Boreogadus saida (Lepechin, 1774)

4.4 | Modeled B. saida View in CoL growth rate

The bioenergetic model provides insights in the sensitivity of B. saida growth to variation in prey fields. It shows that growth rate may be more sensitive to the amount of food ingested, defined in the model through stomach fullness and, to a lesser degree, the energetic content of prey. This is primarily explained by the data distribution of the two parameters in the model having different coefficients of variation, that is, ratio between SD and the mean. Nonetheless, the high importance of stomach fullness in the model could indicate that the benefit of high-energy prey may be counterbalanced by other factors, such as energy expenditure for prey capture or minimizing predation risk. Such findings agree with our microscopic analysis indicating preferred feeding on prey that is abundant and/or easily available and/or prey that can be collected with relatively low effort. Although highest growth rates were found when B. saida fed predominantly on energy-rich prey, such as Calanus spp. or krill (Figure 8), our comparison of prey composition with the abundances of various key prey taxa indicates that the energetic trade-off for such high-energy prey depends strongly on its abundance and the abundance of alternative prey (Figure 5; Table 5).

The effect of subzero temperatures on metabolic rates at sampling could explain the low modeled growth rates. Near-zero temperatures have been experimentally shown to induce low stomach evacuation rates and high assimilation rates in B. saida ( Hop & Tonn, 1998) , resulting in enhanced feed conversion efficiency at low feed intake ( Kunz et al., 2016). This could explain the good body condition of B. saida caught at subzero temperature, while the model indicated low growth rates, suggesting model constrains in its temperature-limitation functions. Unfortunately, we lack crucial data from subzero temperatures to correctly model B. saida adaptation to ice habitat.

Results from model simulations should be interpreted while considering the simplifications and specific assumptions that have been made ( David et al., 2022), including a fixed ratio between active and basal metabolic rates and a constant energetic content of fish and prey types. An averaged energetic content per prey species was used for all sampled fish, although several prey species are known to have varying energy contents over the course of the year or with size (e.g., Kraft et al., 2015; Nowicki et al., 2023; Percy & Fife, 1981). This could reduce the variability in modeled growth rates. However, seasonal variation in the energy content of a species is likely much less than the variation between species.

Although it is not surprising that both stomach fullness and prey energy content influence growth rate, the results indicate that changes in the abundance and catchability of the prey, and thus the amount of prey ingested, may have a larger impact than changes in prey energy content. With the warming of the Arctic Ocean and a shift toward smaller copepods and higher abundances of gelatinous zooplankton species, a number of prey field characteristics will change for B. saida such as the density, the size spectrum, the energy content, and the catchability of prey. The latter is due to potential changes in the density and behavior of the prey, as well as the loss of the under-ice habitat as a major feeding ground. Changes in such characteristics should thus be taken into account when anticipating consequences of environmental changes for B. saida .