated stock abundance were reduced when using the index from the model. The index estimated from the model decreased temporal fluctuation and showed smooth patterns. Especially, the recent index was considerably revised down rather than the nominal index. Additionally, the retrospective bias decreased ca. half compared with the nominal index. Therefore, incorporating species misidentification bias should be an essential process for improving stock assessment.

ecology↗