Description:
About the Data
Dungeness crab (Cancer magister) is an important commercial fishery along the northeastern Pacific coast (California to Alaska). The data show the commercial harvest of Dungeness crabs, excluding sport fishery and non-treaty landings, from 1950 to 1999 off the coast of Washington State. The harvest season typically runs from December through February. The mean catch is about 8.7 million pounds per year, however the time series seems to be composed of two parts, before 1986 (when the catch was lower on average) and after 1986.
There are dramatic variations in harvest size with time, which could be explained in a number of ways. First, organisms often go through natural "boom and bust" cycles; for example, a boom in crabs might create a boom in crab predators, which would then induce a bust in crabs. Second, the crab population might be affected by changes in its environment, "forcing" the crab population to grow or decline. Third, the crab population may be perfectly stable, but the catch may vary due to changes in prices (overharvesting during times of high prices is common) or changes in catch limits or fleet size regulated by governmental agencies.
According to Johnson et al (1986), one of the most important factors in decreasing the crab population is southward-directed winds in the late spring, that drive crab larvae onshore. There is an approximate 5 year lag time between these anomalous winds and the crab bust, corresponding to the 5 years from larvae to maturity. The southward directed winds (and crab cycles) have a period about 8-10 years, as can be seen in the graph.
Students can determine the mean and standard deviation from the mean for these widely varying data. A more challenging exercise would be to model the apparent 8-10 year cycles using trigonometric functions.
