Étude des stratégies d'occupation du temps et de l'espace chez le barbeau fluviatile, Barbus barbus (L.)

This books presents a study on time and space utilization modes and strategies by the common barbel, an aggregative, rheophilous and lithophilous cyprinid, highly representative of the so-called "barbel zone", according to Huet's classification of Western European rivers (1949), although recently described as endangered in most European large river ecosystems. Ecological representativity and population decline both emphasize the need to bring about more information on a species often left apart due to little economic perspectives, and specially on its behavioural ecology, which is often considered as the first key to the understanding of life-history strategies and of their adaptation in modified environments. In order to describe and evaluate strategies in a "physiological" context, the study is conducted in a preserved stretch of the River Ourthe - the main Belgian tributary of the River Meuse - which is typical of the barbel zone and where barbel populations are still abundant and natural (chapter 1). In order to cope with the objectives of the study and the problems of studying fish behavioural ecology in natural surroundings, specific methodologies were developed or adapted, principally as regards underwater telemetry and electrosampling in microhabitats. Methodological concepts, developments and feasibility studies are presented in chapters 2 and 6. The third chapter analyses the use of space by Barbus barbus throughout the annual cycle, on a "macroscopic" level (home range and migrations). The annual mobility cycle is characterized by an hypermobile phase during the spawning period, which is determined by water temperature increase. Migratory tendencies are principally developed by female individuals and followed by precise homing to the resting sites occupied before the spawning journeys. Other seasons are described by a fixation to a well defined home range which is proportional to fish size. Seasonal home ranges were found to be minimal in summer and winter and to correspond to the expression of behavioural strategies developed by barbel in order to cope with temperature variations beyond their comfort (or tolerance) thermal range. These aspects arc enlightened by the study of activity rhythm variations throughout the annual cycle (chapter 4) and of micro-occupation of activity centers (chapter 5). In summer, when mean daily water temperature is ~ JO °C, barbel display a typical dusk and dawn activity rhythm pattern, characterized by the occupation of riffles, these activities being principally triggered by a synergistic action of light intensity (photophobic tendencies) and water temperature. The thermal transitions (spring and autumn) are characterized by consequent rhythm pattern modifications, with the emergence of a diurnal activity period which persists down to 4-5 °C, while auroral and crepuscular activities progressively fade then cease as water temperature decreases. These findings enlighten winter spatial stability and home range behaviour as a direct consequence of physiological limitations by water temperature. The flexibility of activity rhythm patterns and budgets throughout the annual cycle is analyzed as the expression of behavioural thermoregulation through the use of time, barbel developing their activities at times of the daily cycle when water temperature is the closest to their comfort thermal range, this interpretation explaining the phase inversions and the lifting of photophobic tendency observed during thermal transitions. In summer, the triggering role of water temperature is partially masked by the superposition of light and temperature cycles and only apparent at temperatures beyond the 20 °C limit, when activities arc more nocturnal. At these temperatures, barbel were also found to complete this behavioural thermoregulation by the exploitation, in heterothermal environments like confluents, of activity centers where water temperature is the closest to their thermal preferences. Consistent fidelity to summer resting sites is closely related to the permanence throughout this period of the daily activity area, which is exploited in a mixed sequential and radiative pattern, both on macro- and micro-exploitation levels. The activity budget dedicated to journeys between activity centers and diurnal resting sites was found to be substantially reduced in well known environments and by the occupation of nocturnal resting sites in large daily activity areas. As a conclusion, summer home range behaviour is regarded from a strategic point of view as a behavioural compensation of physiological constraints imposed by environmental (principally thermal) conditions, through a more efficient utilization of time and space in a well known but spatially limited environment. Chapters 7 and 8 focus on Barbus barbus preferences for microhabitat variables and analyze their variations depending on activity, season and fish size. The objective is to determine how these preferences actually affect, not only the choice of resting and feeding sites within an activity area, but also the location of this activity area and of its related home range, in order to account for the macrorepartition of barbel populations within the river. Spawning microhabitat is common to ail size classes and corresponds to shallow riffle areas (P₅₀ Depth = 15-24 cm ; P₅₀ CSS (current speed 10 cm above substratum) = 28-43 cm/s). Feeding habitats also belong to lotic mesohabitat types (rapids, riffles) although deeper than those used for spawning ( P₅₀ Depth = 22-40 cm ; P₅₀ CSS = 26-46 cm/s) and are shared by ail size classes above 20 cm FL. On the contrary, diurnal resting sites characteristics vary significantly both along season and size axes, barbel selecting deeper and slower habitat types as size increases and temperature decreases. These variations in habitat preferences result in an apparent spatial partition of distant size classes during daytime and explain the correlation between activity area dimensions and fish size. They also affect the aggregative tendencies displayed by the species and lead to the formation of rather homogeneously-sized shoals, these aggregative tendencies being reinforced by the search for suitable diurnal resting places as close as possible to the activity centers - minimization of activity budget dedicated to journeys and of predation risks - leading to the concept of strategic value for resting sites. When considering sites with equivalent habitat and strategic values, the ultimate choice of the diurnal resting place within the activity area is principally determined by the presence of conspecifics while other components of the ichthyofauna (salmonids, cyprinids) have little or no incidence on its position. On a macroscopic level, comparisons between the distribution of barbel populations and habitat availability on an objective scale (the size of fish activity area determined by radiotelemetry) reveal that the choice of activity area - and home range - location within the river is significantly correlated with the relative availability (%) of highly valuable feeding habitats and thus with subsequent habitat diversity within a river stretch. The last part of the book (chapter 9) investigates the limits of Barbus barbus reproductive "hyperspace" along spatial, temporal, physical, physiographical and behavioural axes. In May-June, large amounts of spawners gather on defined and spatially restricted areas located in shallow riffle habitats, where they form spawning aggregates. Due to these high density conditions and to the sex ratio of the spawning population (up to 600 males/ 9 mature females on the same day), males switch from courting to non-courting strategy in order to meet both specific and individual interests in reproductive success. The probability that a spawning sequence is successfully completed by a female is principally determined by its size and age, leading to take into account the influence of experience on spawning success and thus on the individual contribution to subsequent recruitment. The study of environmental conditions throughout successive spawning periods revealed that barbel spawning is principally triggered by water temperature, which acts in a threshold mode allowing or inhibiting the development of spawning activities, at least under increasing or stable photoperiod conditions. The threshold value was found to be compulsory, necessary and strictly related to daily minimum water temperature (13,5 °C in the River Ourthe). The correspondence between this threshold value and the inferior lethal limit for barbel embryonic development explains the necessary connotation of the threshold while its compulsory aspect is discussed in a general context of population dynamics, including competition and probability of survival to wintering conditions. As regards daily periodicity, spawning activities are exclusively diurnal in natural conditions and principally take place during the morning hours. This rupture on the spawning days of the typical crepuscular activity pattern observed at temperatures above 10 °C represents a potential threat for spawners and is described as the expression of a species strategy aiming at yield maximization since eggs are laid at a time of the day allowing embryos to face the first critical period of their development in optimal thermal conditions. Both seasonal and daily periodicity of spawning are finally discussed in the context of life-history strategies and adaptation to changing environments, through a proposed mechanism involving thermal memory.