2023

Hočevar, Sara

Nature is full of complex phenomena, non-linearity, random chances, and less random changes. These conditions promote diversity among and within species, in their life histories and ecological roles. In this thesis, I untangled some of the evolutionary and ecological advantages that the life history diversity of the iconic Atlantic cod, Gadus morhua, generates and maintains at different levels of biological organisation. I explored the eco-evolutionary dynamics by applying the mechanistic models to empirical systems; one parameterized for Atlantic cod, and another for a newly built Skagerrak food web. Analyses of fitness components revealed that multiple-batch spawning is a risk-spreading strategy of cod, an adaptation to hedge its evolutionary bets when the environmental conditions impacting the survival of egg batches are uncertain and destructive. Because multiple-batch spawning is a size-related trait, the size-selective fishing was found to cause a lingering eco-evolutionary change, not only by selective removal of bigger and older individuals, but also by an unforeseen removal of individuals with the greatest risk-spreading potential. These findings reveal that the risk-spreading benefits multiple-batch spawning provides, are not advantageous under size-selective fishing, considering it does not protect the fitness nor demographic structure of Atlantic cod. I have tackled the role of the co-existing Fjord cod and North Sea cod ecotypes for the food web topology and functioning in a coastal pelagic food web of Skagerrak and found that the intraspecific differences in life-history traits and ontogenetic dietary shifts between the ecotypes led to counterintuitive opposing impact on the fish community. Skagerrak food web proved less robust to the loss of a smaller stationary Fjord cod ecotype, of which absence induced a decline in the biomass of several economically valuable harvested species. Diversity was a silver lining of the four papers, all cautioning about the causes and consequences of fisheries-induced changes that reduce such diversity at any of the three levels: individual, populational, food web.