Volume 6, Number 3, 1997

Counting Delaware's crabs

Numbers. A researcher can spend untold hours collecting, measuring, calculating, recording and analyzing them. But what do they really mean?

A lot, if you have help from the UD's Statistical Laboratory staff, including co-directors and statisticians Lidia Rejtö and Jack Schuenemeyer, faculty members in the Department of Mathematical Sciences, and their two graduate assistants.

For example, fisheries biologists in the Division of Fish and Wildlife of the Delaware Department of Natural Resources and Environmental Control (DNREC) recently sought and received advice on the collection and analysis of data on two important inhabitants of Delaware waters-blue crabs and striped bass.

According to Desmond Kahn, AS '71, '75, '88 PhD, an environmental scientist with the division's fisheries section, the blue crab fishery is the most valuable commercial fishery in Delaware, worth some $4 to $6 million annually. In addition, hundreds of recreational crabbers take part in harvesting Delaware's blue crabs, contributing to the state's tourism economy and taking a heretofore unknown toll on the blue crab population.

"To assess the status of a fish or shellfish stock, you need to know the total harvest, both recreational and commercial," says Kahn, who received his doctoral degree in biological sciences. "The recreational harvest in Delaware was a black hole. We needed to design a study to estimate the number of blue crabs caught by recreational crabbers in the state. But, studies like this can be quite complex, and you always face limitations in the people, time and money available to conduct the study. "

Staff at the Statistical Laboratory helped Kahn and his colleague, Randall Cole, decide how much sampling time to allocate to each of Delaware's three counties. The scientists first conducted a pilot study to estimate the amount of recreational crabbing taking place in each county, which served as the basis for future sampling efforts. Then, during randomly selected sampling periods, the division's research technicians traveled predetermined routes throughout the state, counting the number of recreational crabbers and interviewing them about their harvest. A separate study was designed to estimate the recreational crab harvest taking place from boats.

The results, according to Kahn, were somewhat surprising. "The recreational crab harvest in Delaware last year was less than 5 percent of the commercial harvest," he says. "That was lower than many people expected. By comparison, the recreational harvest in the Chesapeake ranges between 25 and 50 percent of the commercial harvest, according to several studies."

However, because 1996 was a poor year for both commercial and recreational crab fisheries due to the unusually severe winter, scientists will continue to collect recreational harvest data to determine if 1996 was an anomaly.

The Division of Fish and Wildlife also obtains data about the blue crab (and many other aquatic species as well) through research trawl surveys of Delaware Bay and vicinity. In these surveys,
marine organisms are scooped up in a net towed
by a research vessel at 40 different sampling stations. Species of interest are counted, measured and recorded. The division has conducted these surveys monthly from April to October since 1978, gathering what some fisheries scientists consider one of the best data sets on the Atlantic coast.

Kahn, who has worked primarily with the blue crab component of the trawl survey data, once again turned to the Statistical Lab, this time for help analyzing the existing data.

"We wanted to look at the relationship between the size of the spawning population of blue crabs and the number of offspring. The repetitive nature of the study and variations in conditions from the upper to the lower part of the bay, among other factors, introduce a lot of complexity to the analysis. The Statistical Lab helped us select the right model to analyze the data."

The Statistical Lab also assisted Kahn and his colleague, Craig Shirey, with their annual survey of striped bass spawning stock. By the early 1980s, some scientists believed the striped bass was extinct in the Delaware River, where the fish had once migrated upstream in large numbers to spawn each year. Natural resource agencies in both Delaware and Maryland declared a moratorium on striped bass fishing from 1985-89. Since then, strict limits on striped bass harvest were put into effect along the entire East Coast.

As a result, striped bass have made a remarkable recovery. Annual surveys of spawning stock begun in Delaware in 1991 appear to show great improvement in the age structure of the population-that is, from year to year, a wider range of age groups has been present.

However, the biologists didn't feel completely confident in their survey. "We were having some trouble achieving a consistent study design," says Kahn, "because the salinity of the Delaware River varies so much from wet years to dry years. This affects the distribution of the spawning bass, which must be in fresh water to spawn. So, we called on the folks at the Statistical Lab again."

Kahn emphasizes how important sound statistics are in his work at the Division of Fish and Wildlife. "We are biologists with some training in statistics," he says. "But, sometimes it's helpful to consult with someone who is a real expert. By working with the excellent statisticians at the University, we can prevent costly mistakes."

-Elizabeth Chajes