Sakinan Serdar

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Sakinan
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Serdar
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  • Article
    Accounting for seasonal and composition-related variability in acoustic material properties in estimating copepod and krill target strength
    (Wiley, 2019-11-06) Sakinan, Serdar ; Lawson, Gareth L. ; Wiebe, Peter ; Copley, Nancy
    Estimation of abundance or biomass, using acoustic techniques requires knowledge of the frequency dependent acoustic backscatter characteristics, or target strength, of organisms. Target strength of zooplankton is typically estimated from physics‐based models that involve multiple parameters, notably including the acoustic material properties (i.e., the contrasts in density and sound speed between the animal and surrounding seawater). In this work, variability in the acoustic material properties of two zooplankton species in the Gulf of Maine, the copepod (Calanus finmarchicus) and krill (Meganyctiphanes norvegica), was investigated relative to changing season as well as, for the copepod, temperature and depth. Increases in the density and sound speed contrasts of these species from fall to spring were observed. Target strength predictions based on these measurements varied between fall and spring by 2‐3 dB in krill. Measurements were also conducted on C. finmarchicus lipid extract at changing temperature and pressure. The density contrast of the extract varied negatively with temperature, while the sound speed contrast changed by more than 10 % over the temperature and pressure ranges that the organism expected to occupy. C. finmarchicus target strength predictions showed that the combined effect of temperature and pressure can be significant (more than 10 dB) due to the varying response of lipids. The large vertical migration ranges and lipid accumulation characteristics of these species (e.g., the diapause behaviour of Calanus copepods) suggest that it is necessary for seasonal and environmental variability in material properties to be taken into account to achieve reliable measurements.
  • Article
    Insights into the diet and feeding behavior of immature polar cod (Boreogadus saida) from the under-ice habitat of the central Arctic Ocean
    (Wiley, 2024-06-24) Schaafsma, Fokje L. ; Flores, Hauke ; David, Carmen L. ; Castellani, Giulia ; Sakinan, Serdar ; Meijboom, Andre ; Niehoff, Barbara ; Cornils, Astrid ; Hildebrandt, Nicole ; Schmidt, Katrin ; Snoeijs-Leijonmalm, Pauline ; Ehrlich, Julia ; Ashjian, Carin J. ; The MOSAiC team ECO
    Polar cod (Boreogadus saida) is an endemic key species of the Arctic Ocean ecosystem. The ecology of this forage fish is well studied in Arctic shelf habitats where a large part of its population lives. However, knowledge about its ecology in the central Arctic Ocean (CAO), including its use of the sea-ice habitat, is hitherto very limited. To increase this knowledge, samples were collected at the under-ice surface during several expeditions to the CAO between 2012 and 2020, including the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. The diet of immature B. saida and the taxonomic composition of their potential prey were analysed, showing that both sympagic and pelagic species were important prey items. Stomach contents included expected prey such as copepods and amphipods. Surprisingly, more rarely observed prey such as appendicularians, chaetognaths, and euphausiids were also found to be important. Comparisons of the fish stomach contents with prey distribution data suggests opportunistic feeding. However, relative prey density and catchability are important factors that determine which type of prey is ingested. Prey that ensures limited energy expenditure on hunting and feeding is often found in the stomach contents even though it is not the dominant species present in the environment. To investigate the importance of prey quality and quantity for the growth of B. saida in this area, we measured energy content of dominant prey species and used a bioenergetic model to quantify the effect of variations in diet on growth rate potential. The modeling results suggest that diet variability was largely explained by stomach fullness and, to a lesser degree, the energetic content of the prey. Our results suggest that under climate change, immature B. saida may be at least equally sensitive to a loss in the number of efficiently hunted prey than to a reduction in the prey's energy content. Consequences for the growth and survival of B. saida will not depend on prey presence alone, but also on prey catchability, digestibility, and energy content.