122 



Fishery Bulletin 91(1). 1993 



1985 



1986 

 LABORATORY SPAWNING DATE 



Figure 1 



Relationship between mean egg size and laboratory spawn- 

 ing date for Brevoortia tyrannus (•) and B. patronus I ). 

 Observations lf±SDl are derived from approximately 50 eggs. 



at lower incubation temperatures. On the other hand, 

 incubation temperatures in the laboratory did not af- 

 fect the weight of larvae at hatching (Table 1, 3). 



Yolk reserves were larger for Atlantic menhaden at 

 hatching, but oil reserves were greater for gulf men- 

 haden (Table 1, 3). Temperature did not affect the 

 amount of oil or yolk reserves for recently-hatched At- 

 lantic or gulf menhadens. After hatching, both species 

 utilized yolk and oil reserves at an exponential rate 

 (Table 4). Atlantic menhaden utilized yolk reserves at 

 a higher rate than gulf menhaden, but rates of oil 

 depletion did not differ (Table 1, 4). Temperature af- 

 fected rates of yolk and oil utilization. At first-feeding, 

 the oil globule and yolk were virtually depleted 

 (Table 3). Yolk reserves at this time were similar for 

 the two species and were independent of rearing tem- 

 perature. The age at which anatomical features asso- 

 ciated with first-feeding appeared (eye pigment, fore- 

 and midgut, and functional mouth) did not differ, but 

 the interval between the appearance of these features 

 and first-feeding was shorter for Atlantic menhaden 

 (Table 5). 



At first-feeding, Atlantic menhaden were significantly 

 longer and younger than, but of similar weight to, gulf 

 menhaden (Table 1, 3). Although length-at-hatching in- 

 creased with decreasing temperature, temperature did not influ- 

 ence length of larvae at first-feeding. Age-at-first-feeding decreased 

 with increasing temperatures for both species (Table 1, 3). 



First-feeding Atlantic menhaden, which are longer than first- 

 feeding gulf menhaden, may be slightly more resistant to starva- 

 tion (Fig. 3). Atlantic menhaden lived 1-2 d longer than gulf men- 

 haden. The survivorship curves of the two species were similarly 

 shaped at the same temperature. At the highest temperature 

 (24°C) survivorship declined rapidly after the third day, while at 

 the lowest temperature ( 16° C) it was nearly linear with time. 



The amount of biomass gained during the early stages did not 

 differ between species (Table 1, Fig. 4). Population biomass of 

 both menhadens was relatively low at 16° C, as compared with 

 20° C and 24° C when food was not limiting. Food concentrations 

 of <1.0 rotifer/mL similarly limited the growth and survival of 

 both species. Low temperature (i.e., 16° C) affected biomass gained 

 by larvae of both species during this 7d feeding and growth study. 

 Interactions between species and temperature were observed 

 for all growth experiments (Table 1, Fig. 4). Atlantic menhaden 

 growth was lower at 16° C than at 20° and 24° C. Gulf menhaden 

 growth did not appear to differ between temperatures. Atlantic 

 menhaden exhibited higher growth rates at 20° and 24° C, and, 

 in like manner, were larger than gulf menhaden 10 d past first- 

 feeding; however, mouth gape at this time did not differ between 

 species (Table 1, 3). 



Atlantic menhaden may attain its size-threshold for metamor- 

 phosis (i.e., Balon's 1984 concept) earlier than gulf menhaden 

 (Figs. 5, 6). Length-weight relationships expressed during early- 



