Hurst, T.P., Copeman, L.A.,
Haines, S.A., Meredith, S.D.,
Daniels, K. and Hubbard, K.M.
2019
Elevated CO2 alters behavior,
growth, and lipid composition
of Pacific cod larvae.
Marine Environmental Research
145: 52-65.
SUMMARY:
If Hurst et al.'s experiment
had terminated
after two weeks
of pCO2 enrichment,
the logical interpretation
would have been that
ocean acidification
negatively impacts
cod larvae ... but after
three more weeks the
results had reversed !
After three more weeks,
the authors had an
optimistic assessment,
that "the sensitivity
of Pacific cod larvae
to elevated pCO2
varies with developmental
stage," where
"not only did older larvae
not suffer the reduced growth
and energy storage
observed in the early stages,
they actually exhibited
the opposite response,
an increase in growth rate."
Hurst et al. said it is quite possible
that the perceived negative impacts
of ocean acidification measured
at week two of the treatment
is an artifact of "acclimation to
the effects of elevated pCO2."
The researchers note that
"studies of the potential impacts
of OA on marine fish larvae
generally impose elevated pCO2
treatments around the time
of fertilization or hatching
( as done here )."
"As a result,
developmental stage
is often functionally linked
to the exposure duration,
making it difficult
to unambiguously discriminate
ontogenetic effects from
physiological acclimation."
It's possible that an experiment
across multiple generations,
with adult fish reared
in a high pCO2 environment,
and their fertilized eggs as well,
there might have been
no growth-related differences
( or perhaps a growth benefit )
observed in two-week-old larvae.
Until all ocean acidification experiments
implement such protocol, it would be wise
to scale back negative predictions
on this topic.
DETAILS:
Hurst et al. (2019) say
"the lack of a consistent response
among larval fishes to elevated CO2
levels continues to challenge
our ability to draw large-scale
conclusions about the ecosystem
impacts of ongoing ocean acidification."
Hurst et al examined the impacts
of ocean acidification on Pacific cod
( Gadus microcephalus ).
They examined such impacts
on newly hatched larvae
from a captive adult
cod brood stock
that were reared under
normal pCO2 conditions.
Following spawning,
newly-fertilized eggs
were collected and incubated
at ambient pCO2 levels
( 350 µatm ) until hatching.
Then yolk-sac larvae
were transferred into one of two
pCO2 treatment levels
( 502 or 1726 µatm ),
corresponding to seawater
pH values of 7.98 and 7.43,
where their growth and
development were monitored
two and five weeks later
( corresponding to larvae
developmental stages 6 and 9,
respectively).
In addition, half of the larvae
in each pCO2 treatment
were fed a nutritional-enriched diet
(Diet 1) while the other half received
a diet containing no nutritional
enrichment (Diet 2).
Fish in the high pCO2 treatment were
"smaller and had lower lipid levels
at 2 weeks of age than fish at low
(ambient) pCO2 levels."
Three weeks later
"this effect had reversed:
fish reared at elevated pCO2 levels
were slightly (but not significantly)
larger and had higher total lipid levels
and storage lipids than fish reared
at low pCO2."
Fatty acid composition was
significantly different between
fish reared in the low and high
pCO2 levels after two weeks,
but this effect
"diminished by week 5."
