Dissertation - HQ

30 Behaviour in models
1.3 Vertical position
1.3.1 Potential influences
Vertical heterogeneity
in physical variables
The unknown effect
of boundary layers
Any vertical heterogeneity in the current field will interact with the
vertical distribution of larvae to indirectly influence their dispersal, as
demonstrated by modelling 69,70 and empirical 71 studies. And of course,
many things in addition to current velocity vary vertically in the ocean
(temperature, light, food concentrations, etc.). Temperature influences
pelagic phase duration 72 , development rates 73 , and swimming speed 25 .
Food resources are often greater near the thermocline and fish larvae
may accumulate at these depths 74–76 . Conversely, they might use diel
vertical migration to avoid predation near the surface 77 . Larvae may use
sun angle or sound for orientation, so the vertical position of a larva
relative to the surface (sun angle detection) or the thermocline (hearing)
may influence its ability to detect these cues and orient. Overall, the
vertical position of larvae can therefore influence their feeding success,
predation risk, growth, swimming ability, and ability to detect sensory
cues, all of which can influence their trajectories 78 . Of all behaviours,
vertical positionning is the most widely recognised as being influential
and the one most often incorporated into biophysical models.
Furthermore, in coastal waters, larvae may occupy the epibenthic
boundary layer, where current velocity can differ substantially from
that in the water column. Unfortunately, information on the occurrence
of fish larvae in such epibenthic locations is limited because it is very
hard to sample, especially in deep water or where the bottom is very
irregular or hard. Occupancy of the boundary layer not only places the
larvae in a different current regime, but it may also shift their food
regime and expose them to increased risk of predation from benthic
predators. Given the important effect boundary layers potentially have,
further investigation is suited.
1.3.2 When to include this behaviour?
Vertical behaviour
should always
be included
Current velocity, hydrography (e.g. salinity, temperature), and fluorometry
profiles (or their modelled equivalents) over the spatial scale and
depth range where larval fishes occur are required in order to evaluate
the degree of vertical shear in the current, the temperature gradient, and
the depth of chlorophyll maximum. Clearly, if substantial heterogeneity
in the velocity field is detected, vertical distribution of larvae must be
included in a model. Some models integrate water movement over the
surface Ekman Layer, while, in this layer, water velocity often differs
with depth. This means that larvae at different depths within the Ekman
Layer will be subject to different current speeds and directions, and
the model should reflect this and avoid averaging over depth. If some
modelled features (such as survival or growth) explicitly depend on
food availability or temperature and these are not homogeneous on the