Search

In this thesis, I investigated the population dynamics of Atlantic cod (Gadus morhua), and Atlantic herring (Clupea harengus) stocks. These two species are of high commercial interest and play key roles in many ecosystems in the North Atlantic. Understanding their dynamics and recognising the impacts of stressors such as fishing and climate change is instrumental for sustainable management. In the first two papers, we investigated the spawning migration dynamics of the Northeast Arctic (NEA) cod population. NEA cod conducts migrations from its feeding grounds in the Barents Sea to spawning grounds along the Norwegian coast. In Paper I, we investigated a trade-off associated with long-distance migration. The longer the migration, the higher the energetic costs are, compared to spawning closer to the feeding grounds. Thus, in order to justify these additional costs, the longer migration to southern spawning grounds should give a fitness benefit to the population. We tested whether an increase in offspring growth, due to warmer temperatures in the southern spawning grounds can compensate for the energy the parents spent on a longer migration. By using an integral projection model incorporating effects of body length and migration distance, we found that relatively small increases in offspring length were able to counterbalance parental energy costs of long-distance migration. However, the distribution of adult spawners at the spawning grounds has changed over the last decades, with the northern spawning areas now being more important than the southern ones. To explain this change, two competing hypotheses have been explored: First, due to climate warming and a potential northward shift in the distribution inside the Barents Sea, the southern spawning grounds are used less. Second, due to changes in the demography of the stock induced by fishing there are fewer large individuals in the population. Smaller fish are predicted to migrate shorter distances than larger fish. In Paper II, we explored the size of the spawners, based on Norwegian Fisheries data and genetic data, at the spawning grounds. Our results showed that size of the spawners decreased with distance to the feeding grounds in the Barents Sea, thus our results do not support that changed demography of the stock is a main driver of the distribution at the spawning grounds. In the last two papers, we moved beyond spawning migrations and focused on how disturbances through the environment in combination with fishing pressure affect the variability in stock dynamics. In Paper III, we focused on the dynamics of the spawning stock biomass of 14 Atlantic herring stocks distributed across the North Atlantic. By using time series, we investigated whether these stocks underwent abrupt and long-lasting shifts of great magnitude with potentially discontinuous behaviour. Stocks that fit our developed criteria were further tested for discontinuous behaviour in response to fishing pressure in combination with temperature, a proxy for climate change, or predation pressure of Atlantic cod. According to our criteria, of the 14 analysed stocks, three stocks showed true catastrophic and long-lasting changes, where the impact of fishing was modulated by temperature and predation pressure. To compare systematically how species with different life histories respond to fishing and environmental disturbances, we used a theoretical approach in Paper IV. In this paper, we built an age-structured model with vital rates depending on growth rate and asymptotic size. In order to cover a broad spectrum of different life histories, we explored a wide range of growth rates and asymptotic sizes. Environmental variability affected the recruitment. Fishing targeted juvenile and adult biomass. The results indicated that recruitment variability in the tested life histories was largely depending on growth rate and asymptotic size, with larger and faster growing species being less vulnerable to environmental variability than smaller and slow growing species. However, by including fishing in the dynamics, the variability in the populations, visible through variability in recruitment, increases over the whole spectrum of growth rates and asymptotic sizes.