Fishermen
that regularly fish at any of the 4,200 oil and gas production
platforms in the Gulf of Mexico know how good the fishing
can be. Most rig fishermen also know that very often more
fish are found on one side of the platform than another, but
determining a pattern can be difficult.
Until
recently, very little research has been done on how fish are
distributed around these platforms. A project conducted by
Louisiana State University scientists at Gulf platforms has
now produced interesting results. Rather than use nets or
divers, which have problems with gear selectivity, visibility
or diver avoidance, the researchers used sonar transmitters,
which were mounted to send their signals both upward and downward
around the edges of the platforms used in the study. The sonar
signals were able to pick up fish 1.2 inches long and longer,
count the fish and determine the size of the fish.
Sampling
was done at three platforms over a period of several years
at South Timbalier 54 (72 feet deep), Grand Isle 94 (195 feet
deep) and Green Canyon 18 (712 feet deep). The deepest platform
was located off of the gently sloping bottom of the continental
shelf, on the more steeply inclined continental slope.
At South
Timbalier 54, the highest concentrations of fish by far were
found on the north side of the platform. At Grand Isle 94,
highest concentrations were on the north side, closely followed
by the west side. The least fish, by a wide margin, were on
the east side. At Green Canyon 18, far fewer fish were found
than at the two shallower sites, however the most fish were
found on the west side with good numbers also on the north
side of the platform. One possible reason for the high densities
of fish on the north sides of platforms may be the additional
structure on that side due to the location of the well bays.
Distribution
of fish by depth showed even more variation. At the shallowest
platform, South Timbalier 54, fish concentrations were highest
in the upper 16 feet of water. Fish numbers dropped until
32 feet deep, and then increased steadily from that point
to the bottom at 72 feet.
The species
of fish present were identified using a small remote controlled
submarine carrying a video transmitter. It should be noted
that at times, fish identification was difficult because of
the presence of a murky layer of water, most often in the
bottom 15-30 feet of water. At South Timbalier 54, the fish
population was made up of the following species: spadefish
(34%), blue runner (21%), sheepshead (17%), red snapper (11%),
bluefish (2%) and a combination of 13 other species (15%).
The
mid-depth platform, Grand Isle 94, held the most fish, and
had a distribution pattern similar to the shallow platform.
Large concentrations of fish were found near the surface.
By the depth of 15-16 feet, numbers dropped off by about half
and held steady to about 52 feet deep. Then fish numbers increased
rapidly to their highest numbers about 30 feet off of the
bottom. While dropping slightly from there to the bottom (195
ft), fish concentrations were still quite high.
At Grand
Isle 94, by far the most common fish found during the study
period was the blue runner (88%), followed by red snapper
(3%), horse-eye jack (3%), mangrove snapper (1%), amberjack
(1%), barracuda (1%) and 18 other species combined (3%).
The platform
standing in the deepest water, Green Canyon 18, held the least
fish. At this platform, the largest concentration of fish
was at the surface. Fish density showed a spectacular drop
down to a depth of 49 feet deep. From there, fish numbers
were very low, but fairly consistent until 325 feet deep.
Below this depth, down to the bottom at 712 feet, almost no
fish were found. In fact, fish numbers were actually lower
than in the open waters away from platforms up on the continental
shelf.
The break-down
on fish species at Green Canyon 18 was as follows: creole
fish (50%), blue runners (21%), Bermuda chubs (6%), almaco
jacks (6%), amberjacks (4%), horse-eye jacks (4%), barracuda
(2%) and 10 other species (7%). The researchers concluded
that the low fish densities at Green Canyon 18 were probably
due to the location being distant from the highly productive
waters discharged from the Mississippi River. Green Canyon
18’s waters were similar to open ocean waters with low
productivity.
The researchers
made one general observation that applied to all three platforms.
Unlike on natural reefs, fish did not change depths with time
of day. They also noted that at the deepest platform, fish
numbers did not change with the time of the year. At the two
shallower platforms, fish densities were highest in fall and
winter.
One final
finding from this research applies to placing artificial reefs,
which off of Louisiana are built of oil and gas platforms
toppled onto their side. A toppled platform placed in a location
that holds few fish near the bottom, such as near Green Canyon
18, will likely be a very poor artificial reef. In such waters,
the highest point of the platform reef extending up from the
bottom will determine its effectiveness as a fish attractor.
Download:
fishplatforms.pdf
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