Dissertation - HQ

34 Behaviour in models
of measured distributions of larvae that was impossible to
achieve with passively drifting model larvae (Wolanski et al.
1997).
1.4.3 Simple modelling tests
Testing for the importance of un-oriented swimming can be achieved
by increasing the variance of the random walk/flight in the Lagrangian
tracking scheme. This is especially efficient for swimming speeds that
are small compared to ambient current velocities (e.g. one or more
orders of magnitude smaller).
If orientation behaviour is not known, testing extreme oriented
swimming scenarios can give insights on the extent to which swimming
can influence trajectories. Such scenarios can include adding a movement
at full speed, perpendicular or parallel to current direction at each time
step; testing full speed movement relative to whatever cues may exist (e.g.
toward the recruitment location); etc. If these tests lead to the conclusion
that oriented swimming could make a large difference (which it will
in most cases), then information on orientation is needed. Please see
section on orientation below.
1.4.4 How to get the relevant data?
Many measures of
swimming speed . . .
. . . not all are
appropriate
Information on horizontal swimming is becoming more widely available,
but most of it concerns tropical species. In addition, several methods
have been used to estimate swimming speeds of fish larvae (and recently
reviewed 25 ). These are (from high to low): burst speed, which measures
the speed at which larvae flee in response to a stimulus; critical speed
(U crit), which measures the speed of a flow against which larvae can
maintain their position in swimming channels 88 ; maximum sustainable
swimming speed, which measures the speed that can be maintained in a
swimming channel over 24 hours 89 ; in situ speed, in which scuba-divers
follow larvae in the sea and measure their speed 90 ; and routine speed,
which measures swimming speeds of undisturbed larvae in laboratory
containers 91 . These techniques do not actually measure the same thing
and the speed estimates they provide differ. Therefore they are not
equally suitable for use in dispersal models and care must be taken to
ensure that the type of swimming speed measure is adequate to the
purpose.
Routine speed has the advantage of being a measure of swimming
speed undisturbed by divers or any overt forcing by the investigator,
but it carries the disadvantage of being measured in artificial laboratory
conditions. In situ speed has the clear advantage of being measured
in the sea, but with the unknown influence of the observing divers.
U crit is most relevant for comparisons of relative performance, but it is
not a performance measure that can be directly included in dispersal
models as it is almost certainly faster than larvae actually swim in the