Dissertation - HQ

110 Vertical distribution during ontogeny
. . . but induces some
rare retention events
were chosen because an ontogenetic shift was significant and larvae
were abundant enough to estimate the probability density function of
z cms in the three ontogenetic stages. After a larva was released, the
straight-line distance between its starting point and current position
was computed at each time step. These distances were compared to
determine whether shifts in vertical distribution facilitate or impede
drift. After six days, passive particles were on average 80 km away from
their point of origin (past this point, some particles reached the domain
boundaries and biased the estimation of drift distance). In all cases,
vertical migration reduced the mean drift distance, albeit by very little:
2.3 km for Acanthuridae, 3.3 km for Serranidae, 4.7 km for Labridae, and
3.6 km for a vertical movement corresponding to that of the global coral
reef fish community. These differences were even smaller when particles
were allowed to move randomly within the depth range defined by the
PDF, i.e. not enforcing smooth vertical migration.
Passive particles were all advected away more or less at the same
pace. They were only slowed down by their entrapment in transient
eddies. By contrast, some migrating particles were brought at depth,
behind the island, in areas where retention was much higher. As shown
in Figure 5.7, some particles were actually retained within 10 km of the
island for the whole eight days. For those few, the drift trajectory was
very different from that of passive particles.
Figure 5.7 Trajectories of vertically migrating particles advected around an
isolated deep oceanic island for eight days. The island was 20 km in diameter.
Particles were constrained in the upper 100 m. Most particles were advected
away but a few were retained at depth, behind the island.