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Fishery Bulletin 88(1), 1990 



increase vertical mixing, again injecting nutrients into 

 the upper layers (Maze et al. 1986). Additionally, the 

 tilt of the isopycnals or the interleaving of water 

 masses at the front would enhance vertical stability, 

 retaining phytoplankton cells in the euphotic zone 

 (Fournier et al. 1979). 



The increased phytoplankton bioniass along the shelf 

 break supports large concentrations of zooplankton 

 (Hubold 1980a); species on which i^elagnc fish feed (e.g., 

 calanoid copepods) are very abundant (Carreto et al. 

 1981a, b). Species from higher trophic levels also aggre- 

 gate at the front. 



The distribution pattern of anchoita Eugriiulis an- 

 choita. an extremely important prey for hake, is close- 

 ly associated with the shelflireak front (Brandhorst 

 et al. 1971). While hake and anchoita coincidentally 

 migrate offshore and then northward along the slope 

 in autumn, the former actively feed on the latter 

 (Ciechomski and Sanchez 1983). 



During the northward migration, other components 

 may appear in the hake's diet, such as myctophids 

 associated with the Malvinas Current (e.g., Gynuiosco- 

 pelus spp., Lanipnnijctuii spp., and Myctophum spp.) 

 and shortfin squid Illex argerifiin^s (Cordo 1981). Dense 

 concentrations of shortfin squid occur along the shelf- 

 break in late fall (Otero et al. 1981). Squid is increas- 

 ingly found in the stomachs of medium and large hakes 

 between April and July (Angelescu and Cousseau 1969, 

 Cordo 1981). Given their high food assimilation efficien- 

 cy (Caddy 1983), squid probably play an important eco- 

 logical role in rapidly transferring energy from lower 

 trophic levels to hake. 



Wlien hake reach the northernmost end of their feed- 

 ing migration in winter, they liave access to a large 

 abundance of prey from a variety of closely located en- 

 vironments. In addition to the species found along the 

 shelllireak, they may feed on anchoita wintering in mid- 

 shelf waters or on subtropical myctophids from Brazil 

 Current waters. Further south or inshore, in contrast, 

 pelagic fish become rare and the overall abundance of 

 prey decreases rapidly (Angelescu and Cousseau 1969). 



Spawning migration 



The present migratory pattern of hake may have 

 evolved to maximize chances of reproductive, as well 

 as feeding, success. Starvation is a major source of lar- 

 val mortality for most fish species. The choice of spawn- 

 ing sites with adequate concentrations and size ranges 

 of larval food may have a great influence on survival. 

 Hake larvae feed almost exclusively on zooplankton, 

 particularly small calanoid copepods (Ciechomski and 

 Weiss 1974). 



Regional shifts in hake spawning activity may be 

 related to the seasonal pattern of productivity along 



the shelf. The phytoplankton spring bloom begins in 

 the north and progresses southward (Carreto et al. 

 1981a). The spawning activity between 36 °S and 38 °S 

 in early spring, then, may be tied to increased zoo- 

 plankton availability resulting from the bloom. How- 

 ever, satellite imagery shows that postbloom ])hyto- 

 plankton biomass here remains low throughciut late 

 spring and summer and apparently does not support 

 an extended spawning season. Later in the season, the 

 bulk of the spawning activity takes place between 42° S 

 and 44°S, where production possibly may continue dur- 

 ing the summer because of the presence of tidal fronts. 



The tidal fronts appear to be associated with en- 

 hanced phytoplankton biomass: Carreto et al. (1981a) 

 observed integrated chlorophyll concentrations >200 

 mg/m- at the beginning of October. The increased 

 biomass may be due to enhanced nutrient supply by 

 horizontal mixing from the mixed side or by vertical 

 mixing through the pycnocline. Carreto et al. (1986) 

 found high surface concentrations of nitrates (>6 pmi) 

 on the mixed side. 



The existence of the tidal fronts has only recently 

 been noticed and, consequently, there is relatively little 

 information on their biological productivity. Never- 

 theless, high concentrations of zooplankton (express- 

 ed in dry weight) have been reported along the fronts 

 (Angelescu and Prenski 1987). There is additional in- 

 direct evidence for the relatively high productivity of 

 these features: hake larvae in this area are found to 

 be in an excellent nutritional state, better than 

 elsewhere on the shelf (R. Sanchez, INIDEP, C.C. 175, 

 Playa Grande, Mar del Plata, Argentina, pers. com- 

 mun.. Feb. 1989). Additionally, adult hake find abun- 

 dant anchoita and squid in the vicinity of the fronts 

 (Angelescu and Prenski 1987). 



The tidal fronts occur throughout the summer, for 

 as long as the offshore side of the front remains strati- 

 fied. Satellite imagery shows that this area, together 

 with the shelfbreak, are the only places where near- 

 surface phytoplankton biomass remains high through- 

 out the summer. The integrated spring-summer pro- 

 duction of the tidal fronts area, therefore, may possibly 

 be higher than in most parts of the shelf, although this 

 requires confirmation. 



Another factor relevant to the selection of a spawn- 

 ing site is the subsequent transport of eggs and larvae 

 to (or, alternatively, their retention within) areas suit- 

 able for further development. Argentine hake show a 

 wide temporal and spatial spawning range: hake eggs 

 are found on the shelf practically year-round. Never- 

 theless, the gonads do not mature more than once dur- 

 ing the main spawning period of spring-summer (Chris- 

 tiansen and Cousseau 1971), suggesting that a number 

 of groups of spawners release eggs throughout a large 

 area and an extended time interval. Argentine hake 



