What relationship could scientists track in order to see whether or not blue crabs were the main reason that large zebra mussels have declined?

While there were still many zebra mussels in the Hudson River, the overall number of zebra mussels was slightly declining, and they were on average much smaller. Populations of the largest — or oldest mussels — were declining greatly. ... That's because large zebra mussels feed on bigger food particles like zooplankton.

Explanation: I may have over explained...?

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