Masters Thesis

Relationships between eelgrass (Zostera marina) habitat characteristics and juvenile Dungeness crab (Cancer magister) and other invertebrates in southern Humboldt Bay, California, USA

The relationship between eelgrass bed characteristics and animal use is important in better understanding what the optimal habitat features are for specific taxa. The focus of this study was to look at structural and geographical aspects of eelgrass habitat in southern Humboldt Bay, California to determine if presence or abundance and size of common eelgrass invertebrates can be predicted. Animals were selected to represent epibenthic, epifaunal and infaunal groups that are commonly found within the eelgrass habitat. Each month, from May to August 2004, fourteen sites were randomly sampled from the eelgrass beds of southern Humboldt Bay (40º44' to 40º41' N, 124º13' to 124º15' W). The epibenthic decapod crustaceans, Cancer magister, Crangon spp. and hippolytid shrimp were sampled by fishing paired traps at each site for ~ 48 hours. Epifaunal gastropods, Phyllaplysia taylori, were gathered from eelgrass samples and infaunal Macoma nasuta clams were removed from mud cores collected at each site. Above and belowground eelgrass samples were collected from each site in order to describe the vegetation structure in which the animals were captured. Independent variables measured from eelgrass samples included shoot density, shoot biomass, mean shoot length, variation in shoot length, root/rhizome biomass and epiphyte load. Sitespecific variables obtained for each site included distance to the nearest channel, distance iv to the entrance channel and elevation. Multiple regression was used to analyze relationships between animal population numbers and sizes to vegetation structure and location. Logistic regression was used to analyze presence/absence of Crangon. Vegetation and habitat variables differed in the degree to which they were able to predict number and size of C. magister, Crangon spp. and P. taylori, and temporal trends were noted for all of these animals. Dungeness crab were more numerous in areas characterized by high shoot density, homogeneous shoot lengths and proximity to a channel. Mean Dungeness crab size increased over time. Larger crabs were associated with lower shoot density than smaller crabs. Shoot density and distance to the nearest channel were significant predictors of Crangon presence/absence. The addition of sculpin presence/absence to the analysis was significant, and indicated that Crangon were rarely present when sculpin were present. A negative relationship between Crangon size and epiphyte load was found. Phyllaplysia taylori were more numerous in eelgrass beds containing greater shoot biomass. Smaller P. taylori were more prevalent in eelgrass characterized by greater shoot biomass and homogeneous shoot lengths than larger P. taylori. Vegetation structure and location were not found to be important predictors of Macoma nasuta and hippolytid shrimp abundance or size in southern Humboldt Bay. Results of this study indicate that eelgrass habitat structure is important in structuring the size and distribution of some animals in southern Humboldt Bay but not others.

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