FISHERY BULLETIN: VOL. 86, NO. 2 



DISCUSSION 



Menhaden larvae between 3 and 20 mm long 

 collected during wintertime from 3 regions in the 

 northern Gulf of Mexico contained a variety of 

 diet items, including dinoflagellates, tintinnids, 

 copepod eggs, nauplii, copepodids, juvenile pele- 

 cypods, and pteropods (Govoni et al. 1983, 1985). 

 Dinoflagellates and tintinnids constituted the 

 main diet items of larvae <5.0 mm long but were 

 replaced by copepod nauplii, then copepodids and 

 small adult copepods as larvae grew larger. 



In another study, Stoecker and Govoni (1984) 

 found that copepod nauplii were the dominant 

 items in the diet of menhaden larvae 7.0-9.0 mm 

 long, although only 12 fish were examined. Later 

 studies, however, have verified that nauplii are 

 the dominant items in the diet of a wide size 

 range of larval menhaden collected in the 

 boundary between the plume of the Mississippi 

 River and oceanic waters (Govoni and Hoss fn. 3) 



The maximum width of food items in menhaden 

 larvae <5 mm long was about 200 |xm (Govoni et 

 al. 1983). This width increased up to about 400 

 (xm for larvae 10 mm long. In another study 

 (Stoecker and Govoni 1984) maximum prey size 

 was calculated in a different manner; the average 

 width of the largest prey type was used. With this 

 index the estimated maximum width of prey 

 items was 50 ^JLm for larvae <5 mm long and 

 about 140 |jLm for larvae 9-11 mm long. In our 

 study, most of the nauplii were between 40 and 80 

 |jLm body width, and most of the copepodites were 

 between 50 and 200 p-m body width. These cope- 

 pod developmental stages were in the size range 

 of diet items typically found in guts of intermedi- 

 ate size (7-11 mm) larvae of the gulf menhaden 

 and we believe our samples are reasonably repre- 

 sentative of the prey concentrations available to 

 these larvae. Other prey items for larvae in this 

 size range were common only on rare occasions. 



The importance of high prey concentrations to 

 successful feeding and survival offish larvae has 

 frequently been noted (Hunter 1981). Concentra- 

 tions required to give high survival in laboratory 

 experiments are seldom found in the ocean but 

 careful attention to culture techniques can result 

 in reasonably high survival at prey densities that 

 are close to or overlap the maximum natural con- 

 centrations (Houde 1978). At stations near the 

 river delta that are strongly affected by the river 

 plumes, maximum concentrations of nauplii were 

 typically in the range of 20-50 nauplii/L. At sta- 

 tions farther down plume, away from the delta. 



maximum naupliar concentrations were higher, 

 up to 187 nauplii/L at station IV-2 for example. It 

 is not clear whether this is a seasonal pattern, a 

 pattern due to the down-plume development of 

 the food web, or a pattern attributable to some 

 other factors. However, the concentrations ob- 

 served in waters with salinities lowered by the 

 river plumes are mostly within the range of 10- 

 100 nauplii/L that is frequently reported for 

 oceanic and coastal waters for microzooplankton, 

 or the 50-100 nauplii/L range for coastal and es- 

 tuarine areas (see Houde 1978; Hunter 1981 for 

 summaries). The addition of copepodite stages 

 and other potential prey items to the naupliar 

 abundances in this study would increase the esti- 

 mates of available prey somewhat but usually not 

 more than 20%. At least during the wintertime, 

 the waters surrounding the Mississippi River 

 delta do not appear to contain exceptionally high 

 concentrations of copepod nauplii, compared to 

 other coastal and estuarine areas. 



The vertical distribution of nauplii was fre- 

 quently similar to that of chlorophyll. Regres- 

 sions of naupliar concentration on chlorophyll 

 concentration were significant for 4 of the 5 

 cruises although not the same in each case. As- 

 suming chlorophyll is a reasonable indicator of 

 phytoplankton abundance and thus food abun- 

 dance for nauplii, then nauplii appear to be ag- 

 gregated at the depth of highest food availability. 

 Year to year variability in the relationship indi- 

 cates that other factors are also important in de- 

 termining the abundance and distribution of nau- 

 plii. Because eggs released from adult female 

 copepods would sink out of the surface water be- 

 fore hatching, it is probable that active swimming 

 by nauplii plays a part in the aggregation process, 

 perhaps enchanced by physical convergence proc- 

 esses. 



In our study we noted that there were interan- 

 nual differences in naupliar concentrations, there 

 were sometimes significant regional differences, 

 and there were sometimes significant onshore- 

 offshore differences. Although we anticipated 

 finding higher concentrations of nauplii in the 

 region near the Mississippi River delta, this pat- 

 tern was not always observed. This finding might 

 indicate that there is not an important difference 

 between the 3 regions as far as larval food 

 availability is concerned. Alternatively, it might 

 indicate that there are subregions within the 

 larger Mississippi River delta region that contain 

 a food environment for fish larvae that allows for 

 enhanced survival and growi;h, but that we failed 



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