Cod at Cashes Ledge. Copyright Brian Skerry.

Where the Girls Are

In 1895, a Massachusetts fisherman had what must have been one of the better days of his career when he reeled in his line and found a massive 6-foot long cod that weighed over 210 pounds. While this fish was unusual, legends from those days say three foot long fish were so plentiful it was hard to row a boat across a harbor without running in to one.

Since then, large cod have become much less common in New England. Minimum size limits have meant that fishermen target larger fish, driving down the average size of the population and disproportionately removing the oldest fish from the sea. Today, it’s more typical to find cod measuring about 20 inches, and most cod caught are just 3-7 years old. What’s more, some research suggests that fish may actually be evolving to mature at an earlier age and smaller size in response to these fishing pressures.

In addition to fewer big catches for anglers, this decrease in the average size of cod could cause serious trouble for the population of these fish. Scientists have found that the fecundity of fish—or the number of eggs a female produces each spawning season—increases dramatically with age. The scientists have called this the BOFFFF (Big Old Fat Fecund Female Fish) hypothesis.

What the BOFFFF hypothesis means is that populations of smaller fish will have a harder time rebuilding, because these small fish produce just a fraction of the eggs that Big Fat Females would. The eggs produced by smaller fish also have a lower chance of survival to adulthood.

This graph, created by scientists at the Gulf of Maine Research Institute, shows recent research on the number of eggs produced by female cod at various ages. As you can see, the number of eggs produced rises quickly with age, and the oldest females can produce well over 10 million eggs, compared to just 1-5 million for younger fish.

This graph from the Gulf of Maine Research Institute shows cod egg production, in thousands of eggs, as a function of the age of the cod.  The red lines show two different estimates for Georges Bank cod, and the blue line shows Gulf of Maine cod.
This graph from the Gulf of Maine Research Institute shows cod egg production, in thousands of eggs, as a function of the age of the cod. The red lines show two different estimates for Georges Bank cod, and the blue line shows Gulf of Maine cod.

 

The lack of Big Fat Females means that struggling New England cod populations have to depend on the smaller, younger fish to rebuild their numbers—who just can’t match the productivity of their big sisters. To help cod populations recover, the BOFFFFs will have to be protected and restored.

There are a couple of options that could help bring back the Big Fat Females. First, fisheries could implement a maximum catch size for cod. But this would require fishermen to throw back the big fish, and many cod don’t survive being returned to the water once they’ve been caught.

Instead, scientists agree that the best way to bring back the BOFFFFs is to protect their habitat. Marine protected areas—like Cashes Ledge in the Gulf of Maine—provide refuge for cod of all sizes and ages, harboring populations of juvenile cod alongside spawning adults and large females. Protected areas targeted towards aggregations of older fish are the most effective way to ensure that female fish survive to become BOFFFFs and continue to produce the next generation of New England cod.