Anyone
who has fished at one of the many offshore oil and gas platforms
in the Gulf has seen the thousands of blue runners, or as
we commonly call them, hardtails, under and around almost
every platform. Research done at LSU indicates that 10,000
to 30,000 fish may be associated with a platform and that
up to 94 percent of them can be blue runners.
These
12-to 18-inch fish, members of the same family as amberjack
and pompano, are seldom targeted by anglers, except to catch
for use as bait. Their large numbers would certainly indicate
that they are important in the food web however, eating smaller
organisms and themselves serving as food for large predators
such as barracudas, groupers, cobia, other jacks and even
open-water fish such as king mackerel, billfish and tunas
that often visit platforms.
Very little
research has been done on this common fish. Recently, however,
scientists at LSU conducted research on the diet of blue runners
in an attempt to understand the food web associated with platforms
and whether platforms actually produce more fish than open
water/natural bottoms or whether they simply attract fish
from those areas.
The researchers
sampled blue runners from two platforms, Grand Isle 94B (GI
94B) in 208 feet of water and Main Pass 259A, (MP 259A) in
429 feet of water. GI 94B was sampled in June, July and August,
1999 and MP 259A in June, July and September 1999. At each
location, blue runners were caught with rods and reels on
artificial lures and their stomachs removed and preserved.
The food items were later removed, examined with a microscope
and identified.
The researchers
found that blue runners, especially those under 14-inches
long, fed very heavily on zooplankton, rather than on the
plant and animal growth on the platforms. Zooplankters include
tiny free-floating animals and the larvae of bigger marine
life. As blue runners grew larger, fish became a higher percentage
of their diet, but they never stopped eating zooplankton.
Blue runners seemed to feed moderately all day under and near
the platforms, but binge before daylight, between 3 a.m. and
7 a.m. The biologists' theory was that the floodlights on
the platforms allowed these sight-feeding fish to see well
enough to feed at night.
Since
the zooplankton in their diets was likely carried by currents
to the platforms, rather than produced at the platforms, it
would be logical to assume that the platforms do not “produce”
fish. However, platforms may still play an important role.
Previous research has shown that ocean current speeds can
be reduced by 20 percent or more immediately behind a platform
and that the platform legs and casings can break the current
enough to form eddies behind them. The reduced currents and
the eddies can, to a degree, concentrate whatever the currents
carry, such as zooplankton. Also some zooplankters which have
weak swimming ability tend to move towards lights, which would
also tend to concentrate them under lighted platforms at night.
These
factors may provide blue runners with increased concentrations
of food and the ability to feed around the clock. Such conditions
may explain how the large numbers of blue runners and their
predators can be sustained in the waters around platforms.
Questions on whether platforms, and the increasingly popular
artificial reefs, produce more fish to catch or whether they
simply concentrate fish, making them easier to overfish, are
important fisheries management questions.
Download:
hardtails.pdf
(365KB)
