Determinism and plasticity of fish schooling behaviour as exemplified by the South Pacific jack mackerel Trachurus murphyi

Abstract

Pelagic fish schools are thought to show a general pattern of dispersion at night and aggregation within schools during the day. This pattern is often accepted as the major rule driving most of the other physiological, biological and ecological processes. Foraging on mobile prey, for instance, is assumed to be enhanced by schooling behaviour. Current theory assumes then that foraging is only possible for obligatory gregarious predatory fish from dawn to dusk. However, offshore mesopelagic communities perform vertical migrations and are out of reach for most oceanic pelagic predators during the day (with the exception of some apex predators, e.g. swordfish or bigeye tuna). To investigate how fish may overcome this apparent contradiction, we studied the 3-dimensional spatial strategy of the South Pacific jack mackerel Trachurus murphyi according to the abiotic and biotic conditions of the habitat. Data came from acoustic surveys performed in central Chile in 1997Chile in , 1998Chile in and 1999. Our results show that the jack mackerel distribution was driven by prey during the night when foraging, and related to the hydrology when resting during the day in the upper part of the oxycline. Fish were more aggregated at night than during the day, probably because jack mackerel cycles of schooling behaviour depend primarily on prey availability. This 'atypical' behaviour could be an adaptation of gregarious pelagic fish to an oceanic ecosystem. Fish schooling behaviour is not necessarily driven directly by the diel cycle; rather, it can be functional and depends on prey availability.

Schooling behaviour of small pelagic fish: phenotypic expression of independent stimuli

“…The classical view assuming that fish are scattering during the night and in schools during the day (see 'Introduction') has yet to be validated by in situ observations, taking into account that the irradiance level depends on school depth, water turbidity, lunar cycle, season and atmospheric conditions such as cloud coverage. The number of schools detected in our 2 study areas revealed an unusual pattern: fish schools were more numerous during the night than by day, although their internal fish school structure was less organised, as Bertrand et al (2006) observed for Trachurus murphyi off Peru. Feeding (Bertrand et al 2006), reproduction (Nøttestad et al 1996), migration (Fernoe et al 1998), the physiological condition of the fish, their intrinsic sensory perception (Partridge & Pitcher 1980, Bleckmann 1993) and the presence of predators (Massé et al 1996) have all been proposed to influence schooling behaviour (Brehmer 2004).…”

“…Moreover, no noticeable differences were found between the Venezuelan and Senegalese fish school descriptors, except for A min , a descriptor not characteristic of schooling behaviour. The higher A min values in Sénégal could reflect increased prey depth (Bertrand et al 2006) or the effect of local bottom depth configuration in our survey transects. The length of fish schools and its variability was always larger than school height in both populations.…”

“…Nevertheless, in situ measurements of basic behaviour, such as schooling and diel aggregative patterns, are complex to obtain and to interpret. The most common diel schooling pattern of small pelagic species consists of fish scattering during the night and gathering in schools during the day (Shaw 1961, Blaxter & Holliday 1969, Milne et al 2005 although there are numerous exceptions (Bertrand et al 2004(Bertrand et al , 2006. Acoustic survey methods commonly employed in pelagic fisheries can be used to explore fish schooling behaviour, providing standardized in situ samplings of schools (Fréon & Misund 1999, Simmonds & MacLennan 2005.…”

Insight on population structure of the Chilean jack mackerel (Trachurus murphyi)

“…"Patches" of high density can be related to larger abundance of prey, or to non-trophic characteristics. Therefore, knowledge of the diet and the plasticity of T. murphyi behaviour is important (see Konchina 1981;Konchina and Pavlov 1996;Bertrand et al 2004Bertrand et al , 2006). …”

“…Two of them, conditions 3 and 16, are actually in some way contradictory with Levin's definition. Indeed, the different populations cannot be considered as discrete and it is impossible to draw clear borders between them with the existing data (condition 16): T. murphyi has a high plasticity (Bertrand et al 2004(Bertrand et al , 2006, and can be encountered in the whole "Pacific belt". Condition 3 should also be discussed because we observed contradictory results: on the one hand, local differences occurred in the demographic structures but, on the other hand, there is not complete independence between the major dynamics of the different populations, as synchrony of variations in abundance, migrations, etc.…”