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Fishery Bulletin 98(2) 



ronus (the Atlantic and gulf menhaden, respectively) 

 are well understood. Both species are typical repre- 

 sentatives of estuarine dependent species that spawn 

 in the marine environment (Lawler et al., 1988; Day 

 et al., 1989) in contrast to the Brazilian menhaden, 

 which spawns in an estuarine environment. 



Since 1983 the coastal resources of the Argen- 

 tine—Uruguayan Common Fishing Zone have been 

 monitored by a number of cruises, and results per- 

 taining to menhaden form the basis for this paper. 

 Our objectives were to describe the timing and spa- 

 tial occurrence of spawning in relation to the major 

 hydrographic features of the region in order to gain 

 insight into the spawning habitat requirements of the 

 Brazilian menhaden. Whenever possible, comparative 

 analyses with other species of the estuary and men- 

 haden of the Northern Hemisphere were performed. 



Materials and methods 



Samples from 47 cruises from 1983 through 1998 

 were analyzed in our study. Twenty-two of these 

 cruises were on research vessels of the National 

 Institute for Fisheries Research and Development 

 (INIDEP), covering the Rio de la Plata estuary and 

 adjacent coastal waters, throughout which stations 

 were randomly distributed. Twenty-five cruises took 

 place in Samborombon Bay, on small fishing boats 

 using a systematic sampling design. Monthly distri- 

 bution of the sampling effort is shown in the insert of 

 Figure 1. During 1983 and 1987, nineteen cruises in 

 Samborombon Bay were performed every 30-45 days 

 during the entire year. The remaining six cruises in 

 this bay correspond only to spring months (October, 

 November, and December). Cruises were made with 

 several objectives, hence covering different periods, 

 but sampling effort was higher in spring (October- 

 November) when cruises for stock assessment were 

 performed. The high number of plankton samples 

 in May was due mainly to one cruise, designed to 

 study physical variables and plankton in the estuary. 

 Cruises on the small boats and on the research ves- 

 sels were not simultaneous. All data were employed 

 as a composite representing mean conditions. 



Plankton was collected at 980 sampling stations by 

 oblique tows of 60-cm bongo nets, 20-cm bongo nets, 

 or a Nackthai sampler ( a modified Gulf V high-speed 

 plankton net, see Nellen and Hempel, 1969). All nets 

 were equipped with flowmeters. The volume filtered 

 in each tow ranged from 10 to 400 m''. Sampling 

 depth was estimated from measurements taken with 

 an angle indicator (inclinometer) and a wheelout 

 meter. All samples were preserved in 5*^ buffered 

 formalin. Plankton samples were sorted and ana- 



lyzed in the laboratory. Brevoortia aurea eggs were 

 identified following the description of de Ciechomski 

 (1968). 



Estimates of egg density from the different sam- 

 plers were not intercalibrated. They were arranged 

 into four broad density classes (<10, 10 to 99, 100 to 

 999, >1000 eggs/m'^), and marked on a map to delin- 

 eate the geographic location of the spawning area. 

 Monthly distribution of the average number of eggs 

 per tow (catch per unit of effort, CPUE) and the per- 

 centage of positive stations for menhaden eggs were 

 plotted to identify the spawning season. Stations with 

 no catch were included in the CPUE estimates. 



Data from the November 1995 cruise were useful 

 for gaining insight into the vertical distribution of 

 menhaden eggs. During that cruise, plankton was 

 sampled at five stations in a small area near Mon- 

 tevideo, where schools of spawning B. aurea were 

 previously detected. In each of those stations, two 

 tows were conducted with the Nackthai net, one 

 to sample the surface and the other to sample the 

 bottom layers as defined by a strong halocline. The 

 depth of the halocline was previously measured with 

 a conductivity-temperature-depth (CTD) profiler. 



On all cruises, menhaden were collected with bot- 

 tom trawls, and the presence of females with hydrated 

 oocytes (macroscopic examination) was considered 

 evidence of imminent spawning. The percentage of 

 females with hydrated oocytes for each tow was also 

 indicated on the map to identify the location of the 

 spawning area. In addition, the mean monthly per- 

 centage of females with hydrated oocytes was used 

 to determine the spawning season. 



Temperature and salinity information was taken 

 from the oceanographic database created by Guer- 

 rero et al. (1997a), which included all the CTD sta- 

 tions sampled during the cruises that we performed. 

 Bottom salinity and temperature data for spring- 

 summer were used because they could be matched 

 to the vertical distribution of the eggs and the main 

 reproductive period. Salinity is expressed as psu 

 (practical salinity units; Anonymous, 1981). Hori- 

 zontal and vertical salinity contour lines were made 

 to compare the spawning area with the salinity field. 

 Egg density was plotted into a temperature-salinity 

 diagram to show the environmental ranges of the 

 spawning habitat of the Brazilian menhaden. 



Results 



Eggs in the plankton 



Brazilian menhaden eggs were detected virtually all 

 year round (Fig. 1). The highest CPUE occurred from 



