Red snappers are easily the most-studied fish in the Gulf of Mexico, so a lot is known about their biology.

They are a surprisingly long-lived fish, growing to more than 50 years old. While the average size for red snappers increases with age, it is very difficult to estimate the age of an individual fish based on its size.

The IGFA world-record red snapper, caught by Doc Kennedy off of Grand Isle in 1996, was 50 pounds, 4 ounces. From its huge size, a person would expect the fish to be an ancient fish; yet, when the fish was aged by scientists, it was found to be slightly less than 20 years old.

Both sexes grow relatively rapidly and at the same rate until about 8 years old and 28 inches in length.

Then two things happen: The growth rate for both genders begins to slow, and the growth rate becomes slower for males than for females. 

The growth gap continues to widen until about age 25, and then it stabilizes. At that age, males average less than 36 inches in length and females average 38 inches.

Very little growth occurs after age 25, even out to over 50 years of age.

These are average numbers. Some individual fish grow much faster than others.

In one Louisiana study, one 8-year-old red snapper was 17 inches long and another was 35 inches long. In that study, a 16-inch fish could be anywhere from 2 to 7 years old, a 24-inch fish could be 3 to 9 years old and a 32-inch fish could be from 5 to more than 35 years old. 

Also, in that study the two oldest fish were 52.6 and 51.7 years — but were only 34 and 34.5 inches in length, and weighed a modest 17.3 and 20.2 pounds.

That makes Kennedy’s fish interesting.

On average, red snapper become sexually mature at age 5 (although some as young as 2 will spawn) and right around 17 inches long.

Like many offshore species, red snappers are “batch spawners.” This means not all of the eggs in their ovaries are laid at one time; some eggs mature for one spawning, some more for the next spawning and so forth through the spawning season.

Red snappers spawn 21 to 35 times per year at four- to six-day intervals. Spawning begins in May and goes into October, with a peak in June, July and August.

Spawning seems to take place at early evening or night. 

Hours before a spawn, the mature eggs in the ovary that will be laid will hydrate. This simply means those eggs swell with water to several times their original size.

“Batch fecundity” is a term biologists use to describe the number of eggs laid per spawn. It can be very low for small fish — as few as 13 hydrated eggs have been counted in a 4-year-old fish.

But the number of eggs laid per batch increases at a very rapid rate as red snappers become older and larger. A 12-year-old fish was calculated to produce of 1.7 million eggs per spawn. For the whole season, such a fish could conceivably produce 60 million eggs. 

One of the most interesting “secrets” about red snapper is what they eat. The species is a reef fish, meaning it is attracted to complex hard bottoms.

Most anglers, logically enough, assume red snappers hang around reefs because that is where their food source is.

Wrong!

Scientists have found that red snappers feed on animals not associated with reefs, preferring instead to feed on animals found on soft bottoms.

A research project done with fish taken off the Alabama coast produced pretty typical results. Researchers divided identifiable food items from the snapper stomachs into the general categories of fish, stomatopods (sea lice or king shrimp), crabs, shrimp and other. The other category was mostly made up of tiny, pinhead-size zooplankters and bottom worms.

The most-commonly eaten fish were pipefish, snake eels, sea robins, pinfish, striped anchovies, cusk eels and pigfish. Most of these are not species that one would expect to find on a reef. 

By volume, over the whole year of the study, fish was the largest category at nearly 40 percent, followed by stomatopods at close to 30 percent and then crabs (six species) at 15 percent.

Shrimp made up only 1 or 2 percent by volume or number.

The researchers also found definite shifts by season. Fish were always important, but crabs were the most-common category in spring, and stomatopods were most important in the winter. A lot of stomatopods were also eaten in the summer, but almost none in the spring or fall. 

Diet changed with fish size. As red snappers grew larger, they ate more fish. Also, the largest snappers — those 24 inches long and longer — ate far more stomatopods than did smaller fish.

The scientists also found the day and night diets of red snappers, at least in the summer, were very different. Fish increased from 22 percent in the day to 45 percent at night. Stomatopods dropped from 36 percent in the day to 5 percent at night. 

No rock shrimp were found in the day, but they made up 19 percent of the nighttime diet. Crabs stayed about the same, at 25 percent in the day and 18 percent at night. 

The researchers also noted a trend in night feeding by measuring the fullness of the fishes’ stomachs. They were most empty from 7 p.m. until 3 a.m.

The fish began feeding heavily at 3 a.m. and quickly reached a peak at 4 a.m., although heavy feeding continued until 6 a.m.