Marine environments are facing several threats from climate change. The threats include changing salinity, temperature and acidity. Changes in these environmental aspects can have an influence on individual foraging and patterns of growth in marine fish and ultimately entire food webs. Understanding the foraging biology of marine fish is important because their feeding habits and their diet selection are vital characteristics of trophic relationships in marine ecosystems. In this thesis, I explored the food web structure of the Arabian Gulf Marine Fishery, the growth rate of estuarine threespine stickleback (Gasterosteus aculeatus) fish, and their diet choice. The focus for all three studies was the potential threat caused by the elevation of salinity, temperature and associated changes in productivity and diet.
In Chapter 2, I present a study on diet selectivity in larval and juvenile G. aculeatus along a salinity gradient of how this environmental parameter along with temperature, prey species richness, prey diversity and productivity affected the diet choice of these fish. I found that prey selectivity, which I defined as the difference between size in gut and size in pond, declines with salinity but that selectivity was unaffected by temperature, productivity, prey species richness, and prey diversity.
In Chpater 3 I present a study of how salinity, temperature, productivity and zooplankton food size affects the growth rate of larval and juvenile threespine sticklebacks. I found that salinity, zooplankton prey size and zooplankton species richness all affected the growth rate of these fish, while no effect was observed for temperature, productivity and other features of the environment.
In Chapter 4 I present the largest marine food web ever constructed, for the Arabian (=Persian) Gulf. The Arabian Gulf is experiencing increased salinity in spawning ground areas due to alterations of freshwater influx. I examined the structure of this community in comparison to other previously reported marine food webs and found it to be similar. I also explored how the structure of the food web changes when only species were considered (taxonomic food webs) versus of the consideration of life stages as species (disaggregated food webs). I also explored the impact on the food web structure due to the removal of all commercial species from both webs. One of the major findings was that copepods, mainly calanoids, were the most common prey. Thus, their productivity is vital to the fishery.
My work in this thesis on larval and juvenile fish growth and on diet choice shows that larval and juvenile fish are sensitive to the changes in environmental parameters, particularly salinity. My work also shows that calanoid copepods are a major prey in a large fishery. In general, this suggests that it is very important to understand the sensitivity of larval and juvenile fish to environmental characteristics and the management of their prey (e.g. copepods).
