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On the Trail of the Blue Crab

Lesson Plan

Students will practice subtraction skills by tracking the movements of blue crabs in the Hudson River estuary.


Students will solve word problems that require them to:

  • subtract using data from tagged crabs to calculate distance traveled and elapsed time;
  • understand that blue crabs migrate.

Grade level:

Elementary (Grades 3-4)

Subject Area:

Math, Social Studies (Geography), Science

New York State Learning Standards:

Mathematics, Science, & Technology Standards 3, 4


  • Use whole numbers to identify locations and measure distances.
  • Subtract two digit whole numbers.
  • Apply mathematics in real world settings.
  • Reason mathematically.


Preparation time: 5 minutes
Activity time: 40 minutes


Each student should have:


The blue crab's life cycle involves migration between regions of high and low salinity in estuaries. This migration is aided by back legs, shaped like paddles, that enable the animal to swim rapidly. To track crab migration, scientists attach numbered tags to blue crabs. People who catch tagged crabs contact the scientists using a phone number on the tags. The data they provide adds to knowledge of how far and how fast the crabs migrate.

Also called blue claw crabs, these crustaceans are born in the saltiest parts of the estuary around New York Harbor. They then migrate upriver into less salty areas of the Hudson which serve as nurseries, providing plenty of food and shelter from predators. Male crabs tend to go further north than females, entering fresh water and sometimes reaching the Federal Dam at Troy. Females stay downriver, closer to the high salinities that their eggs will need to develop properly. As winter approaches, blue crabs move back downriver to New York Harbor.

Distances on the Hudson are often measured in Hudson River Miles. Hudson River Miles start at the southern tip of Manhattan. This spot, called The Battery, is River Mile 0. The estuary part of the Hudson ends at the dam in Troy at River Mile 153. Only two digit milepoints are used in this lesson, though blue crabs do travel more than 100 miles upstream.


  1. In preparation for this lesson, have students do the Readings in Hudson River Natural History lesson titled "Blue Claw!"
  2. Discuss the concept of migration and how it fits into the blue crab's life cycle.
  3. Introduce the Hudson River Miles system; show students the Hudson River Miles map.
  4. Go over the worksheet with the class or hand out as an in-class or homework assignment.

Vocabulary List and Answer Key:

Available in the pdf version of this teacher's section and in the package that bundles all of the readings.


  • Have students share answers to questions from worksheets, or collect and grade sheets.
  • Make up similar elapsed time/distance problems for quiz.


  • There are many websites about blue crabs, among them "Blue Crab Info". Use the links leaving DEC's website on the right side of this page to visit the Blue Crab Info website.
  • While much of the research and information available on crabs focuses on the Chesapeake Bay, a brochure and technical reports about Hudson River crabs are available from DEC on the Blue Crab Fishery Monitoring page.