Impact des facteurs anthropiques et naturels sur le fonctionnement de quelques populations halieutiques

Researcher since 1974 at CNEXO (National Centre for Exploration of the Oceans), then at IFREMER (French Institute for Exploitation of the Sea), I have at the same time brought to the work of researching the dynamics of exploited ichthyologic populations actions for enhanced improvement and transfer of knowledge to help managers and stakeholders administer these resources and restore them if needed. This activity led me to play a double role: that of the expert who is asked to set up "a truth judgment" on a given subject or according to a specific question, and that of the scientist faced with the judgment of his peers who strives not to go against his knowledge (i.e., that which is subject to the criticism of the scientific community). This change of status is not obvious to the expert—i.e., "well-versed in the knowledge of things thanks to having practiced them"—and can only be understood if the job of expert is not permanent. In that case, the expert rejoins his discipline's field in order to establish his credibility as an expert. It is partly this coming and going that characterises the manner of my activity. In essence, my course of action may be compared to what is currently called "ecosystemic approach for fitting-out of fisheries". The latter led me to lead an IFREMER program called: "Ecosystemic Course of Action for the Integrated Management of Halieutic Resources" within IFREMER's theme number 4: "Halieutic Resources, Sustainable Exploitation and Enhanced Improvement". The theme of my scientific work is identified with the Biology (Ecology) of Populations, which covers the whole of investigations concerned with the structure and processes that govern the adaptation of a population to a given environment3 subject to natural variations (changes of climate, for example) or anthropic variations (linked to Man's actions on the biocoenoses and biotypes). Population Dynamics is my benchmark discipline and implementing quantitative Ecology has allowed me to study a few factors that determine the unstable equilibrium of 3 stocks or ichthyologic populations characterised by fast or slow life cycles and by mechanisms that regulate their very different population kinetics: The common anchovy (Engraulis encrasicolus), the Atlantic salmon (Salmo salar) and the European eel (Anguilla anguilla). Using the model as "conceptual probes that we can plunge into reality" is widely used to better synthesize the effect of anthropic and natural factors over the biogenetic capacity of colonised habitats or over the demographic evolution of the populations themselves. Starting from 2 simple equations established on the one hand by Lotka-Volterra and on the other by Verhulst-Pearl, the parameters r and K are introduced. The first one defines the intrinsic rate of growth of the population, the second one, the biogenetic capacity of the habitat occupied by that population. This allows us to introduce in fine the concept of type r and type K strategists. In the first case, the population manages its growth rate, in the second case, the population regulates its size according to the trophic capacities of the environment it is colonising. This distinction certainly has the merit of simplicity4, but does not account for the complexity of the demographic strategies that a given population brings into play during its life cycle5. It is for this reason that from this simple classification in which the species or biological stages of a specific species are placed, the modelling of the life cycle or of the cycle of some of the biological stages of the chosen target species is developed. To illustrate some of the regulating mechanisms involved and their modulations under the effect of various natural and anthropic factors, a first comparison is made between the Atlantic salmon, defined as a type K strategist and the anchovy, considered as a type r strategist. For the first one, its demographic strategy allows it to adjust the size of its population of juveniles to the size of the available production surface and not to the number of genitors. Hence the link that exists between the size of the stock of genitors and the size of juvenile production zones. However, the productivity of the population (expressed by the number of recruits per genitor) has nothing to do with the size of the production zones, but is strongly dependent on the quality of these zones. The development of a stochastic model makes it possible to predict, from various scenarios of fitting out the environment and regulating exploitation, the future of a salmon population. The results obtained show the limited effects of merely monitoring fishing when the continental production habitats of this species have been damaged. For the second type, with a high fertility rate, the strategy is to scatter the eggs and larvae within a large area whose environmental characteristics are unpredictable. Whereas with the type K strategist, like the Atlantic salmon, it is the genitors who carefully prospect the initial production environment, with the type r strategists, like the anchovy, it is the larvae that are the first level of environmental sampling.For this strategist, the sampling seems random or at least synchronised by unpredictable environmental factors, while for the first one, it is determined and conditioned by the phenomenon of "homing". The impact on the stochasticity and prediction of the size of these short-cycle populations are analysed. From the analysis, a profound difference emerges from the variability of the production environment's "fertility" for the two species. The Atlantic salmon favoured a reproduction strategy characterised by a burying of the eggs in specific places with physical characteristics that the female looks for in order to deposit her clutch of eggs. These habitats stay the same in time and space and allow the larvae to hatch in a protected environment and to benefit from an environment where food is brought by the drifting of organisms from upstream to downstream. Regulating the population's size is then done by means of intra- and interspecific competition for the acquisition of territory. The anchovy has favoured a reproduction strategy of dispersing its eggs and larvae. The latter sample an environment as vast as its reproductive biomass is significant. Within this reproduction zone, the favourable habitats do not stay the same in time and space and are dependent for their size and number on essentially climatic conditions that occur during the summer and autumn periods. The question of predicting the population's size: Myth or reality? is in fact posited for short-cycle species. It remains possible for type K strategists or at least for regulated life stages following a type K mode ("managing the asymptote or the optimum"). It is much more random, indeed difficult to imagine for type r strategists for whom it is difficult to find appropriate descriptors to characterise recruitment. The study of the third species, Anguilla anguilla, makes it possible to show the need for implementing an ecosystemic approach in order to restore a population currently in steep regression in the majority of the continental habitats that it frequents. A panmitic species that reproduces in the Sargasso Sea, its larvae, known as "leptocephalus", are disseminated by means of ocean circulation towards the catchment areas located between Mauritania and the Polar Circle. The population's tree structure6 makes it possible to understand that restoring this population can only be done by taking into account the geographic units constituted by the catchment areas at least at a regional level: Mediterranean Sea, Gulf of Gascony, Baltic Sea... Analysis of the natural and anthropic factors acting upon the productivity of the stock shows that restoring this resource and its habitats requires a partnership framework to define the context composed of different realities: Biological, economic, social and cultural and perceived from different angles, which means that one person's reality is not necessarily others' reality. It is the recognition of this context that has led to proposing a project of knowledge transfer and enhancement as well as of know-how about this species in the context of the INTERREGIIIB-Atlantic Area programme. This project, whose acronym is INDICANG, has as its objective the implementation of a network of partnerships in order to adapt and make available to managers a set of colonisation and size indicators of the European eel in the central part of its area of distribution. The concluding section of this report raises again the problem of the heteronomy of the field of discipline that makes up "environmental research". It mentions the difficulty of reconciling heteronomy and autonomy in the context of interdisciplinary research, whose objective is to contribute to sustainable development, i.e., the implementation of a "more ecological dialogue for more humane conservation in order to avoid allowing a consensus to be established at Nature's expense or allowing certain practices of strict conservation to cause social conflicts".