Grabowski TB & JH Grabowski. 2019. Early life history. Pages 133-168 in GA Rose (ed.). Atlantic Cod: the bio-ecology of the fish. John Wiley and Sons, New York.
Like all marine fishes that undergo a planktonic life history stage, Atlantic cod must overcome a daunting set of challenges as part of their early life history. Atlantic cod must develop the morphological and physiological characteristics that will enable them to transition to a demersal, predatory existence as juveniles and adults, while retaining the ability to deal with the immediate and pressing challenges of a planktonic existence, e.g., avoiding predators while finding and subduing prey. At fertilization, an individual Atlantic cod embryo is cast adrift simultaneously with potentially millions of its siblings and half-siblings. Each embryo will take with it all of the parental investment it will ever receive in the form of energy reserves in its yolk sac provided by its mother and genetic material with a proven record of success from both parents. The parents of each embryo have themselves successfully completed the journey that this new generation of cod is about to undertake. The parents may have traveled hundreds of kilometers to release their offspring at the time and place where the most favorable conditions possible exist for success. Even so, the odds of any individual possessing the “winning” combination of pure luck and genetic fitness to overcome the specific hazards faced are infinitesimally small. Only the sheer number of embryos set adrift guarantees that there will be individuals capable of running the gauntlet. The truly amazing thing about cod is not the tremendous amount of mortality that occurs during this stage, but the fact that so many individuals survive. From a management perspective, understanding the factors influencing the high levels of mortality that occur during the early life history of cod is a critical component of predicting the future population size of cod stocks. This chapter details the early life history of the Atlantic cod starting at fertilization through larval development and continues with the descent of a juvenile cod from the plankton to suitable nursery habitat on the seafloor. This chapter also covers the early years, from habitat use to growth, diet, mortality and behavior, and ends as the juvenile cod begins to shift to adult habitats and behaviors.