Dissertation - HQ

The balance between feeding and predation 131
of Acanthuridae 236 . Therefore, the larval phase is divided into three time
periods, to account for the changes in swimming abilities. The limits
of those three periods are defined approximately at ontogenetic shifts:
end of yolk sac period and end of flexion. Swimming speed values
are estimated from published records 60,236,258 . Most studies measured
critical swimming speed (maximal speed of a current against which a
larva can maintain its position). These speeds are probably greater than
actual swimming speeds in the field 258 . Therefore, we choose lower
swimming speeds for both species in each time period, while retaining
the difference observed between them: 0, 13, and 36 cm s -1 for the
Acanthuridae and 3, 10, and 20 cm s -1 for the Pomacentridae.
During the first period, larvae extract energy from their yolk sac
reserves, hence do not need to forage. In the model, their energy resources
are constant and maximal. Afterward, they lose one energy
unit per time step. As they have a maximum resource of five units they
can only swim four time steps (24 h) before food is required. Coral
reef fish larvae can swim for longer periods of time before starving (up
to 194 h for Acanthuridae for example) 236 . Nevertheless, in the field,
larvae are likely to avoid starvation and keep their energy resources
level as high as they can. Furthermore, most studies about the swimming
endurance of reef fish larvae focus on the time that a larva can
swim against a current before starving or being completely exhausted,
without any consideration about maintaining growth rate or integrity
of metabolic pathways. However, the daily food intake of fish larvae
needed to maintain their growth rate is high (50% of body weight per
day is a general mean 232 ), especially for fast-growing, warm-water fish
larvae 232 . Therefore, fish larvae should eat often, probably on a daily
basis, during dispersal.
. . . and their swimming
abilities
Energy intake is
limiting in fish larvae
Finally, the parameters for this more elaborate model can be summarised
along the guidelines set by the modelling framework.
Time 6 hours time step; horizon of 50 days for Acanthuridae and
20 days for Pomacentridae
State Energy reserves (five levels, one consumed per 6 h of swimming)
and three-dimensional position in a 100 km × 50 km × 100 m
domain of 720 m horizontal mesh size and 50 m vertical mesh
size.
Environment Spatially explicit survival and feeding probabilities; concentration
around the island is described by factor f; f = 1 means
homogenous repartition; as f increases, the island effect is stronger
and food as well as predators are more concentrated near shore;
f = 1, 1.1, 1.2 and 1.4 are tested. Static current field with incoming
flow speed of 10 cm s -1 .