Where do you come from, where do you go: early life stage drift and migrations of cod inferred from otolith microchemistry and genetic population assignment

Abstract

This study investigates stock mixing of genetically distinct Atlantic cod (Gadus morhua) stocks in the Kattegat, an area geographically located between the North Sea and the Baltic Sea, by combining genetic population identification with habitat assignments from hatch to capture from otolith microchemistry. Cod captured in Kattegat were genetically assigned to either the North Sea or the endemic Kattegat population. Otolith chemical fingerprints differed significantly between populations during the larval and pelagic juvenile stage with higher strontium and lower barium and manganese concentrations in the North Sea population than the Kattegat population, indicating that North Sea cod are spawned in the North Sea or Skagerrak and drift into the Kattegat during the early life stages. Individual cod of both populations undertook frequent migrations to other areas, with < 25 % of individuals remaining resident within the Kattegat throughout their life. Across seasons and age classes, the two populations were most frequently distributed in the Kattegat (67 %), with approximately 25 % distributed in the western Baltic Sea and < 10 % in the Skagerrak/North Sea.

Into the wild: coupling otolith and archival tag records to test assumptions underpinning otolith chemistry applications in wild fish

“…Thus, one would expect higher otolith Sr concentrations at depth both as a result of salinity and size differencesyet we observed the opposite. In other Atlantic cod stocks, no relationship between fish size and otolith Sr was observed (Hüssy et al, 2021b). It is likely that some of the contradictions in otolith Sr concentrations in this study have occurred because ambient Sr concentrations (or Sr/Ca ratios) do not relate consistently to salinity.…”

Connectivity of Greenland halibut in the northwestern Atlantic Ocean inferred from otolith chemistry

“…In another paper, (Xiong et al, 2021) evaluated the core-to-margin Sr concentrations of otoliths from yellow croaker (Larimichthys polyactis) for samples collected in 2003, 2012, and 2013. The collected otoliths had similar patterns of Sr along the core-to-margin transects showing nonsignificant interannual differences between all samples (Xiong et al, 2021). Finally, Hüssy et al (2022) and Albertsen et al (2021) both used otolith chemistry to assess the population structure of cod (Gadus morhua) within a large-scale, multiyear design and observed no apparent seasonality among their samples. The abovementioned studies suggest that the temporal variability of chemical markers with large-scale studies, such as ours, is sufficiently stable to provide inter-regional inferences.…”

Mercury Sources and Fate in a Large Brackish Ecosystem (the Baltic Sea) Depicted by Stable Isotopes

“…This likely results from both their more pronounced benthic feeding that leads to the accumulation of both pelagic and benthic MMHg that is less or not photodegraded and their migration strategy. It has been evidenced that most individuals from the population in Kattegat (Fladen) undertake spawning migrations toward the North Sea, and it is well established that a lower slope is imprinted during MMHg photodegradation in low DOM marine waters. ,,,, …”

“…This likely results from both their more pronounced benthic feeding that leads to the accumulation of both pelagic and benthic MMHg that is less or not photodegraded and their migration strategy. It has been evidenced that most individuals from the population in Kattegat (Fladen) undertake spawning migrations toward the North Sea, 91 and it is well established that a lower slope is imprinted during MMHg photodegradation in low DOM marine waters. 18,28,29,72,92 There is a good separation of the various fish populations in the δ 202 Hg − Δ 199 Hg space (Figure 2b), suggesting that they could be discriminated based on their Hg isotopic signatures as previously demonstrated for juvenile Seabass at the European scale.…”