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Sanctuary Rocky
Intertidal Open
Ocean & Endangered
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Mapping Rockfish Habitats of the Sanctuary Rockfishes (Sebastes sp.; often referred to as red snapper or rock cod in fish markets) have been declining at alarming rates along much of the U.S. West Coast. Concern by sport and commercial fishers, government scientists and managers, and the general public has led to an increased effort to seek solutions for sustaining and conserving these economically important fisheries. Because many rockfish species are commonly associated with high, rugged seafloor relief, it is essential to identify and quantify areas with these characteristics. Marine benthic habitats are identified and mapped using a suite of geophysical remote sensing tools. Unlike terrestrial habitats, which are defined by flora and fauna in relationship to altitude and climate, deep water (>30 meters) marine benthic habitats initially are defined by substrate type, relief, and depth or by their seafloor morphology as imaged by the various geophysical mapping tools. All of these tools rely on sound to produce the images. Side scan sonographs that exhibit backscatter signals and shadows form an image that looks much like a photograph. Another recently-developed tool is the multibeam echo sounder, which also produces a photographic-like image of the seafloor that can resolve features on the order of one to three meters. Geophysical surveys are followed by on-site examination of the habitats using remotely-operated vehicles (ROVs) and manned submersibles. This approach allows confirmation of interpretations of the geophysical data and observations of rockfish assemblages associated with the habitats. These modern habitat characterization methodologies have evolved
from studies undertaken in the Monterey Bay National Marine Sanctuary.
In the early 1990s a multidisciplinary approach to characterizing
marine benthic habitats began with biologists and geologists from
government, academic, and private scientific agencies and institutes
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As progress in habitat characterization and mapping advanced, the team expanded to keep pace with the demand for delineating benthic fish habitats. In the mid-1990s the Center for Habitat Studies at MLML was formed. Seafloor mapping continued, with funds from Monterey Bay National Marine Sanctuary, UC California Sea Grant, and California Department of Fish and Game (CDFG), in the vicinity of the Big Creek Ecological Reserve. Techniques developed in the Sanctuary were used to map marine benthic habitats in Southeastern Alaska. In the late 1990s, the Monterey Bay team organized and participated in several national and international workshops to classify and characterize marine habitats, reporting on the success of their mapping activities. The Seafloor Mapping Laboratory of CSU Monterey Bay (CSUMB) was formed and convened a regional workshop on marine benthic habitat characterization and mapping in 1999, which was supported by CDFG and NOAA Special Projects. Most recently the team, supported by CDFG and the National Sea Grant Program, has been actively involved in digitally compiling offshore geological information and recently-released industry geophysical data for the construction of marine benthic habitat maps, not only within the Sanctuary but throughout offshore California. CSUMB's Seafloor Mapping Lab used grants from the U.S. Department of Defense and CDFG to purchase a 27-foot boat with technologically-advanced multibeam and ROV systems that are being used to map benthic habitats in the Sanctuary. In addition, the excellent deep-water multibeam data collected by MBARI are being used to define deeper water habitats. In the past year we have identified many areas within the Sanctuary
that are probable deep-water rockfish habitats. Potential rockfish
habitats exist at the heads of submarine canyons and on the continental
shelf where eroded granitic and sedimentary rocks are exposed.
During the past year extensive marine benthic habitat maps have
been produced through the conversion of the California Continental
Margins Geological Map series, published by the California Division
of Mines and Geology, into geographical information systems (GIS).
-- H. Gary Greene1, Mary Yoklavich2, Rikk Kvitek3,
Mud and sand covering the continental shelf underlying the Monterey Bay National Marine Sanctuary are relatively young, geologically speaking. During the glacial age, ending about 18,000 years ago, the level of the oceans was much lower due to the amount of water locked up in continental ice sheets, and a large portion of today's Sanctuary was dry land. As the glaciers melted, the level of the oceans began to rise and sand and mud carried offshore from rivers began to accumulate on the continental shelf, filling in depressions and covering the once-dry surface. The process of land erosion and transport to the Sanctuary environment continues today, and recent studies are shedding light on where the mud is coming from and where it is accumulating.
The Sanctuary floor north of the Monterey Peninsula today is covered at water depths between about forty and ninety meters by a nearly continuous blanket of mud as much as thirty meters thick. The shape of this mud deposit is like a long stretched-out pancake from central Monterey Bay toward the northwest; although it may reach a maximum thickness of thirty meters, it thins both toward the land and farther offshore to thicknesses of a few meters or less. Past calculations of sediment erosion rates and river discharges by researchers at UC Santa Cruz have shown that the offshore muds may be explained by the three rivers that empty into Monterey Bay: the Salinas, Pajaro, and San Lorenzo. The amount of sediment contributed annually from sediment washed out of eroding cliffs and from gullies of smaller streams and creeks is minor in comparison. Samples of this mud were collected from the seafloor to determine its source and to measure how rapidly it is accumulating. The rate of accumulation was determined by measuring profiles of the isotope 210Pb (Pb is lead, which has a half-life of twenty-two years and hence can be used to date relatively young sediment on the seafloor). The surprising finding was that accumulation rates for the muds are higher than would be expected from the rates of supply from the three rivers and other mud sources. Therefore it appears that most, if not all, of the mud delivered to the Sanctuary by the rivers is captured and deposited on the midshelf and very little escapes to deeper areas across the shelf break or down the canyon. To better understand the movement of particles in the Sanctuary and the ultimate origin of the mud deposits, current meters and other instruments were anchored to the seafloor and suspended in the water column over the belt of mud between Davenport and Santa Cruz. Measurements over a one-year period during 1997-98 at these moorings found that the mud is being transported northwestward, as far as Año Nuevo and perhaps farther, toward San Francisco. Certain areas of the continental shelf have escaped inundation by this mud blanket. Near shore in shallow water, where the pounding surf and wind-induced currents produce high-energy bottom conditions, mud that may be deposited by winter floods is rapidly resuspended and transported away. Seaward of the mud accumulation zone, on the far outer shelf where water depth exceeds ninety meters, lies a zone of outcropping bedrock, sands, and hardgrounds that probably are all remnants from an environment when sea level stood at a position much lower than it is today. Coincidentally, that outer shelf area is also a zone of high energy conditions resulting from horizontally moving water masses and possibly breaking internal waves (very long-period waves that occur below the surface and oscillate at boundaries of very slight density differences deep in the water column). Surrounding the Monterey Peninsula and southward, there is little
evidence of significant mud accumulation on the shelf. In these
areas coarse sand deposits (mostly very coarse sand and perhaps
some gravel) are found out to depths beyond 100 meters. The intricate
and curvilinear patterns of the boundaries around these coarse
sand deposits indicate that dynamic processes, not fully understood,
are responsible for maintaining the geometry of these deposits.
Stephen L. Eittreim, Michael E. Field,
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For comments or question please refer to the Webmaster Last modified on: Jan 15, 2000 |
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