On the Trail of the Blue Crab
Educator Lesson Plan
Students will practice subtraction skills by tracking the movements of blue crabs in the Hudson River estuary.
Objectives:
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, Geography, Science
Standards:
Mathematics, Science, & Technology Standards 3, 4
Skills:
- Use whole numbers to identify locations and measure distances.
- Subtract two digit whole numbers.
- Apply mathematics in real world settings.
- Reason mathematically.
Vocabulary:
estuary, life cycle, migration, recapture, river mile
Duration:
Preparation time: 5 minutes
Activity time: 40 minutes
Materials:
Each student should have:
-
Hudson River Miles map (190 kb pdf) * Note: This is set for legal (8.5 inches x 14 inches) paper to maximize legibility, but it will work on letter (8.5 inches x 11 inches) paper as well. To obtain legal size copies, you may need to use the options in your computer's print menu to specify printout on legal size paper.
-
Pencil
Background:
The blue crab's life cycle involves migration between regions of high and low salinity in estuaries. This migration is aided by an adaptation for swimming: back legs-shaped like paddles-that can move the animal through the water at impressive speeds. 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 Hudson River estuary in spring and early summer. They soon migrate upriver into less salty areas of the estuary. These regions of brackish water are excellent nursery areas for young crabs, offering plenty of food and shelter from predators. Male crabs tend to go further north than females, swimming into fresh water and sometimes reaching the Federal Dam at Troy. Females stay downriver, closer to the high salinities that their eggs will require to develop properly. It appears that as winter nears, the crabs move 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.
Activity:
- Discuss the concept of migration and how it fits into the blue crab's life cycle.
- Introduce the Hudson River Miles system.
- Go over the worksheet with the class or hand out as an in-class or homework assignment.
Assessment:
- Have students share answers to questions from worksheets, or collect and grade sheets.
- Make up similar elapsed time/distance problems for quiz.
Answers:
1. 9 days; 20 miles; south 2. 9 days; 11 miles; north 3. 7 days; 17 miles; north 4. 6 days; 28 miles; south Challenge question: The crab would probably not have its tag. It grows by shedding its shell and, with it, the tag.
Resources:
Readings in Hudson River Natural History lesson titled "Blue Claw!"
There are many websites about blue crabs, among them "Blue Crab Info" at www.bluecrab.info. 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 at Blue Crab Fishery Monitoring.
