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Populations of aquatic vertebrates, mainly fish, have dramatically decreased since the last century. Collapsed fish populations often show slow recovery or remain at the same level, due to associated changes in food web interactions and population characteristics. Most depleted fish populations are commercially exploited and play an important ecological role, and their recovery is a priority for conservation biology and the fishing industry. Restocking programs aim to restore populations and often habitats and ecosystems by releasing captive-bred individuals into their natural habitat. Releasing fish at early life stages, such as larvae, avoids the negative effects of captive breeding and reduces costs. The Eastern Baltic cod (EBC) is ecologically and genetically divergent from the Atlantic cod (Gadus morhua). In a few decades, the population size has dramatically decreased, and EBC now attains sexual maturity at half the size it used to. In parallel, the Baltic Sea is becoming warmer, more anoxic and fluctuating in salinity. Project ReCod aims to strengthen the EBC population through releases of yolk-sac larvae. This has further allowed close study of early life stages and their performance, as well as to better understand the situation of EBC. In this thesis, I increase our knowledge of EBC for restocking purposes and investigate its present and future situation. In Paper I, I found that neutral buoyancy plays a crucial role in determining salinity preference in first-feeding larvae. Paper II showed that yolk-sac larvae are able to adjust to a low salinity environment when released. In Paper III, I found that the elusive large and healthy cod from the Åland Sea (northern Baltic Proper) is not different from southern EBC and locally adapted to the low environmental salinity of the region; however, EBC eggs appear to be able to develop on sediment. Paper IV showed that, contrary to expectations, the smallest fish produced the largest and most buoyant eggs, and the eggs from small-sized fish were buoyant enough to avoid the anoxic bottom in the Baltic Sea. In Paper V, I found that climate change-induced increased water temperature and decreased salinity can be detrimental for the early stages of EBC.