> NOAA Technical Memorandum NMFS-SEFSC-530

![](media/image1.png){width="1.25in" height="1.2395833333333333in"}

THE USE OF SIDESCAN SONARS, MAGNETOMETER, AND VIDEO CAMERAS IN DETECTING
ILLEGAL ARTIFICIAL REEFS IN WATERS

OF THE FLORIDA KEYS

BY

REX C. HERRON

![](media/image2.jpeg){width="4.996527777777778in" height="2.96875in"}

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Southeast Fisheries Science Center

Mississippi Laboratories

Bldg. 1103, Rm. 218

Stennis Space Center, MS 39529

May 2005

NOAA Technical Memorandum NMFS-SEFSC-530

![](media/image3.png){width="1.3in" height="1.3in"}

THE USE OF SIDESCAN SONARS, MAGNETOMETER, AND VIDEO CAMERAS IN DETECTING
ILLEGAL ARTIFICIAL REEFS IN

WATERS OF THE FLORIDA KEYS

BY

REX C. HERRON

NOAA Fisheries

Bldg. 1103, Rm. 218

Stennis Space Center, MS 39529

U.S. DEPARTMENT OF COMMERCE

Carlos Gutierrez, Secretary

National Oceanic and Atmospheric Administration

Conrad C. Lautenbacher, Jr., Under Secretary for Oceans and Atmosphere

National Marine Fisheries Service

William T. Hogarth, Assistant Administrator for Fisheries

May, 2005

This Technical Memorandum series is used for documentation and timely
communication of preliminary results, interim reports, or similar
special-purpose information. Although the memoranda are not subject to
complete formal review, editorial control, or detailed editing, they are
expected to reflect sound professional work.

NOTICE

The National Marine Fisheries Service (NMFS) does not approve, recommend
or endorse any proprietary product or material mentioned in this
publication. No reference shall be made to NMFS or to this publication
furnished by NMFS, in any advertising or sales promotion which would
imply that NMFS approves, recommends, or endorses any proprietary
product or proprietary material mentioned herein which as its purpose
any intent to cause directly or indirectly the advertised product to be
used or purchased because of this NMFS publication.

This report should be cited as follows:

Herron, Rex C. 2005. The use of sidescan sonars, magnetometer, and video
cameras in detecting illegal artificial reefs in waters of the Florida
Keys. NOAA Technical Memorandum NMFS-SEFSC-530, 23 p.

This report will appear on the SEFSC web site at URL:
<http://www.sefsc.noaa.gov/>

Copies may be obtained by writing:

Rex C. Herron or National Technical Information Service

NOAA Fisheries 5258 Port Royal Road, Springfield VA 22161

Bldg. 1103, Rm. 218 800-552-6847 or 703-605-6000

Stennis Space Center, MS 39529 <http://www.ntis.gov/numbers.htm>

Copies may be obtained by email from:

<rex.c.herron@noaa.gov>

ii

**Abstract**

Four instruments were assessed in 2004 to determine their suitability
for detecting and mapping illegally placed artificial reefs in and near
the Florida Keys National Marine Sanctuary (FKNMS). The instruments
were: 1) Datasonics SIS-1500 Chirp Side Scan Sonar System, 2) Geometrics
G-882 Marine Magnetometer, 3) Imagenex\'s Sportscan Side Scan Sonar, and
two locally manufactured video cameras attached to a towed body. The 42
foot RV HST-1 was deployed from Pascagoula to Key West as a platform to
test the Equipment. The project was conducted from mid-June through the
end of June in 2004. All instruments were towed simultaneously along
five transects in a study area encompassing approximately 602 km². All
transects were north of the keys between Key West and Big Pine Key. Two
transects generally followed the 9.1 m (30 ft) and 12.2 m (40 ft)
contours. Three transects ran from shallow water and continued
perpendicular to bottom contours to a maximum depth of 14 m (46 ft). A
large number of manmade structures were detected by all instruments
along 4 of the 5 transects. Divers checked approximately 15 sites to
verify targets. Based on the number of suspected illegal reefs detected
with sidescan sonars, the estimated number of illegal reefs within the
study area was 1463 (Illegal structures may number in the thousands when
the study area is expanded).

Given the complexity and expense of the chirper and magnetometer (along
with the large number of returns from the magnetometer), they are not
recommended for further use for detecting and mapping illegal artificial
reefs. The smaller and less expensive Sportscan sonar, which performed
nearly as well as the chirper, and the video cameras are recommended for
further illegal reef studies (Scuba divers will also be required to
verify some of the targets). These instruments have proven to be
excellent tools for detecting illegal reefs in the addition to other
underwater features such as coral heads.

iii

> **Introduction**

The Caribbean spiny lobster (*Panulirus argus*), also commonly referred
to as the Florida spiny lobster, is highly exploited by fishermen in its
shallow shelf water habitat off the southern U.S., Bermuda, and the
Caribbean (Harper and Muller 2001). Spiny lobster larvae spend an
estimated 6-9 months in the planktonic stage (Herrnkind et al. 1988).
Swimming postlarvae move inshore and settle in seagrasses, thick
macroalgae, and other bottom habitats where they transform into
bottom-dwelling juveniles. At about 15-20 mm carapace length, the
juveniles take up residence in crevices, dens, and other sheltered
places. They begin to roam several hundred meters at night at about
1-1.5 years of age, but still seek shelter during daylight hours. Adding
artificial shelters enhances juvenile recruitment at these sites when
postlarvae are plentiful. As adults, lobsters begin seasonal migrations
from shallow to deeper waters (Herrnkind 1985) possibly to escape the
shallower waters that become disturbed by weather conditions (e.g.,
cooler temperatures, turbulence, etc.). These migrations are regional in
nature and do not appear to be related to reproduction.

The spiny lobster is exploited primarily from Palm Beach, Florida to the
Dry Tortugas and into the Gulf of Mexico. It is the second ranked
economically most important commercial fishery in Florida after pink
shrimp (Harper and Muller 2001). Commercial landings in 1999 were 4.9
million pounds (Muller et al. 1999), and the number of recreational
lobsters harvested was 1,190,000 individuals. Their ex-vessel value was
over \$22 million U.S. dollars in 1998.

Materials such as concrete blocks have often been used to create
artificial habitat for spiny lobster. Many types of materials, such as
concrete blocks, car hoods, etc., have been placed in shallow marine
waters illegally by fishermen to attract lobsters where they are easy to
capture. With global positioning satellite systems (GPS), these
materials can be easily relocated by fishermen. The Florida Keys
National Marine Sanctuary (FKNMS) has a program to find and remove these
illegal habitats (Weekly Situation Report, FKNMS, July 31, 2001).
Between July 16 and July 31, 2001, about 150 structures were removed at
69 waypoints within the Sanctuary. In Oct. 2000, two men were fined
\$2,000 for dumping materials meant to attract lobsters (U.S. Department
of Commerce Press Release, Oct. 2000), and in July of 2001, two men were
assessed a combined civil penalty of \$75,000 (U.S. Department of
Commerce Press Release, July 2001) for creating these illegal habitats (
commonly referred to as 'lobster condos').

Possibly thousands of these artificial reefs have been installed on the
ocean floor in the Florida Keys to attract and exploit spiny lobster.
Most of these structures are believed to be in the waters of the Gulf of
Mexico off the Lower Keys. Dumping trash, and other materials, to create
'lobster condos' violates FKNMS regulations that prohibit depositing
materials into the Sanctuary without a permit. Dumping materials in
excess of 5000 pounds or 100 cubic feet or any materials for commercial
purposes is illegal under Florida law. It is also illegal to transport
materials for artificial reef-construction without a permit. These
structures may have impacted the natural spawning migration of the
Florida spiny lobster, and there is very limited scientific information
on the extent and impact of these artificial structures on migration
patterns, reproductive behavior, catch-rates of lobsters. These 'condos'
cause lobsters to congregate in large numbers in known locations where
they are subject to intense recreational and commercial fishing
pressures. It should also be noted that many of these 'lobster condos'
are made with industrial and other materials (e.g., refrigerators, oil
barrels, car hoods, dumpsters, etc.) that contaminate and impact the
coral reef environment and cause habitat (seagrass, coral) damage,
particularly during storms. Additionally, the allocation of lobster
catch amongst fishers is being impacted (i.e. those using illegal
'lobster condos' are much more efficient). Dumping these structures may
be having an impact on the migration patterns, as well as the population
size, of Florida Spiny Lobsters. The first step in addressing the
'lobster condo' problem is the development of a method for accurately
detecting and mapping these structures so that accurate density
estimates of these illegal reefs can be obtained.

The objective of this project was to evaluate the effectiveness of a
towed array of optical, acoustic, and magnetic sensors for detecting and
mapping illegal artificial reefs inside and near the FKNMS. Results from
this project will assist resource managers and law enforcement personnel
in the Florida Keys to remove existing illegal structures, and will
support follow-up studies on 1) spiny lobster population impact, and 2)
the socio-economic impact on the spiny lobster fishery. Results will
also provide support for compliance and educational and outreach
campaigns.

**Study Area**

The study area encompassed a rectangular area of about 602 km² (176
nmi²) in relatively shallow water north of the Florida Keys (Figure 1).
This area was selected based on information provided by the Florida Fish
and Wildlife Conservation Commission and contained known and suspected
locations of illegal artificial reefs. The southwest corner of the study
area began at Calda Bank (24° 36.82′ N, 81° 50.41′ W) and extended
northeasterly for approximately 42.1 km (22.8 nmi) to the southeast
corner (24° 48.49′ N, 81° 50.41′ W) at Content Keys. The east and west
sides of the area extended toward the northwest for approximately 14.3
km (7.7 nmi)(northwest corner at 24° 43.36′ N, 81° 54.76′ W and
northeast corner at 24° 55.21′ N, 81° 33.27′ W) into water approximately
15.3 km deep (about 50 ft). Most of the study area lies within the
boundaries of the Florida Keys National Marine Sanctuary (FKNMS). The
study area also overlaps the Great White Heron National Wildlife Refuge.

Five transects were in the study area to evaluate the instruments. Two
of the transects (transects 2 and 5) were selected because they were
generally parallel to bottom contours and because of known or suspected
illegal reefs in the area (Figure 2). Three transects (transects 1, 3,
and 4) were selected because little was known about the presence of
illegal reefs in these areas. These three transects were oriented
perpendicular to bottom contours. This allowed the instruments to be
tested in shallow to deeper water to assess their performance at various
depths. Additionally, a site was selected and materials were placed on
the bottom to test instruments. The test materials were later removed by
divers. Another site where a known illegal artificial reef existed was
also examined to evaluate the instruments.

![](media/image4.jpeg){width="5.99375in" height="3.810416666666667in"}

Figure 1. Location of study area in waters north of the Florida Keys.

![](media/image5.jpeg){width="5.99375in" height="4.102777777777778in"}

Figure 2. Locations of transects 1 through 5.

**Methods and Materials**

Four instruments were tested during this project to assess their
suitability for detecting illegally placed artificial reefs that attract
lobsters for easy harvesting. Datasonics' SIS-1500 chirp sidescan sonar
system was tested (the use of trade names does not imply endorsement by
the National Marine Fisheries Service). This chirp digital system
operates in the 200 kHz band and provides high resolution imagery.
Signals were processed and displayed by a Windows NT workstation
provided with the instrument. This instrument was rented from Ashtead
Technology Rentals at a cost of \$300 per day, assessed while the
instrument was in use and for the time the instrument was being shipped
to and from the site, and shipping costs (estimated cost to purchase
this or a similar system is approximately \$65,000). The SIS-1500 was
deployed directly behind the vessel and slightly off center to
starboard. The instrument was set to image 50 m per side for an
effective swath width of 100 m.

The second instrument evaluated during the study was Geometrics' G-882
Marine Magnetometer. The G-882 is a cesium vapor high performance system
that can output data to any IBM personal computer. A workstation was
provided with the instrument to process and display the signals. This
system was rented from Ashtead Technology Rentals at a cost of \$200 per
day plus in transit rental time and shipping costs. The magnetometer was
deployed slightly off center of the vessel's trackline and opposite the
SIS-1500 sonar.

A third instrument assessed was Imagenex Technology Corporation's Model
881 Sportscan single or dual frequency digital sidescan sonar (maximum
depth rating of 30 m or about 100 ft). This system was purchased for
approximately \$7000 along with the software necessary to operate it.
The signal is processed with proprietary Windows software. The system
allows an operating frequency setting of high (330/800 kHz) or low (330
kHz). When high frequency is selected, the overall range is limited to
about 60 m (200 ft) per side. The instrument was connected to an onboard
laptop computer for signal processing and displaying. The Sportscan
sonar was deployed off the portside of the vessel near midship. It was
set to image 25 m per side maximum so the swath width was 50 m. Although
the sonar did not image 90 degrees downward, the video cameras were used
to detect illegal reefs directly beneath the boat so the effective swath
width was considered to be 50 m.

Two Bowtech·Ros Model BP-L3C-HR video cameras were attached to a
tow-body and towed near the bottom for use in verifying targets. These
cameras were designed for water depths as deep as 4000 m and cost
approximately \$1500 each. The video was displayed on shipboard monitors
and recorded on video cassettes. The tow-body with attached cameras was
deployed off the starboard side of the vessel near midship.

The 12.8 m (42 ft) RV HST-1 was deployed from Pascagoula, Mississippi to
Key West, Florida as a platform to test the equipment. All instruments
were connected to the shipboard GPS system. GPS coordinates were
referenced to the location of the ship and not the location of the towed
instruments.

A site was chosen to test and calibrate the instruments with known
materials. The site was selected at the western edge of the study area
near the outlet of Northwest Channel in 9.1 m (30 ft) of water with
little coral and sea grasses on the bottom (24° 39.998′ N, 81° 52.523′
W). Materials were placed on the bottom by divers for instrument testing
and were removed at the conclusion of the field study. A 1.2 m (4 ft) by
1.5 m (5 ft) sheet of concrete "Wonder Board" was placed on four 23 cm
(9 in) concrete blocks, one at each corner, and four more concrete
blocks were placed on top (one per corner) to hold the board in place
(Figure 3). In addition, two metal pipes about 2.4 m by 10.2 cm (8 ft by
4 in) were placed on the bottom about 0.6 m (2 ft) apart, and parallel
to each other, and a 0.6 m (2 ft) by 0.9 m (3 ft) steel plate 9.5 mm
(0.375 in) thick was placed on top of the metal pipes and perpendicular
to them. The three instruments, SIS-1500 sonar, Sportscan sonar, and
magnetometer were towed within a few meters of the test material from
different directions to assess their detection capabilities.

Several locations were provided by the Florida Fish and Wildlife
Conservation Commission where illegal reefs had been found and
destroyed, but the rubble had not been removed. Two of these sites,
located at approximately 24° 44.208 N, 81° 46.877′ W, were subsequently
examined with the test instruments.

Transect 1 began in shallow water approximately 3 m deep (about 10 ft)
just north of the Lower Harbor Keys (24° 40.205′ N, 81° 44.630′ W) and
continued perpendicular to bottom contours for a distance of 12 km
(6.483 nmi) in a northeasterly direction and ending in approximately 14
m (46 ft) of water (24° 46.026′ N, 81° 48.350′ W). Transect 1 was imaged
primarily by the SIS-1500, so the total area imaged was 1.2 km² (Table
1).

Transect 2 roughly followed the 9.1 m (30 ft) bottom contour
(approximately 8.2 to 10 m depths (27-33 ft)) beginning at a point
roughly north northwest of Key West (24° 40.144′ N, 81° 52.127′ W) and
ending roughly north northwest of Content Keys (24° 51.117′ N, 81°
31.427′ W). The 9.1 m contour was not followed exactly. Instead the
vessel followed a straight line course from the starting point to the
end of the transect. Total distance of transect 2 was approximately 40.7
km (22 nmi). Transect 2 was imaged primarily with the SIS-1500 so that
the total area imaged was about 4.07 km². Vessel speed varied from about
1.6 knots to 3.5 knots. As many as three sea anchors were deployed
behind the vessel to try to maintain vessel speed at under 2 knots.

Transect 3 began near Sawyer Key (24° 45.914′ N, 81° 35.050′ W) in about
6 m (20 ft) of water and continued northwesterly, perpendicular to
bottom contours, for about 7.41 km (4 nmi) and ended at a depth of about
10.7 m (35 ft) (24° 49.350′ N, 81° 37.199′ W). Vessel speed was about
3.5 knots. A tail wind reduced the effectiveness of the sea anchors to
slow the vessel to an optimal speed for sidescan sonar imaging. The
Datasonics SIS-1500 stopped functioning so the transect was completed
with the Sportscan sonar, magnetometer, and video cameras. The total
area imaged was about 0.37 km².

Transect 4 began in water approximately 12.2 m (40 ft) deep (24° 47.083′
N, 81° 44.004′ W) north of Mud Keys and continued southeasterly for
approximately 10.73 km (5.79 nmi) ending in about 4.6 m (15 ft) of water
in the Marvin Keys Channel (24° 42.701′ N, 81° 40.144′ W). Only the
Sportscan sonar, magnetometer, and video cameras were used on this
transect. The total area imaged was about 0.54 km².

![](media/image6.jpeg){width="6.0in" height="4.5in"}

Figure 3. Image of temporary experimental reef consisting of a sheet of
cement

board supported by concrete blocks with additional concrete blocks

on top to anchor it in place.

**Table 1. Dimensions of transects and number of sidescan sonar targets
detected**

**during each transect.**

Transect Length(km) Width (m) Area (km²) Targets Targets/km²

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

1 12.00 100 1.20 2 1.7

2 40.70 100 4.07 13 3.2

3 7.41 50 0.37 0 0.0

4 10.73 50 0.54 3 5.6

5 27.80 50 1.39 3 2.2

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Transect 5 began in 12.2 m (40 ft) of water approximately 15.3 km (8.3
nmi) north of Barracuda Keys (24° 51.412′ N, 81° 37.265′ W). The
transect roughly followed the 12.2 m (40 ft) bottom contour (actual
depth varied from about 11.3 m (37 ft) to 14.6 m (48 ft)) in a
southeasterly direction for about 27.8 km (15 nmi) and ended at a point
approximately 19.4 km (10.5 nmi) north of Mule Key at 24° 44.059′ N, 81°
52.020′ W. The Model 881 Sportscan sidescan sonar and video cameras were
the only instruments tested along transect 5 because the Datasonics
chirp sidescan sonar system was inoperable and the magnetometer detected
so many false and real returns that its usefulness was limited. The
total area imaged was about 1.39 km².

**Results**

The instruments did not perform as well as expected at the test site.
The Sportscan sonar imaged the concrete blocks as very small targets,
but the metal plate did not image well (Figure 4). The metal pipes and
associated metal plate were not detected at all. The SIS-1500 sonar did
not image the test materials. The magnetometer gave a signal at the test
site, but it continued returning signals away from the test site.
Initial results from the test site suggested that metal plates might be
difficult to image using sidescan sonar, but later results proved this
assumption to be false. The sites where illegal artificial reefs had
been located by the Florida Fish and Wildlife Conservation Commission
were subsequently imaged by both sonars and video cameras, with
excellent results from the Datasonics SIS-1500, Sportscan sonar (Figure
5), and the video cameras (Figure 6).

Two clusters of illegal artificial reefs were imaged along transect 1,
but the sites were not verified by divers or in the video images (Table
2, Figure 7). The Sportscan sidescan sonar functioned well, but was
towed too far above bottom to image well. The magnetometer functioned
well, but the numerous returns made interpretation difficult.

Transect 2 was a target rich environment. A total of 13 clusters of
suspected illegal artificial reefs were imaged by the Datasonics
SIS-1500 chirp sidescan sonar system set at 15 pulses per second (Figure
8). The Imagenex Model 881 Sportscan was towed too near the surface to
image well because of the limitation of the 30 m tow cable, vessel
speed, and to avoid interference with the SIS-1500 instrument. A 60 m
cable was ordered to replace the 30 m cable so the Sportscan could be
towed at 1-2 m above bottom. Site 12 was verified with video cameras as
a corrugated metal sheet, and divers further verified the target with a
digital camera (Figure 9). A second target was verified by divers in the
water who took digital images of the illegal reef (Figure 10). The reef
consisted of two adjacent rows of iron or steel plates with 4 plates per
row. The plates were approximately 1.2 m (4 ft) per side, 6.35 mm (0.25
in) thick, and placed about 15 cm (6 in) above bottom on concrete
blocks. The reef was in 9.1 m (30 ft) of water. Another target was
subsequently verified by divers as a flat metal sheet (Figure 11)

No targets were identified by the sonar for verification along transect
3. The magnetometer continued to return too many signals to be useful.
The vessel speed may

![](media/image7.jpeg){width="6.0in" height="7.0in"}

Figure 4. Image of the sea bottom from the Sportscan sidescan sonar (top
image)

depicting the experimental artificial reef (at tip of arrow in top
center

of the image). The area within the box is magnified (bottom image) for

better detail. The bean shapes represent concrete blocks.

![](media/image8.jpeg){width="5.995833333333334in"
height="6.991666666666666in"}

Figure 5. Image of the sea bottom from the Sportscan sidescan sonar (top
image)

depicting an illegal artificial reef (at tip of arrow) that the Florida
Fish

and Wildlife Conservation Commission had destroyed. The area within

the box is magnified (bottom image) for better detail.

![](media/image9.jpeg){width="6.0in" height="4.5in"}

Figure 6. Video camera image of plates on the bottom used to attract
lobsters.

Table 2. Approximate locations of suspected illegal artificial reefs.

Site Transect North Latitude West Longitude

1 1 24° 44.216′ 81° 47.204′

2 1 24° 43.783′ 81° 46.901′

3 2 24° 41.222′ 81° 50.255′

4 2 24° 41.457′ 81° 49.793′

5 2 24° 43.030′ 81° 46.860′

6 2 24° 44.480′ 81° 44.990′

7 2 24° 44.590′ 81° 43.790′

8 2 24° 45.080′ 81° 42.810′

9 2 24° 45.150′ 81° 42.670′

10 2 24° 45.285′ 81° 42.425′

11 2 24° 45.553′ 81° 41.942′

12 2 24° 45.828′ 81° 41.349′

13 2 24° 47.401′ 81° 38.305′

14 2 24° 47.438′ 81° 38.232′

15 2 24° 50.203′ 81° 32.997′

16 4 24° 45.358′ 81° 42.452′

17 4 24° 45.095′ 81° 42.206′

18 4 24° 43.777′ 81° 41.049′

19 5 24° 44.190′ 81° 51.712′

20 5 24° 50.057′ 81° 39.838′

21 5 24° 50.338′ 81° 39.225′

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

![](media/image10.jpeg){width="5.99375in" height="3.7895833333333333in"}

Figure 7. Locations where sidescan sonars imaged possible illegal
artificial reefs

(red) north of the Florida Keys.

![](media/image11.jpeg){width="6.0in" height="7.0in"}

Figure 8. Image of the sea bottom from the SIS-1500 chirp sidescan sonar
(top

image) depicting a possible illegal artificial reef (at tip of arrow)
consisting

of at least four rectangular objects. The area within the box is
magnified

(bottom image) for better detail.

![](media/image12.jpeg){width="6.0in" height="4.5in"}

Figure 9. Image of a corrugated metal sheet taken by diver with a
digital camera.

![](media/image13.jpeg){width="6.0in" height="4.5in"}

Figure 10. Image of an illegal artificial reef comprised of metal plates
atop concrete

blocks (image was taken by diver with digital camera).

![](media/image14.jpeg){width="6.0in" height="4.5in"}

Figure 11. Image of metal sheet taken by diver with digital camera.

have been too great for the Sportscan sonar to properly image illegal
reefs or no reefs may have been placed along the transect.

Targets identified as possible illegal reefs were detected in three
general locations along transect 4. A curved shaped object detected by
the Sportscan sonar at site 17 was initially overlooked as a possible
target, but divers later confirmed the object as a metal car hood and
functioned as an illegal reef. Car hoods are sometimes placed on the
bottom to attract lobsters. Targets at site 18 were verified with video
cameras and divers. The objects at site 18 were spread over a large area
and consisted of several types of materials including corrugated sheets
of metal, small concrete slabs which were probably crab trap weights,
and flat aluminum sheets.

Three target sites were detected along transect 5 and classified as
possible illegal reefs. One site was at the western end, and the two
other sites were relatively close together at the eastern end of the
transect. All three sites were in water approximately 12.2 m (40 ft)
deep.

Data from the three transects perpendicular to bottom contours
(transects 1, 3, and 4) were used to estimate illegal reef density and
total number of illegal reefs over the entire study area. Data from
these three transects were used for the estimate because the transects
were spaced approximately equal distances apart across the study area
and there was no prior knowledge of the number and distribution of
illegal reefs in the area. Transect 2 (9.1 m bottom contour) contained
biased data because information provided by the Florida Fish and
Wildlife Conservation Commission suggested that illegal reefs were
concentrated at this depth. The average number of suspected illegal
reefs from the three transects was 2.43 (s = 2.8711) per km². When
extrapolated to the entire study area of 602 km², this results in an
estimated 1463 illegal reefs for the area. When data from transects 2
and 5 (roughly parallel to bottom contours) were used, even though the
data from transect 2 are considered to be biased, the estimate changes
only slightly to 1529

(s = 2.0659) illegal reefs over the entire study area.

**Discussion**

This project was performed primarily to determine if towed sidescan
sonars, magnetometers, and video cameras could be used to detect and map
locations of illegal artificial reefs and, secondly, to provide a
preliminary estimate of the number of illegal reefs in the study area.
Both sidescan sonars performed well. The SIS-1500 chirp sidescan sonar
performed well for the first two transects and then ceased to function
and was not used for the remainder of the survey. A loose screw in the
circuitry was later found to have caused the system to fail. Advantages
of the SIS-1500 sonar include its wider swath width and its overall
quality imaging capability. Its disadvantages are that it is more costly
(\$65,000) than the Sportscan sonar, and it requires more training to
operate. Its larger size and weight can also be considered a
disadvantage when the unit is towed with smaller vessels.

The Sportscan sonar works well when towed near the bottom and is set on
high frequency although it has a narrower swath than the SIS-1500. The
lower cost of the unit and its associated software (approximately
\$7,000) and its simplicity of operation makes it an ideal candidate
detect illegal reefs especially if video imagery is simultaneously
acquired to confirm the identify of targets. Both sonars imaged
different types of materials (metal and concrete).

The magnetometer was not suitable for this type of effort. Although the
instrument successfully detected iron plates used in the construction of
some illegal reefs, it is not capable of detecting non-ferrous
materials, and the numerous returns makes interpretation difficult.

Video cameras are useful at verifying some targets if they can be towed
directly over the targets. They are most useful when towed with a
sidescan sonar. Their limitations are that they need to be towed very
near the bottom for clear pictures and their swath width is narrow
because of their proximity to the bottom.

The divers had difficulty locating some of the targets. The instruments
were referenced to the GPS location of the vessel and not to the
locations of the instruments attached to the end of the tow cables. The
targets were therefore not at the exact locations given by from the GPS
coordinates. Follow-up studies with towed instruments should include
inserting equipment with associated software that will reference the
actual location of the towed instrument so that relocating targets for
verification and removal will be easier.

Enumerating illegal reefs is difficult because of the variety of
construction styles and materials used. Some illegal reefs consisted of
metal or concrete plates arranged adjacent, and in close proximity, to
each other. These reefs can be counted as a single unit, and a nearby
arrangement can be counted as a second unit even though both may have
been constructed by the same person(s). Other illegal reefs consisted of
materials, such as metal sheets, scattered over a wide area. This type
of illegal reef was counted as a single unit for this study.

Illegal artificial reefs are widespread in the study area north of the
Florida Keys, as well as in areas outside the study area, and are found
in a variety of depths (Figure 12). Materials used to form these reefs
are varied. The number of reefs is estimated to be in the hundreds in
the study area alone. These illegal reefs are thought to be placed in
other areas near the Florida Keys as well. The impact of their use on
the spiny lobster population is unknown as is their impact on bottom
habitat. Further efforts are needed to more thoroughly and accurately
map these reefs within and beyond the study area and to determine their
impacts on bottom flora and fauna. Given the number of illegal reefs
estimated within the study area from this effort, total number of
illegal reefs in the entire Florida Keys waters could easily be in the
thousands.

Illegal artificial reefs are a real and widespread problem inside and
outside the FKNMS, not only because of the potential damage it causes
lobsters, surrounding seagrasses, and corals, but also because they are
a management problem of extreme proportions. Catching people in the act
of placing and using these structures is extremely difficult because of
the large area involved and because law enforcement officials are spread
so thin on their patrols. One way of controlling this problem is to find
the illegal reefs and remove, or otherwise, destroy them. This
assessment project demonstrated that appropriate technology can be a
cost-effective and efficient way to detect and map these illegal
structures.

![](media/image15.jpeg){width="5.99375in" height="3.810416666666667in"}

**Figure 12. Sites of possible illegal artificial reefs found with
sidescan sonars (red)**

**combined with sites provided by the Florida Fish and Wildlife**

**Conservation Commission (black).**

**Acknowledgments**

The author wishes to thank the following people whose efforts made this
project possible: staff members of the Florida Keys National Marine
Sanctuary, law enforcement officers of the Florida Fish and Wildlife
Conservation Commission, Travis Burke and Kenneth Wilkinson for their
engineering and technical expertise, and Thomas Leming and Nelson May
for reviewing this manuscript and offering useful suggestions for its
improvement.

**References**

Chapman, P., D. Wills, G. Brooks, and P. Stevens. 1999. Visualizing
underwater environments using multi-frequency sonar. *In* IEEE Computer
Graphics and Applications, pp. 61-65.

Harper, D.E., and R.G. Muller. 2001. Spiny lobster fisheries of the
United States of America. FAO Fisheries Report (619): 258-278.

Herrnkind, W.F. 1985. Evolution and mechanisms of mass single-file
migration in spiny lobster: synopsis. Contributions in Marine Science
27: 197-211.

Herrnkind, W.F., M.J. Butler IV, and R.A. Tamkersly. 1988 The effects of
siltation on recruitment of spiny lobsters, *Panulirus argus*. Fish.
Bull. 86: 331-338.

Muller, R.G., W.C. Sharp, T.R. Matthews, R. Bertelsen, and J.H. Hunt.
1999. The 1999 update of the stock assessment of the Florida Keys spiny
lobster, *Panulirus argus*. Florida Fish and Wildlife Conservation
Commission, Marathon, Florida, USA. 12 p.

Muller, R.G., W.C. Sharp, T.R. Matthews, R. Bertelsen, and J.H. Hunt.
2000. The 2000 update of the stock assessment for spiny lobster,
*Panulirus argus*, in the Florida Keys. Florida Fish and Wildlife
Conservation Commission, Marathon, Florida, USA. 12 pp.