Nonaka et al,: Seasonal variation in larval fish assemblages off eastern Brazil 



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Discussion 



The warm and salty tropical water which covers the entire 

 sui-vey area, is transported southward by the Brazil Cur- 

 rent. The cold South Atlantic Central Water occupies the 

 subsurface layer beneath the Brazil Current but owing to 

 the deep thermocline (80-120 m), the nutrient rich SACW 

 does not come up to the euphotic zone in the survey 

 area, with exception of the coastal region of Cape Frio- 

 Cape Sao Tome. Consequently this area is known to be 

 oligotrophic with a low concentration of nitrates, chloro- 

 phyll-a biomass, and primary production (Gaeta et al., 

 1999). The primary production of the nutrient-depleted 

 surface layer is supported principally hy autotrophic and 

 heterotrophic picoplankton which use the recycled nutri- 

 ents in the water. The heterotrophic dinoflagellates and 

 ciliates are responsible for the transfer of energy pro- 

 duced by picoplankton to the upper trophic levels (Susini- 

 Ribeiro,1999). 



There was a time gap between the first two cruises 

 (June and November-December in 1978) and the last one 

 (April-May in 1995); therefore the seasonal variations of 

 hydrographic conditions and larval distribution in this 

 region could have been the result of interannual variation. 

 The El Nino year in the tropical Pacific normally causes 

 a drought in the northeastern Brazil and a flood in the 

 south. Bearing in mind that 1978 and 1995 were not atypi- 

 cal El Nino years, we assumed that the interannual varia- 

 tion in hydrographic conditions between these two periods 

 could be ignored. We had no any other information on the 

 interannual variation in oceanographic conditions in this 

 region. 



The fish larvae in the Abrolhos Bank region off the east- 

 ern coast of Brazil were produced by a diverse collection of 

 fish species that can be divided into two dominant groups; 

 mesopelagic fish and tropical reef fish. The high diversity 

 of taxa (77 families and 6 orders) is characteristic of the 

 tropical marine ecosystem and similar taxonomic diversity 

 of lai-val fish is known from the eastern tropical Pacific 

 (Ahlstrom, 1971; 1972) and the Gulf of Mexico (Richards et 

 al., 1993). 



The most surprising results were the occurrence of 

 many reef-fish larx'ae along the shelf break areas. Assum- 

 ing that the reef fish larvae found along the shelf break 

 area off the Abrolhos Bank are transported southward by 

 the main axis of the Brazil Current, most of them should 

 be trapped in the Vitoria eddy after passage through the 

 Vitoria Channel. Consequently they can be recruited at 

 the southern margin of the bank. In order to prove this 

 hypothesis, an intensive sampling program, coupled with 

 an application of aging technique for postsettlement juve- 

 niles, is necessary in this region. 



The dominant taxonomic groups (family) from the three 

 cruises showed a distinct seasonal pattern in abundance. 

 The overall larval abundance of the summer cruise was 

 the highest. Among the Myctophidae, the larval abundance 

 of Myctophum affine was highest in summer and insig- 

 nificant in other seasons. Other species iDiaphus spp., 

 Lepidophanes giientheri, Myctophum iiitidiiluin, M. obtu- 

 sirostre, Hygophum reinhardtii) also showed relatively 



high values in summer Many dominant species concen- 

 trate their spawning activities during the austral summer. 

 Myctophid fish in the temperate and subtropical seas are 

 known to spawn mainly from late winter to summer, coin- 

 ciding with the seasonal peak of zooplankton production in 

 the area (Clarke, 1973; Doyle et al., 1993). The mean dis- 

 placement volume of macrozooplankton in the open ocean 

 was 33-50% higher in summer than during winter and 

 autumn, suggesting that many fish species have evolved 

 spawning patterns that are synchronized with zooplank- 

 ton production in this region. 



The composition of dominant myctophid larvae in this 

 region is different from that obsei-ved on the other side 

 of the South Atlantic, i.e. the Lampanyctodes hectoris, 

 a commercially exploited lanternfish, is the most abun- 

 dant in the Benguela Current, followed by Syrnbolophorus 

 and Diaphiis (Ahlstrom et al., 1976; Olivar and Shelton, 

 1993). Meanwhile, the four abundant genera (Diaphus, 

 Myctophum. Lepidophanes, and Hygophum ) in the eastern 

 tropical Pacific (Ahlstrom, 1971; 1972) were also found in 

 the survey area. Most of the Maurolicus muelleri lai-vae 

 were collected from south of Abrolhos Bank in winter and 

 summer, but their occurrence during the autumn cruise 

 was at a minimum. Because the highest densities of M. 

 muelleri larvae were recorded in oceanic waters from 

 south Brazil Bight (23-29°S) (Ribeiro, 1996), those found 

 in the survey area may represent only a northern exten- 

 tion of the southern population. 



Multivariate analysis of the dominant taxa in the region 

 suggests the existence of geographically distinct larval 

 fish assemblages that show significant seasonal variation. 

 Observed distribution patterns of dominant gi'oups are 

 the result of synchronous and geographically coherent 

 spawning activities of different groups of adult fish. The 

 Abrolhos Bank assemblage was characterized by coral- 

 reef-associated fish, predominantly Gobiidae, on the three 

 cruises. Small pelagic fish were the dominant group in 

 the neritic assemblage. The transitional assemblage was 

 characterized by a mixture of coral-reef-associated fish 

 and mesopelagic fish and a seasonal change of dominant 

 groups. The oceanic assemblage was dominated by the 

 presence of mesopelagic fish, especially the Myctophidae 

 in all seasons. 



The general geographic positions of the four lai-val 

 fish assemblages showed similar distribution patterns on 

 winter and autumn cruises, but those of the summer 

 cruise were slightly different. The oceanic assemblage, 

 which extended in the offshore area from the Minerva Sea- 

 mount to Cape Sao Tome in autumn and winter, was lim- 

 ited to the area between Vitoria and Cape Frio during the 

 summer cruise. The offshore area between the Minerva 

 and Vitoria Seamounts was occupied by the transitional 

 assemblage in summer The Abrolhos Bank and neritic 

 assemblages consistently occupied most parts of the conti- 

 nental shelf during the three cruises, but different larval 

 fish assemblages occupied the offshore area south of the 

 Abrolhos Bank. This finding can be interpreted as a result 

 of the seasonal variation of hydrographic conditions in 

 this specific area, such as the formation of the Vitoria eddy 

 and the intensity of coastal upwelling. 



