Aquaculture and feeding ecology: Feeding behaviour in turbot larvae

The period of first feeding, characterized by the shift from internal (yolk-sac) to external food sources, is considered particularly critical for the survival of marine fish, but the underlying causes are still unknown. The larval stage, characterized by high mortality rates, is particularly challenging for larval rearing.

After the start of exogenous feeding, another intense and likely critical period of change occurs in the early life stages of fish. This stage is the metamorphosis, during which the larvae transform organs and body morphology to become juveniles. Compared to other teleosts, larvae of flatfishes undergo a particularly evident and dramatic metamorphosis, because flatfishes completely reprogram their body to move from the pelagic habitat, in the water column, to the benthic habitat, on the sea floor.

Due to the complex morphological and physiological changes, in aquaculture metamorphosis in flatfish species is often unsuccessful, resulting in different types of abnormal development. The objective of this thesis was to analyse the feeding behaviour of the flatfish species turbot (Psetta maxima L.) larvae during the two crucial life periods, 1) first feeding and 2) metamorphosis.

To analyse whether these two periods are critical for the correct development and survival of turbot, feeding behaviours of larvae during the period of first feeding and during the first stages of metamorphosis was studied using video recordings. This provided qualitative and quantitative descriptions of behavioural parameters, including modal action patterns (e.g.

Pause, S-shape, and Attack), attack rate, capture success rate, and swimming speeds of predator and prey. The feeding behaviour of turbot larvae could also be influenced by prey swimming behaviours. To test the effect of two common types of copepod swimming behaviours, turbot larvae were exposed to two species of copepod nauplii.

The results presented in this thesis indicate that neither the period of first feeding nor the period of transition from an upright to a tilted swimming position seem to be critical for turbot larvae. First feeding and metamorphosis rather represent periods of adjustments to new morphological and physiological conditions, such as the depletion of yolk-sac during first feeding, and the migration of one eye when metamorphosis starts.

Moreover, the behaviour of prey appears to have limited or no influence on larval turbot capture success. This thesis is part of a large international project aimed at improving the rearing techniques of high value fish species larvae fed with calanoid copepods, their natural prey, to achieve high levels of survival and quality.

In fact, fish aquaculture is becoming increasingly important as source of food. However, several bottlenecks during the larval stage still affect its production efficiency. The larvae of turbot were chosen as a model species because turbot is a highly prized flatfish species, which is particularly difficult to rear.