Geology topics

James R. Gilbert

Publications and source records attributed to James R. Gilbert.

3 recordsLinked to original sources

USGS2006

Comparison of methods used to estimate numbers of walruses on sea ice

The US and former USSR conducted joint surveys of Pacific walruses on sea ice and at land haul-outs in 1975, 1980, 1985, and 1990. One of the difficulties in interpreting results of these surveys has been that, except for the 1990 survey, the Americans and Soviets used different methods for estimating population size from their respective portions of the sea ice data. We used data exchanged between Soviet and American scientists to compare and evaluate the two estimation procedures and to derive a set of alternative estimates from the 1975, 1980, and 1985 surveys based on a single consistent procedure. Estimation method had only a small effect on total population estimates because most walruses were found at land haul-outs. However, the Soviet method is subject to bias that depends on the distribution of the population on the sea ice and this has important implications for interpreting the ice portions of previously reported surveys for walruses and other pinniped species. We recommend that the American method be used in future surveys. Future research on survey methods for walruses should focus on other potential sources of bias and variation.

Chukchi Sea↗

USGS1997

Adaptation of a stage-projection model for species with multiple year reproductive cycles

We apply stage projection matrices to multiple age and reproductive categories, deriving the stable age - reproductive category distribution for populations with multi-year reproductive cycles and identifying conditions for the existence of a dominant latent root. Given the stable distribution of age and reproductive category and the number of female young for each reproductive category, the age specific fecundity rates can be calculated. For species with multi-year reproductive cycles, the fecundity rate associated with the stable age distribution is often not a smooth function of age, but fluctuates as a larger or smaller fraction of females becomes available to breed. We suggest that fecundity rates for species with multi-year reproductive cycles be defined in terms of litter size and probabilities of conception and successful gestation rather than average fecundity.

Ecological Modelling↗

USGS1978

Evaluation of an aerial survey of Pacific walruses

An aerial survey of Pacific walruses ( Odobenus rosmarus divergens ) was evaluated to determine the reliability of estimates of population abundance. The probability of detecting groups of walruses on the pack ice remained uniform to at least 0.93 km from the flight line, whereas the probability of detection decreased significantly beyond 0.23 km for walruses in the water. Walruses were more abundant along the ice-edge zone between 162 and 165°W than in other areas of the Chukchi Sea during September 1975. Few walruses were observed in consolidated pack ice north of the ice-edge zone or in ice-free water to the south. More walrus groups and larger mean group size were observed on September 8 than on other days. We estimated abundance for each day and all days combined using methods based on sample area and numbers of strip samples. Estimates varied among days by over an order of magnitude; this variation is attributed to the combined effect of chance sampling of an aggregated population and variation in the fraction of walruses hauled out. The coefficient of variation of the estimates ranged between 0.25 and 0.99. This imprecision was due to the aggregated distribution of walruses and the large variation in group size. Using the survey data as a basis for stratification, we calculated that, due to the high variability within strata, a sample size of 40% of the total area or 56% of the total available strips would be required to obtain 95% confidence limits within 10% of the estimate of total abundance. Variation contributed by observer error in estimating group size also is relatively unimportant to the precision of abundance estimates. Studies of natural history, particularly those oriented toward activity and habitat selection, would help investigators estimate bias due to the variable fraction hauled out and design surveys based on meaningful strata. Estimates of total abundance based on limited survey efforts will provide information of little reliability.

Journal of the Fisheries Research Board of Canada↗