Red snapper
are easily the most popular offshore bottomfish in the northern
Gulf of Mexico. Because of their popularity, they became overfished.
Monitoring the restoration of red snapper populations demands
good data. Some of the most important data is on age and growth
of the fish. LSU scientists have recently conducted the largest
ever age and growth study on the species.
Researchers
from LSU and the Louisiana Department of Wildlife and Fisheries
sampled 3,791 red snappers from commercial and recreational
catches landed between the Mississippi River delta and Galveston,
Texas, although most of the fish were from landings at Grand
Isle and Port Fourchon, Louisiana. The fish were sampled over
an eight-year period. Each fish was weighed and measured,
sex was determined where possible, and both otoliths (earbones)
were removed. Each otolith was weighed, embedded in an epoxy
resin, and then cut in a thin cross-section with a special
saw. Age was determined by reading the annuli (rings) in the
otolith. Each otolith’s annuli were counted separately
by two readers. When they didn’t agree, each counted
the annuli again. With this method, agreement was reached
on 99.3% of the otoliths.
Of the
fish for which sex could be determined, 1,438 were male and
1,542 were female. Males ranged in size from 9.8 - 37.8 inches
and 0.4 - 30.1 pounds. Females’ sizes were 9.7 - 41.6
inches and 0.4 - 50.1 pounds. Not until age 5 and 16.8 - 17.6
inches in length were 100 percent of the fish mature.
The vast
majority of the fish examined were 2 to 5 years old and only
1.2 percent were older than 15 years old. However, between
the ages of 16 and 52, the only ages that were not represented
in the study were 24, 28, 31, 34, 39, 40, 42-46, 49 and 50.
Red snapper age 0 (under one year old) and age 1 were not
available in any numbers for the study because the minimum
size limit prevented their landing. The scientists speculated
the large number of 2-5 year old fish in the sample might
be partly due to the preference of commercial fishermen and
wholesalers for smaller “plate-size” fish.
The biology
of the fish may also have played a role. Age 1 and younger
red snapper, besides being mostly undersized, tend to be found
on open bottoms where few people fish, rather than on reefs.
After age 1 they move to reef type habitats, including offshore
platforms. After age 6, they move away from structures and
again spend more time on less-fished open-bottom areas.
The study
results showed that male and female red snapper grow rapidly
and at about the same rate until about 8 years old and about
28 inches in length. Then two things happen. The growth rate
for both genders begins to slow down, 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 and females
average 38 inches in length. Very little growth occurs after
age 25, even out to over 50 years of age.
These
are average numbers. The researchers pointed out that some
individual fish grow much faster than others do. For example,
at age 8, one fish was 17 inches long and another was 35 inches
in length. A 16-inch fish could be anywhere between 2 and
7 years old, a 24-inch fish could be 3 to 9 years old, and
a 32-inch could be from 5 to over 35 years old.
The two
oldest fish in the study were 52.6 and 51.7 years old, but
were only 34 inches and 34.5 inches long and weighed a modest
17.3 and 20.2 pounds. The biologists also aged the IGFA world
record red snapper, caught by Doc Kennedy off Grand Isle,
Louisiana, in 1996. Given its huge size of 50 lb, 4 oz, a
person would expect it to be an ancient fish. However, after
it was aged, the fish proved to be slightly less than 20 years
old.
This
was easily the largest and most detailed study done on red
snapper age and growth. However, the scientists still expressed
concern that more data on older, larger fish is needed.
Download:
redsnappergrowth.pdf
(440KB)
