Dissertation - HQ

158 Oceanography vs. behaviour
6.4.6 Discussion
The difference between
active vs. passive larvae
may be milder but
is probably real
Low swimming
speeds matter
Early swimming may
be more constrained . . .
One important result of this study is the tremendous difference between
active and passive trajectories, even with very low swimming speeds.
The combined effect of downward movements to avoid surface advection
and horizontal movements to move between water masses enhances
self-recruitment in this model. The second important finding is that
early swimming decisions, even though they would appear expensive
energetically, seem to be key in determining the fate of larvae. As a
consequence, conditions which enhance swimming abilities, in particular
early on, (such as an increase in temperature) result in increased selfrecruitment
rate for larvae displaying optimal strategies. Surprisingly,
this is not accompanied by greater retention of larvae around recruitment
areas, but by an increase in the distance roamed by particles from their
release point.
A nuance to the difference between passive and active trajectories is
that the rough advection scheme we use here is probably not capturing
the full potential for passive retention. With a finer time step, particles
would follow streamlines more closely and may be retained more inside
eddies for example. Similarly, the inclusion of diffusion would increase
the probability to encounter a retentive area from any release point.
Yet, both those effects would also be relevant for active trajectories. In
addition, it is not likely that an increase in self-recruitment rate from
0% to 95% could be achieved purely passively, only because of the
inclusion of those two refinements. Therefore, the difference observed
here strongly advocates for the inclusion of swimming in all early life
history models of fish, or of other organisms whose swimming speeds
may be as low as a few centimetres per second.
Indeed, the environments considered here are not those where swimming
would be expected to make a large difference: stratification is not
particularly strong (no returning flow at depth, only a slow down) and
current speeds are rather high (up to 60 cm s -1 at the surface). Nevertheless,
mean swimming speeds of 2 cm s -1 are sufficient to completely
shift the system from nearly no self-recruitment to return rates of 95%
in the case of the Pomacentridae. Furthermore, the temperate larvae
were very weak swimmers (maximum swimming speed of 5 cm s -1 at
the end of the larval phase) and still achieved self-recruitment rates
over 40%. Both results suggest that weakly swimming organisms could
have an impact on their dispersal, provided that their swimming is
oriented and exploits the heterogeneities in the current field.
Early swimming may, however, be favoured intrinsically in this model.
First, because no energy budget is explicitly represented, no recovery
after swimming is allowed. As a consequence, strategies that result in
as little swimming as possible during the whole larval life are favoured.
This translated in much swimming early on at, or close to, the maximum
speed, and less later in larval life, often at only a fraction of maximum
speed. With recovery, it might have been better to space swimming