Submarine landslides : selected studies in the U.S. Exclusive Economic Zone

Submarine Landslides: An Introduction 1 By RIo Lee, W.C. Schwab, and J.S. Booth U.S. Atlantic Continental Slope Landslides: Their Distribution, General Anributes, and Implications 14 By J.S. Booth, D.W. O'Leary, Peter Popenoe, and W.W. Danforth Submarine Mass Movement, a Formative Process of Passive Continental Margins: The Munson-Nygren Landslide Complex and the Southeast New England Landslide Complex 23 By D.W. O'Leary The Cape Fear Landslide: Slope Failure Associated with Salt Diapirism and Gas Hydrate Decomposition 40 By Peter Popenoe, E.A. Schmuck, and W.P. Dillon Ancient Crustal Fractures Control the Location and Size of Collapsed Blocks at the Blake Escarpment, East of Florida 54 By W.P. Dillon, J.S. Risch, K.M. Scanlon, P.C. Valentine, and Q.J. Huggett Tectonic and Stratigraphic Control on a Giant Submarine Slope Failure: Puerto Rico Insular Slope 60 By W.C. Schwab, W.W. Danforth, and K.M. Scanlon Slope Failure of Carbonate Sediment on the West Florida Slope 69 By D.C. Twichell, P.C. Valentine, and L.M. Parson Slope Failures in an Area of High Sedimentation Rate: Offshore Mississippi River Delta 79 By J.M. Coleman, D.B. Prior, L.E. Garrison, and H.J. Lee Salt Tectonics and Slope Failure in an Area of Salt Domes in the Northwestern Gulf of Mexico 92 By B.A. McGregor, R.G. Rothwell, N.H. Kenyon, and D.C. Twichell Slope Stability in Regions 01 Sea-Floor Gas Hydrate: Beaufort Sea Continental Slope 97 By R.E. Kayen and H.J. Lee Mass Movement Related to Large Submarine Canyons Along the Beringian Margin, Alaska 104 By P.R. Carlson, H.A. Karl, B.D. Edwards, J.V. Gardner, and R. Hall Comparison of Tectonic and Stratigraphic Control of Submarine Landslides on the Kodiak Upper Continental Slope, Alaska 117 By M.A. Hampton Submarine Landslides That Had a Significant Impact on Man and His Activities: Seward and Valdez, Alaska 123 By M.A. Hampton, R.W. Lemke, and H.W. Coulter Processes Controlling the Style of Mass Movement in Glaciomarine Sediment: Northeastern Gulf of Alaska 135 By W.C. Schwab and H.J. Lee Contents V VI Contents Liquefaction of Continental Shelf Sediment: The Northern California Earthquake of 1980 143 By M.E. Field A Submarine Landslide Associated with Shallow Sea-Floor Gas and Gas Hydrates off Northern California 151 By M.E. Field and J.H. Barber, Jr. Sur Submarine Landslide, a Deep-Water Sediment Slope Failure 158 By C.E. Gutmacher and W.R. Normark Seismically Induced Mudflow in Santa Barbara Basin, California 167 By B.D. Edwards, H.J. Lee, and M.E. Field Submarine Landslides in a Basin and Ridge Setting, Southern California 176 By M.E. Field and B.D. Edwards Giant Volcano-Related Landslides and the Development of the Hawaiian Islands 184 By W.R. Normark, J.G. Moore, and M.E. Torresan Submarine Slope Failures Initiated by Hurricane Iwa, Kahe Point, Oahu, Hawaii 197 By W.R. Normark, Pat Wilde, J.F. Campbell, T.E. Chase, and Bruce Tsutsui (PDF contains 210 pages)

Greenland turbot Reinhardtius hippoglossoides of the eastern Bering Sea and Aleutian Islands region

Greenland turbot (Reinhardtius hippoglossoides) is a commercially important flounder in both the North Atlantic and North Pacific Oceans. In the latter, its center of abundance is in the eastern Bering Sea and along the Aleutian Islands chain where its population is managed as a single stock. Harvest levels in this region of the North Pacific during the period 1970-81 were comparable with those in the northwest and northeast Atlantic, with annual average catches of 53,000 metric tons (t). However, the catch in 1984 dropped sharply to 23,100 t, in part because of reduced quotas arising from concern over continued poor recruitment and declining catch-per-unit-effort. Recruitment failure was manifested in 1) the sharp decline in the catch rate of young flsh in annual research trawl surveys on the continental shelf of the eastern Bering Sea and 2) an increasing proportion of older and larger fish in the commercial catch from the continental slope of both the eastern Bering Sea and Aleutian Islands. The cause ofthe decline in recruitment could not be clearly identifled. Greenland turbot of the Bering Sea-Aleutian Islands share certain distributional features with the North Atlantic form. There is an apparent bathymetric change in the size and age of fish, with younger animals occupying continental shelf depths and the older individuals residing at depths of the continental slope. At shallow depths the young are exposed to temperature fluctuations, whereas older animals along the slope are exposed to relatively stable temperatures. A hypothesis is proposed for describing the temporal and spatial paths by which young animals reach the mature or spawning portion of the population. (PDF file contains 38 pages.)

Net phytoplankton and zooplankton in the New York Bight, January 1976 to February 1978, with comments on the effects of wind, Gulf Stream eddies, and slope water intrusions

Results are given of monthly net phytoplankton and zooplankton sampling from a 10 m depth in shelf, slope, and Gulf Stream eddy water along a transect running southeastward from Ambrose Light, New York, in 1976, 1977, and early 1978. Plankton abundance and temperature at 10 m and sea surface salinity at each station are listed. The effects of atmospheric forcing and Gulf Stream eddies on plankton distribution and abundance arc discussed. The frequency of Gulf Stream eddy passage through the New York Bight corresponded with the frequency of tropical-subtropical net phytoplankton in the samples. Gulf Stream eddies injected tropical-subtropical zooplankton onto the shelf and removed shelfwater and its entrained zooplankton. Wind-induced offshore Ekman transport corresponded generally with the unusual timing of two net phytoplankton maxima. Midsummer net phytoplankton maxima were recorded following the passage of Hurricane Belle (August 1976) and a cold front (July 1977). Tropical-subtropical zooplankton which had been injected onto the outer shelf by Gulf Stream eddies were moved to the inner shelf by a wind-induced current moving up the Hudson Shelf Valley. (PDF file contains 47 pages.)

Length-weight relationships of demersal fishes from the upper continental slope off Colombia

Parameters of the length–weight relationship of the form W=aLb are presented for 45 demersal fish species caught on the upper continental slope of the Caribbean Sea off Colombia. The b values varied between 2.13 and 4.97, with the mean b = 3.042 (95% CI, 2.887- 3.196).

Snail control by fish: an explanation for its failure

Systematic studies on the performance of fish as snail eaters in waterbodies are rare. Here, trials with a promising African cichlid (Astatoreochromis alluaudi) revealed that the fish were only successful at reducing snail populations if there was nothing better to eat - and this is hardly the case. The fish need solid jaws to crush the snails but do not develop such jaws if they can find other, preferables foods. The prospects for this form of control are also discussed in the broader context.

Length-weight relationship of some deep-sea fish inhabiting the continental slope beyond 250m depth along the west coast of India

The length–weight relationships of 22 species of deep-sea fishes inhabiting the continental slopes beyond 250 m depth along the West Coast of India are presented. The parameters a and b of the equation W=a Lb were estimated. The fish samples were collected from trawl surveys during 1999 to 2001 on board the FORV Sagar Sampada at a depth range of 250 to 600 m in the area between 7°N and 20°N latitude. The value of b ranged from 1.94 to 3.36.

Chartered Trawling on the Brazilian Slope

Commercial trawling on the Atlantic slope areas off Brazil intensified in the late 1990’s owing to the expansion of coastal trawling areas and the operations of a chartered foreign fleet. Between 2000 and 2003, 59 fishing trips conducted by 10 chartered trawlers were intensely monitored by observers and satellite vessel monitoring systems, totaling 9,069 tows and 30,085.2 trawling hours. Fishing operations occurred in northern, northeastern, southeastern, and southern sectors of the Brazilian coast in 60–1,173 m depths. Total retained and processed catch were 8,074.6 t and 6,479.8 t, respectively. Argentine hake, Merluccius hubbsi; and Argentine shortfin squid, Illex argentinus, were the primary species taken contributing to 41.1% and 28.6% of the overall catch, respectively. The silver John dory, Zenopsis conchifera; monkfish, Lophius gastrophysus; Brazilian codling, Urophycis mystacea; and the black grouper, Epinephelus nigritus, composed 23% of total processed catch, and the remaining 7.2% was composed of deep-sea shrimps (family Aristeidae) and other teleosts and elasmobranches. The occupation of slope areas included an early exploratory phase, followed by directed phases of the upper slope (300–500 m), aiming principally at the Argentine hake, and the lower slope (>700 m), targeting valuable concentrations of deep-sea aristeid shrimps. The role of chartering for slope trawling development was critically addressed. We conclude that chartered vessels were efficient explorers and were particularly important in areas not available to the technologically limited national fleet. Because the charters were market-oriented and had elevated profit demands, however, those vessels quickly turned from exploration to exploitation and competed with national trawlers in shallower areas and produced significant impacts on Brazil’s modest deep-sea resources.

Effects of Gear Modifications on the Trawl Performance and Catching Efficiency of the West Coast Upper Continental Slope Groundfish Survey Trawl

Since 1984, annual bottom trawl surveys of the west coast (California–Washington) upper continental slope (WCUCS) have provided information on the abundance, distribution, and biological characteristics of groundfish resources. Slope species of the deep-water complex (DWC) are of particular importance and include Dover sole, Microstomus pacificus; sablefish, Anoplopoma fimbria; shortspine thornyhead, Sebastolobus alascanus; and longspine thornyhead, S. altivelis. In the fall of 1994, we conducted an experimental gear research cruise in lieu of our normal survey because of concerns about the performance of the survey trawl. The experiment was conducted on a soft mud bottom at depths of 460–490 m off the central Oregon coast. Treatments included different combinations of door-bridle rigging, groundgear weight, and scope length. The experimental design was a 2 ´ 2 ´ 2 factorial within a randomized complete-block. Analysis of variance was used to examine the effects of gear modifications on the engineering performance of the trawl (i.e. trawl dimensions, variation in trawl dimensions, and door attitude) and to determine if catch rates in terms of weight and number of DWC species and invertebrates were affected by the gear modifications. Trawl performance was highly variable for the historically used standard trawl configuration. Improvements were observed with the addition of either a 2-bridle door or lighter ground gear. Changes in scope length had relatively little effect on trawl performance. The interaction of door bridle and ground gear weight had the most effect on trawl performance. In spite of the standard trawl’s erratic performance, catch rates of all four DWC species and invertebrates were not significantly different than the 2-bridle/heavy combination, which did the best in terms of engineering performance. The most important factor affecting DWC catch rates was ground gear. Scope length and the type of door bridle had little effect on DWC catch rates. Subsequent revisions to survey gear and towing protocol and their impact on the continuity of the slope survey time series are discussed.

Spatial and temporal patterns in the demersal fish community on the shelf and upper slope regions of the Gulf of Alaska

We analyzed data from National Marine Fisheries Service bottom trawl surveys carried out triennially from 1984 to 1996 in the Gulf of Alaska (GOA). The continental shelf and upper slope (0–500 m) of the GOA support a rich demersal fish fauna dominated by arrowtooth flounder (Atheresthes stomias), walleye pollock (Theragra chalcogramma), Pacific cod (Gadus macrocephalus), Pacific halibut (Hippoglossus stenolepis), and Pacific Ocean perch (Sebastes alutus). Average catch per unit of effort (CPUE) of all groundfish species combined increased with depth and had a significant peak near the shelf break at 150–200 m. Species richness and diversity had significant peaks at 200–300 m. The western GOA was characterized by higher CPUEs and lower species richness and diversity than the eastern GOA. Highest CPUEs were observed in Shelikof Strait, along the shelf break and upper slope south of Kodiak Island, and on the banks and in the gullies northeast of Kodiak Island. Significant differences in total CPUE among surveys suggest a 40% increase in total groundfish biomass between 1984 and 1996. A multivariate analysis of the CPUE of 72 groundfish taxa revealed strong gradients in species composition with depth and from east to west, and a weak but significant trend in species composition over time. The trend over time was associated with increases in the frequency of occurrence and CPUE of at least eight taxa, including skates (Rajidae), capelin (Mallotus villosus), three flatfish species, and Pacific Ocean perch, and decreases in frequency of occurrence and CPUE of several sculpin (Myoxocephalus spp.) species. Results are discussed in terms of spatial and temporal patterns in productivity and in the context of their ecological and management implications.

Distribution and co-occurrence of rockfishes (family: Sebastidae) over trawlable shelf and slope habitats of California and southern Oregon

The rockfishes of the sebastid genus Sebastes are a very important fishery resource off the coasts of California and southern Oregon. How-ever, many of the 54 managed stocks of west coast rockfish have recently reached historically low population levels, leading fishery managers to re-examine current management practices. Management of rockfish stocks as multispecies aggregates, as opposed to independent stocks within the ground-fish fishery, can be more desirable when nontargeted bycatch, discard, and management complexity are considered. Rockfish assemblage structure and species co-occurrences were determined by using data from the Alaska Fisheries Science Center triennial continental shelf bottom trawl survey. The weight of rockfish species in trawl catches was expressed as a catch-per-unit-of-effort (CPUE) statistic, from which species spatial distributions, overlaps, diversity, and richness were analyzed. Multidimensional scaling of transformed CPUE data was employed in indirect gradient and multivariate partitioning analyses to quantify assemblage relationships. Results indicated that rockfish distributions closely match the bathymetry of coastal waters. Indirect gradient analysis suggested that depth and latitude are the principal factors in structuring the spatial distributions of rockfish on trawlable habitat. In addition, four assemblages were identified through the joint evaluation of species’ distributions and multivariate partitioning analyses: 1) deep-water slope; 2) northern shelf; 3) southern shelf; and 4) nearshore. The slope, shelf, and near-shore groups are found in depth ranges of 200–500 m, 100–250 m, and 50–150 m, respectively. The division of northern and southern shelf assemblages occurs over a broad area between Cape Mendocino and Monterey Canyon. The results of this analysis are likely to have direct application in the management of rockfish stocks off the coasts of southern Oregon and California.