Traditionally, population estimates
of fished species have been based on the fishermen's
catch and on data from trawl surveys conducted
by government agencies. In other words, the
customary method of assessing fish populations
has been to go fishing for them. Trawl surveys
have several disadvantages. They do not representatively
sample all species and habitats, and may even
contribute to the depletion of some species.
As the populations of many fished
species have declined, the need for better information
about population status and distribution has
become increasingly critical. With the reauthorization
of the Magnuson-Stevens Fishery Conservation
and Management Act in 1996, NOAA was given a
clear mandate to identify and characterize essential
fish habitats, and to include habitat management
in fisheries programs. For many species, this
means studying the range of habitats and ecosystems
on the seafloor.
Groundfish assemblages, those
fish and macroinvertebrate species associated
with the seafloor, may be particularly well
suited to habitat-based resource management.
They are among the most diverse and economically
valuable fishery resources on the U.S. West
Coast. A number of rockfish species have declined
dramatically over the past three decades, to
the point where 6 species are currently listed
as overfished by the Pacific Marine Fisheries
Council, with at least one, boccacio, estimated
to be at 2-3% of its pre-fishery level. In general,
rockfishes prefer rough, rocky seafloor with
abundant places to shelter and hide, and trawling
in such areas is not an ideal strategy. However,
because of the strong association between rockfish
species and specific seafloor habitats, the
distribution of habitats provides a rational
framework for studying and managing this diverse
Seafloor habitat research within
NOAA typically has concentrated on estuarine
and relatively shallow marine waters. Part of
the reason this has not been extended to deeper
areas is the fundamental difficulty and expense
of making detailed, deep-water observations
over an area large enough to be relevant to
the dynamics of fish populations. Below SCUBA
depth (i.e., > 30 meters), visual observations
must be made either from human-occupied submersibles
or from cameras on a remotely operated vehicle
(ROV). This approach can be time-consuming and
expensive, and usually is limited to "representative"
small areas. Acoustic surveys by sidescan or
multibeam sonar can cover much broader areas
and are useful in a number of fishery habitat
applications, but they have much lower resolution
than direct observations. They can define the
distribution of different types of seafloor,
i.e., mud, smooth sand, rippled sand, rock outcrops,
canyons, etc., but they cannot show the relationship
of fish and invertebrates to these habitats.
Also, interpretations of the acoustic images
must be checked by visual observations, usually
from a submersible or ROV.
A Northrup-Grumman SM-2000 monochrome laser line
scan integrated with a McCartney Underwater Technology FOCUS
tow body, owned and operated by Science Applications International
The challenge is to find a more
efficient technique -- one that provides high-resolution
information on both the variety of seafloor
habitats and the associated marine life, and
can cover a large enough area to be statistically
Laser line scan (LLS), an electro-optic
imaging technique, may be able to bridge the
gap. It provides the efficiency and spatial
coverage of a remote survey system, at an image
resolution approaching that of visual observations.
LLS produces high contrast underwater light
field images, at millimeter to centimeter scale
resolution and at two to five times the range
of conventional video and photographic systems.
Resolution and area covered by the images (swath
width) vary with water clarity and tow height
above the bottom.
To evaluate the capabilities and
effectiveness of LLS technology for fisheries
habitat research, NOAA's
Undersea Research Program's (NURP) West Coast
& Polar Regions Center and NOAA's
Office of Ocean Exploration co-sponsored
a field test of a commercial LLS system for
imaging a range of seafloor habitats. The inter-disciplinary,
inter-agency group of investigators was headed
by Mary Yoklavich and Churchill Grimes (NMFS,
Southwest Fisheries Science Center)*.
The test was conducted in the
vicinity of the Big Creek Ecological Reserve
off the coast of central California, in November,
2001. Water depth in the survey area ranged
from 40 to 100 meters. This area had been previously
surveyed by 100/500 kHz sidescan sonar and submersible
video transects, and contained 4 different types
of seafloor (rock outcrop, coarse sediment,
fine sediment, sand) and two small canyons.
The LLS system was deployed for
a total of 45 hours, towed at 2-3 knots at 3-9
meters above the seafloor. The LLS system was
deployed at a depth range of 40-100 meters,
but it is capable of operating to a water depth
of 1500 meter. Under these conditions, the system
imaged a swath of seafloor 4 to 13 meters wide.
To evaluate the LLS's ability to detect disturbance
to these habitats by fishing trawl gear, the
sites outside the reserve were imaged both before
and after trawling. Seafloor video from an ROV
also was collected for comparison with the LLS.
Because resolution of the LLS images varies
with clarity of the water, the scientists also
measured temperature, salinity and optical properties
of the water column, to evaluate the performance
of the LLS under various environmental conditions.
Typical performance of monochrome laser
line scan system (Science Applications
Preliminary results from this
successful field test show that LLS technology
does have potential for improving the efficiency
and accuracy of fish habitat assessment. The
LLS system did an excellent job at imaging details
of the low relief sediments such as sand waves
and ripples, as well as rock outcrops. These
are vertical incidence (down-looking) images.
One example shows an isolated rock outcrop with
patches of large white sea anemones and groups
of fishes, in about 60 m of water inside the
Big Creek Ecological Reserve. Other laser images
show several species of fishes, salp chains,
sea anemones, sea pens, drift kelp and other
macro-algae, etc., in relation to the seafloor.
Preliminary identification of the fishes is
based on image profile and fishes observed and
collected in the vicinity. This type of detail
is not possible with acoustic techniques (sidescan
and multibeam sonar mapping), and affords the
opportunity for scientists to investigate spatial
relationships among fish, invertebrates, and
the seafloor substrata to a degree that was
not possible with other remotely sensed data.
(top left) Laser line
scan image of a large group of fish congregating
around an isolated rock outcrop. The investigators
speculate that these fishes are young
widow rockfish and bocaccio, based on
their ROV observations in the vicinity.
The rock has white sea anemones (Metridium
giganteum) on it, and is surrounded by
soft sediment (light grey with ripples).
The dark region surrounding the outcrop
is an artifact resulting from a rapid
increase in the FOCUS vehicle's altitude
as it passed over the rock outcrop. The
site is in about 60 meters water depth
inside the Big Creek Ecological Reserve.
Swath width (side to side of image) is
about 7.5 meters; rock outcrop is about
3.7 meters high.
(bottom left) Laser
image of a sharp boundary between sand
waves (top left corner) and smooth seafloor.
Dark objects in the area of sand waves
are pieces of drift kelp. Salp chains
(lower left) were relatively common in
laser survey (mirror image is salp shadow
-- objects above the seafloor are seen
as double images).
(top right) Laser line
scan images of (A) drift kelp at 45 m
water depth, swath width 2.7 m; (B) sea
pen (arrow) and ratfish in water depth
90 m; (C) California halibut, swath width
4.3 m; and (D) juvenile lingcod over sand
bottom (based on swath width, fish is
about 20 cm). Fish identification is a
best guess based on laser image profile
and fishes collected in the vicinity.
- *Principal Investigators:
- Mary Yoklavich and Churchill Grimes (NMFS,
Southwest Fisheries Science Center, Santa
- Waldo Wakefield (NMFS, Northwest Fisheries
Science Center) and Gary Greene (Moss Landing
- Additional participants:
- Science Applications International Corporation
(SAIC); National Undersea Research Center
at University of North Carolina Wilmington;
Oregon State University COAS Ocean Optics
Group; University of New Hampshire Center
for Coastal and Ocean Mapping