Date of Award
Open Access Dissertation
Ryan R. Rykaczewski
The natural mortality of fishes is an important component for understanding population dynamics. The larval stages of pelagic fishes living in the open ocean are particularly vulnerable to high rates of mortality, and fluctuations in these rates are thought to exert a large influence on the number of fish maturing into the spawning population. Early stage larval fishes are thought to undergo a critical period after hatching when they must find food or succumb to starvation. While the availability of suitable food for larval fishes is, on average, too low in the open ocean to support survival, patchy distributions of planktonic food can be vital habitat. The stable ocean hypothesis describes how wind mixing can influence this habitat through dilution of plankton patches and increase the mortality of larval fishes.
In this dissertation, I reexamined the stable ocean hypothesis from three perspectives to understand how it might influence several key components of the California Current Ecosystem. First, I calculated larval mortality rates for five fishes (northern anchovy, Pacific sardine, Pacific hake, Pacific mackerel, and jack mackerel) from 1979 to 2015. Then I compared the mortality rates to indices of storm events and calm periods as a proxy for water column mixing and planktonic patchiness. Contrary to expectations, storm events did not negatively influence the survival of any of these species. In fact, mortality for Pacific hake decreased with an increased number of storms. Next, I examined the effect that storm events had on the vertical distribution of chlorophyll layers. I found that storms tended to decrease the occurrence of high-concentration chlorophyll layers in the water column. Finally, changes to the planktonic community composition within the chlorophyll layer was assessed. Significant variability associated with years and regions was found among sampled community compositions. Furthermore, a proxy for larval fish food exhibited considerable variability; however, the variability was not explained by the region, season, or year. Moreover, there were two peaks in biomass much higher the median value of the food proxy biomass and these events were dominated by potentially noxious algal species that may have negative implications for larval fish survival.
Overall, evidence was found to support the notion that winds may influence the plankton communities larval fish rely upon, but, little support was found for a direct link to larval mortality. The stable ocean hypothesis may be important for fish recruitment in the California Current Ecosystem, but its influence appeared to be minimal on seasonal, regional, and interannual scales.
Turley, B. D.(2019). Larval Fish Mortality and Vertical Chlorophyll Structures: Reexamination of the Stable Ocean Hypothesis in the Southern California Current. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/5599