Podest^ Migratory pattern of Merluccius hubbsi 



173 



latitude of the Brazil-Malvinas confluence along the 

 shelf break. Shifts in the position of the confluence 

 plausibly could have been responsible for the differ- 

 ences in hake distribution between 1978 and 1979. 



Hydrographic stations occupied by the RV Walther 

 Herwig during July 1978 placed the Brazil-Malvinas 

 confluence along the shelf break at about 36° S. In 

 August 1978, the confluence must have been located 

 north of 37 °S, as temperatures characteristic of Brazil 

 Current waters -were not observed during a cruise car- 

 ried out by the RV Sh inkn i Ma.ru that reached approx- 

 imately that latitude (data not shown). 



No hydrographic data were available to locate the 

 confluence in July- August 1979. Instead, I used weekly 

 GOSSTCOMP (global ocean sea surface temperature 

 computation) charts for the period 3 July- 28 August 

 1979. The charts were produced from satellite data by 

 the U.S. National Oceanic and Atmospheric Adminis- 

 tration (Brower et al. 1976). The 10°C and 12°C surface 

 isotherms were considered to represent the Brazil- 

 Malvinas confluence at this time of the year. The ap- 

 proximate latitudes at which those isotherms occurred 

 along the shelf break are displayed in Figin-e 7. 



In early July 1979, the confluence was located at 

 about 37°S, only slightly further south than in July 

 1978. During the last 2 weeks of July 1979, however, 

 it moved to the south, reaching about 42 °S by the end 

 of the month. During August 1979, the confluence was 

 generally located around 40°S. The deployment of 

 fishing effort during July- August 1979 coincided 

 roughly with the confluence locations deduced from the 

 GOSSTCOMP charts. This seems to support the hypoth- 

 esis linking shifts in the position of the Brazil-Malvinas 

 confluence and hake distribution in 1978 and 1979. 



Discussion 



Feeding migration 



The need to find an adequate food supply for main- 

 tenance, growth, and reproduction may have contrib- 

 uted to shape the migi'atory pattern of Argentine hake. 

 Adult hake feed actively after spawning (Hart 1946). 

 The postspawning offshore migi-ation of adults towards 

 the shelf break, then, may correspond to a search for 

 abundant food sources. 



The western edge of the Malvinas Current, from 

 about 40° S to its confluence with the Brazil Current, 

 plays an important role in the northward migration of 

 hake, as the fish are closely associated with it for 5-6 

 months of the year. The boundary between shelf and 

 Malvinas Current waters can be characterized as a 

 shelfbreak front. Shelfl^reak fronts elsewhere have 

 been associated with high biological productivity and 

 Ashing intensity (Fournier et al. 1979). 



Figure 7 



Approximate latitude of the Brazil and Malvinas currents (.'(jniluence 

 along the shelf break, July- August 1979, derived from GOSTCOMP 

 charts. Each vertical line corresponds to a weekly sea-surface tem- 

 perature chart. Shaded area represents projection of the 10- and 12°C 

 isotherms along the shelf break, assumed to represent the position 

 of the confluence. 



The high productivity of the Malvinas front has been 

 described from ship data (Hubold 1980a,b and refer- 

 ences therein). More recently, remote sensing tech- 

 niques have confirmed this feature: images collected 

 by the Coastal Zone Color Scanner (CZCS) were userl 

 to monitor nearsurface concentrations of phytoplank- 

 ton pigments and, indirectly, phytoplankton liiomass. 

 High phytoplankton concentrations occurred along the 

 shelfbreak through spring and summer; in contrast, 

 phytoplankton biomass across most of the shelf de- 

 creased rapidly folk)wing the late September-early 

 October spring bloom. The shelfbreak, therefore, pos- 

 sibly represents one of the few places where forage is 

 abundant during the summer, following the hake's 

 spawning. 



After a second bloom ends on the shelf in the fall (late 

 March-early April), phytoplankton biomass along the 

 shelfbreak remains high into late April. This may in 

 turn support elevated zooplankton concentrations in 

 subsequent winter months (Hubold 1980b), although at 

 that time phytoplankton biomass is low. 



The large phytoplankton biomass along the shelf 

 break is probably the result of enhanced supply of 

 nutrient-rich MaKnnas Current waters into the euphotic 

 zone, which could happen through a variety of pro- 

 cesses. Small-scale eddies along the edge of the Mal- 

 vinas Current may upwell nutrient-rich waters. This 

 has been shown to occur along the edge of the Gulf 

 Stream, resulting in an increase in phytoplankton and 

 zooplankton biomasses (Paffenhoffer et al. 1984). Dick- 

 son et al. (1980) suggested that the interaction between 

 coastally trapped waves propagating along the slope 

 and bottom topography at the shelfbreak could enhance 

 upwelling. The generation of internal tides at the 

 shelfbreak, coupled with episodic wind stress, may 



