Dissertation - HQ

Oriented swimming and passive advection 143
of critical speed (U crit) 266 with age and size. They found size to be a
better predictor of speed than age. Yet, when age was expressed in terms
of developmental age (i.e. days since hatching/larval duration) and
when speed was normalised per species (speed equals zero at hatching
and one at settlement), a unique log 10 (age) · log 10 (speed) relationship
held for all ten perciform species studied by Fisher 57
log 10 (U − U hatch )
log 10 (U settlement − U hatch ) = log 10 (A)
log 10 (A settlement )
(6.8)
where A is age in days post-hatch (so A settlement is what is commonly
measured as pelagic larval duration from otoliths: number of daily
increments between hatching and settlement marks), and U is U crit in
cm s -1 (U hatch is speed at hatching, and U settlement is speed at settlement).
In a more useful form, this equation provides speed in function of age
post-hatch
log 10 (A)
log
U = U hatch + 10 10 (A settlement ) log 10 (U settlement −U hatch ) (6.9)
It requires U crit and age at settlement, values of which can be found in
the literature, and U crit at hatching, which is more scarcely reported.
Given the ranges of variation and units of age and speed (days and cm s -1
respectively), this is approximately equivalent to a linear relationship,
as depicted by the solid curve in Figure 6.12.
To measure critical swimming speed, larvae are made to swim in
a flume inside which current speed is increased every few minutes.
Therefore, it measures “forced” swimming and probably overestimates
speeds at which larvae would swim when not constrained. Actually,
U crit was found to be approximately five times the speed at which larvae
were observed to swim freely in a large tank (routine speed) 91 , and twice
the speed they go in situ 25 . The development of in situ speed with size
has also been described in at least two studies 190,229 . Yet, in this model,
larvae are given the possibility to choose between several swimming
speeds. So, what we really need is the maximum potential speed larvae
could attain, discarding burst speed (burst speed is fuelled anaerobically
and is only relevant to behaviours such as escape from predators 92 ).
And U crit is precisely that: a measure of potential speed 25,92 .
The direct effect of temperature on swimming speed is twofold: first
water temperature affects muscle efficiency because fishes are ectotherms;
second, water viscosity increases when temperature drops, which could
slow larvae down 25 . These two effects explain in part why tropical fish
larvae are in general much better swimmers than temperate ones 25
(section I.5.2, page 22). Finally, temperature also affects development,
and larvae develop faster — hence acquire swimming abilities faster —
in warmer waters 37,267 . In a brilliant meta-analysis, O’Connor et al. 37
derived a single relationship between temperature and pelagic larval
duration (PLD – i.e. the duration of ontogeny), that is valid for an
extremely wide range of marine taxa (crustaceans, Annelids, fishes,
Critical speed is the
right measure of
potential speed
Temperature affects the
metabolism, physics,
and development
of swimming