1 National Institute of Aquatic Resources, Technical University of Denmark2 Centre for Ocean Life, National Institute of Aquatic Resources, Technical University of Denmark
With emphasis on Atlantic cod (Gadus morhua)
During their early life stages, the fishes face a range of challenges. To sustain the development of the pelagic eggs and larvae spawning needs to occur where the offspring is retained in optimal environmental conditions. The larvae rely on a spatial and temporal match to their prey in order to ensure fast growthand avoid starvation. Inshore areas may provide such favorable conditions, as these are sheltered from offshore currents and act as physical barriers that restrict dispersal and drift of eggs and larvae. Even though inshore spawning grounds of e.g. Atlantic cod (Gadus morhua) has been reported from numerous fjords in Norway, Canada and Greenland, very little is known about their early life history and the physical-biological mechanisms impacting their survival in such inshore areas. To better understand the processes of importance to the early life success of fish, I carried out a seasonal study on a West Greenlandic inshore population of Atlantic cod. I followed the distribution, development and mortality of eggs and larvae in relation to the physical conditions and the prey availability. The spawning of this population of cod was restricted to a shallow inshore area located at the innermost part of the fjord branch Kapisigdlit, in the Godthåbsfjord system. This shallow area was characterized by elevated temperatureswithin the upper part of the water column, which increased the development rate of the pelagic eggs. The mortality in the immediate spawning area was low and egg dispersal from the area was limited. Later, the seasonal breakup of ice in a connecting river resulted in an increased freshwater outflow, then enhancing the dispersal of the older larvae. Throughout the study, marked spatial and temporal differences in the zooplankton community composition were observed. The distribution of larvae generally overlapped the distribution of their preferred prey. Although no direct relationship could be found between prey availability and cod larval growth, the otolith growth rate was significantly improved in larvae that dispersing away from the spawning area. This was despite lower temperatures, suggesting that prey availability had some influence on the growth pattern. The relatively low mortality rates of eggs and larvae,and high larval growth rates compared to other studies, indicate that this fjord affords especially favorable conditions for the early life stages of cod. These conditions may result in a strong recruitment, which again might be the background of the relatively high cod spawning stock biomass found in Kapisigdlit. Since different species of fish may vary in their spawning strategies and adaptations to physical and biological conditions, the larval assemblages in inshore areas with variable physical and biological characteristics are likely to be strongly structured. Therefore, I assessed the distributional patterns of larval fish assemblages comparing distributions to the variability in hydrography and prey availability. This was carried out across the West Greenlandic Godthåbsfjord system and the offshore Fyllas Bank, which encompass different oceanographic regimes, and which harbor different zooplanktoncommunities in specific regions. Significantly different patterns of distribution were shown for the fish larvae and these appeared linked to the specific hydrographical conditions and the availability of preferred prey. Generally few larvae were found close to the glacier suggesting that this was a poor habitat for most species, while habitats at the entrance and in other branches of the fjord was relatively rich in species of fish larvae. With changes in wind regimes, relative inflowof Atlantic water, temperature increase, glacial melting and runoff from land, the environment off West Greenland will undergo significant changes in the future. This thesis points out that in fjord systems, where such processes might change the timing and magnitude of freshwater outflow and circulation patterns the distributional patterns and life of the early life stages of fishes can be markedly affected.