Macchi et al.: Seasonal egg production of M/cropogonias furnien 



341 



From the duration of the breeding season, batch fecun- 

 dity, and spawning frequency, it is possible to get a rough 

 estimate of the annual egg production for whitemouth 

 croaker of the Rio de la Plata. Therefore, assuming a 

 spawning season of about six months, we estimated that 

 a 40-cm-TL female could produce between 3,300,000 and 

 7,300,000 eggs, depending on whether batch fecundity and 

 spawning frequency were estimated near the end of the 

 season or during the spawning peak, respectively. This an- 

 nual egg production is within the range of that estimated 

 for M. furnieri from the West Indies (from 2.4 to 13.5 mil- 

 lion eggs); (Manickchand-Heileman and Ehrhardt, 1996). 



During February-March, the percentage of whitemouth 

 croaker females with active ovaries decreased significantly 

 and oocyte production was half of that estimated for the 

 spawning peak. Furthermore, the plankton data showed 

 that M. furnieri egg diameters and dry weights decreased at 

 the end of the breeding season. The seasonal decrease in egg 

 size has been reported for other species (Ware, 1975; DeMar- 

 tini and Fountain, 1981; Kjesbu et al., 1996). Explanations 

 for this change in marine batch spawners include variations 

 in temperature or nutritional state of the female (Hinckley, 

 1990; McEvoy and McEvoy, 1991). Some authors have sug- 

 gested that larger eggs are advantageous because they result 

 in larger hatchlings that are able to avoid predators more 

 effectively (Hinckley, 1990; Wootton, 1994). In this case, in 

 addition to the decrease in fecundity and spawning activity 

 observed during the end of breeding season, the quality of 

 the eggs produced for the last batches could be lower than 

 in the spawning peak. These results suggest that the annual 

 recruitment of M. furnieri in the Rio de la Plata area depends 

 to a great extent on females spawning during November. 



Acknowledgments 



This work is part of the INIDEP's Coastal Project. We 

 thank Teresa Carle and Haraldo E. Christiansen for the 

 preparation of histological sections and Marcela Tobio for 

 the photographic procedures. We would also like to thank 

 Olav S. Kjesbu for a critical reading of our manuscript. 



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