François Gerlotto

SCALE-INVARIANT MOVEMENTS OF FISHERMEN: THE SAME FORAGING STRATEGY AS NATURAL PREDATORS

We analyzed the movement of fishing vessels during fishing trips in order to understand how fishermen behave in space while searching for fish. For that purpose we used hourly geo-referenced positions of vessels, provided by a satellite vessel monitoring system, for the entire industrial fleet (809 vessels) of the worlds largest single species fishery (Peruvian anchovy, Engraulis ringens) from December 1999 to March 2003. Observed trajectories of fishing vessels are well modeled by Lévy random walks, suggesting that fishermen use a stochastic search strategy which conforms to the same search statistics as non-human predators. We show that human skills (technology, communication, or others) do not result in the fishermens spatial behavior being fundamentally different from that of animal predators. With respect to probability of prey encounter, our results suggest that fishermen, on average, evolved an optimal movement pattern (mu = 2.00) among the family of Lévy random walks. This Lagrangian approach opens several perspectives in terms of operational management of the pelagic fish stock.

SCHOOL TYPOLOGY AND 3D DYNAMICS: STUDY OF THE BEHAVIOURAL MECHANISMS ALLOWING ADAPTATIONS OF COLLECTIVE STRUCTURE TO ENVIRONMENTAL CHANGES

Being the collective interface with the environment and the exploitation, schools and their typology represent an important life history trait of gregarious fish. Changes in the characteristics of the environment sensu largo induce changes in the school typology, which can be used as indicator of stock adaptive patterns. In order to understand the interactions between school structure and environment patterns, a first step consists in studying the school internal structure and the functioning of fish aggregation inside it and in its surroundings. The paper describes the spatial morphology and dynamics changes in Peruvian Anchovy (Engraulis ringens) schools in Peru using multibeam sonar observation on a station. Different cases of school types are extracted from a data base of around 200 recordings: from small circular well individualized schools to large aggregation surrounded by scattered targets. The internal structure of the aggregations is described at a 10 cm definition scale. The shape and dynamics of internal structures (nuclei) are described. The school edges are measured in shape and dynamics, and the relationship between the school and the fish surrounding the school are measured, using geostatistics and fractal dimensions approaches