Keywords, Research Introduction

Research fields and specialities

Marine ecology, Marine biology, Fisheries biology, Fisheries oceanography
Small pelagic fish, Population dynamics, Recruitment mechanism, Growth and survival during early life stage

Topics

Biological mechanisms of species alternations

Marine ecosystems are notorisously complex. Despite such a nature, small pelagic fish have exhibited cyclic patterns of population dynamics worlwide. A typical example is the phenomenon of out-of-phase population oscillations between anchovy and sardine. These species alternations have been associated with climate changes; however, the biological processes have remained unresolved. We have been exploring simple and direct potential mechanisms to explain the biological processes of multi-species regime shifts of small pelagic fish, with a focus on species-specific temperature optima.

Key questions:

Why do even subtle environmental changes sometimes trigger dramatic fish regime shifts?
Why do anchovy flourish and sardine collapse or vice versa under the same ocean regime?
Why are anchovy and sardine alternations synchronous or asynchronous among different ecosystems?

Key contents:

"Optimal growth temperature" hypothesis (Takasuka & Aoki 2006 in FO, Takasuka et al. 2007 in CJFAS)
Contrasting spawning temperature optima (Takasuka et al. 2008 in PO)
Multi-species regime shifts (Takasuka et al. 2008 in MEPS)
Spawning overlap (Takasuka et al. 2008 in MEPS)
Characterizing spawning habitats (Oozeki et al. 2007 in CalCOFI)
Mechanism review (Drinkwater et al. 2009 in JMS)
Expansion/contraction (Barange et al. submitted)

Growth-based survival mechanisms during early life stages

Fish experience severe challenges to survival during the early life stages. Growth rates dominate survival potential suring those stages, playing a key role in survival dynamics. The "growth-survival" paradigm posits that faster growing individuals will have a higher probability of survival than slower growing conspecifics. This theory had been explained indirectly by two concepts: size and time (the "bigger is better" and "stage duration" hypotheses, respectively). Predation is the major and direct source of mortality. Nonetheless, no direct evidence had yet been obtained to support the relationship between growth rates and predation mortality. In our field studies, larval anchovy and their potential predators were captured simultaneously by the same tows. Through otolith microstructure analysis, we directly examined growth rates of the ingested larvae from the stomach contents of predatory fish and compared them with those of the surviving larvae from the original populations to demonstrate direct impacts of growth rates on predation mortality.

Key questions:

Is a slower growing fish larva actually removed by predation at a given moment in the sea?
If so, is the mortality size-dependent?
Is there any difference among predatory species?

Key contents:

"Growth-selective predation" hypothesis (Takasuka et al. 2003 in MEPS)
Three synergistic growth-related mechanisms (Takasuka et al. 2004 in MEPS)
"Growth-selective predation" hypothesis revisited (Takasuka et al. 2004 in MEPS)
Predator-specific "growth-selective predation" (Takasuka et al. 2007 in MEPS)
Growth effect on the otolith and somatic sizes (Takasuka et al. 2008 in FS)
Growth in the Kii Channel (Yasue & Takasuka 2009 in JFB)
Predation dynamics of mackerel (Robert et al. submitted)
Dynamics of growth-based survival mechanisms (Takasuka et al. in prep.)

Geographical variations in biological parameters

Unexpoited offshore populations of small pelagic fish may migrate and spawn beyond our view in the various regions of the world. If any differences exist in the ecological features between the inshore and offshore populations, these should be clarified for a better understanding of the population dynamics of the species. Japanese anchovy are distributed mainly in inshore waters during their low-biomass phases; however, their distribution and spawning areas expand to offshore waters during their high-biomass phases. We compared biological factors such as reproductive parameters and stable isotope traits of Japanese anchovy between inshore and offshore waters.

Key questions:

Is there any difference in biological factors between inshore and offshore waters?
What factor is resposible for such differences?

Key contents:

Temperature impacts on reproductive parameters (Takasuka et al. 2005 in FR)
Geographical variations in trophic ecology (Tanaka et al. 2008 in MB)

Other topics of collaborations

Patchiness structure and mortality

Key contents:

Patchiness structure and mortality of Pacific saury (Oozeki et al. 2009 in FO)

This field work presents patchness structure and mortality rate of Pacific saury larvae based on repeated samplings. In the Discussion section, you can also find generalization of differences in mortality estimates between methods.

Specific keywords

"Growth-selective predation" hypothesis

A mechanism linking growth rate to predation mortality (Takasuka et al. 2003 in MEPS) under the theoretical framework of the "growth-survival" paradigm. Slower growing larvae are more vulnerable to predation mortality than faster growing conspecifics, even if they are the same size (i.e. non-size-related), at a given moment (i.e. non-time-related) in the sea. This means that the level of growth rate per se has direct impacts on vulnerability to predation, independently of both size (negative size-selective mortality) and time (stage duration). We proposed the "growth-selective predation" hypothesis (mechanism), which is theoretically independent of and synergistic with the existing hypotheses based on size and time ("bigger is better" and "stage duration" hypotheses, respectively) under a general concept of the "growth-mortality" hypothesis.
The first demonstration was conducted by direct comparison of growth rates between the larvae actually ingested by the predators and surviving larvae from the original populations in Sagami Bay (Takasuka et al. 2003 in MEPS). The hypothesis was then tested based on the characteristics of the survivors versus original populations (Takasuka et al. 2004 in MEPS) and revisited for larval anchovy in offshore waters (Takasuka et al. 2004 in MEPS). A further study demonstrated predator-specific "growth-selective predation" on larval anchovy with multiple samples (Takasuka et al. 2007 in MEPS).

"Optimal growth temperature" hypothesis

A potential biological mechanism for anchovy and sardine alternations (Takasuka et al. 2007 in CJFAS). A mystery of the ocean is the phenomenon of out-of-phase population oscillations between anchovy and sardine. Why do even subtle environmental changes sometimes trigger dramatic alternations? Why do anchovy flourish and sardine collapse or vice versa under the same ocean regime? We propose a simple "optimal growth temperature" hypothesis, in which anchovy and sardine alternations are caused by differential optimal temperatures for growth rates during the early life history stages. The "growth-survival" paradigm, direct temperature impacts, and differential optimal temperatures for growth rates constitute the bottom line of this concept. Subsequently, the hypothesis was extended to the multi-species regime shifts of small pelagic fish in the western North Pacific (Takasuka et al. 2008 in MEPS) and synchronous alternations during certain phases between the opposite sides of the North Pacific (Takasuka et al.2008 in PO), focusing on species-specific spawning temperature optima.

Keywords

Fish

Pelagic fish, Larva, Juvenile, Japanese anchovy, Engraulis japonicus, "Shirasu" (larval anchovy in Japanese), Japanese sardine, Sardinops melanostictus, chub mackerel, Scomber japonicus, spotted mackerel, Scomber australasicus, Japanese jack mackerel, Trachurus japonicus, Predatory fish (barracuda Sphyraena pinguis, Japanese sea bass Lateolabrax japonicus, greater amberjack Seriola dumerili, white croaker Pennahia argentatus, Japanese jack mackerel, Pacific round herring Etrumeus teres, skipjack tuna Katsuwonus pelamis, chub mackerel, spotted mackerel, and Japanese anchovy), Raptorial feeding predator, Filter feeding predator, Particulate feeding predator

Fields

Western North Pacific, Sagami Bay, Tosa Bay, "Shirasu" fishing ground, Kuroshio Extension region, Kuroshio-Oyashio transition region, East China Sea

Sampling gears

"Shirasu" trawl, ORI (Ocean Research Institute) net, NORPAC net, Frame-type midwater trawl, Issacs-Kidd midwater trawl (IKMT), Matsuda-Oozeki-Hu-Trawl (MOHT), Neuston net, Drift net

Tools and techniques

Otolith microstructure analysis, Gut content analysis, Stable isotope ratio, RNA/DNA ratio, Long-term data set of egg and larval surveys

Topics

Growth rate, Growth history, Growth trajectory, Predation mortality, Vulnerability to predation, Survival, Survival potential, Survival strategy, Early life history stages, Recruitment mechanism, Population dynamics, "Growth-survival" paradigm, "Growth-mortality" hypothesis, "Bigger is better" hypothesis (mechanism), "Stage duration (growth-dependent)" hypothesis (mechanism), Size-selective mortality, Non-size-related mortality, Environmental factors, Sea temperature, Food availability, Prey item, Salinity, Feeding ecology, Feeding habit, Prey-predator interaction, Anti-predator behavior, Prey selectivity, Cannibalism, Trophic relationship, Trophic level, Nutritional condition, Field research, Research cruise, In situ demonstration, Metamorphosis, Northward migration, Dispersal, Transport, Sagittal otolith, Back-calculation, Biological intercept method, Uncoupling between otolith and somatic growth rates, Growth effect, Optimal foraging theory, Optimal growth temperature, Spawning temperature optimum, Stock assessment, Egg and larval survey, Spawning ecology, Reproductive parameter, Daily egg production method (DEPM), Batch fecundity, Spawning frequency, Interspecific interaction, Competition, Fish regime shift, Ocean regime shift, Species alternation, Spawning ground, Global warming

The website of current researches by Akinori Takasuka (髙須賀明典/高須賀明典).

Biological mechanisms of species alternations

Marine ecosystems are notorisously complex. Despite such a nature, small pelagic fish have exhibited cyclic patterns of population dynamics worlwide. A typical example is the phenomenon of out-of-phase population oscillations between anchovy and sardine. These species alternations have been associated with climate changes; however, the biological processes have remained unresolved. We have been exploring simple and direct potential mechanisms to explain the biological processes of multi-species regime shifts of small pelagic fish, with a focus on species-specific temperature optima.

Key questions: Why do even subtle environmental changes sometimes trigger dramatic fish regime shifts? Why do anchovy flourish and sardine collapse or vice versa under the same ocean regime? Why are anchovy and sardine alternations synchronous or asynchronous among different ecosystems?
Key contents: "Optimal growth temperature" hypothesis (Takasuka & Aoki 2006 in FO, Takasuka et al. 2007 in CJFAS) Contrasting spawning temperature optima (Takasuka et al. 2008 in PO) Multi-species regime shifts (Takasuka et al. 2008 in MEPS) Spawning overlap (Takasuka et al. 2008 in MEPS) Characterizing spawning habitats (Oozeki et al. 2007 in CalCOFI) Mechanism review (Drinkwater et al. 2009 in JMS) Expansion/contraction (Barange et al. submitted)

Growth-based survival mechanisms during early life stages

Fish experience severe challenges to survival during the early life stages. Growth rates dominate survival potential suring those stages, playing a key role in survival dynamics. The "growth-survival" paradigm posits that faster growing individuals will have a higher probability of survival than slower growing conspecifics. This theory had been explained indirectly by two concepts: size and time (the "bigger is better" and "stage duration" hypotheses, respectively). Predation is the major and direct source of mortality. Nonetheless, no direct evidence had yet been obtained to support the relationship between growth rates and predation mortality. In our field studies, larval anchovy and their potential predators were captured simultaneously by the same tows. Through otolith microstructure analysis, we directly examined growth rates of the ingested larvae from the stomach contents of predatory fish and compared them with those of the surviving larvae from the original populations to demonstrate direct impacts of growth rates on predation mortality.

Key questions: Is a slower growing fish larva actually removed by predation at a given moment in the sea? If so, is the mortality size-dependent? Is there any difference among predatory species?
Key contents: "Growth-selective predation" hypothesis (Takasuka et al. 2003 in MEPS) Three synergistic growth-related mechanisms (Takasuka et al. 2004 in MEPS) "Growth-selective predation" hypothesis revisited (Takasuka et al. 2004 in MEPS) Predator-specific "growth-selective predation" (Takasuka et al. 2007 in MEPS) Growth effect on the otolith and somatic sizes (Takasuka et al. 2008 in FS) Growth in the Kii Channel (Yasue & Takasuka 2009 in JFB) Predation dynamics of mackerel (Robert et al. submitted) Dynamics of growth-based survival mechanisms (Takasuka et al. in prep.)

Geographical variations in biological parameters

Unexpoited offshore populations of small pelagic fish may migrate and spawn beyond our view in the various regions of the world. If any differences exist in the ecological features between the inshore and offshore populations, these should be clarified for a better understanding of the population dynamics of the species. Japanese anchovy are distributed mainly in inshore waters during their low-biomass phases; however, their distribution and spawning areas expand to offshore waters during their high-biomass phases. We compared biological factors such as reproductive parameters and stable isotope traits of Japanese anchovy between inshore and offshore waters.

Key questions: Is there any difference in biological factors between inshore and offshore waters? What factor is resposible for such differences?
Key contents: Temperature impacts on reproductive parameters (Takasuka et al. 2005 in FR) Geographical variations in trophic ecology (Tanaka et al. 2008 in MB)

Patchiness structure and mortality

Key contents: Patchiness structure and mortality of Pacific saury (Oozeki et al. 2009 in FO) This field work presents patchness structure and mortality rate of Pacific saury larvae based on repeated samplings. In the Discussion section, you can also find generalization of differences in mortality estimates between methods.