2.2.4 Long-term depuration experiments: examining the allocation
of released body carbon by labeling fish bodies with14C
To examine the release (/efflux) rate of fish body carbon and the
allocation of released fish body carbon as DOC, CO2, and
PC, the
marine
medaka were 14C-labeled by feeding the fish14C-labeled rotifers for 15 days (0.7–1.1 Bq/µg C).
Then, three to five fish were collected for 14C
measurement in fish at time zero, and other fish were individually
transferred into 50-mL feeding beakers containing 25 mL clean FSW for
8-day depuration. During the depuration period, the fish were fed
nonradioactive rotifers at two rations of1000 and 2000 ind./fish/day,
corresponding to 2.2% and 4.3% of the fish DW, respectively.
These
rations are within the typical range of daily food consumption for wild
mesopelagic fish (Davison et al., 2013). During depuration, fish, feces
and water samples were collected every 24 h for the measurement of the14C retained in fish and released into seawater in the
2000 ind./fish/day ration treatment. The samples were collected on days
1, 3, 6, and 8 in 1000 ind./fish/day ration treatment. All the samples
were handled and measured by the same methods as described for the
short-term depuration experiments.
The release (/efflux) rate of fish body carbon was calculated as the
slope of the linear regression of the natural logarithms of the14C retained in fish with the time of depuration. Only
the data on the second day and afterward were used for the regression.
Based on the mass balance, the apparent body carbon release
(B 14C release) was
calculated as the difference between the amount of 14C
in fish at the beginning and the 14C retained in the
fish at the end of depuration. The sum of the collected14CO2, DO14C and
P14C during depuration was taken as the measured body
carbon release
(B 14C ’release). The
ratio ofB 14C ’release/B 14C releasewas used to indicate the 14C recovery of the total
released body 14C in seawater.
2.3Scenarios
for
e stimating
carbon release from mesopelagic fish in the global open ocean
Carbon release model scenarios were separately established for four
groups of mesopelagic fish classified on the basis of their ocean
latitudes (40°N–40°S vs. 40–70°N/S) and
fish behavior (diel vertically
migrating (DVM) vs. nonmigrating (NM)) (Table 1). The carbon release
parameters of marine medaka were extrapolated to wild mesopelagic fish.
Other parameters, such as the mesopelagic fish biomass and the daily
food intake of mesopelagic fish, were derived from the literature.
We assumed that the wild fish would release the same proportions of
their daily food carbon to seawater in the forms of DOC,
CO2, and PC as
marine
medaka. The food carbon release parameters measured after 24-h
depuration were used in the model scenarios for two reasons. First, the
release rates of DOC, CO2, and PC from food decreased to
relatively constant low values during the 16–24 h depuration phase,
indicating that 24 h or less was enough for the fish to completely
allocate the ingested food carbon to assimilation or food carbon
release. Second, the feeding behavior of DVM mesopelagic fish usually
has a diel rhythm. Therefore, the mean proportions of ingested food
allocated to AE (38.9%) and released as DOC (32.7%),
CO2 (20.9%), and PC (7.5%) during 24-h depuration were
used in the model scenarios (Table 1).
The body carbon release (/turnover) rate (K e)
of mesopelagic fish was
extrapolated from the carbon release (/efflux) rate of marine medaka by
using the
power-law
function reported in Gillooly et al. (2001) and Davison et al. (2013).
That is, K e is proportional to 1924 ×WW 0.75 × e(-5020/K ),
where WW is the wet weight of fish (g), and K is the
absolute temperature. Based on this formula, theKe s of 0.5-g mesopelagic fish living at different
temperatures could be derived from the K e of
marine medaka. The K e (0.053
d-1) of marine
medaka
at the daily food ration of 4.3% fish DW was used as the basis for
estimating the K es of mesopelagic fishes (0.5 g
in WW) (Table 1). Following the pattern of marine medaka, 40.4%, 42.6%
and 16.9% of the released body carbon of wild mesopelagic fish was
assumed to be in the forms of DOC, CO2, and PC,
respectively. Carbon released through reproduction was not considered in
the current estimation.
Variations in the body carbon release rate during different activities,
such as swimming, feeding and resting, were not considered in the
present estimation. As a simplification, DVM mesopelagic fish were
assumed to feed and live in surface waters at night (for 12 h) and
inhabit mesopelagic depths during the daytime (for 12 h). NM mesopelagic
fishes were assumed to feed and live at mesopelagic depths all day. That
is, the DVM mesopelagic fish have two differentK es, depending on the mean temperatures of the
surface waters and of the mesopelagic waters. In contrast, the NM
mesopelagic fish have only one K e, depending on
the mean temperature of the mesopelagic waters.
The biomass of the mesopelagic fish in the open ocean was assumed to be
constant over time. The total WW of the mesopelagic fish in the open
ocean between 40°N and 40°S was assumed to be
109–1010t, and the WW of the mesopelagic fish
in
other regions between 40°N–70°N and 40°S–70°S was assumed to be
0.3×109–1010 t (Lam & Pauly, 2005;
Irigoien et al., 2014). Furthermore, 30%–50% of the mesopelagic fish
were assumed to undergo diel vertical migration (Davison et al., 2015;
Klevjer et al., 2016).
For mesopelagic fishes living in the open ocean, the ratio of fish DW to
WW was 19.1%, and the carbon content was 43.8% of the fish DW
(Childress
& Nygaard, 1973), which is similar to that (46.7%) of marine medaka.
The mean WW of individual fish was assumed to be 0.5 g (Davison et al.,
2013, 2015).
The mean temperatures in the surface and mesopelagic waters in the open
ocean between 40°N and 40°S were assumed to be 25°C and 9°C,
respectively (Davison et al., 2013; Irigoien et al., 2014), whereas the
mean temperatures in the surface and mesopelagic waters at high
latitudes, 40°N–70°N and 40°S–70°S, were assumed to be 8°C and 3°C,
respectively (Kaeriyama & Ikeda, 2004; Max et al., 2012). The daily
food ration for a 0.5-g mesopelagic fish was assumed to be
temperature-dependent according to Davison et al. (2013), i.e., the
daily food ration was 10%, 5%, 5%, and 4% of the fish WW at 25°C,
9°C, 8°C, and 3°C, respectively. All mesopelagic fishes were assumed to
be zooplanktivorous, and the carbon content of zooplankton was assumed
to be 0.12 mg C/mg WW (Harris et al., 2000).
The annual rate of carbon release in each form was calculated by
multiplying the daily rate by 365.