Shelf Break Off Research Articles

Diet and feeding of the cutlassfish trichiurus lepturus in the subtropical convergence ecosystem of southern brazil

the diet and feeding of the cutlassfish trichiurus lepturus were studied based on analyses of the stomach contents of larvae, juveniles, and adults collected along the continental shelf and shelf break off southern brazil, from the cape of santa marta grande (28°36′s) to chuí (34°45′s). the larvae and pre-juveniles with a total length of <5 cm were found to have fed almost exclusively on calanoid copepods, whereas juveniles (5 to 30 cm) fed on small zooplanktonic crustaceans such as lucifer faxoni and promysys atlantica. on the other hand, sub-adults (30 to 70 cm), fed on euphausiids, mainly euphausia similis, and small fish, mostly anchovy engraulis anchoita. adults (70 to 160 cm) fed on a wide range of larger prey, such as anchovy, sciaenid fish, cephalopods and coastal shrimps, as well as euphausiids. the juveniles and sub-adults fed more intensively than adults during the warm-water season whereas adults fed more intensively during the cold-water season. feeding activity was more intense at night, while cannibalism was more frequent during the warm season, when biological productivity was lower. finally, diet diversity was higher in coastal waters, during the warm season. the abundance of cutlassfish off southern brazil may be explained by the fact that it is adapted to feed on a wide size-range of both pelagic and demersal prey.

Methane in the western part of the Sea of Okhotsk in 1998–2000

We measured oceanic methane in the western part of the Sea of Okhotsk in 1998, 1999, and 2000. An anomalously high methane concentration was found in near‐bottom water above the shelfbreak (∼200 m) off northeast Sakhalin every year: up to 488 nmol kg−1 in 1998, 981 nmol kg−1 in 1999, and 556 nmol kg−1 in 2000. This anomalously high concentration can be used to trace the water with density range 26.6 to 26.8 σθ in the upper Dense Shelf Water. In the shelf off east Sakhalin, a strong stratification caused by freshwater from Amur River controlled the upward transport of methane through the suppression of vertical convection. The calculated methane flux was largest in the northeastern shelf region of Sakhalin (88 mol CH4 km−2 d−1). In the western part of the Sea of Okhotsk (0.73 × 106 km2, 51% of total Sea of Okhotsk area), the emission rate of methane was 11 Gg CH4 yr−1.

Larval Fishes in Relation to Water Masses of the Central North Pacific Transitional Areas, Including the Shelf Break of West-Central Alaska

Larval fishes and corresponding physical data were sampled along two transects from Seward, Alaska to Oahu, Hawaii in February 1991. The first transect was west along the shelf break (SH) off Kodiak Island, 57°38′ to 55°20′ N, between 149 and 156°W. The second transect, North Hawaii (NH), was south along 158°W from 54°40′N, at Shumigan Islands off the Alaskan Peninsula, to 22°00′ N, near the island of Oahu, Hawaii. No other cruise has specifically sampled fish larvae along a similar transect from Alaska to Hawaii in the winter. Myctophidae, Gonostomatidae and Sternoptychidae dominated our catches. A variety of methods have been used to assign faunal provinces and to define distribution of fishes, from descriptive to empirical to statistical to physical. The result is that zoogeographic zones in the North Pacific have been defined in several ways, depending on the taxa and area studied. The multiplicity of nomenclature complicates comparisons, however the difference may be a matter of semantics. We used Roden's (1991) salinity and temperature limits as an objective method for determining water mass boundaries and analyzing faunal provinces. The subtropical domain had the highest diversity, as expected; however, the subarctic front was a close second. Diversity was the lowest in the subarctic domain and the transition zone. The counter-intuitive low values of diversity and evenness in the transition zone are explained by the vertical distributions of fish in these water masses. When temperature divisions are used to calculate diversity, a true “transition zone” appears between 8 and 19°C. The “transition” of taxa apparently occurred north of the physically derived transition zone. This may be a situation unique to the relatively wide transition zone of February 1991. If not, then future analysis should include frontal zones as part of one large transition zone to reflect the broader subarctic-subtropical transition zone. Regardless of the label or the method used to assign faunal provinces, the distribution of larval fishes is regulated by water mass characteristics.

Temporal and spatial variability of the sea surface salinity in the Nordic Seas

In this paper, the temporal and spatial variability of the sea surface salinity (SSS) in the Nordic Seas is investigated. The data include a Russian hydrographical database for the Nordic Seas and daily to weekly observations of salinity at Ocean Weather Station Mike (OWSM) (located at 66°N, 2°E in the Norwegian Sea). In addition, output from a medium‐resolution version of the Miami Isopycnic Coordinate Ocean Model (MICOM), forced with daily National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data, is used to complement the analysis of the temporal and spatial fields constructed from the observational data sets. The Nordic Seas show a strong seasonal variability in the vertical density stratification and the mixed layer (ML) depth, with a weak stratification and a several hundred meters deep ML during winter and a well‐defined shallow ML confined to the upper few tens of meters during summer. The seasonal variability strongly influences the strength of the high‐frequency variability and to what extent subsurface anomalies are isolated from the surface. High‐frequency variability has been investigated in terms of standard deviation of daily SSS, calculated for the different months of the year. From observations at OWSM, typical winter values range from 0.03 to 0.04 psu and summer values range from 0.06 to 0.07 psu. Results from the model simulation show that highest variability is found in frontal areas and in areas with strong stratification and lowest variability in the less stratified areas in the central Norwegian Sea and south of Iceland. Investigation of the interannual variability over the last 50 years shows a marked freshening of the Atlantic Water in the Norwegian and Greenland Seas. Moreover, the strength of the southern sector of the Polar front, as defined by the 34.8–35.0 psu isohalines along the western boundary of the inflowing Atlantic Water, undergoes significant interannual variability with gradient stretching reaching up to 300 km. In comparison, the variability in the strength of the eastern front and northern sector of the Polar front, seemingly controlled by the shelf break off Norway and the ridge between the Norwegian and the Greenland Seas, typically undergoes stretching only between 60 and 80 km. The investigation also demonstrates that the low‐frequency variability in the upper ocean density field in the Greenland Sea, a key factor for the deep water convection, is governed by the variability in the sea surface field. Since the early 1960s, there has been a negative trend in the salinity, probably contributing to the observed decrease in the deep water production in that period.

Yellowfin tuna (Thunnus albacares) aggregations along the shelf break off south-eastern Australia: links between inshore and offshore processes

In May 1996 the biological oceanography of the main yellowfin tuna longline fishing waters off southern New South Wales, Australia, was examined in relation to the catch by the fishery. A warm-core eddy was identified directly east of Eden with a temperature at 250 m depth of 15˚C. At the western edge of this eddy, relatively high levels of fluorescence (chlorophyll a) were recorded together with pigments typical of diatoms, a feature of upwelling communities. The biomass of zooplankton and micronekton was also significantly higher at the western edge of the eddy. Similarly, acoustic data showed relatively high concentrations of backscatter at the margins of the eddy, particularly at the shelf break and slope. These areas had the greatest potential prey biomass; a fact supported by the presence of shelf-and slope-associated prey species in the stomachs of yellowfin tuna caught at the same time. Fishery data for yellowfin tuna (Thunnus albacares) captures showed that catch per unit effort was highest along the shelf and shelf break. Examination of yellowfin tuna catch records from 1988 to 1998 from south-eastern Australia showed highest catches in 1996, ~75%of the catch coming from the western edge of the eddy. We conclude that the presence of a warm-core eddy in the area at this time provided a localized but productive area to which the yellowfin were attracted.

The marine mammals of the Gulf of Mexico

Recorded knowledge of marine mammals of the Gulf began with commercial whaling of sperm whales, short-finned pilot whales, and Risso’s dolphins in the 1700s and 1800s, progressed to natural history observations and one of the first volunteer stranding organizations, the Texas Marine Mammal Stranding Network, in the 1970s. The vegetarian sirenian, the West Indian manatee (Trichechus manatus), occurs mainly in Florida. All others are cetaceans, members of the whale and dolphin clades. There are no porpoises, sea lions, fur seals, or true seals in the Gulf. The most common cetacean in the Gulf numerically is the pantropical spotted dolphin (Stenella attenuata), and the one with most biomass is the sperm whale (Physeter macrocephalus), common in mid-depth waters off Louisiana and the shelf break off Texas. Major anthropogenic threats exist for all marine mammals but do not appear to be as intensive in the Gulf as in several other oceanic basins. Threats to marine mammals in the Gulf include prey depletion, incidental mortality, injury due to fisheries, intentional and direct takes, vessel strikes, disturbance, acoustic (noise) pollution, chemical contamination, ingestion of solid debris, oil spills, and aspects of ecosystem change.

Reproduction of the cutlassfish <i>Trichiurus lepturus</i> in the southern Brazil subtropical convergence ecosystem

The reproduction of the cutlassfish Trichiurus lepturus was studied from samples collected in bottom trawl surveys on the continental shelf and shelf break off southern Brazil. The mean total length (LT) at first gonadal maturity was 63.9 cm for males and 69.3 cm for females. Batch fecundity ranged from 3,917 in a specimen of 70 cm LT to 154,216 in a specimen of 141 cm but the number of spawning batches in each season was not established. Spawning occurs in late spring and summer in the continental shelf and probably year round over the shelf break. High surface temperatures and moderate stratification of the water column were associated with the final stages of gonadal maturation. Seasonal and spatial differences in sex ratios, sizes, maturity stages and feeding intensities suggest that adult female T. lepturus remain during the cold season in the continental shelf of southern Brazil to feed, while adult males move to warmer waters northward or offshore.

Metamorphosis of the final phyllosoma and secondary lecithotrophy in the puerulus of Panulirus cygnus George: a review

The final phyllosoma of Panulirus cygnus metamorphoses to a non-feeding puerulus that lives on energy reserves accumulated in the final larva, and the metamorphic moult occurs mainly in the slope region adjoining the shelf-break off Western Australia. A review of the literature on field studies, laboratory rearing and nutritional studies of phyllosomal and other decapod zoeal larvae provided no evidence that metamorphosis in P. cygnus (or other shallow-water palinurids) is triggered by a direct environmental cue. It did indicate that metamorphosis results from the culmination of sustained nutrition and reserve energy levels through the later larval phase. Therefore, since the puerulus is secondarily lecithotrophic, it is considered that metamorphosis occurs only after the final phyllosoma has reached some critical, specific, level of stored energy reserves. Appropriate food for later larval development and successful metamorphosis of P. cygnus is more abundant in the shelf-break region (than further offshore) because this is a region of high plankton and micronekton biomass dominated by the Leeuwin Current. It also explains why metamorphosis occurs mainly in the shelf-break region. This review indicates research necessary for evaluation of the present interpretation and of larval recruitment processes in this species.

Morphology of a large meandering submarine canyon system on the Peru-Chile forearc

A SeaMARC II side-scan sonar, single-channel seismic reflection, and bottom sampling survey across the Peru-Chile forearc mapped a large submarine canyon system offshore of southern Peru and northern Chile. The main branch of this canyon system extends over 160 km from the shelf break off southern Peru to its termination in the trench off northern Chile. The canyon is 350 to 1100 m wide, and has relief of 150 to 250 m through most of this distance. Within the East Arequipa Basin, the canyon is intricately meandering and has an overall sinuosity of 1.95, comparable to highly meandering subaerial rivers. Meandering is apparently controlled by the basin slope and the characteristics of the turbidity currents that form the canyons. The upper threshold of basin slope for meandering in this canyon is about 15 m/km, which is approximately 10 times greater than that observed for subaerial rivers of comparable size. Bordering terraces at several levels above the canyon floor are remnants of meander belts that developed during early stages of canyon formation. Several cutoff meander loops are preserved on these arcuate terraces. Other sedimentary structures observed along the canyons include levees, overbank deposits, and crevasse splays. The canyon exits the East Arequipa Basin by breaching the forearc structural high along a fault. Downslope of the structural high, the canyon makes a sudden, nearly right-angle bend to the west and continues across the lower forearc slope to the trench axis. This lower stretch of canyon may have formed by headward erosion that pirated the upslope portion of the southward flowing canyon. The lower slope canyon, south of the sudden bend in the East Arequipa Basin canyon, may be a remnant of the lower portion of this ancestral canyon.

On a Sugimoto-Whitehead effect in the Mediterranean Sea: sinking and mixing of a bottom current in the Bari Canyon, southern adriatic sea

In this paper we present some data gathered during two cruises in the southern Adriatic Sea. They concern the sinking and mixing of a cold, dense and relatively fresh bottom current into a submarine canyon crossing the shelf break off Bari. This dense water is formed during the winter months in the northern Adriatic Sea through surface cooling and evaporation due to strong northeasterly winds. The resulting cold and dense water mass flows southward following approximately the isobaths. When bottom irregularities, such as the submarine canyon or the southern Adriatic Basins's shelf break, are encountered, impressive sinking and mixing processes occur. We observed these processes through CTD casts and dissolved oxygen measurements. Similar phenomena have been recently observed in tank experiments by Sugimoto and Whitehead.

Signatures of surface wave/internal wave interactions: Experiment and theory

This paper summarizes results from an extended investigation of the physics of surface wave/internal wave interactions. During summer conditions of stratification, internal solitary waves are generated by the semidiurnal tide at the continental shelf break off New York. The waves evolve as packets of solitons having rank-ordered amplitudes, wave spacings, and crest lengths. They propagate toward shore and are ultimately dissipated in the region where the mixed layer intersects the bottom. We have studied their hydrodynamic properties, their interactions with surface waves and with each other, and the scattering of electromagnetic radiation from their accompanying surface roughness variations. Measurement using a towed thermistor chain. CTD casts, and moored current meter arrays have allowed a detailed characterization of the subsurface and near-surface hydrodynamics. Quantitative surface signatures were obtained using: (a) TV Stilwell methods, (b) calibrated airborne synthetic aperture radar at L- and X-bands, and (c) the L-band SIR-B synthetic aperture radar on the space shuttle Challenger. The scattering data have been analyzed using the wave action equation for calculating internal current/surface wave interactions. Hughes' description of the wave relaxation rate has been used to obtain the perturbation to the surface wave spectrum from the internal currents and current straining. Radar backscatter variations from the perturbed surface wave roughness were then calculated using a variant of the Kirchhoff approximation to the scattering integral. Both Bragg scattering and specular point scattering are important in this case. Excellent agreement is obtained between the theory and the observations.

Crustal Structure and Tectonic Development of Gulf of Guinea Cul-de-Sac from Integrated Interpretation of New Aeromagnetic and Existing Geophysical Data: ABSTRACT

Data-acquisition difficulties and propriety restrictions on industry data have necessitated liberal extrapolations and generalizations in previous tectono-structural studies of the Gulf of Guinea cul-de-sac. This region is the locus of a postulated Late Cretaceous triple junction whose arms were the transform-dominated Equatorial Atlantic, the northward-propagating South Atlantic, and the Benue Trough aulacogen. Oceanic crust has been inferred to underlie most of the thick sedimentary wedge of the oil-prolific Niger Delta basin. Integrated interpretation of new aeromagnetic data of the Geological Survey of Nigeria and existing geophysical data corroborates previous work on the general structure of the marginal basins. New aeromagnetic data, however, reveal a detail structure more complex than previously known. Low-frequency magnetic anomalies over the Niger delta indicate that oceanic crust extends northward to about Onitsha. From Onitsha, the edge of oceanic crust trends southwestward along the Benin hinge line (an apparent continental continuation of either the Chain fracture zone or a new Okitipupa fracture zone) and also wiggles southeastward (adjoined by a wide margin of transitional crust) toward the shelf break off Cameroon. Linear magnetic anomalies trending northeast indicate about 7 fracture zones b neath the Niger Delta basin. The region of high-frequency magnetic anomalies west of the Niger delta represent the Okitipupa basement ridge complex, which separates the Niger Delta basin from the Dahomey embayment. In this embayment, 2 wide bands of intervening high- and low-frequency aeromagnetic anomalies are interpreted to represent a basement high or ridge and a fault-bounded trough, respectively. The change in the directions of fracture-zone trends (from east-northeast in the southeast, to northeast in most of the Niger delta area, and then to a more easterly direction over most oceanic areas) indicates 3 phases of tectonic evolution recorded as changes in sea-floor spreading directions in the Gulf of Guinea cul-de-sac. End_of_Article - Last_Page 235------------

Progradation of the late Quaternary Gediz delta, Turkey

Detailed 3.5-kHz seismic-reflection profiling and coring have shown that the continental shelf break and slope off Izmir Bay, Turkey, are formed by prograding deltaic silty mud sequences. The muds contain shallow brackish-water benthic and cold-water planktonic foraminifera. They were deposited when sea level was about 110 m below its present position, during the last Pleistocene glaciation. Slumping took place on the prodelta slope, leaving rotational slump scars and imbricate slump blocks downslope. Delta-top truncation took place in shallow water by erosive coastline retreat after distributary abandonment. There has been little or no sedimentation since that time: outer Izmir Bay is thus floored by relict sands. Following rapid post-glacial transgression, the Gediz delta was reestablished in Izmir Bay. Archaeological evidence shows it has prograded during the last 3000 yrs. At least six shifts in the position of the Gediz mouth are distinguished during this time, and abandoned delta-top channels can be correlated with submarine delta-lobe sequences.

Current response at the shelf break to transient wind forcing

An analysis of current meter and hydrographic data collected from an array of moorings situated at the shelf break off Nova Scotia is presented. In particular, the focus is on the ocean response to moderate (10 to 20m s−1) transient winds of two or more days duration blowing parallel to the local bathymetry. Under these conditions, upwelling occurs at the shelf break from depths of 400 m and with peak vertical velocities of about 2 mm s−1. The upwelling appears to be confined to within 10 km of the slope. Acceleration, Coriolis, and pressure‐gradient terms are important for the along‐ and cross‐slope momentum balances. Local topographic variations contribute to the anomalously large bottom currents on the shoreward side of the shelf break. Both coastal sea level and the shelf break circulation respond coherently to synoptic wind forcing, but quantitative agreement with two‐layer shelf models is poor.

Low-Frequency Circulation at the Edge of the Scotian Shelf

Abstract Records of current, temperature and salinity from a two-year mooring program at the shelf break off Nova Scotia are examined to determine the low-frequency oceanic responses to the driving surface wind field and fluctuating offshore currents associated with the Gulf Stream. The seasonal mean and subtidal variance (at periods of 2–10 days) of the cross-shelf currents reflect the strong annual cycle in the wind field measured at Sable Island. The mean vertical shear suggests a simple Ekman response to winter increases in the longshore wind component, but this model fails quantitatively because 1) the inferred surface-layer (20 m) transport is much too large and 2) the deep (150 m) “return” flow shows no annual signal. The excessive offshore near-surface transport in winter must be reconciled with the relatively stationary position of the shelf/slope-water boundary (SSB) by invoking intense cross-frontal mixing and/or a seasonal mean alongshore pressure gradient. The seasonal mean longshore currents...

Factors Influencing Sediment Transport at Shelf Break: ABSTRACT

Recent investigations of sediment dynamics at the shelf break suggest that the factors which control the transport of sediment there can be grouped into two major categories: oceanographic and geologic. Oceanographic factors include tides, surface waves, internal waves, fronts, and meteorlogically driven currents that create a wide range of unsteady and quasi-steady water motions whose influences on sediment dispersal is poorly understood. These phenomena affect sediment along the shelf break in zones that have a large spatial and temporal variability. Geologic factors include shelf geometry and physiography, tectonism, and sediment type and supply. These factors influence shelf-break processes either by modifying oceanographic processes or by controlling the geologic set ing of the shelf break. An example of the former is that the physical structure of the bottom boundary layer depends strongly on the shape of the sea floor. An example of the latter is that sediment supply partly determines whether the shelf break is a net depositional or an erosional zone. Whereas both oceanographic and geologic factors interact to influence sediment transport processes at the shelf break, a particular continental margin may be dominated by one of these factors. For instance, such oceanographic agents as the Gulf Stream and winter storms control sediment dynamics over large segments of the east United States coast, whereas such geologic factors as large sediment supply and active tectonism overshadow oceanographic phenomena in the eastern Gulf of Alaska. Many intermediate conditions also exist. An example is at the complex shelf break off southern California, where active tectonism and oceanographic phenomena nearly equally influence shelf-edge processes. End_of_Article - Last_Page 943------------

Geophysical transects of the Labrador Sea: Labrador to southwest Greenland

In 1977 the Federal Institute for Geosciences and Natural Resources, Hannover, carried out a large scale multichannel reflection seismic survey in the Labrador Sea. This survey provided an opportunity for the direct comparison of the geologic structure of the Labrador and Greenland margins. The seismic records across the Labrador Shelf show a thick, prograding sedimentary wedge consisting of several seismic sequences onlapping an acoustic basement that dips steeply seaward. The surface of the acoustic basement is irregular below the continental slope, indicating Late Cretaceous—Early Tertiary faulting. The thick sedimentary section below the slope is divided by an unconformity, tentatively identified as Late Tertiary in age, into two seismic megasequencies which can be subdivided. The acoustic basement on the Greenland side is also strongly faulted but is overlain, in the south, by a thin sedimentary section. The sediment cover thickens on the Greenland Shelf to the north as the shelf becomes wider. As with more southerly parts of the western Atlantic margin, a positive free-air anomaly (30–50 mgal) lies landward of the shelf break off Labrador and a smaller negative anomaly follows the base of the slope. Similar, but generally narrower features are observed along the Greenland margin. West of the negative anomaly off the Greenland slope a narrow band of lower amplitude positive anomalies tends to be associated with an acoustic basement high observed in the reflection profiles. A landward negative gradient in the simple Airy isostatic anomaly across this margin suggests that the ocean—continent boundary is related to this high. Detailed magnetic measurements across the northern Labrador margin show that well-developed oceanic anomalies trending north-northwest lie east of the large Labrador Shelf gravity high, beyond the 2000 m isobath. Landward of these magnetic anomalies is a quiet magnetic zone within which the linear gravity high is parallel to the shelf break and correlates with a deep, sediment-filled basin. It is inferred that oceanic-type crust or greatly-attenuated continental crust underlies this basin and that continental crust thickens markedly westward of the gravity high over a distance of about 50 km.

Influence of the Shelf-Break Front Off Nova Scotia on Phytoplankton Standing Stock in Winter

Observations made on the shelf-break front off Nova Scotia during winter on 2 successive yr show the concentration of chlorophyll a in the euphotic zone to be correlated to the inclination of the subsurface front. The steepness of the front influences the depth to which phytoplankton are mixed which in turn controls the average amount of illumination received by a given cell. The two winter observations showed the subsurface front at each of two extremes: vertical with little phytoplankton biomass and almost horizontal with significant quantities of chlorophyll. Calculations were made as to the amount of production enchancement that could result from shallowing of the mixed layer by decreased frontal inclination. They show that during the winter under ideal conditions as few as 35 d of intermittent growth increase annual production in the frontal region by 25%. Key words: front, winter production, phytoplankton, shelf-break, chlorophyll a, frontal oscillations, growth enhancement, stability, stratification

Interleaving at the subsurface front in the slope water off Nova Scotia

It has been known for some time that a frontal zone exists in the slope water, which lies between the Gulf Stream and the shelf break off Nova Scotia, where the temperature jumps from 7°C to 11°C and the salinity from 34.8‰ to 35.2‰ in a horizontal distance of a few kilometers. Data taken from this area with a yo‐yoing CTD (conductivity‐temperature‐depth probe) have been analyzed and show that at the front the two water masses interleave with a vertical scale of the order of 10 m. The interleaving layers are coherent for several kilometers in the cross‐frontal direction and for at least 17 km in the long‐frontal direction. The data suggest that considerable vertical mixing, through double diffusive processes, and horizontal mixing, through interleaving processes, occur in the frontal zone. Vertical eddy coefficients of 8 × 10−4 m2/s for heat and 3 × 10−4 m2/s for salt are calculated for the water column between 40 and 160 m at the front by using Turner's (1965, 1967) formulae to calculate the double diffusive fluxes across the interfaces of the layers. The horizontal eddy diffusion coefficients at the front are calculated to be 10 m2/s for heat and 9 m2/s for salt. The estimated annual vertical heat flux at the front into the coastal water is 15% of the solar input to the shelf water, and the salt flux is 10% of that needed to produce shelf water from the freshwater runoff. Horizontal mixing across the front is shown to be large enough to explain the observed annihilation of the Labrador Slope Water during the time that it takes to travel along the Scotian Shelf. The high vertical mixing at the front may provide a significant vertical nutrient flux.

Measurement of internal waves of tidal frequency near a continental boundary

We have measured the internal wave field seaward of the continental shelf break off Vancouver Island, British Columbia, with an array of four moored temperature sensors and numerous STD casts. These measurements have been analyzed for their energy content at the semidiurnal tidal frequency, and the results compared with a theory of generation of internal tides at a shelf break. A peak in amplitude is observed at a depth slightly greater than that predicted by the theory. The observed phase change across this peak is consistent with seaward propagation of energy from the shelf break.