Masters Thesis

Correlations between fish abundance and physicochemical parameters in Humboldt Bay, California

Eelgrass habitats, within estuaries, are known to be important nursery zones for several fish species. However, they are considered to be harsh environments due to wide fluctuations in physicochemical factors. The objectives of this study were to determine the extent of correlations between fish and their physical surroundings, to minimally explore interactions between climatic and physicochemical parameters, and to investigate the use of geostatistical interpolation tools to elucidate physicochemical changes spatially and temporally. This research was conducted in Humboldt Bay, Humboldt County, California from June 2006 to August 2008. Three eelgrass sites (Samoa, Indian Island and North Bay) were sampled monthly using an epibenthic otter trawl. In addition to fish populations, physicochemical and climatic parameters were assessed. These included water temperature, salinity, dissolved oxygen, pH, turbidity, average monthly precipitation and average monthly wind speed. A Canonical Correspondence Analysis was run to evaluate fish community/environment relationships. To examine fish species/environment relationships, an empirical distribution function was constructed using the physicochemical parameters and weighted by fish species abundances. Then a randomization test using the Kolmogorov-Smirnov test statistic indicated the strength of the cdf correlations. Universal Krig estimates of water temperature and salinity were iv calculated using Geographic Information Systems geostatistical analyst to transform data into spatially referenced surfaces. A total of 16,261 fish, representing 21 families and 40 species were captured in this study. Fish abundance was greatest during 2007. Twelve fish species accounted for 94% of the total fish catch. Annually, species richness and diversity were significantly different; each being greatest in 2008. Within each year, species diversity and fish abundances were highest during the spring and summer months. Fish were strongly correlated with environmental parameters. Juvenile Cymatogaster aggregata preferred warmer water temperatures, greater than 16°C, while adults were found at lower water temperatures. Pholis ornata exhibited the highest abundances during 2007 warmer water temperatures and lower precipitation rates. Gasterosteus aculeatus and Pholis ornata showed strong associations to both water temperature and precipitation. Gasterosteus aculeatus adults appeared to prefer warmer water temperatures than juveniles, but both life stage abundances decreased with slight increases in precipitation. Sebastes melanops and Embiotoca lateralis showed strong associations with high turbidity, while Osmerid spp. showed a strong correlation with high dissolved oxygen readings. In general, the warmer summer water temperatures at the North Bay location attracted juvenile fish, while adult fish were more common at the Samoa location. Interpolated water temperature and salinity surfaces successfully showed general patterns in physicochemical parameters over space and could prove very useful in simplifying complex spatial physicochemical and fish distribution patterns.

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