Claude Roy

Climate Variability, Fish, and Fisheries

Abstract Fish population variability and fisheries activities are closely linked to weather and climate dynamics. While weather at sea directly affects fishing, environmental variability determines the distribution, migration, and abundance of fish. Fishery science grew up during the last century by integrating knowledge from oceanography, fish biology, marine ecology, and fish population dynamics, largely focused on the great Northern Hemisphere fisheries. During this period, understanding and explaining interannual fish recruitment variability became a major focus for fisheries oceanographers. Yet, the close link between climate and fisheries is best illustrated by the effect of “unexpected” events—that is, nonseasonal, and sometimes catastrophic—on fish exploitation, such as those associated with the El Nino–Southern Oscillation (ENSO). The observation that fish populations fluctuate at decadal time scales and show patterns of synchrony while being geographically separated drew attention to oceanograph...

Climate change and small pelagic fish

Foreword Preface 1. History of international co-operation in research Jurgen Alheit and Andrew Bakun 2. A short scientific history of the fisheries Alec D. Maccall 3. Habitats Dave Checkley, Patricia Ayon, Tim R. Baumgartner, Miguel Bernal, J. C. Coetzee, Robert Emmett, Renato Guevara, Larry Hutching, Leire Ibaibarriaga, Hideaki Nakata, Yoshioki Oozeki, Benjamin Planque, Jake Schweigert, Yorgos Stratoudakis and Carl D. Van der Lingen 4. Variability from scales in marine sediments and other historical records David B. Field, Tim R. Baumgartner, Vicente Ferreira, Dimitri Gutierrez, Hector Lozano-Montes, Renato Salvatteci and Andy Soutar 5. Decadal-scale variability in populations Jurgen Alheit, Claude Roy and Souad Kifani 6. Biophysical models Christophe Lett, Kenneth A. Rose and Bernard A. Megrey 7. Trophic dynamics Carl D. Van der Lingen, Arnaud Bertrand, Antonio Bode, Rick Brodeur, Luis Cubillos, Pepe Espinoza, Kevin Friedland, Susana Garrido, Xabier Irigoien, Todd Miller, Christian Mollman, Ruben Rodriguez Sanchez, Hiroshige Tanaka and Axel Temming 8. Impacts of fishing and climate change explored using trophic models Lynne Shannon, Marta Coll, Sergio Neira, Philippe Cury and Jean-Paul Roux 9. Current trends in the assessment and management of stocks Manuel Barange, Miguel Bernal, Maria Cristina Cergole, Luis A. Cubillos, Carryn L. Cunningham, Georgi M. Daskalov, Jose A. A. de Oliveira, Mark Dickey-Collas, Dave J. Gaughan, Kevin Hill, Larry D. Jacobson, Fritz W. Koster, Jacques Masse, Hiroshi Nishida, Miguel Niquen, Yoshioki Ooozeki, Isabella Palomera, Suzana A. Saccardo, Alberto Santojanni, Rodolfo Serra, Stylios Somarakis, Yorgos Stratoudakis, Andres Uriarte, Carl D. Van der Lingen and Akihiko Yatsu 10. Global production and economics Samuel F. Herrick, Jr., Jerrold G. Norton, Rognvaldur Hannesson, U. Rashid Sumaila, Mahfuzuddin Ahmed and Julio Pena-Torres 11. Human dimensions of the fisheries under global change Rosemary E. Ommer, Astrid C. Jarre, R. Ian Perry, Manuel Barange, Kevern Cochrane and Coleen Moloney 12.Mechanisms of low-frequency fluctuations in anchovy and sardine populations Alec D. MacCall 13. Research challenges in the twenty-first century Andrew Bakun 14. Conjectures on future climate effects on marine ecosystems dominated by small pelagic fish Pierre Freon, Francisco Werner and Francisco Chavez 15. Synthesis and perspective Dave Checkley, Andrew Bakun, Manuel A. Barange, Leonardo R. Castro, Pierre Freon, Renato Guevara-Carrasco, Samuel F. Herrick, Jr., Alec D. MacCall, Rosemary Ommer, Yoshioki Oozeki, Claude Roy, Lynne Shannon and Carl D. van der Lingen Index.

Abrupt environmental shift associated with changes in the distribution of Cape anchovy Engraulis encrasicolus spawners in the southern Benguela

Cape anchovy Engraulis encrasicolus spawners in the southern Benguela showed an eastward shift in their distribution on the Agulhas Bank that occurred abruptly in 1996 and has since persisted. We assessed whether this shift was environmentally mediated by examining sea surface temperature data from different regions of the Agulhas Bank, which showed that in 1996 the inner shelf of the Agulhas Bank to the east of Cape Agulhas abruptly became 0.5°C colder than in previous years and has since remained that way. In addition, signals, coherent with the 1996 shift recorded in sea surface temperatures, were also found in atmospheric surface pressure and zonal wind data for that region; interannual coastal SST variability is also shown to be correlated with zonal wind-stress forcing. As a result, increased wind-induced coastal upwelling east of Cape Agulhas is proposed as the main driver of the observed cooling in the coastal region. The synchrony between the environmental and biological signals suggests that the eastward shift in anchovy spawner distribution was environmentally mediated and arose from a change in environmental forcing that altered the relative favourability for spawning between regions to the west and east of Cape Agulhas. The results highlight how a relatively minor change in environmental conditions can lead to a drastic spatial reorganisation of the life history of one species in an ecosystem.

Generation of cyclonic eddies by the Agulhas Current in the lee of the Agulhas Bank

Anti-cyclonic rings are shed from the Agulhas Current at its retroflection. They subsequently drift off into the South Atlantic. Smaller, cyclonic eddies have also been observed in this region. The origin of these latter eddies has remained unknown. We present model results that indicate that the configuration of the southern Agulhas Current and the poleward termination of the continental shelf of Africa allows shedding of cyclonic lee eddies by a flow detachment process. Hydrographic data, thermal infrared satellite images and altimetric observations are furnished that show that this model simulation is consistent with the characteristics and the behaviour of cyclonic eddies in the region.

Simulation of a coastal jet retention process using a barotropic model

In the southern Benguela, sardine and anchovy spawn on the Western Agulhas Bank. Eggs and larvae are transported to the West coast nursery ground by a coastal current. Once the larvae reach the West coast, retention within the favorable coastal domain is key to larvae survival and recruitment. The Cape Columbine-St Helena Bay area is the main nursery ground along the West Coast. In this area, the interaction between upwelling and topography results in the formation of an upwelling plume. The plume, and the associated circulation patterns, is thought to be the mechanism providing retention within the coastal domain. The dynamics of the plume are studied using a set of numerical experiments. It is shown that an equatorward wind forcing produces a cyclonic eddy in the lee of Cape Columbine. A balance between advection and bottom friction regulates the evolution of this eddy. The eddy contributes to create a dynamic boundary between the coastal and the offshore domains. Retention induced by the eddy is quantified using a tracer. The magnitude of retention is positively related to the wind for weak to moderate wind intensities. For strong wind intensity, retention reaches a maximum or slightly decreases. This is in agreement with previous results relating recruitment success with wind speed.

Cross-shelf structure of coastal upwelling : a two - dimensional extension of Ekman's theory and a mechanism for inner shelf upwelling shut down

Sea-surface temperature images of the coastal upwelling regions off Northwest Africa show that the core of upwelling is sometimes located far from the coast. This has been documented in three regions that share a common feature, namely a wide and shallow continental shelf. This upwelling feature plays a key role in the ecology of the Canary Current System. It creates an innerfront which provides retention for biological material, e.g. fish eggs and larvae, in the highly productive nearshore environment. An analytical model has been developed based on a two dimensional extension of Ekmans solution. The linear and steady response of a homogeneous ocean forced by an upwelling-favorable wind provides a mechanism for the upwelling separation from the coast. The merging of the surface and bottom Ekman layers induces a very weak cross-shore circulation and a “kinematic barrier” for the Ekman transport divergence. In the case of an alongshore wind, the barrier is located near the isobath h ≈ 0.4D, where D is the thickness of Ekman layers. This yields an upwelling cell which is essentially concentrated in the region 0.5D < h < 1.25D, with upwelling occurring preferentially near the isobath h ≈ 0.6D. It turns out that the cross-shore width of upwelling scales with D/S, the ratio of Ekman depth to bottom topographic slope. The application of this solution to real bathymetric profiles rationalizes, not only the offshore upwelling observations in Northwest Africa, but also the influence of topography on the cross-shelf structure of a wind-driven coastal upwelling. The model also quantifies the effect of the cross-shore wind component showing how it drives the nearshore pressure gradient adjustment and how it affects the upwelling. A linear numerical experiment reproduces the theoretical steady solution, thereby allowing investigation of the transient regime. Relaxation of the hypothesis in the numerical model validates the linear assumption of the theory and then allows investigation of the sensitivity to friction parameterizations and the influence of stratification. The latter leads to an “oscillation” of the upwelling cell with seaward migration driven by outcropping and homogeneization of the water column, and, coastal incursion driven by a “boundary layers splitting” process caused by shoreward advection of the isopycnal dome and stratification of the inner shelf.

Modelling the shear edge eddies of the southern Agulhas Current

Observations have revealed the presence of cyclonic eddies embedded in the landward border of the southern Agulhas Current. These shear edge eddies are most prevalent in the Agulhas Bank shelf bight, have a diameter of about 50– 100 km and are represented by a thermal dome and a surface warm plume inshore. In an attempt to better understand their generation and behavior, we have used a high-resolution model designed for the general region. The ocean model employed is the Regional Ocean Modeling System that solves the free surface, hydrostatic, primitive equations over variable topography using stretched, terrain-following coordinates in the vertical, orthogonal curvilinear coordinates in the horizontal, and featuring high-order schemes. The model simulates the creation of cyclonic shear edge eddies and their attendant surface plumes of warm water along the edge of the Agulhas Bank with a high degree of verisimilitude. The dimensions of the model features, their hydrographic structure and their velocities bear a strong resemblance to what has been observed. The simulation suggests that shear edge eddies remain trapped in the Agulhas Bank shelf bight and that eddies that subsequently travel downstream represent leakages from the resident shear eddy. This intermittent but frequent leakage of cyclonic motion downstream propagates at roughly 8 km day � 1 at intervals of roughly 20 days,

ENSO related modulation of coastal upwelling in the eastern Atlantic

An index of ENSO in the Pacific during early boreal winter is shown to account for a significant part of the variability of coastal SST anomalies measured a few months later within the wind driven West African coastal upwelling region from 10°N to 26°N. This teleconnection is thought to result from an atmospheric bridge between the Pacific and Atlantic oceans, leading to warm (cold) ENSO events being associated with a relaxation (intensification) of the Atlantic trade winds and of the wind-induced coastal upwelling. This ENSO related modulation of the wind-driven coastal upwelling appears to contribute to the connection observed at the basin-scale between ENSO and SST in the north Atlantic. The ability to use this teleconnection to give warnings of large changes in the West African upwelling several months in advance is successfully tested using data from the 1998 and 1999 ENSO events.

Climate Change and Small Pelagic Fish: Decadal-scale variability in populations

1 TOCA Summary 00 2 TOCA Introduction 00 3 TOCA Decadal-scale regime shifts 00 4 TOCA California Current ecosystem (CCE) 00 5 TOCB Changes in the biota 00 6 TOCB Regime shifts 00 7 TOCB Mechanisms linking climate to decadal-scale 8 population dynamics 00 9 TOCA Kuroshio Current ecosystem (KCE) 00 10 TOCB Changes in the biota 00 11 TOCB Regime shifts 00 12 TOCB Mechanisms linking climate to decadal-scale 13 population dynamics 00 14 TOCA Humboldt Current ecosystem (HCE) 00 15 TOCB Changes in the biota 00 16 TOCB Regime shifts 00 17 TOCB Mechanisms linking climate to decadal-scale 18 population dynamics 00 19 TOCA Benguela Current ecosystem (BCE) 00

Modeling the structure and variability of the southern Benguela upwelling using QuikSCAT wind forcing

The structure and patterns of variability of the southern Benguela coastal upwelling system are investigated with a high-resolution regional model forced by QuikSCAT winds over 1999-2003. The relevance of this global wind product is tested, at first, for the specific nearshore southeast Atlantic; then, the wind products are spatially or temporally degraded from the original 0.5 degrees daily fluxes and are used to diagnose the main scales of the surface dynamical forcing variability. Time resolution appears as a crucial factor in the wind stress to retrieve the patterns of interannual anomalies in sea surface temperatures in a good agreement with independent NASA Pathfinder observations. Various upwelling indices are also calculated in the model to study specific warm or cold coastal events whose circulation schemes are detailed from a Lagrangian interpretation of the model time-varying three-dimensional velocity field. Our study emphasizes the connections established between the Benguela upwelling and the large-scale circulation in relation with, for instance, the Agulhas Current retroflection and associated ring shedding.