The cod (Gadus morhua) population in the Baltic Sea is drastically depleted due to decades of overfishing, unsustainable management and habitat degradation. Severe oxygen deficiency and decreased salinity in their spawning grounds further limited successful spawning to the Bornholm basin, which caused a decline in recruitment. To facilitate the populations’ recovery, a range of measures are necessary, including the implementation of restocking programs. Baltic cod, which are genetically distinct from other cod populations, are however not successfully cultivated yet, creating the need for the development of breeding efforts. We therefore investigated the quality and quantity of the reproductive output from Baltic cod in relation to broodstock size. The results of this study will give insight into reproduction of Baltic cod, which is valuable for broodstock management to achieve high breeding success.
Material and methods
Broodstock fish of the eastern Baltic cod stock were caught between February 2021 and March 2023. At the research station Ar, Gotland, Sweden, they were separated into three different tanks according to capture time, in which they were kept in recirculating seawater (17 psu, 7 ˚C) and followed a natural light cycle. The study is carried out in 2023, from April to August, when the fish naturally spawn. Quantity of produced eggs is measured daily in each tank. Quality of produced eggs in each tank is assessed weekly, focusing on deformities, developmental stage, size and thiamin content. For each tank, 100 eggs are then transferred into a 400 mL beaker and incubated for a total of 14 days. Water change occurs daily, along with counting of dead eggs/larvae and hatched larvae. At the end of the incubation period, total survival, hatching success and quality of produced larva is assessed, including presence of deformities, length, and yolk-sac size.
With this study, we expect to be able to describe the reproductive output of Baltic cod, a threatened population of small fish in bad condition (Ero et al., 2020), possible under controlled conditions in captivity. We will further compare the relation between Baltic cod female size and quality of reproductive output to known ratios of other cod populations (e.g., Chambers & Waiwood, 1996; Kjesbu et al., 1996). Additionally, we aim to find out whether even small sized broodstock can produce good quality reproductive output in a controlled environment with sufficient nutrient-rich feed supply.
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