Dissertation - HQ

Predator avoidance 41
intervals that are more closely aligned with the perceptual abilities of
the larvae; (2) empirical characterisation of the perceptual fields of fish
larvae on different prey, under different conditions (light, turbulence),
and at different sizes.
1.7 Predator avoidance
1.7.1 Potential influences
Traditional aquatic food webs place plankton at the base of the food
chain, often with fishes as top predators. However, during ontogeny,
fishes go through a phase as an important (albeit transient) member of
the plankton. At this small size, fish larvae are subject to predation by
other plankters: carnivorous copepods such as Paraeuchaeta norvegica,
chaetognaths, gelatinous zooplankton, other ichthyoplankon, etc.
As early larvae, fishes have only a limited capability to perceive and
escape from predators. In contrast, many adult invertebrates, and/or
older larval or juvenile fishes, are formidable predators against which
early-stage fish larvae would have virtually no chance. There are almost
no empirical observations of such interactions (although see Yen &
Okubo 123 , Browman et al. 124 ). For more swimming capable larvae, the
probability of escape is highly dependent on the type of predator. For
example, if an aggregation of gelatinous zooplankton sweeps through a
population of larvae, high mortality could result.
As with foraging, predator avoidance occurs on a relatively small
spatial scale and is therefore not likely to influence dispersal trajectories,
except through indirect influences via vertical or horizontal repositioning
in different water masses.
Early fish larvae are
predated upon . . .
. . . and have little
chance to avoid it
1.7.2 When to implement this behaviour?
There is currently no data available to parameterise larvae-predators
interactions. In addition, for a model that is designed to predict larval
trajectories (and not mortality or recruitment abundance), predator
avoidance would only need to be incorporated if there were evidence
that these movements result in displacement into different water masses.
Knowledge is lacking
1.8 Schooling
1.8.1 Potential influences
Field observations, net sampling, and acoustic trace indicate that some
fish larvae undergo a near-bottom schooling phase prior to recruitment
125,126 . The size of these larvae may be intermediate between the
sizes of larvae collected in plankton tows and of metamorphosed juveniles
collected from the benthos 125 . Rearing experiments also demonstrated
that schooling is developed early during ontogeny among pelagic
Fish larvae school
prior to settlement