Akinori Takasuka

We investigated the occurrence pattern of Salpa fusiformis , broad‐spectrum filter‐feeding zooplankton that intercept small particles around the Kuroshio Current, off southern Japan and inferred in situ distribution of near‐surface small microplastics (plastic particles < 330 μ m) based on their gut contents. First, we examined 98 neuston net samples to determine the distribution of S. fusiformis . While no S. fusiformis were found in the daytime samples, they occurred in 75% of the night samples, indicating active diel vertical migration. Their nocturnal density in the surface layer was 0.026–0.82 ind. m ⁻³ and was particularly high around the cold‐core ring within the large meander of the Kuroshio Current. Microplastics were found in 98% of the guts of S. fusiformis (mean: 5.23 ± 4.40 particles/ind.). The in situ density of near‐surface small microplastics estimated from their gut content analysis ranged from 194 ± 138 to 7093 ± 923 particles m ⁻³ . The estimated small microplastics density was negatively correlated with surface current velocity, while small microplastic size and polymer compositions were spatially variable. This suggests that the small microplastic distribution was influenced by oceanographic conditions and location of pollution sources. The maximum small microplastic ingestion rate by S. fusiformis in the surface layer (< 0.75 m depth) was 34.5 particles m ⁻³ d ⁻¹ , corresponding to 1.05% of the in situ small microplastic density. Overall, the uptake efficiency of small microplastics from the water column may be substantial owing to the active diel vertical migration and production of large fecal pellets by S. fusiformis .

Growth and survival rates during the early life stages are key vital parameters driving population dynamics of fish. Growth rates generally present a pattern of autocorrelation. Growth autocorrelation is stronger when faster and slower growing individuals continue to grow faster and slower. Thus, the extent of growth autocorrelation can be a tool for considering potential effects of early growth rates on subsequent growth rates in the life history of fish. In the present study, we applied a group‐level growth autocorrelation analysis to Atlantic bluefin tuna (Thunnus thynnus ) larvae in the northwest Mediterranean (MED) and the Gulf of Mexico (GOM). Based on the otolith increment width data compiled mainly from published datasets, the pattern of growth autocorrelation was described for the species and compared between the MED and GOM populations. Atlantic bluefin tuna showed the highest levels of growth autocorrelation during the early life stages compared with various fish species. Their characteristics supported the general hypothesis that the species and populations with higher growth rates have stronger growth autocorrelation, extending the hypothesis to cover large pelagic piscivorous fish. Therefore, the maternal effects and the environmental variability that larvae encounter right after hatching would be even more critical in survival dynamics and useful for predicting the recruitment dynamics than previously recognized.

Species in the genus Engraulis show extensive intraspecific as well as interspecific morphological and genetic diversity. Since morphological differences do not necessarily correspond to genetic differences, it is necessary to clarify the relationship between morphological differences and genetic differences for a better understanding of the population structure. Fish morphology at a given standard length differs between cohorts of Japanese anchovy Engraulis japonicus during the early life stages in the Kii Channel, but it is unknown whether the differences are caused by genetic differences or not. The Kii Channel includes the boundary between the Pacific (southern side of the Kii Channel) and the Seto Inland Sea stocks (northern side), but stock separation is based primarily on demographic characteristics. In the present study, genetic analyses were conducted to examine the genetic differences among samples (month and area) based on mitochondrial DNA cytochrome b region (Cyt b), control region (CR) and microsatellite markers. AMOVA revealed that the percentage of genetic variation among samples was low at 0.11% for Cyt b, 0.30% for CR and 0.00% for microsatellite, and no significant genetic variation was observed among samples. Although two clades were identified in the unrooted neighbour-joining tree for Cyt b and CR, both Cyt b and CR sequences were similar between months and between areas. Accordingly, the morphological differences among cohorts can be attributed to phenotypic plasticity. Additionally, there were no genetic differences between samples from the southern side and the northern side of the Kii Channel, suggesting strong genetic connectivity in these areas.

The linkage between growth rate and feeding success has been shown to strengthen the effects of early growth rate on later growth rate in the early life history of fish. However, the growth–feeding linkage largely remains to be tested at the individual level within the same populations/cohorts. We examined the growth–feeding linkage for multiple populations/cohorts in Japanese anchovy Engraulis japonicus larvae and Pacific round herring Etrumeus micropus larvae, through otolith microstructure analysis, based on samples collected from the commercial fishery for larval fish in the Kii Channel, Japan. The three growth–feeding mechanisms, which are based on the respective potential advantages of larger somatic size, higher growth rate, and earlier morphological development for achieving feeding success, were tested to understand how growth rate relates to feeding success. The “somatic size” mechanism was supported for all of six samples for anchovy larvae and three of four samples for round herring larvae. The “growth rate” mechanism was supported for two of six samples for anchovy larvae and three of four samples for round herring larvae. The “morphological development” mechanism was supported for three of four samples for anchovy larvae and all of three samples for round herring larvae. Overall, the present analysis supported the growth–feeding linkage but revealed the dynamics of the growth–feeding mechanisms. All the mechanisms were shown to operate at least for certain populations/cohorts, but none of them were universally effective over all populations/cohorts across the two species. Understanding the dynamics of the growth–feeding mechanisms would provide precious hints for considering strategies of predicting recruitment dynamics.

Early life survival is critical to successful replenishment of fish populations, and hypotheses developed under the Growth‐Survival Paradigm (GSP) have guided investigations of controlling processes. The GSP postulates that recruitment depends on growth and mortality rates during early life stages, as well as their duration, after which the mortality declines substantially. The GSP predicts a shift in the frequency distribution of growth histories with age towards faster growth rates relative to the initial population because slow‐growing individuals are subject to high mortality (via starvation and predation). However, mortality data compiled from 387 cases published in 153 studies (1971–2022) showed that the GSP was only supported in 56% of cases. Selection against slow growth occurred in two‐thirds of field studies, leaving a non‐negligible fraction of cases showing either an absence of or inverse growth‐selective survival, suggesting the growth‐survival relationship is more complex than currently considered within the GSP framework. Stochastic simulations allowed us to assess the influence of key intrinsic and extrinsic factors on the characteristics of surviving larvae and identify knowledge gaps on the drivers of variability in growth‐selective survival. We suggest caution when interpreting patterns of growth selection because changes in variance and autocorrelation of individual growth rates among cohorts can invalidate fundamental GSP assumptions. We argue that breakthroughs in recruitment research require a comprehensive, population‐specific characterization of the role of predation and intrinsic factors in driving variability in the distribution and autocorrelation of larval growth rates, and of the life stage corresponding to the endpoint of pre‐recruited life.

Johan Hjort's “critical period” hypothesis, which postulates that year‐class strength is determined in the short period following the onset of exogenous feeding, has rarely been supported by empirical data. Instead, the current understanding is that recruitment is determined by cumulative mortality throughout early life. Recent studies relied on the measure of growth autocorrelation derived from otolith daily increment widths to test the link between growth rate achieved during the post‐hatch period and during subsequent phases of the larval stage. Based on this approach, we revisit the role of larval growth in driving survival potential in three clupeoid species: Japanese sardine Sardinops melanostictus, Japanese anchovy Engraulis japonicus, and Pacific round herring Etrumeus micropus throughout the larval stage, using a combination of published datasets of otolith increment widths. Strong growth autocorrelation was detected for all three species throughout the larval stage, suggesting that initial growth determines to some extent growth rates achieved later in life. The extent of autocorrelation was reduced in sardine relative to anchovy and round herring at older ages. This interspecific difference could be attributed to differences in sensitivity to variability of environmental factors such as water temperature and food availability. The present findings suggest that the effect of early growth rate persists into later life stages for driving survival potential, which could reconcile the classic concept of “critical period” and the current “growth–survival” paradigm.

Proportionality between spawning stock biomass (SSB) and annual total egg production (TEP) has been an inherent assumption in spawner–recruitment models for fisheries management and studies on recruitment mechanisms in fish. However, recent evidence of density dependence in egg production has led to a need to reconsider the validity of recruitment per spawning stock biomass (RPS), which is a SSB‐based survival index from hatching to recruitment, in recruitment studies. Here we revisit sardine recruitment hypotheses based on recruitment per egg production (RPE), which is a TEP‐based survival index, by reanalyzing published data for the Pacific stock of Japanese sardine (Sardinops melanostictus) in the Kuroshio Current system at a multidecadal scale. By adopting RPE, we detected statistically significant relationships of the survival from hatching to recruitment to large‐scale ocean climate anomaly, ambient temperature in the nursery grounds and growth rate during the early life stages. None of these relationships were detected by RPS. We propose the adoption of TEP‐based survival indices rather than SSB‐based ones in future studies on recruitment mechanisms of fish in relation to environmental and biological factors. Developing egg‐production‐based survival indices has the potential to improve understanding of recruitment mechanisms of fish under climate variability.