The out-of-phase population oscillations between anchovy and sardine have been attributed to climate changes. However, the biological processes causing these species alternations have remained unresolved. Here we propose a simple "optimal growth temperature" hypothesis, in which anchovy and sardine regime shifts are caused by differential optimal temperatures for growth rates during the early life stages. Dome-shaped relationships between growth rate and sea temperature were detected for both Japanese anchovy (Engraulis japonicus) and Japanese sardine (Sardinops melanostictus) larvae based on otolith microstructure analysis. The optimal growth rate for anchovy larvae occurred at 22.0 °C, whereas that for sardine larvae occurred at 16.2 °C. Ambient temperatures have historically fluctuated between these optima, which could lead to contrasting fluctuations in larval growth rates between the two species. This simple mechanism could potentially cause the shifts between the warm anchovy regime and the cool sardine regime in the western North Pacific. Although retrospective analysis suggested synergistic effects of other factors (e.g., trophic interactions and fishing), the optimal growth temperature concept would provide a possible biological mechanism of anchovy and sardine regime shifts.

Growth-selective predation mortality was demonstrated for postlarval Japanese anchovy Engraulis japonicus in field research. The larval anchovy and their predatory fish were simultaneously captured by a trawl in Sagami Bay during October to November 2000. The growth rates analyzed by otolith microstructure were compared between the larvae from the stomach contents of the predators (prey larvae) and those from the population of origin (surviving larvae). The mean growth rates of the prey larvae collected on 28 October and 2 to 4 November (mean ± SD: 0.57 ± 0.07 mm d-1) and on 23 November (0.50 ± 0.06 mm d-1) were significantly lower than those of the corresponding surviving larvae (0.63 ± 0.07 and 0.54 ± 0.06 mm d-1 , respectively). Such significant differences were not explained by size-selective predation, but were due to variations in the mean growth rates at the same larval size (i.e. non-size-related). The mean growth rates of the prey larvae were different among predatory species (barracuda Sphyraena pinguis, Japanese sea bass Lateolabrax japonicus, white croaker Pennahia argentatus, Japanese jack mackerel Trachurus japonicus, Pacific round herring Etrumeus teres and juvenile anchovy). Comparisons of back-calculated daily growth rates showed that the decrease in growth rates of the prey larvae were consistent from directly after hatching up to predation. The larvae with lower growth rates were more vulnerable to predation, owing to the cumulative decline in growth rates from hatching to each encounter with predators, compared to the larvae with higher growth rates, even if they were the same size, at a given moment in the sea. Therefore, the level of growth rates itself had direct impact on vulnerability to predation for larval anchovy, independently of both size (size-selective mortality) and time (stage duration). In addition, such impacts could be predator specific. We propose the 'growth-selective predation' hypothesis (mechanism), which is theoretically independent of and synergistic with the existing hypotheses based on size and time under the general 'growth-mortality' concept for the survival process during the early life history of marine pelagic fish.

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The 'growth-mortality' hypothesis, which holds that larger and/or faster growing individuals will have a higher probability of survival, currently includes 3 functional mechanisms (hypotheses) in its theoretical framework: 'bigger is better', 'stage duration' and the recently proposed 'growth-selective predation', which are based on size, time and per se growth rate, respectively. Through otolith microstructure analysis, we tested these 3 synergistic growth-related mechanisms according to growth characteristics of the survivors vs the original population in the short-term (ca. 2 wk) survival process of larval Japanese anchovy Engraulis japonicus in the 'shirasu' (larval anchovy) fishing ground in Sagami Bay, Japan. Back-calculated standard length (growth trajectory) and growth rate (growth history) were compared between the survivors (SV) captured on 18 July 2001 and the presumed original population (OP) captured on 1 and 5 July 2001. The larvae from SV were consistently smaller than the larvae from OP until at least the start of the ca. 2 wk survival process (1 July). Daily growth rates, however, were higher for SV than for OP at least at the start of the survival period. Therefore, faster growing individuals survived even if they were smaller than slowergrowing conspecifics. This was probably mediated by predation. Growth histories were generally similar between the metamorphosing larvae and non-metamorphosing larvae older than 40 d, the minimum age for metamorphosis, except for the period immediately after hatching. As such, we failed to detect a clear relationship between growth rates and the timing of metamorphosis (stage duration) as a whole. The results supported and extended the 'growth-selective predation' hypothesis but not the 'bigger is better' hypothesis. The 'stage duration' hypothesis was not unequivocally supported by the present findings.KEY WORDS: Growth rate · Otolith microstructure analysis · Short-term survival · Growth-selective predation hypothesis · Larval Japanese anchovyResale or republication not permitted without written consent of the publisher

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Predator-specific growth-selective predation on larval Japanese anchovy Engraulis japonicus was demonstrated by comparing growth rates between the larvae ingested by predators and the larvae from the corresponding original populations through otolith microstructure analysis, based on original data and reanalyzed data from previous studies. Ingested larvae from the stomachs of small pelagic predators (juvenile Japanese anchovy, round herring Etrumeus teres, jack mackerel Trachurus japonicus and white croaker Pennahia argentatus) had significantly lower growth rates than the larvae from the original populations in general. For large piscivorous predators (sea bass Lateolabrax japonicus, greater amberjack Seriola dumerili and skipjack tuna Katsuwonus pelamis), no measurable differences in the growth rates were observed between ingested larvae and larvae from the original populations. Small pelagic fish were therefore identified as growth-selective predators, whereas large piscivorous fish were identified as non-growth-selective predators. Exponential declines in the relative predation mortalities of larvae with higher growth rates suggest the potential for growth rate to exert a great effect on recruitment variability. However, the predator field would regulate selection for growth characteristics of survivors.KEY WORDS: Growth-selective predation hypothesis · Growth rate · Otolith · Early life stage · Japanese anchovy · Relative predation mortality · Optimal foraging theory Resale or republication not permitted without written consent of the publisher

A paradigm of fisheries science holds that spawning stock biomass (SSB) is directly proportional to total egg production (TEP) of fish stocks. This “SSB–TEP proportionality” paradigm has been a basic premise underlying the spawner–recruitment models for fisheries management and numerous studies on recruitment mechanisms of fish. Studies on maternal effects on reproductive potential of a stock have progressed during the last few decades, leading to doubt concerning the paradigm. Nonetheless, a direct test of the paradigm at multidecadal scales has been difficult because of data limitations in the stock assessment systems worldwide. Here, we tested the paradigm for marine fish based on a novel combination of two independent 38‐year time series: fishery‐dependent stock assessment data and fishery‐independent egg survey data. Through this approach, we show that the SSB–TEP proportionality is distorted by density dependence in total egg production per spawner individual (TEPPS) or spawner unit weight (TEPPSW) at a multidecadal scale. The TEPPS/TEPPSW exponentially declined with biomass and thus was density‐dependent for Japanese sardine, a small pelagic species exhibiting a high level of population fluctuation, in the western North Pacific. By contrast, the TEPPS/TEPPSW was sardine‐density‐dependent for Japanese anchovy, another small pelagic species exhibiting a moderate level of population fluctuation well‐known for being out of phase with sardine. Our analysis revealed intraspecific (sardine) and interspecific (anchovy) density dependence in TEPPS/TEPPSW, which was previously unaccounted for in spawner–recruitment relationships. Such density‐dependent effects at the time of spawning should be considered in fisheries management and studies on recruitment mechanisms.

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