Dissertation - HQ

38 Behaviour in models
In situ studies
are required to test
whether larvae orient
Information about
cues and detection
thresholds is scarce
The first step is to detect whether fishes orient or not. Such information
can be provided by field studies involving the release of larvae
followed by divers 90,109–111 , or in situ orientation chambers 112 .
The second step involves testing experimentaly the ability of larvae
to detect environemental cues 100,102,103,113–116 . The potential for detecting
a cue can be demonstrated in the laboratory by testing the preference
of larvae toward a given environmental signal for example (e.g. coastal
vs. oceanic water, reef sounds vs. random sound). Which cue is actually
used in situ is currently unknown, due to the lack of experimental
means of testing cues separately in the pelagic environment.
The last step would be to describe thresholds for detection. This
directly relates to the spatial scale over which cues can be detected
and used for orientation. Knowledge in this regard is currently mostly
lacking and is difficult to obtain, yet this is essential information for
incorporation into models.
1.5.4 Suggested implementation
Orientation as a
parameter or an
emergent property
The implementation of orientation is closely associated with that of
swimming (both horizontal and vertical): orientation is simply a choice
among the set of possible swimming vectors. Once again, two approaches
can be taken: (1) behavioural rules in response to the environment
can be defined a priori, based on observations and experimental
work; (2) these behavioural rules can emerge from the model by defining
the set of possible swimming vectors, a biologically sensible “goal” for
larvae (e.g. settlement), and letting an algorithm choose the suite of
decisions to achieve this goal (see Irisson et al. 117 for an example of the
use of stochastic dynamic programming to solve such an optimisation
problem).
In both cases, orientation is a function which associates a swimming
decision to the current state of a particle, such as
f : state × time × environment → swimming(speed,direction)
The amount of detail in the orientation behaviour is determined by
what is incorporated in each of the left hand side variables. In the most
simple model, when orientation is observed but the clues are unknown,
orientation depends only on the position of larvae and on time. When
responses to sensory clues are involved, the environment can include
temperature, food, predators, current fields, land associated chemical
concentrations, sun orientation, etc. If some kind of energy budget is
present, the state of larvae can also encompass energetic reserves. This
formalisation is therefore very scalable.