During eleven cruises conducted off the Oregon continental slope from April 1973 to March 1975, approximately 1600 specimens of the majid crab Chionoecetes tanneri Rathbun were collected. A time sequence of size frequency histograms and bivariate plots of allometric measurements were used to determine size classes, population structure, growth and age at maturity of C. tanneri from the Oregon coast. Adult sex ratios and sex ratios for each of the juvenile size classes were computed. Bivariate plots of carapace width and cheliped length for males and carapace width and abdomen width for females defined nine size classes for juvenile males and eight size classes for juvenile females. The morphogenic relationship between differential cheliped length and gonad maturity in males and differential abdomen width and gonad maturity in females (Watson, l970a; Brown and Powell, 1972) allowed determination of minimum size at maturity of 118 mm carapace width for male C. tanneri and 85 mm carapace width for female C. tanneri. Mean carapace widths for adult males (142.7 mm) and adult females (102.5 mm) collected for this study compare well with those given by Pereyra (1972). Mean carapace width increase for each molt for all juvenile size classes was computed to be 26.14%. When size classes were equated to molt groups a linear regression (Time = -2.94 + 0.81 (Carapace width)), based on progression of carapace width modes through time, predicts age at maturity to be from five to six years. A highly significant departure from a 1:1 sex ratio for adults, of 2.41 females per 1 male, is consistent with reports for other members of the genus Chionoecetes (Hilsinger, 1975). / Graduation date: 1976

The Hemigrapsus oregonensis population at Coquille Point in the Yaquina Bay Estuary on the Central Oregon Coast was studied from April, 1972 through May, 1973. The population was found to be vertically stratified from the 1 ft level to the 5 ft level. Population densities were found to be most dense in the upper regions. Greatest population density (about 20 crab/m²) was found to be in the 3-4 ft interval above MLLW (0.0 ft level). The population sex ratio was biased in favor of the females (53.3%) and did not vary appreciably during the year. The reproductive season, as determined by the percentage of berried females, was from February through May with a peak (32.8%) during March. Brooding females were found every month during the study, indicating a continuous, low level egg production throughout the year. A model for estimating potential egg production is given. The minimum carapace width of brooding females was found to be 0.86 cm. Biomass values were determined from carapace width measurements. A conversion equation is given. Biomass values generally increased as tidal height increased. The average biomass value for the area was 8.47 g/m². The average dry weight per crab decreased as tidal height increased, The average dry weight per crab at each tidal height (about 0.5 g) did not significantly increase during the study, suggesting a stable population. The average monthly production showed an over-all negative rate of -1.23 g/m² per month. No significant differences were found between tidal heights. The net production rate at each tidal height could not be shown to be different from a zero net production rate, again suggesting a stable population. Monthly distributional patterns indicated an high degree of population mobility. Crabs tested for locomotory activity patterns in the laboratory showed rhythms influenced by both the light regime and the tidal regime. Weak endogenous displays were found for a light component with increased activity during dark periods. Greatest activity generally occurred during dark-high tide periods. It is suggested that the locomotory activity patterns of H. oregonensis are influenced by both a tidal cycle and a light cycle. Under constant experimental conditions, the endogenous rhythmicity decayed within 3-9 tidal cycles and resulted in more or less continuous random movements. Only about 50% of the tested crabs, however, displayed an endogenous locomotory rhythm. / Graduation date: 1974

Typescript. Includes vita and abstract. Bibliography: Includes bibliographical references (leaves 45-47). / Rhithropanopeus harrisii is an introduced Atlantic crab in Coos Bay, Oregon. In Coos Bay, it occurs only in the uppermost estuary where salinities are seasonally low (<1 0/00). Rhithropanopeus occurs at higher salinities in its native range, and tolerates higher salinities in the laboratory. A native crab, Hemigrapsus oregoneos1s, occurs in the estuary, but not as far up into the oligohaline and freshwater zones as Rb1tbcopanopeus. In laboratory experiments, adult Hem1grapsus consume juvenile Rhithropanopeus. Hemjgrapsus is significantly more aggressive than Rhithropanopeus. The absence of juvenile Rhithropanopeus at field sites where both species occur suggests that field interactions may be occurring. Taken together, these results and observations suggest that there may be a dynamic zone in the estuary where the distribution of Rhithropanopeus is restricted by Heroigrapsus. Rhithropanopeus may find a refuge from Hemigrapsus in the oligohaline-freshwater Zones of the estuary, such that Hemigrapsus may be exerting selective pressure on the Rhithropanopeus population favoring freshwater adapted individuals.