Malin Shelf Research Articles

Static Acoustic Monitoring of Harbour (Phoca vitulina) and Grey Seals (Halichoerus grypus) in the Malin Sea: A Revolutionary Approach in Pinniped Conservation

Harbour and grey seals rely on acoustic signals to mate, socialise and defend their territory. Previous studies have focused on their behaviour, movements and communication from the coast, leaving a knowledge gap in the offshore environments, and therefore being unable to determine the proper use they give to those areas and the risks they face around them. Acoustic data collected with a SoundTrap were analysed to assess the detectability of both species in the Malin Sea. Vocalisations were classified based on aural and visual features, as well as using non-parametric classification trees. Differences in the vocalisation rate of grey seals per diel, season and tidal state were also assessed through Generalised Linear Mixed Models, obtaining significant results, and finding similarities in the vocalisations of grey seals with the Scottish and Irish populations. A small sample of adult and pup harbour seals was detected, and differences in call type and number of detections per type were found across the seasons. These results show the importance of the area for both species, and lay the foundations for future studies, which will help to implement proper conservation measures such as Marine Protected Areas.

Exploring environmental and biological drivers of cetacean occurrence in the cross-border region of the Malin Shelf using data from a European fishery survey

Irish and Scottish waters are important habitats for cetaceans in Europe. Yet, little data is available for the region of the Malin Shelf, north of Ireland. Despite a rich species diversity, relative cetacean abundance appears low compared to hotspots documented west of Scotland and Ireland. Whether this perceived low prevalence accurately portrays an ecological discontinuity or arises from a lack of published results and low survey effort in that transborder area remains unclear. Here, we used sighting records from a multi-disciplinary fisheries survey, the Western European Shelf Pelagic Acoustic Survey (WESPAS), to explore cetacean habitat preferences over the Malin and Hebridean shelves. Northern minke whale and common dolphin occurrence was modelled within a Bayesian Additive Regression Trees (BART) framework, against selected environmental and biological variables. No correlation was observed between cetacean presence and in-situ prey biomass. Minke whales distribution was better explained by oceanography, notably proxies for frontal activity, and primary productivity. Common dolphins similarly showed preferences for shelf waters within 5-25km of fronts, but also affinities for fine substrates. Favourable habitats identified by the models were consistent with literature around the Hebrides and shed light on potentially important areas along the Islay front and north of Donegal, so far unreported due to data deficiency. Results will contribute towards informing future monitoring, strategic management and conservation efforts in this cross-border region.

Exploring controls of the early and stepped deglaciation on the western margin of the British Irish Ice Sheet

ABSTRACTNew optically stimulated luminescence dating and Bayesian models integrating all legacy and BRITICE‐CHRONO geochronology facilitated exploration of the controls on the deglaciation of two former sectors of the British–Irish Ice Sheet, the Donegal Bay (DBIS) and Malin Sea ice‐streams (MSIS). Shelf‐edge glaciation occurred ~27 ka, before the global Last Glacial Maximum, and shelf‐wide retreat began 26–26.5 ka at a rate of ~18.7–20.7 m a–1. MSIS grounding zone wedges and DBIS recessional moraines show episodic retreat punctuated by prolonged still‐stands. By ~23–22 ka the outer shelf (~25 000 km2) was free of grounded ice. After this time, MSIS retreat was faster (~20 m a–1 vs. ~2–6 m a–1 of DBIS). Separation of Irish and Scottish ice sources occurred ~20–19.5 ka, leaving an autonomous Donegal ice dome. Inner Malin shelf deglaciation followed the submarine troughs reaching the Hebridean coast ~19 ka. DBIS retreat formed the extensive complex of moraines in outer Donegal Bay at 20.5–19 ka. DBIS retreated on land by ~17–16 ka. Isolated ice caps in Scotland and Ireland persisted until ~14.5 ka. Early retreat of this marine‐terminating margin is best explained by local ice loading increasing water depths and promoting calving ice losses rather than by changes in global temperatures. Topographical controls governed the differences between the ice‐stream retreat from mid‐shelf to the coast.

Interaction of multiple ice streams on the Malin Shelf during deglaciation of the last British–Irish Ice Sheet

ABSTRACTThe Malin Shelf, off north‐west Ireland, was an important zone of confluence for marine‐based ice streams of the former British–Irish Ice Sheet (BIIS). Legacy geophysical datasets are used to construct models of the seismic character, relative age and distribution of shelf sediments and landforms. Buried and surface landform assemblages provide evidence that during deglaciation of the Late Devensian BIIS, the region was occupied not by a single Hebrides Ice Stream as previously proposed, but by four discrete ice streams, here referred to as the Sea of the Hebrides (SHIS), Inner Hebrides, North Channel and Tory Island ice streams. Our observations of stratigraphic relationships between the deposits of these ice streams indicate physical interactions between them during shelf deglaciation. We interpret an initial dominant cross‐shelf flow along the SHIS impeding cross‐shelf ice flow from other ice sheet sectors. Following withdrawal of the SHIS grounding line from the shelf edge to mid‐shelf bathymetric highs during deglaciation, a reconfiguration of ice sheet flow paths allowed the expansion of smaller cross‐shelf ice streams draining central Scotland and north‐western Ireland. This internal dynamic behaviour provides a possible physical analogue for time‐transgressive flow patterns reported for outlets draining the West Antarctic Ice Sheet.

Final deglaciation of the Malin Sea through meltwater release and calving events

During the last glacial maximum, the British–Irish Ice Sheet (BIIS) extended to the shelf edge in the Malin Sea between Ireland and Scotland, delivering sediments to the Donegal Barra Fan (DBF). Analysis of well-preserved, glacially derived sediment in the DBF provides new insights on the character of the BIIS final deglaciation and palaeoenvironmental conditions at the Younger Dryas. Chaotic/laminated muds, ice-rafted debris (IRD)-rich layers and laminated sand–mud couplets are interpreted as respectively mass transport deposits, plumites and turbidites of BIIS-transported sediments. Peaks in IRD, constrained by radiocarbon dating to after 18 cal ka BP, indicate discrete intervals of iceberg calving during the last stages of deglaciation. Glacially derived sedimentation on the slope occurred until c . 16.9 cal ka BP. This is interpreted as the last time the ice sheet was present on to the shelf, allowing glacial meltwater to reach the fan. Bioturbated and foraminifera-rich muds above glaciomarine sediments are interpreted as interglacial hemipelagites and contourites, with the presence of Zoophycos suggesting restoration of bottom currents at the transition between stadial and interstadial conditions. During the Younger Dryas, Neogloboquadrina pachyderma sinistral abundances and an isolated peak in IRD indicate the temporary restoration of cold conditions and the presence of icebergs in the region. Thematic collection: This article is part of the Early Career Research collection available at: https://www.lyellcollection.org/cc/SJG-early-career-research

Extent and retreat history of the Barra Fan Ice Stream offshore western Scotland and northern Ireland during the last glaciation

During the Last Glacial Maximum (LGM) the marine-terminating Barra Fan Ice Stream (BFIS), a major conduit of the British Irish Ice Sheet (BIIS), drained much of western Scotland and northwest Ireland with ice streaming onto the continental shelf of the Malin Sea. The extent and retreat history of this ice stream across the shelf, until now, is not well known. In particular, geochronological constraints on the history of this ice stream have thus far been restricted to deep-sea cores or terrestrial cosmogenic nuclide dating onshore, with ages across the shelf absent. To understand the possible external forcing factors acting on this marine terminating ice stream during retreat, improved geochronological constraint on its deglaciation is necessary. Here, we present new geophysical data, marine sediment cores and over forty radiocarbon dates to provide important constraints on maximum extent of the BFIS, as well as the timing and pattern of retreat back across the Malin Shelf. Dated moraines and grounding-zone wedges (GZW) seen in seafloor sub-bottom profiles provide evidence that the BFIS reached the Malin Shelf edge during the LGM and was at its maximum extent around 26.7 ka BP. The presence of two sets of GZWs suggests that the style of retreat was episodic. The new radiocarbon chronology shows that retreat from the shelf edge was underway by 25.9 ka BP, with the majority of the continental shelf ice free by 23.2 ka BP, and that glacimarine conditions were present in the Sea of Hebrides by 20.2 ka BP at the latest. Collectively, these results indicate that the majority of the Malin Shelf was free of grounded ice by ∼21.5–20 ka BP, which is up to 4000 years earlier than previously reconstructed. We attribute this early deglaciation to high relative sea level caused by glacial isostatic depression when the BIIS reached its maximum extent promoting ice shelf and grounding line instability. Two deep troughs, forming reverse bed slopes, aided the continued retreat of the BFIS. This suggests that local ice loading and bed morphology can be significant controls on the destabilisation of a marine-terminating ice stream and can override the influence of ocean and atmospheric temperatures.

Cross-slope flow in the Atlantic Inflow Current driven by the on-shelf deflection of a slope current

We identify a newly described current, the Atlantic Inflow Current (AIC), a persistent pathway of Atlantic Water from the European Slope onto the Malin Shelf. Using drifters and gliders we examine the vertical and horizontal structure of the AIC and use Lagrangian statistics to quantify lateral mixing along its path. We estimate this current to have a transport of approximately 0.2 Sv, advecting 2.8 TW of heat onto the shelf (referenced to 7 °C) and 2.4 kT/day of the limiting nutrient, nitrate. This nutrient-rich AIC joins the Irish Coastal Current, continuing into the Minch and the outer Hebridean Shelf before ultimately entering the North Sea. The biological consequences of the influx of water masses and nutrients onto the shelf can range from altering primary production, and subsequent food web dynamics, to recruitment of fish larvae from oceanic water. A better understanding of the current dynamics described here is crucial for the assessment of shelf sea primary production and its impacts on carbon draw-down as well as recruitment for commercially important fisheries.

A downslope propagating thermal front over the continental slope

AbstractIn the ocean, internal frontal bores above sloping topography have many appearances, depending on the local density stratification, and on the angle and source of generation of the carrier wave. However, their common characteristics are a backward breaking wave, strong sediment resuspension, and relatively cool (denser) water moving more or less upslope underneath warm (less dense) water. In this paper, we present a rare example of a downslope moving front of cold water moving over near‐bottom warm water. Large backscatter is observed in the downslope moving front's trailing edge, rather than the leading edge as is common in upslope moving fronts. Time series observations have been made during a fortnight in summer, using a 101 m long array of high‐resolution temperature sensors moored with an acoustic Doppler current profiler at 396 m depth in near‐homogeneous waters, near a small canyon in the continental slope off the Malin shelf (West‐Scotland, UK). Occurring between fronts that propagate upslope with tidal periodicity, the rare downslope propagating one resembles a gravity current and includes strong convective turbulence coming from the interior rather than the more usual frictionally generated turbulence arising from interaction with the seabed. Its turbulence is 3–10 times larger than that of more common upslope propagating fronts. As the main turbulence is in the interior with a thin stratified layer close to the bottom, little sediment is resuspended by a downslope propagating front. The downslope propagating front is suggested to be generated by oblique propagation of internal (tidal) waves and flow over a nearby upstream promontory.

Tidally induced residual current over the Malin Sea continental slope

Tidally induced residual currents generated over shelf-slope topography are investigated analytically and numerically using the Massachusetts Institute of Technology general circulation model. Observational support for the presence of such a slope current was recorded over the Malin Sea continental slope during the 88-th cruise of the RRS “James Cook” in July 2013. A simple analytical formula developed here in the framework of time-averaged shallow water equations has been validated against a fully nonlinear nonhydrostatic numerical solution. A good agreement between analytical and numerical solutions is found for a wide range of input parameters of the tidal flow and bottom topography. In application to the Malin Shelf area both the numerical model and analytical solution predicted a northward moving current confined to the slope with its core located above the 400m isobath and with vertically averaged maximum velocities up to 8cms−1, which is consistent with the in-situ data recorded at three moorings and along cross-slope transects.

Bottom trapped internal waves over the Malin Sea continental slope

Three-dimensional dynamics of internal tides on the continental slope of the Malin Sea is studied numerically and validated with the observational data collected on the 88-th cruise of the RRS “James Cook” in June–July 2013. Observed in-situ bottom intensification of baroclinic tidal currents was reproduced in a series of numerical experiments using the Massachusetts Institute of Technology general circulation model. The baroclinic tidal dynamics is explained in terms of superposition of two wave processes developing in the area. One of them is a tidal beam generated at the shelf break and radiating tidal energy downward into abyss. The second important process identified here was a bottom trapped internal wave, generated by the tidal flow over a local prominence. This internal wave was trapped by the topography at the depths below 800m and propagated northward along the slope.

Nitrogen dynamics in the Irish Sea and adjacent shelf waters: An exploration of dissolved organic nitrogen

Relatively little is known about dissolved organic nitrogen (DON) in the marine environment because research has historically focused on dissolved inorganic nitrogen (DIN). In this study we combine measurements of dissolved organic matter (DOM), DIN, particulate organic nitrogen (PON), dissolved inorganic phosphorus (DIP) and silicon (DIS), with temperature and salinity data from the western shelf region of the UK and Ireland, and with inorganic and organic nitrogen (N) data from the western Irish Sea to develop an understanding of N dynamics in the Irish Sea and adjacent shelf waters, and investigate the role of DON in the nitrogen budget of the seasonally stratifying western Irish Sea. In January 2013, the sampling area was divided by density fronts into 4 regions of distinct oceanography and homogeneous chemistry. DON concentrations accounted for 25.3 ± 1.8% of total dissolved N (TDN) across all regions. DOM concentrations generally decreased from the freshwater influenced water of Liverpool Bay to the oceanic waters of the Celtic Sea and Malin Shelf. Urea and dissolved free amino acids (DFAA) together made up 27.3 ± 3.1% of DON. Estimated concentrations in the rivers discharging into Liverpool Bay were 8.0 and 2.1 μmol N L−1 respectively: at the high end of reported riverine concentrations. Oceanic nutrient inputs to the Irish Sea only have a small influence on N concentrations. Riverine N inputs to the Irish Sea are substantial but are likely removed by natural N cycling processes. In the western Irish Sea, DON and PON concentrations reached maxima and minima in midsummer and early spring respectively. DIN followed the opposite trend. DON accounted for 38% of the yearly internal N cycling and we estimated that as much as 1.4 ± 1.2 μmol N L−1 of labile DON was available as an N source at the start of the spring bloom. Our study supports the view that DON plays an important role in N cycling in temperate shelf and coastal seas and should be included more often in biogeochemical measurements if we are to have a complete understanding of N dynamics in a changing world.

Constraints on a shallow offshore gas environment determined by a multidisciplinary geophysical approach: The Malin Sea, NW Ireland

During the Irish National Seabed Survey (INSS) in 2003, a gas related pockmark field was discovered and extensively mapped in the Malin Shelf region (NW Ireland). In summer 2006, additional complementary data involving core sample analysis, multibeam and single-beam backscatter classification, and a marine controlled-source electromagnetic survey were obtained in specific locations.This multidisciplinary approach allowed us to map the upper 20 m of the seabed in an unprecedented way and to correlate the main geophysical parameters with the geological properties of the seabed. The EM data provide us with information about sediment conductivity, which can be used as a proxy for porosity and also to identify the presence of fluid and fluid migration pathways. We conclude that, as a whole, the central part of the Malin basin is characterized by higher conductivities, which we interpret as a lithological change. Within the basin several areas are characterized by conductive anomalies associated with fluid flow processes and potentially the presence of microbial activity, as suggested by previous work. Pockmark structures show a characteristic electrical signature, with high-conductivity anomalies on the edges and less conductive, homogeneous interiors with several high-conductivity anomalies, potentially associated with gas-driven microbial activity.

Internal Wave Parameter Inversion at Malin Shelf Edge Based on the Nonlinear Schrödinger Equation

We established a deep-sea internal wave detection model based on the nonlinear Schrödinger (NLS) equation and Synthetic Aperture Radar (SAR) images, and applied the model to the Malin Shelf edge, located at UK Continental Shelf, west of Scotland, to retrieve internal wave parameters. We selected the SAR images of internal waves at Malin Shelf edge, combined NLS equation with the action spectrum balance equation and Bragg scattering model, retrieved the amplitudes and phase velocities of the internal waves at Malin Shelf edge, and compared these data with those retrieved by the model based on KdV equation and those observed at the same period. The results show that the error between the data retrieved by our model and the measured data is very small, while the difference between the data retrieved by the detection model based on KdV equation and the measured data is significant. In addition, the phase velocities, calculated in our model and the model based on KdV equation, are both close to the measured data. Consequently, our model is valid and more accurate for the parameter inversion of internal waves in deep-sea area.

Growth and decay of a marine terminating sector of the last British–Irish Ice Sheet: a geomorphological reconstruction

The boundary conditions that govern ice sheet dynamics can change significantly with the development of marine margins. This paper uses the glacial landscape in western Scotland to reconstruct changes in the British–Irish Ice Sheet that accompanied the growth and decay of a marine sector over the Malin Shelf. Ice advanced from a restricted mountain ice sheet with tidewater margins after ∼35 ka BP, and reached the continental shelf in ∼7 ka (average rate of ∼30 m a−1). Early ice flow had been directed through north-south, geologically controlled, over-deepened fjords that were carved during previous ‘restricted’ glaciations. This flow regime was abandoned with development of the Malin Shelf ice sheet sector; ice flow direction switched by ∼90° and was drawn westwards towards the shelf edge. The marine ice sheet phase saw episodes of west-east ice divide migration by up to 60 km over west central Scotland, possibly linked to ice streaming and calving events at the ice sheet margin. However, permanent and stationary ice divides and zones of cold-based ice, associated with subglacial topographic highs, also characterised the marine glacial stage over western Scotland. The North Channel ice divide remained a constant, though migratory feature while the BIIS occupied the Malin Shelf; it finally collapsed at the end of the Killard Point Stadial when the Irish Ice Sheet began to rapidly decay ∼16.5 ka BP. This permitted the Scottish Ice Sheet to temporarily advance over north-east Ireland (previously identified as the East Antrim Coastal Readvance) before it too retreated, at rates in the order of 102 m a−1. Although the imprint of extensive shelf-edge ice sheet glaciation exists in the coastal landscape of western Scotland, the dominant landscape features relate to a restricted, marine-proximal mountain ice sheet with markedly different flow configurations. Similar first-order geomorphological features, relating to ‘restricted’ glacial conditions, are likely to be preserved in subglacial highlands under interior parts of modern ice sheets.

Glacier response in western Scotland following collapse of the last British-Irish Ice Sheet over the Malin Shelf

Glacier response in western Scotland following collapse of the last British-Irish Ice Sheet over the Malin Shelf

Geophysical and geochemical survey of a large marine pockmark on the Malin Shelf, Ireland

Marine pockmarks are a specific type of seabed geological setting resembling craters or pits and are considered seabed surface expressions of fluid flow in the subsurface. A large composite pockmark on the Malin Shelf, off the northern coast of Ireland was surveyed and ground truthed to assess its activity and investigate fluid related processes in the subsurface. Geophysical (including acoustic and electromagnetic) data confirmed the subsurface presence of signatures typical of fluids within the sediment. Shallow seismic profiling revealed a large shallow gas pocket and typical gas related indicators such as acoustic blanking and enhanced reflectors present underneath and around the large pockmark. Sulphate profiles indicate that gas from the shallow reservoir has been migrating upwards, at least recently. However, there are no chimney structures observed in the sub‐bottom data and the migration pathways are not apparent. Electromagnetic data show slightly elevated electrical conductivity on the edges of the pockmarks and a drop below regional levels within the confines of the pockmark, suggesting changes in physical properties of the sediment. Nuclear Magnetic Resonance (NMR) experiments were employed to characterize the organic component of sediments from selected depths. Very strong microbial signatures were evident in all NMR spectra but microbes outside the pockmark appear to be much more active than inside. These observations coincide with spikes in conductivity and the lateral gas bearing body suggesting that there is an increase in microbial activity and biomass when gas is present.

An assessment of persistent organic pollutants in Scottish coastal and offshore marine environments

Concentrations of persistent organic pollutants (POPs), including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) were determined in sediment and biota (fish liver) from around Scotland. The concentrations were investigated using assessment criteria developed by OSPAR and ICES. Organic contaminant concentrations, PAHs, PCBs and PBDEs in sediment, and PCBs and PBDEs in fish liver, were significantly higher in the Clyde compared to all other sea areas. This is mainly due to historic industrial inputs. Highest PCB and PAH concentrations were found in the strata furthest up the Clyde estuary, with concentrations of POPs in these strata being at levels such that there is an unacceptable risk of chronic effects occurring in marine species. Furthermore, for PAHs in Clyde sediment there was a significant negative gradient going from north to south towards the open sea. PAH and PCB concentrations in sediment and biota in all other Scottish sea areas (except for PCBs in sediment from East Scotland) were unlikely to give rise to pollution effects, being below relevant assessment criteria. Although no assessment criteria are available for PBDEs, the concentrations observed in Scottish sediments were low with all congeners below the limit of detection (LoD; 0.03 µg kg(-1) dry weight) in 140 out of a total of 307 samples analysed. Where PBDEs were detected, the dominant congeners were BDE47 and BDE99. PBDEs were detected in fish livers, although concentrations were less than 150 µg kg(-1) lipid weight in all sea areas except the Clyde where concentrations ranged between 8.9 and 2202 µg kg(-1) lipid weight. Few trends were detected in contaminant concentrations in biota or sediment at any Scottish site with more than five years data. Downward trends were detected in PAHs in sediment from the Clyde, Irish Sea and Minches and Malin Sea and PCBs in fish liver from the Moray Firth. Rules were developed for the aggregation of the contaminant data across a sea area. An overall assessment for each sea area was then assigned, looking at the frequency of sites or strata within each sea area that were above or below the relevant assessment criteria. Overall the status of the various sea areas, with respect to the assessed POPs, can be considered to be acceptable in that they were below concentrations likely to result in chronic effects for all sea areas except the Clyde.

Marine geophysical evidence for ice sheet extension and recession on the Malin Shelf: New evidence for the western limits of the British Irish Ice Sheet

Multibeam swath bathymetry and backscatter data collected as part of the Irish National Seabed Survey and Integrated Mapping for the Sustainable Development of Ireland's Marine Resource programmes is used to investigate the glacial geomorphology of the Malin Shelf. The data provides direct evidence that the former British Irish Ice Sheet was grounded as far out as the shelf break in this sector on more than one occasion. Drumlins on the outer shelf indicate the region was a zone of confluence where ice flowing onto the shelf from northwest Ireland converged with ice flowing across the shelf from western Scotland. Ice berg furrows located on the outer shelf record ice sheet break up by a calving event that may have been triggered by rising sea levels and northwest–southeast aligned moraines indicate the ice retreated north-eastwards across the shelf towards Scotland. The absence of direct dating control from moraines on the Malin Shelf precludes a definitive assessment of the timing of ice sheet events. However, consideration of the stratigraphic relationship of the glacial landforms on the shelf with age constrained ice sheet events in the wider region indicates that the former British Irish Ice Sheet reached the outer shelf in this region on at least two occasions during the last cold stage. Moraines that have been overprinted by drumlins may possibly belong to an early phase of glaciation that occurred c 45 ka B. This was subsequently followed by another episode of shelf extensive glaciation where converging ice flow from northwest Ireland and western Scotland reached the shelf break during the LGM that subsequently retreated from the shelf edge around 23 ka BP.

Spatial and temporal variations in parasite prevalence and infracommunity structure in herring (Clupea harengusL.) caught to the west of the British Isles and in the North and Baltic Seas: implications for fisheries science

Herring Clupea harengus L. viscera were examined for endoparasitic infections as part of a multidisciplinary stock identification project (WESTHER, EU Contract no. Q5RS-2002-01 056) which applied a range of stock discrimination techniques to the same individual fishes to obtain comparable results for multivariate analysis. Spawning and non-spawning adults, and juvenile herring were caught, over 3 years, by commercial and research vessels from numerous locations to the west of the UK and Ireland, along with control samples of spawning fish from the eastern Baltic Sea, and juveniles from sites in the eastern and western North Sea, and the north of Norway. The metacercariae of two renicolid digeneans (Cercaria pythionike and Cercaria doricha), one larval nematode (Anisakis simplex s.s.) and one larval cestode (Lacistorhynchus tenuis) were selected as tag species. Results were compared with those from herring collected between 1973 and 1982, which suggested remarkable stability in the parasite fauna of herring in the study area. These species were used to compare the parasite infracommunities of spawning herring. A significant variation in infracommunity structure was observed between different spawning grounds. These results suggest that the parasite fauna of herring are spatially variable but remain temporally stable in both the short and long term. Significant differences in prevalence and abundance of infections and comparisons of parasite infracommunity enabled the separation of putative herring stocks west of the British Isles. Distinctive patterns of parasite infection in two different spawning groups off the north coast of Scotland suggest that this area is occupied by two spawning populations, one recruiting from the west of Scotland, the other from outside this area, and most likely from the eastern North Sea. The distribution patterns of L. tenuis, C. doricha and C. pythionike suggest the potential for fish that spawn in three distinct International Council for the Exploration of the Seas (ICES) management units to be present in mixed aggregations found over the Malin Shelf, with significant implications for management in this area.

Topographic generation of internal waves by nonlinear superposition of tidal harmonics

A numerical study of topographic generation of internal waves by superposition of the semidiurnal and the diurnal tidal harmonics was motivated by the experimental data collected in the area of the Malin Shelf. In situ measurements and synthetic aperture radar (SAR) images revealed the evidence of the packets of internal waves propagating on-shelf. The tidal periodicity of these waves and their location (between the isobaths 160 m and 1000 m, i.e. far from the shelf break) as well as the spatial orientation of wave fronts suggest that they were not locally generated but came from some distant source. A detailed analysis performed with the help of a two-dimensional nonlinear nonhydrostatic numerical model shows that the most probable place of wave generation is the eastern flank of the Rockall Bank located 150 km off the Malin Shelf. The result demonstrates that a nonlinear superposition of diurnal and semidiurnal barotropic tidal harmonics was capable of generating the waves even though each of harmonics separately was too weak to produce a substantial baroclinic response. Such a case of efficient superposition of two tidal forcings takes place only over relatively steep bottom topography when fundamentally unsteady lee waves are excited. The same mechanism of generation can take place in some other parts of the World Ocean where the barotropic tides comprise several harmonics.