Monitoring the upper tropic

The past decade has generated significant progress in understanding ocean processes and the coupling of ocean and atmosphere in regulating Earth’s climate. These accomplishments were made possible by the concurrent development of new technology and instrumentation, as well as substantial progress in numerical modelling (ocean general circulation models and new conceptual models of lower trophic level food webs). Despite the increase complexity when considering all the pelagic ecosystem, a similar approach, closely associating observation and modelling, seems the most appropriate to investigate the dynamics of upper trophic levels (from macroplankton to higher predators).

It is proposed in the present project to use existing technologies, and also to develop new instrumentation for monitoring the upper trophic levels of the pelagic ecosystem. Observation will combine both extensive studies at ocean basin-scale and intensive studies in some sub-areas and key sites. Extensive studies aim at building ocean data sets for micronekton biomass and large pelagics biomass or individual records, using acoustic (micronekton biomass), sonar (tuna biomass), and electronic tracking (individuals) devices. Intensive studies will focus on important processes and behavior (e.g., prey-predator interaction, habitat, schooling and aggregation of tunas, reproduction, composition and dynamics of micronekton, etc).

Food web structure in pelagic ecosystems

Production at higher trophic levels (usually exploited species) depends on the production at lower levels (bottom-up control) and may be modulated by the physical forcing and the structure of the marine food webs. Ecological concepts suggest for instance that the structure of the food web can be controlled by the biodiversity within the system and/or by higher predators (top-down control). However, concerning pelagic ecosystems, there is very little observation to illustrate such controls. In association with the data collected by the monitoring component of this project, it is essential for modelling the pelagic ecosystem to identify the functional groups, how energy and matter flow through these groups and how they are affected by physical and biological changes as well as by human activities (fisheries).

Two kinds of analyses will be helpful in this task. A classical approach based on the study of stomach contents to establish the prey-predator interactions, and the more recent isotope-ratio approach, that appears a promising way for describing the energy transfer through the food web. The success of these approaches also relies on the multiplicity of studies in different regions of the ocean(s) and in different periods of time. The comparative study necessitates developing standardized protocols, reference databases and controlled laboratory experiments. Retrospective analyses based on the numerous diet studies published or still in archives of many institutes should be also encouraged. Information obtained from these studies and from the monitoring will be used in individual energetics models (IBM), mass-balance models (ECOPATH-ECOSIM) and spatial ecosystem models (SEPODYM).

Close association between observation and modeling has been a permanent guide in conceptualization of this project. Recognizing the diversity of space-time scales processes overlapping in pelagic ecosystem dynamics, a second key idea is that a general framework is needed to integrate studies at different time and space scales with potential connections between them. There is a large range of models represented in the project covering global to individual scales. At global or basin scales, predictions from three different coupled physical-biogeochemical models will be used over the period 1950-present. The global model will also provide predictions for the next century using a scenario of greenhouse warming. These predictions will be used to run the ecosystem models of upper trophic levels on which the economical and social analyses rely. At least one of the physical-biogeochemical models should provide prediction at high resolution in one or a few identified sub-regions where intensive process studies are conducted. A similar approach will be investigated for the spatial ecosystem models. This would allow connections between large and small scales (low and high frequencies) processes and testing the mechanisms that control the system when moving from one scale (frequency) to the other.

Socio-economical impacts

The interannual climate variability due to ENSO events has important socio-economic impacts on tuna fishery and industry at the global scale, that in turn may affect the tuna populations (e.g., higher/lower catch) and the pelagic ecosystem (by-catch, interaction between species, top-down effects). Several causes drive the fluctuations of tuna stocks and catches. While economic rather than biological reasons limit (today) the catch increase of the most productive tuna species (skipjack) in the Pacific, the intense fishing effort on the highly valuable bluefin tuna, perhaps combined with environmental forcing, has led to a decline in this population from the 1960’s to the eighties.

Interactions amongst species and between the multiple and diverse fisheries, as well as potential cascade effects in the ecosystem raise important questions for management with potential strong socio-economic repercussions. Based on existing model, investigations of these interactions and effects occurring with ENSO would help to assess the vulnerability and impacts in a scenario of global warming, and to eventually propose adaptations and/or mitigation measures for the future.

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