Fish seminar 22.09.2025: characterisation of the carbon export potential of commercially important fish species in the Oslo fjord
background:
human induced climate change
increased carbon emissions from the Anthropocene
affecting function
increased ocean temp
biodiversity loss and species shifts
climate and ecosystems are technically linked
impacts on ecosystem services
ocean carbon sink
absorbs CO2
carbon sequestration
organic carbon taken up by organisms including zooplankton and transported actively or passively.
passive carbon sink
POC and DOC
DVM (diel vertical migration)
role of fish poorly quantified + unexplored
few recent studies believes that it contributes significantly
human impacts and fisheries
sinking fecal matter and dissolution
differences across species
nearly 40% of the world’s fish stocks are depleted
habitat damage
fishing down the food wed
fisheries target large individuals over small individuals
weakening carbon export potential due to larger ones being taken out
large research gaps
aims and objectives:
characterisation of the carbon export potential of commercially important fish species and heavily exploited fish species in the oslo fjord, where the focal species are atalntic cod, saithe, whiting, and herring, as well as investigating what infleunces this
obj 1: to optimize methodology for extracting fish fecal matterial
hypothesis 1: different sampling methods yields different amounts and vary in consistency
obj 2: to test how sinking rates differ among species, size classes and seasons (sinking rate experiment)
hypothesis 2: larger individuals compared to a species exhibit faster sinking rates due to pellet size, diet, seasonal shifts, and there are expected to be higher rates in the summer
obj 3: to test how faeces carbon content differentiates between species, size classes, and season (dissolution rate experiment)
hypothesis 3: in smaller individuals from lower trophic species exhibit higher dissolution rates larger higher trophic species. this is also due to the pellet size and the rates being higher during the summer.
methods:
sampling methods:
stations DK1 (inner oslo fjord) and IM2 (outer oslo fjord)
fish divided into summer and winter catches
dissection of stomach, took out intestines, emptied feces into sterile petri dish (most optimal as it yielded the highest amount of faeces)
experimental design:
sinking rate experiment (obj 2)
dissolution rate experiment (obj 3)
together these experiments will give an insight into how much carbon is dissolved and weighed down in the water column and then returned to carbon in the carbon cycle and how much remains, which can then be sequestered on the sea floor
cod, saithe, and whiting samples
both experiments were possible
herring samples
needed to have enough for both experiments, had to choose it separately for the experiments
the temperature of the experiments were at 4 and 15 degrees reflecting the summer and winter months
distributing material based on species:
totally random → herring
block design → cod, saithe, and whiting
sinking rate experiment
looked at how faeces sank over a predetermined distance, and it differs among the samples
distilled seawater was used to represent the oslo fjord
size of pellet measured before and after
if the samples broke during descent → fastest decending pieces
touched the wall → discarded
bouyant → dicarded
descent rate:
recorded on phone for accuracy
converted to meters per day
disolution rate experiment:
did an incubation experiment over four different time points
for cod, saithe, and whiting there was enough fecal material to distribute them acorss the four time points from one single sample so was able to get a continuous reading for CO2
for herring, each indivdual sample had to be chosen for one incubation treatment only. these samples had a higher variability which we will com back to
placed in their incubation bottle, which contained filtered seawater, but kept the natural bacteria of the sea water.
the incubation was done in both 4 and 15 degrees celsius
results:
sinking rate experiment:
effect of fecal pellet size on sinking rate
a positive trend between fecal pellet size and sinking rate (goes against H2?)
looking at species separately
cod had fastest sinking rate
herring was the second fastest in sinking rate
looking at size classes
large individuals generally exhibit faster rate, but was not giving a significant result
looking at temperature of treatment
4˚C and 15˚C
had a significant effect on sinking rates, which basically reflect hte seasonal temperature differences
slower rates in the winter
season alone did not have an affect on sinking rates
dissolution rate experiment:
standardized CO2 overtime - showing the CO2 production for the incubation experiment
whiting + saithe exhibit more production of CO2 making them more susceptible to microbial breakdown, less sequenstration compared to cod and herring
species:
saithe had highest dissolution rate
herring had lowest dissolution rate (not expected, but can be explained by the high variability)
size class (log scale):
small individuals had a big difference compared to large individuals
so supports H3, this is due to smaller individuals producing smaller fecal pellets which then has a higher surface area to volume ratio, therefore more easily broken down
seasonal effect (log scale):
winter had higher dissolution rate
much cannot be said due to dietary differences between seasons, and did not do any dietary analysis
discussion:
there was a considerable difference in the amount of carbon that was lost back into the water column, and how much was left in the ocean floor between size classes, and across species
characterizing carbon exponential:
several assumptions
mean sinking rate and temp influence
carbon content
previous studies:
his were generally higher compared to other sinking rate experiments, probably due to his faecal pellet were much larger than others.
fecal pellet with size 0.5 to 0.75 cm were comparable
findings confirm that temp and associated density effects sinking rates
phytoplankton sinking rate: 0.2-3.17 m/d
zooplankton sinking rate: 50-900 m/d
fish fecal pellets sink much faster compared to phytoplankton and zooplankton
plankton biomass is far greater than fish biomass but fish proves that they can deliver a significant amount of carbon tot he seafloor despite their smaller biomass
the sampling method didnt give him the most realstic consistency or size of hte pellets
future research:
limitations and improvements
more accurate parameter and less assumptions
integrate POC and DOC measuremnts
larger dataset and spatial average
characterising fish distribution and fecal pellet
characterize carbon export with a larger dataset with more smaples form more fish species as well as larger spatial covverage would be benficial to see if the carbon export we see is a phenomenon for the oslo fjord or if it can be applied to a larger system such as skagerrak
knowing the distribution of the fish are very important parameters for future attempts at tryning to characterise carbon export
conclusion:
large individuals deliver greater amount of carbon to the seaafloor
size selected fishing pressure may leave a negative effect on carbon sequestration efficiency, and fishing down the food web has a negative effect.
fish fecal pellets represent importnat contribution to the carbon export
by handloing fish populations more cautiosly int he future, allowing for popultion recovery and stability, fish may provide ecosystem services that beenfit both natural kind of sollution, as well as ecological and economical roles in society
this may prove to be a natural climate solution in the future