Mathematician Says Mathematical Models Failed
The author of the commentary quoted below is a professor of mathematics at the Baltimore County campus of the University of Maryland.
(p. 4) . . . , in a fishery, the maximum proportion of a population earmarked each year for harvest must be set so that the population remains sustainable.
The math behind these formulas may be elegant, but applying them is more complicated. This is especially true for the Chesapeake blue crabs, which have mostly been in the doldrums for the past two decades. Harvest restrictions, even when scientifically calculated, are often vociferously opposed by fishermen. Fecundity and survival rates -- so innocuous as algebraic symbols -- can be difficult to estimate. For instance, it was long believed that a blue crab's maximum life expectancy was eight years. This estimate was used, indirectly, to calculate crab mortality from fishing. Derided by watermen, the life expectancy turned out to be much too high; this had resulted in too many crab deaths being attributed to harvesting, thereby supporting charges of overfishing.
In fact, no aspect of the model is sacrosanct -- tweaking its parameters is an essential part of the process. Dr. Thomas Miller, director of the Chesapeake Biological Laboratory at the University of Maryland Center for Environmental Science, did just that. He found that the most important factor for raising sustainability was the survival rate of pre-reproductive-age females. This was one reason, in 2008, after years of failed measures to increase the crab population, regulatory agencies switched to imposing restrictions primarily on the harvest of females. . . .
The results were encouraging: The estimated population rose to 396 million in 2009, from 293 million in 2008. By 2012, the population had jumped to 765 million, and the figure was announced at a popular crab house by Maryland's former governor, Martin O'Malley, himself.
Unfortunately, the triumph was short-lived -- the numbers plunged to 300 million the next year and then hit 297 million in 2014. Some blamed a fish called red drum for eating young crabs; others ascribed the crash to unusual weather patterns, or the loss of eel grass habitat. Although a definitive cause has yet to be identified, one thing is clear: Mathematical models failed to predict it.
For the full commentary, see:
(Note: ellipses added.)
(Note: the date of the online version of the commentary is MAY 2, 2015.)