Engraulis Japonicus Research Articles

Fishery Dynamics, Status, and Rebuilding Based on Catch-Only Data in Coastal Waters of China

China has become the largest contributor to marine fisheries in the world with its fishing fleets explosively increasing their fishing effort and resulting catch, but its fishery composition and sustainability have deteriorated. Limited information on fishery exploitation status encumbers effective resource management. In this study, a data-poor Monte Carlo method, the Catch-Maximum Sustainable Yield (CMSY) method, was used to estimate the historical exploited dynamics and current stock status of ten Chinese economic marine fish stocks, includingTrichiurus lepturus,Larimichthys crocea,Larimichthys polyactis,Thamnaconus modestus,Scomberomorus niphonius,Ilisha elongate,Decapterus maruadsi,Scomber japonicus,Engraulis japonicus, andClupea pallasii, which accounted for about 50% of total fish catches in the coastal waters of China and covered five functional groups (i.e., large, medium benthopelagic, large, medium, and small pelagic). SpeciesL. croceaandL. polyactishad been subjected to overfishing since the 1950s. The others showed a decreasing trend in biomass along with the explosively increasing fishing efforts since the 1990s. Benthopelagic fish experienced overfishing pressure about a decade earlier than pelagic species. All the fish stocks investigated in this study were depleted (current biomass lower than the biomass capable of producing maximum sustainable yields, i.e.,B<Bmsy) in 2019, and most species were still facing high-fishing pressure (current fishing mortality higher than the mortality capable of producing maximum sustainable yields, i.e.,F>Fmsy). Also, a Schaefer model was used to assess stocks rebuilding status until 2030 under four exploitation scenarios, i.e., fishing mortality equals 0.5, 0.6, 0.8, or 0.95 timesFmsy. Most species stocks will likely recover to theBmsy, which indicates that reduction of fishing pressure is probably the most effective way for fishery recovery.

サバ類資源によるカタクチイワシ太平洋系群の捕食死亡率の推定

サバ類資源によるカタクチイワシ太平洋系群の捕食死亡率の推定

Development of species‐specific multiplex real‐time PCR assays for tracing the small pelagic fishes of North Pacific with environmental DNA

AbstractEnvironmental DNA (eDNA) has become a popular non‐invasive method to study the presence of organisms in the environment. To study the ecological behavior and migration routes of small pelagic fishes that are difficult to trace with traditional capturing methods, we developed multiplex real‐time PCR assays to measure the eDNA of chub mackerel, blue mackerel, Japanese jack mackerel, Japanese anchovy, Japanese sardine, and Pacific saury. To test the validity, eDNA samples collected from the water near fisherman set nets in Northeast Japan between 2018 and 2019 were analyzed. Warm‐climate species including blue mackerel, jack mackerel, and anchovy were prominent in autumn, while cold‐climate species such as chub mackerel and sardine were richly present in winter. Pacific saury is an off‐coast species and was not detected in the coastal set net area. The eDNA data were partially correlated to the fishery catch records, suggesting that the eDNA quantities are relevant to the biomass of the fish schools, and our PCR assays can quantify the biological traces left by the small pelagic fish schools. Different patterns of eDNA occurrence were observed between the chub and blue mackerels, indicating that the specificity of the multiplex assay is applicable to field samples to measure the presence of the two closely related species separately. The methods are useful to evaluate the eDNA samples collected from the open water in the North Pacific to elucidate the life cycles and monitor the sustainability of the small pelagic fishes, which not only are important food sources, but also carry a role in the Japanese food culture.

Incorporating thermal niche to benefit understanding climate‐induced biological variability in small pelagic fishes in the Kuroshio ecosystem

AbstractClimate‐induced fluctuations in marine fish populations have been raising great scientific concerns. Previous studies generally dedicate to parameterization of climatic, environmental, and biological variability patterns with focuses on their empirical or incompletely mechanistic relationships. Little consideration on physiological characteristics of organisms prevents from exploring mechanisms behind the apparent relationships. Therefore, incorporating physiological information into investigation on climate–environment–biology relationships is essential to understand climate‐induced biological variability. Small pelagic fishes play unparalleled roles in the Kuroshio ecosystem and show dramatic fluctuations in response to environmental changes, serving as ideal research targets in studies on climate‐induced biological variability. This study focuses on Japanese sardine, chub mackerel, and Japanese anchovy, three key small pelagic fishes in the Kuroshio ecosystem with distinct physiological characteristics, with attempts to incorporate thermal niches into investigations on their responses to climate variability. The recruitment suitable index (RSI) is developed considering both sea surface temperature (SST) and species thermal niche. Results show that the RSI significantly increase explanations on variations in resource abundance indices of the three key small pelagic fishes. Responses of small pelagic fishes to climate variability show differences by species and stocks. In addition, variations in the Siberian High pressure system and Atlantic Multidecadal Oscillation contribute highly to variations in resource abundance indices through affecting the SST and further the RSI. This study demonstrates importance of physiological information for understanding climate‐induced biological variability and effectiveness of combining thermal niches with the investigation on mechanistic climate–environment–biology relationships, which should be extensively considered in the future studies.

Retraction notice to 'A dynamic energy budget–integral projection model (DEB-IPM) to predict population-level dynamics based on individual data: a case study using the small and rapidly reproducing species <i>Engraulis japonicus</i>'

Retraction notice to 'A dynamic energy budget–integral projection model (DEB-IPM) to predict population-level dynamics based on individual data: a case study using the small and rapidly reproducing species <i>Engraulis japonicus</i>'

Characteristics of catch losses in stow nets with finless porpoise excluder devices and a strategy to reduce them

Finless porpoise excluder devices have been developed to prevent bycatch of finless porpoise by stow net in Korea; however, fishermen are hesitant to use them owing to concerns of associated catch losses. To promote their use, we aimed to identify the characteristics of catch losses associated with finless porpoise excluder devices used along the Korean west coast and suggest a strategy to reduce these losses. Therefore, we analyzed the selectivity of the separator net in excluder devices, the catch loss rates in stow net with upward- or downward-facing exit holes, and the size distribution between the lost and caught fish for significant species of stow net. As a result, it was confirmed that the selectivity of the separator was best expressed by the bi-normal function for Japanese anchovies (L50 = 8.1 to 10.9 cm) and golden cuttlefish (L50 = 6.7 to 8.7 cm) and by the constant function for hair tails and swimming crabs. In addition, the size distribution between lost and caught fish species was not significantly different except for Japanese anchovies. Analysis results of the catch loss rate by the position of exit of the excluder devices were as follows. Upward-facing exit holes showed relatively high losses of Japanese anchovies (upward exit 14.40 vs. downward exit 1.11%) and hair tails (3.38 vs. 0.26%). In contrast, downward-facing exit holes showed high losses of golden cuttlefish (3.58 vs. 7.55%) and swimming crabs (0% vs. 15.72%). However, these losses significantly decreased when the exit hole direction was reversed, suggesting that changing the exit hole direction depending on the species could reduce catch losses, increasing fishermen’s willingness to use finless porpoise excluder devices and reducing bycatch.

Spring Ichthyoplankton Assemblage Structure in the Yangtze Estuary Under Environmental Factors

Estuaries, where fresh and salty water converge, provide abundant nutrients for ichthyoplankton. Ichthyoplankton, including fish eggs, larvae, and juveniles, are important fishery recruitment resources. The Yangtze Estuary and its adjacent waters comprise a typical large-scale estuary and supply many important fish spawning, feeding, and breeding areas. In this study, 1,291 ichthyoplankton individuals were collected in the Yangtze Estuary in spring, from 2013 to 2020. The aims of the study were to provide detailed information on characteristics of the ichthyoplankton assemblage, explore interannual variation, and evaluate the effects of environmental variables on the temporal variation in assemblage structure. Twenty-six species in seventeen families were identified. The dominant species were Coilia mystus, Chelidonichthys spinosus, Engraulis japonicus, Hypoatherina valenciennei, Larimichthys polyactis, Salanx ariakensis, Stolephorus commersonnii, and Trachidermus fasciatus. The ichthyoplankton assemblage changed significantly over time, and Chelidonichthys spinosus became one of the dominant species. Canonical correspondence analysis showed that temperature and chlorophyll a were the key factors affecting the assemblage structure in the Yangtze Estuary in spring.

Experimental evidence for population variability of reproductive traits in Japanese anchovy Engraulis japonicus

Small pelagic fishes show spatiotemporal variations in reproductive traits due probably to responses to varying environmental conditions, but potential genetic contributions to such variations have received little attention. This study compared several reproductive traits of Japanese anchovy between two geographically isolated populations, from the central Seto Inland Sea and Omura Bay, western Japan, under common environmental conditions over three generations. There were no significant differences in age and standard length at sexual maturation between populations. Temperature-mediated variation in egg size was found in the two populations with decrease in egg size as temperature increased. However, regression analysis showed that the relationship between egg size and temperature differed significantly between the two populations over three generations. There was also a significant difference in the relative batch fecundity of the F1 and F2 generations between populations. Hence, females from the central Seto Inland Sea could produce a larger number of smaller eggs than females from Omura Bay. Variation in egg size appears to reflect significant differences in larval size at hatch and the onset of feeding in waters of different temperatures. These findings suggest that the thermal reaction norms of egg size observed in the two populations could have genetic contributions and that their egg traits may mediate a possible difference in offspring viability during early life stages.

Heterogeneity of fork length-weight relationship for juvenile Engraulis japonius based on linear mixed-effects models

In order to examine the growth heterogeneity of juvenile Engraulis japonicus, we conducted heterogeneity analysis of fork length-weight relationships of the species based on the samples of juvenile E. japonius from April to June 2019 collected from the coastal waters of Zhejiang Province by using the generalized linear model and 9 linear mixed-effect models. The results showed that the fork length of juvenile E. japonius was between 14-74 mm, with an average of 33 mm. The dominant fork length group was 21-50 mm. The weight range was 0.01-2.96 g, with an average of 0.28 g, and the dominant body weight group was 0.01-0.50 g. According to Akaike information criterion (AIC), the growth-fitting model with both months and regions random effects on the growth parameters a and b of juvenile E. japonius fitted best.The reliable prediction results was testified by the cross-validation. In the best model, the fixed value of growth parameter a was 0.24×10-5 and its estimated value did not fluctuate significantly, while the fixed value of b was 3.246 and the estimated value ranged from 3.206 to 3.272, indicating that juvenile E. japonius was under positive allometric growth. Our results suggested that month and region significantly affected the fork length-weight relationships of juvenile E. japonius.

Temperature increase drives critical slowing down of fish ecosystems.

Fish ecosystems perform ecological functions that are critically important for the sustainability of marine ecosystems, such as global food security and carbon stock. During the 21st century, significant global warming caused by climate change has created pressing challenges for fish ecosystems that threaten species existence and global ecosystem health. Here, we study a coastal fish community in Maizuru Bay, Japan, and investigate the relationships between fluctuations of ST, abundance-based species interactions and salient fish biodiversity. Observations show that a local 20% increase in temperature from 2002 to 2014 underpins a long-term reduction in fish diversity (∼25%) played out by some native and invasive species (e.g. Chinese wrasse) becoming exceedingly abundant; this causes a large decay in commercially valuable species (e.g. Japanese anchovy) coupled to an increase in ecological productivity. The fish community is analyzed considering five temperature ranges to understand its atemporal seasonal sensitivity to ST changes, and long-term trends. An optimal information flow model is used to reconstruct species interaction networks that emerge as topologically different for distinct temperature ranges and species dynamics. Networks for low temperatures are more scale-free compared to ones for intermediate (15-20°C) temperatures in which the fish ecosystem experiences a first-order phase transition in interactions from locally stable to metastable and globally unstable for high temperatures states as suggested by abundance-spectrum transitions. The dynamic dominant eigenvalue of species interactions shows increasing instability for competitive species (spiking in summer due to intermediate-season critical transitions) leading to enhanced community variability and critical slowing down despite higher time-point resilience. Native competitive species whose abundance is distributed more exponentially have the highest total directed interactions and are keystone species (e.g. Wrasse and Horse mackerel) for the most salient links with cooperative decaying species. Competitive species, with higher eco-climatic memory and synchronization, are the most affected by temperature and play an important role in maintaining fish ecosystem stability via multitrophic cascades (via cooperative-competitive species imbalance), and as bioindicators of change. More climate-fitted species follow temperature increase causing larger divergence divergence between competitive and cooperative species. Decreasing dominant eigenvalues and lower relative network optimality for warmer oceans indicate fishery more attracted toward persistent oscillatory states, yet unpredictable, with lower cooperation, diversity and fish stock despite the increase in community abundance due to non-commercial and venomous species. We emphasize how changes in species interaction organization, primarily affected by temperature fluctuations, are the backbone of biodiversity dynamics and yet for functional diversity in contrast to taxonomic richness. Abundance and richness manifest gradual shifts while interactions show sudden shift. The work provides data-driven tools for analyzing and monitoring fish ecosystems under the pressure of global warming or other stressors. Abundance and interaction patterns derived by network-based analyses proved useful to assess ecosystem susceptibility and effective change, and formulate predictive dynamic information for science-based fishery policy aimed to maintain marine ecosystems stable and sustainable.

An Analysis of Consumer Evaluations of the “Tagonoura Shirasu” Certification under the Japan Geographical Indication Protection System

Over time, the Geographical Indication (GI) protection system has been independently adopted in more than 100 countries worldwide. In Japan, the GI protection system has been operating since 2015 in response to a growing interest in local brands. Therefore, based on a questionnaire survey, we econometrically analyze the consumer evaluation of Shizuoka Prefecture’s Shirasu “Tagonoura Shirasu,” which is registered under the GI protection system. Shirasu is a collective term for long and narrow larvae such as anchovy (Engraulis japonicus), sardine (Sardinops melanostictus), or red-eye round herring (Etrumeus sadina). Based on the contingent valuation method, we clarified that consumers' additional willingness to pay (WTP) for Tagonoura Shirasu was approximately 8.2% compared to the Shirasu produced in Shizuoka Prefecture, which has not been certified under the GI protection system. Moreover, eight characteristics of consumers with a high WTP were revealed: they have purchased Shirasu within the last year, attach importance to freshness when buying seafood, have a greater awareness of regional production standards when purchasing seafood, purchase foodstuffs linked to the revitalization of production areas and regions, show little regard for buying products as cheaply as possible when purchasing foodstuffs, know about the GI protection system in greater detail, are male, and have a high annual household income. To further enhance the economic value of Tagonoura Shirasu, it would be effective to target consumers with all eight characteristics for a high WTP.

The Diet Composition and Ingested Plastics of Laysan and Black-Footed Albatrosses Incidentally Captured by the Pelagic Longline Fishery in the Western North Pacific

The stomach contents (food and ingested plastics) of Laysan Albatross Phoebastria immutabilis and Black-footed Albatross P. nigripes were examined by necropsy analysis of birds caught as bycatch in the pelagic longline fisheries in the Western North Pacific. The contents were classified separately for the proventriculus and gizzard. Undigested fish and cephalopods were found in the proventriculus, while hard objects such as cephalopod beaks, plastics, and pebbles were found in the gizzard. This indicates that the retention time of soft tissues in fish or cephalopods differs from that for hard objects. The main prey of both albatrosses consisted of mesopelagic cephalopods such as Cranchiidae, Gonatidae, Histioteuthidae, and Onychoteuthidae species. Laysan Albatrosses also foraged on small teleosts (Japanese Anchovy Engraulis japonicus and some Myctophidae fishes) as major prey items. The estimated dorsal mantle length of cephalopods preyed upon by the albatrosses was below 200 mm, which was smaller than the mature sizes of those cephalopods and the size class mainly preyed upon by cetaceans. This implies that the albatrosses may forage on immature cephalopods floating in the surface layer. Ingested plastics were found in 71.8% of Laysan and 31.8% of Black-footed Albatrosses and plastic fragments were the most abundant.

Fish environmental DNA in Tokyo Bay: A feasibility study on the availability of environmental DNA for fisheries

Environment DNA (eDNA) is a nonlethal sampling method that has potential for evaluating fisheries resources. To evaluate whether eDNA information is useful for fisheries, we collected and analysed eDNA samples from 14 sites in Tokyo Bay from April 2018 to March 2020. Using MiFish universal primers, fish mitochondrial 12S rRNAs were amplified from the eDNA by polymerase chain reaction (PCR) and then sequencing. A total of 535 eDNA samples yielded 44.5 million reads, with 197 of teleosts and 16 of elasmobranchs identified. Of those, 42 identified species were commercially important, with the most abundant being Engraulis japonicus (Japanese anchovy), followed by Konosirus punctatus (dotted gizzard shad) and Lateolabrax japonicusi (Japanese seabass). Mitochondrial cytochrome b copy numbers of K. punctatus and L. japonicus showed a seasonal trend, although catch per unit effort for L. japonicus was not correlated with copy number. Although eDNA is a powerful tool that can detect the presence of fish species from a water sample, caution should be used when interpreting fish biomass based on eDNA dynamics until the method has been better studied.

Short-term change in fish assemblages after the passage of a typhoon in a temperate, coastal bay

To study the effects of a typhoon on a temperate, coastal bay community, the species composition, catch amount, and diversity of epipelagic fish assemblages were investigated. Fish samples were taken from catches of a purse seine fishery in Tachibana Bay, Japan between May and July 2011, covering before and after the passage of a typhoon in the area. Although major changes in total catch amount were not observed before and after the passage of the typhoon, the abundance of the Japanese anchovy, Engraulis japonicus Temminck et Schlegel, 1846, markedly decreased and bycatch of species increased, accompanied by increasing levels of diversity of the fish assemblage. Multivariate analysis showed that community differences before and after the passage were quantitative rather than qualitative. Comparisons in total length frequencies between the two periods indicated that specimens of the species compared were bigger in size for Trachurus japonicus (Temminck et Schlegel, 1844) and smaller for E. japonicus in the “after” period. These results suggest that the passage of the typhoon triggered not only interspecific faunal change but also intraspecific recruitment shifts in and around the bay.

Evidence of bottom-up effects of climate on Japanese anchovy (Engraulis japonicus) in the western North Pacific

Evidence of bottom-up effects of climate on Japanese anchovy (Engraulis japonicus) in the western North Pacific

Food web in jellyfish-shrimp-shellfish polyculture pond

To analyze the feeding habits and trophic level of jellyfish (Rhopilema esculentum), Chinese shrimp (Fenneropenaeus chinesis), grass shrimp (Penaeus monodon), and clam (Ruditapes philippinarum), and the food web structure in marine aquaculture pond, we measured the δ13C and δ15N values of the four species and different feed from May to September in 2017. The average proportional contribution of different feed to the four species were analyzed using the IsoSource linear mixture model. The results showed that zooplankton was the main food source to jellyfish, Engraulis japonicus was the main food source to the two shrimp species, and the phytoplankton, benthic diatoms and dejecta of the two shrimp species were the main food source to the clam. The trophic level of the clam ranged from 2.64 to 2.95, with a mean value of 2.84. The trophic level of jellyfish ranged from 2.78 to 3.27, with a mean value of 3.06. the grass shrimp ranged from 3.03 to 3.54, with a mean value of 3.25. The trophic level of Chinese shrimp ranged from 3.76 to 4.40, with a mean value of 3.95. Results of comprehensive analysis showed that the clam was the primary consumer, jellyfish was the secondary consumer, and shrimps were the predators. Jellyfish filtered the dejecta of the two kinds of prawns and improved water quality of the polyculture pond to a certain extent.

Feeding ecology of Engraulis japonicus based on stomach contents and stable isotope

Engraulis japonicus, an important fishery resource, is a key species in ecosystem trophodynamics studies. In this study, we examined stomach content of E. japonicusby stable isotope analyses, with samples collected from the East China Sea in 2008-2009 and 2020. The aim of this study was to demonstrate their diet composition, diel and ontogenetic changes in feeding habits and trophic level. Results of the stomach content analysis showed that E. japonicus mainly fed on planktonic crustaceans and small fish. The main prey species were Euphausia pacifica [index of relative importance (IRI)=87.6%; frequency(F)=57.6%], Paracalanus parvus (IRI=3.2%, F=15.3%), and Themisto gracilipes (IRI=2.1%, F=13.1%). Results of the stable isotope analysis showed that Copepoda were the main food source of E. japonicus, followed by Euphausiacea, and the contribution rate of Amphipoda was the least, which was less than 1%. There was significant diel change in diet composition. Feeding intensity was higher in the daytime than at night, with the highest in the dusk and the lowest at midnight. Ontogenetic change in feeding habit occurred when fork length reached 90 mm, over which the fish fed both zooplankton and small fishes. The δ13C of E. japonicus was between -21.66‰ and -18.14‰, with an average of (-19.92±0.86)‰. The δ15N of E. japonicus ranged from 4.07‰ to 10.78‰, with an average of (8.14±2.48)‰. Both δ13C and δ15N values were positively correlated with fork length. Trophic level of the fish was 3.4 with stomach content analysis and 2.7 with stable isotope analysis. The results would provide important reference for understanding nutritional status of pelagic small fish, and offer some basic data to establish ecopath model.

Assessment of coastal anthropo-ecological system dynamics in response to a tsunami catastrophe of an unprecedented magnitude encountered in Japan

On 11 March 2011, a catastrophic earthquake and subsequent tsunami hit the Pacific coast of northern Japan, devastating many of the towns, villages and coastal ecosystems located along the shoreline. To assess the impacts of the disaster, we investigated temporal dynamics of fish and epibenthic megafaunal community structure in relation to changes in a range of physical, biological and anthropogenic variables between 2007 and 2018 in Onagawa Bay. Commercially important fish such as greenlings, Japanese anchovy, flatfishes, rockfishes were consistently abundant in both larval and adult fish assemblages. While abundance, species richness, and Shannon index H′ for adult fish and epibenthic megafaunal assemblages increased significantly soon after the disaster to peak values towards the end of the study period, the same metrics did not change accordingly for larval fish assemblages. Temporal dynamics of larval fish community clearly demonstrated significant seasonal variation along with changes in large-scale environmental conditions such as temperature and nutrients. However, anthropogenic components such as decline in human population, reduction in fishing pressure and the recovery of aquaculture operations significantly explained the observed post-disaster change in adult fish and epibenthic megafaunal communities. The pelagic and benthic components of Onagawa Bay appeared to have responded to the 2011 disaster very differently, and this study suggests the post-disaster recovery and dynamics of the coastal ecosystems may be regulated by how human societies respond to the impacts of a tsunami catastrophe through their influences on benthic habitat of ecosystems.

Length–length and Length–weight Relationships for Four Dominant Small Pelagic Fishes in the Kuroshio–Oyashio Current System

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.

Temporal variations in hatch date and early survival of Japanese anchovy (Engraulis japonicus) in response to environmental factors in the central Seto Inland Sea, Japan

AbstractReproductive success can be influenced by the match or mismatch between the spawning season and optimal environmental conditions for larval survival. However, it is challenging to understand how temporal factors affect population dynamics. In the central Seto Inland Sea, Hiuchi‐nada, western Japan, the recruitment of Japanese anchovy (Engraulis japonicus) has markedly decreased in the last decade. To explore the causes of this decline, egg survey and commercial catch data from 1994 to 2019 (26 years) between April and August were used to examine egg production, hatch date, and early survival in relation to environmental factors. Otolith microstructure analysis indicated that most landed larval and juvenile fish (recruits) hatched in May and June. Egg and recruit abundances were negatively correlated over the study period. The survival indices from hatching to recruitment were higher in waters at 16–24°C in a given season until the mid‐2000s. Thereafter, they decreased over the studied temperature range and remained relatively low over the last five years. The copepod nauplii densities from 2015 to 2019 were significantly higher than those from 2002 to 2005 due possibly to the effect of increasing temperature, but the ratio of copepod nauplii density to larval density in May and June from 2015 to 2019 was approximately 34% and 66% of those from 2002 to 2005, respectively. These findings and the hydrodynamic features of these waters suggest that the recent decline in recruit abundance is partly due to a decrease in larval survival in response to increasing intracohort competition for prey in early‐life stages.