FISHERY BULLETIN: VOL. 84, NO. 4 



Rearing Systems 



The striped bass and hybrid larvae were reared 

 from 6 to 30 d after hatching in 36 L, rectangular 

 glass aquaria. Each aquarium was lighted by two 

 61 cm, 40-W fluorescent lights 25 cm overhead on 

 a 12-h light-12-h dark cycle. Immersion heaters con- 

 trolled the temperature. For additional control, the 

 aquaria sat in a shallow, refrigerated waterbath. An 

 airstone in each aquarium provided oxygen and kept 

 food dispersed. 



Temperature was maintained at 19° ± 1°C. Salin- 

 ity was held at 3°/oo by diluting 5 ^m filtered Patux- 

 ent River water with well water. All larvae were 

 fed Artemia nauplii, eggs of which originated from 

 Shark Bay, Australia. Water quality was maintained 

 by replacing half of the water in each aquarium on 

 alternate days. Feces, dead Artemia, and dead lar- 

 vae were siphoned off each day. Ammonia levels 

 were checked on 13 May (16 d after hatching) and 

 were <0.25 ppm in all tanks. The pH in the nine rear- 

 ing tanks ranged from 8.0 to 8.4 on 11 May (14 d 

 after hatching) and from 8.3 to 8.4 on 26 May (29 

 d after hatching). 



Food Levels, Larval Densities, and Sampling 



Two Artemia nauplii levels, 100 L _1 and 500 

 Lr 1 , were tested. The lower level is similar to zoo- 

 plankton densities in Chesapeake Bay subestuaries 

 where striped bass larvae occur (Miller 1978). For 

 each of the larval types duplicate experiments were 

 run at the 500 L _1 level but only a single experi- 

 ment was run at 100 Lr 1 . Food was first offered at 

 6 d after hatching when the experiments started. 

 Artemia nauplii concentrations in each aquarium 

 were checked twice daily by counting the num- 

 ber in pipetted 100 cc aliquots. Food levels were 

 maintained and adjusted by adding suspensions 

 of Artemia of known concentration to the aquar- 

 ia. 



In each aquarium, 144 larvae were stocked at an 

 initial, relatively low density of 4.0 Lr 1 . Some 

 larvae were preserved in 5% Formalin 4 at the start 

 of experiments (6 d after hatching). Three or four 

 larvae from each aquarium were sampled and 

 preserved on days 8, 10, 13, 16, 19, and 25 for 

 growth rate determination. Samples (15-27 larvae) 

 of survivors were preserved at 30 d when ex- 

 periments were terminated. Preserved larvae were 



4 Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service. 



measured and wet- weighed (nearest 0.1 mg after 

 blotting). 



Analysis 



The expected number of survivors in each experi- 

 ment is the number that would have survived had 

 no larvae been sampled and preserved during the 

 experiments. If Z = F + M, where Z is instanta- 

 neous total mortality and F is preservation mortal- 

 ity, then M is mortality from all other causes. The 

 expression N t = N e~ (F+M)t applies, where N t is 

 number of survivors at age t (30 d) and N is initial 

 number of stocked larvae (144 at 6 d). Knowing N , 

 N t , Z, and F, we solved for M and then estimated 

 expected survivors, if no larvae had been preserved, 

 as N' t = N e~ m . Analysis of variance was used to 

 test for survival differences among types of larvae 

 and between food levels. 



Lengths and weights of the three types of larvae 

 were compared at 6 d after hatching and when ex- 

 periments terminated. In addition, lengths and 

 weights at the 100 L _1 and 500 L _1 food levels 

 were compared to determine if food concentration 

 affected mean sizes. Comparisons were carried out 

 using analysis of variance followed by the SNK 

 multiple comparison test. 



Growth in length was described by linear regres- 

 sions of standard length on days after hatching, l t 

 = a + bt, where l t is estimated length (mm) at age 

 t and b is daily growth rate (mm day -1 ). Growth in 

 weight was determined from the exponential regres- 

 sion of wet weight (mg) on days after hatching, W t 

 = W e Gt , where W t is estimated weight at age t 

 and G is the instantaneous daily growth coefficient 

 (day -1 ). Percent daily weight gains were calculated 

 as 100 (e G - 1). Weight-length relationships were 

 obtained from the power function, W = aL b , where 

 Wis wet weight (mg), I is standard length (mm), and 

 a and b are coefficients from the fitted regression. 



Enclosure Experiments 



Cubic enclosures, 1.32 m on each side, open at the 

 top, and constructed of wood frames and 500 yon 

 Nitex mesh, were submerged to a depth of 1.12 m 

 in a 1-acre, freshwater pond of 1.5 m mean depth 

 at the Manning Hatchery. The nine enclosures, each 

 holding 2 m 3 , were placed in the pond from 3 to 5 

 d before larvae were stocked. Enclosures were 

 assigned to the striped bass and two hybrids using 

 a linearized Latin-square design (Steel and Torrie 

 1960) with three replicates for each type of larva. 

 The larvae were progeny of a single 10.4 kg female 



906 



