BOSTON – Fishermen say it’s bunk. Researchers have trouble explaining what it’s about. But the science of determining the number of fish in the sea has rarely been more important, or scrutinized.

At issue are looming restrictions to stop overfishing that are based on the science and, if enacted next August as planned, could put hundreds of fishermen out business.

Fishermen have long said the science is flawed and undercounts fish, and their doubts were magnified after researchers in September acknowledged a major mistake in how they collect the data.

A faulty cable on a research trawler that collects fish samples set the net askew and threw two years of research into doubt. Scientists scrambled to determine the error’s impact, though their damage to their credibility was already done.

But the science of counting fish involves much more than two years of survey trawls.

The complexities of fishery science are tough to grasp, and misunderstandings between fishermen and researchers are exacerbated by their fundamentally different goals at sea: Fishermen cast nets intending to catch as many fish as possible; scientists just want the net to perform consistently, even if it’s not particularly efficient, so various population formulas will work.

“We’re starting out with two different purposes, and that causes misperceptions,” said Steve Cadrin a fisheries research biologist with the National Marine Fisheries Service.

Sam Novello, a lifelong Gloucester fisherman, acknowledges he doesn’t understand the intricacies of fishery science. What’s clear to him is scientists who can’t properly set their nets are making rules that make or break him.

“I just want a true picture. You get a net that works, that catches fish, you get a true picture,” said Novello, who was among the six fishermen who went to sea with scientists to investigate the effects of the faulty net.

Estimating fish stock health all starts with understanding the past, which is the foundation for predictions about future fish populations. To do this, scientists combine the results of ongoing survey trawls with 40 years of historical population data on each stock.

Since the natural life span of a groundfish is 20 years, at most, scientists have been able to determine what past populations have been, essentially by counting the ones that have been killed or died over a species’ expected life span. Certain information is vital to do this.

The age of a fish when it’s caught is provided by nature. With cod, for instance, researchers can tell the age by counting the rings on an ear bone, which reveal the age in the same way as rings on a tree trunk.

The total fish in each species killed by fishermen is detailed in mandatory catch reports that seafood distributors make to the government.

The estimated numbers of certain species that die of natural causes – disease, old age, predators – is determined using long-established biological studies.

Combining this information gives researchers an estimated fish count. For instance, scientists know how many haddock were born in 1968 because they can add the catch numbers of the class of haddock throughout its estimated 20-year life span to 1988, then figure in the mortality rate.