Graduate Thesis Or Dissertation

 

Influence of Regional Oceanography on the Distributions, Trophic Interactions, Growth, and Survival of the Early Life History Stages of Fishes Public Deposited

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  • Most marine fishes experience high rates of mortality during their early life history stages with far reaching consequences for adult population dynamics. Within a few weeks of hatching, relatively small changes in larval growth and mortality rates can lead to orders of magnitude variability in year-class strength. Growth and survival during this phase are contingent upon the ability of larvae to find food and avoid predation in a physically and biologically heterogeneous environment. Here, we coupled biological sampling and fine-scale in situ plankton imaging to examine the influence of regional oceanography on larval fish distributions, feeding, and growth in the context of their zooplankton prey and predators in the northern California Current (NCC). Larval fish were strongly affected by two highly dynamic regional oceanographic features in the NCC: coastal upwelling and the Columbia River Plume. While the NCC supports major fisheries whose larval and juvenile stages depend on upwelling driven primary and secondary production, coastal upwelling is highly variable in space and time. In Chapter 2, diet and otolith microstructure analysis showed that the condition of a dominant myctophid (Stenobrachius leucopsarus) reflected the prevailing upwelling conditions. During reduced upwelling, recent growth was substantially slower, guts less full, and diets dominated by low trophic level prey. In contrast, during active upwelling, faster-growing northern lampfish fed on higher quality copepod prey. Yet, larvae exhibited reduced feeding and growth in the most intense upwelling, revealing a dome-shaped relationship with the fastest growth occurring in moderate upwelling conditions. Further, high zooplanktivorous predation pressure on larval northern lampfish led to above average growth, which may indicate the selective loss of slower-growing larvae. Chapter 3 revealed that upwelling also influenced the larval growth of another forage fish, northern anchovy (Engraulis mordax). Northern anchovy otolith-derived recent growth was spatially variable and related to the location of the summer upwelling front. When the front was restricted nearshore, inshore larval anchovy grew significantly faster than offshore. Conversely, when a period of prolonged active upwelling pushed the front to the edge of the continental shelf, offshore anchovy larvae grew significantly faster than inshore. Larval anchovy growth may be constrained by cross-shelf temperature differences and the distribution of nutritious copepod prey that are restricted to the nearshore environment off Oregon in summer. Embedded within the highly dynamic NCC, the tidally modulated Columbia River Plume is an important spawning and nursery ground for many fishes (e.g., northern anchovy, E. mordax). In Chapter 4, data illustrated how the strength and location of the plume front exposed larval fishes to a diversity of unique prey and predator fields over the progression of a tidal cycle. While the plume region provided a substantially higher concentration of prey that are important for the feeding of young fishes occupying this area, this region was also characterized by enhanced spatial overlap of larval fishes and their zooplankton predators relative to oceanic waters. In a separate study, and in partial fulfillment of the National Science Foundation Research Traineeship (NRT) program at Oregon State University, Chapter 5 switched focus from larval fishes to another component of the plankton: Dungeness crab larvae. Mortality during the early life history stages of Dungeness crab is considered a bottleneck for fishery production, but information on the offshore distribution of the most vulnerable pelagic larval stages is lacking. Fine-scale depth discrete biological sampling over two years revealed that Dungeness crab larvae were not uniformly distributed in time or space, but exhibited distinct spatial distributions within the water column, over the continental shelf, and across latitudes, with larval abundance significantly negatively correlated with in situ temperature and salinity throughout ontogeny. Taken together, this body of work demonstrates that local and regional oceanographic features contribute to variable zooplankton distributions as well as growth and mortality patterns for larval fishes by affecting their trophic interactions. This dissertation illustrates the importance of incorporating food-web dynamics and local and regional oceanographic processes when predicting the response of fish and crab populations to ecosystem variability.
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