Feeding and growth of Atlantic cod (Gadus morhua L.) in the eastern Baltic Sea under environmental change

Abstract

Five decades of stomach content data allowed insight into the development of consumption, diet composition, and resulting somatic growth of Gadus morhua (Atlantic cod) in the eastern Baltic Sea. We show a recent reversal in feeding level over body length. Present feeding levels of small cod indicate severe growth limitation and increased starvation-related mortality. For young cod, the low growth rate and the high mortality rate are manifested through a reduction in size-at-age. The low feeding levels are likely the result of a decrease in benthic prey abundance due to increased hypoxic areas, while decreasing abundances of pelagic species in the area of cod distribution have prevented a compensatory shift in diet. Our study emphasizes that environmental forcing and the decline in pelagic prey caused changes in consumption and growth rates of small cod. The food reduction is amplified by stunted growth leading to high densities of cod of smaller size competing for the scarce resources. The average growth rate is negative, and only individuals with feeding levels well above average will survive, though growing slowly. These results suggest that the relation between consumption rate, somatic growth and predatorprey population densities is strongly environmentally mediated.

Compensatory Feeding in Eastern Baltic Cod (Gadus morhua): Recent Shifts in Otolith Growth and Nitrogen Content Suggest Unprecedented Metabolic Changes

“…If the increased N-content is indicative of an increased feeding rate, it is clear that the feeding rate has increased mostly in larger and older fish. Thus, the results do not support the hypothesis that the lower productivity of EBC is related to food limitations (e.g., Eero et al, 2012;Svedäng and Hornborg, 2014, Casini et al, 2016Karlson et al, 2019;Neuenfeldt et al, 2019;Orio et al, 2019). Neither does our study corroborate the conjectures of lower metabolic rates due to physiological stress, as a consequence of encounter hypoxic or low oxygen-saturated water Casini, 2018, 2019), reducing the rate of digestion and hence food consumption in EBC on a systemic level (Brander, 2020).…”

“…There is no common understanding on why and how the productivity of EBC has declined (Eero et al, 2015). Several hypotheses have been put forward connecting the decreased productivity to a reduced feeding rate, which could stem from (a) regionally unbalanced fishing (Eero et al, 2012), (b) densitydependent feeding competition due to selective fishing Hornborg, 2014, 2017), (c) the spread of hypoxic seafloor leading to density-dependent growth and lower condition (Casini et al, 2016), and substantially higher natural mortality (Neuenfeldt et al, 2019), or (d) that spread of hypoxic or low oxygen-saturated water may affect metabolism and thereby reducing the rate of digestion and hence food consumption in EBC Casini, 2018, 2019;Brander, 2020). Other hypotheses instead point at the increase of seal parasites, impairing growth and health status (e.g., Horbowy et al, 2016), or at the occurrence of thiamin deficiency affecting EBC among other Baltic animal populations (Engelhardt et al, 2020).…”

“…Moreover, recent modeling work by ICES (2019b) suggests that exceptional increases in natural mortality might also result in truncated size distributions, assuming in the modeling that individual growth and survival rates are inversely related to each other. Because high, or normal, growth must be weighed against lower survival rates and vice versa in the modeling, it is unknown to what degree the truncated size distribution is related to reduced growth or higher natural mortality, or at which stage of the life cycle these processes may have been altered (ICES, 2019a;Neuenfeldt et al, 2019). As most of the proposed factors behind lower productivity in EBC suggest metabolic changes, it is crucial to determine when and how such metabolic alterations may have occurred in the stock.…”

“…By obtaining new insights into EBC metabolism and feeding rates over the period between the capture years of 1995 and 2015, we may confront hypotheses that relate the reduced productivity of EBC to lower feeding rates (Eero et al, 2012;Hornborg, 2014, 2017;Casini et al, 2016;Casini, 2018, 2019;Karlson et al, 2019;Neuenfeldt et al, 2019;Orio et al, 2019;Brander, 2020). Finally, by finding when a shift in feeding rates has occurred, it is also possible to evaluate if the spread of seal parasites is a potential driver behind changed metabolism in EBC (e.g., Horbowy et al, 2016).…”

Combined climate change and nutrient load impacts on future habitats and eutrophication indicators in a eutrophic coastal sea

“…The projected changes in salinity and O 2 concentrations will have major impacts on the productivity and eventually the biomass of cod in the Baltic Sea, even if exploitation of marine resources is maintained at the maximum sustainable or lower levels (Lindegren et al ; Niiranen et al ). For example, cod feeding and condition during the past 20 yr have been linked to the spatial extent of bottom layer hypoxia, and cod in poor condition often have lower survival and less economic value to fisheries (Svedang and Hornborg ; Casini et al , b ; Eero et al ; Neuenfeldt et al ). As cod is a key predator in the food webs of the central Baltic Sea (Casini et al ; Niiranen et al ), a decline in its biomass will have important impacts on other trophic levels (e.g., zooplanktivorous fish; benthic consumers) and the overall dynamics of food webs and fisheries.…”