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.
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DOBA, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UILJ, UKNU, UL, UM, UPUK
•Effects of growth rate on recruitment changed over time.•Baseline recruitment level also changed over time.•Time-varying relationship model was more statistically plausible than time-constant ...models.•Growth–recruitment relationship is more dynamic than previously recognized.
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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Spatiotemporal patterns in spawning of fish 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 transition 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 onward. 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.
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DOBA, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UILJ, UKNU, UL, UM, UPUK
Environmental DNA (eDNA) is increasingly used to noninvasively monitor aquatic animals in freshwater and coastal areas. However, the use of eDNA in the open ocean (hereafter referred to OceanDNA) is ...still limited because of the sparse distribution of eDNA in the open ocean. Small pelagic fish have a large biomass and are widely distributed in the open ocean. We tested the performance of two OceanDNA analysis methods-species-specific qPCR (quantitative polymerase chain reaction) and MiFish metabarcoding using universal primers-to determine the distribution of small pelagic fish in the open ocean. We focused on six small pelagic fish species (Sardinops melanostictus, Engraulis japonicus, Scomber japonicus, Scomber australasicus, Trachurus japonicus, and Cololabis saira) and selected the Kuroshio Extension area as a testbed, because distribution of the selected species is known to be influenced by the strong frontal structure. The results from OceanDNA methods were compared to those of net sampling to test for consistency. Then, we compared the detection performance in each target fish between the using of qPCR and MiFish methods. A positive correlation was evident between the qPCR and MiFish detection results. In the ranking of the species detection rates and spatial distribution estimations, comparable similarity was observed between results derived from the qPCR and MiFish methods. In contrast, the detection rate using the qPCR method was always higher than that of the MiFish method. Amplification bias on non-target DNA and low sample DNA quantity seemed to partially result in a lower detection rate for the MiFish method; the reason is still unclear. Considering the ability of MiFish to detect large numbers of species and the quantitative nature of qPCR, the combined usage of the two methods to monitor quantitative distribution of small pelagic fish species with information of fish community structures was recommended.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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•We developed the integrated model delta-GLM-tree to derive standardized catch rate.•Abundance indices for recruitment of Pacific chub mackerel have increased recently.•Optimal area ...stratification suggested a range shift of recruits to high-sea areas.•Delta-GLM-tree basically outperform GAM and improved a stock assessment model.•The obtained information will be useful for assessment and management.
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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Abstract 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 ( mt DNA) cox 2 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 mt DNA cox 2 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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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:
L
t
= 401{1 − exp−0.275 (
t
+ 1.149)} (0.8 <
t
< 6.9), where
L
t
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.
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CEKLJ, DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Inter-annual variation in the feeding habits and food sources of Japanese sardine and mackerel at age-0 and age-1+ caught in the Kuroshio-Oyashio transition zone of the Western North Pacific were ...investigated based on analyses of bulk stable isotopes (
δ
13
C,
δ
15
N) and amino acid nitrogen isotopes (
δ
15
N
AA
). Differences in
δ
13
C and
δ
15
N between Japanese sardine and mackerel were small for age-0, and inter-annual variation trends were similar, suggesting they depend on similar food sources in the same food web at this age. In contrast, inter-annual variation in
δ
13
C and
δ
15
N were significantly different between both species at age-1+, and both
δ
15
N of phenylalanine (
δ
15
N
Phe
: an indicator of nitrogen source) and trophic position estimated from
δ
15
N
AA
(TP
AA
) were higher in mackerel, suggesting that the two species depend on distinct food webs as they age. Inter-annual variations in
δ
15
N
Phe
were considered to have different causes for the two species; differences in food web structure due to the degree of southward intrusion of the Oyashio Current for Japanese sardine, compared to a shift in migration area and depth for mackerel. Furthermore, competition for food due to the recent increases in the population densities of both fishes appeared to be reflected in increased TP
AA
of mackerel. Although they are caught in the same region, the mechanism of variation in food sources differs because of differences in migration area, depth, and feeding habits. Differences in the feeding habits of Japanese sardine and mackerel may affect trophic status and spawning characteristics, potentially leading to different shifts in stock abundances.
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 15-22degC sea surface temperature (SST), and mature spotted mackerel was observed in the area of 17-25degC 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.
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CEKLJ, DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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