EGGS AND YOLK-SAC LARVAE OF YELLOWFIN MENHADEN 



95 



OVliarton, 1954). The pens were made of juniper 

 boards and cedar slats (fig. 1). The dmiensions 

 were as follows: 18 inches high, 62 inches long, 

 and 32 inches wide, with a 5-inch flange on all 

 four sides. The flange regulated the submergence 

 depth of the pen and provided stabilit}-. 



Compartments, made of woven-mesh nylon 

 netting, were used to confine the fertilized eggs 

 and larvae within the pens. Two compartments 

 were made of mesh with an average opening of 

 0.5 mm., and two compartments were made of 

 mesh with an average opening of 1.0 mm.^ All 

 seams reinforced with nylon bmding tape. The 

 compartments were enclosed, except for access 

 along one side (fig. 2). Because the access slit 

 gaped during rough weather and allowed debris to 

 enter and eggs to escape, a plastic slide fastener 

 later was added. Each pen held two compart- 

 ments that were kept in place by a tie at each 

 corner. 



Yellowfin menhaden used in the fertilization 

 trials were obtained from gill-net catches made 

 Anthin sight of the temporary field laboratory. 

 Usually, the nets were set at dusk and picked up 

 about an hour later. Eipe females with extruded 

 ova, or greatly distended abdomens, were removed 

 and set aside while the net was being recovered. 

 Males producing milt with viable sperm commonly 

 occurred in the catch. Ripe females, on the 

 other hand, were rare, for only 25 gravid females 

 were found among approximately 4,000 fish ex- 

 amined. For all attempted fertilizations, fish 

 were dead less than two hours. 



Fertilization was accomplished by mixing ova 

 and sperm obtained by pressing the sides of the 

 fish, or by cutting open the ovary or testes to 

 free the mature sex products. The ova and sperm 

 were mixed "dry," i.e., without sea water. Dock- 

 side water was filtered through cotton to remove 

 organisms including fish eggs and then added. 

 The criterion of fertilization was the formation 

 of a wade perivitelline space. 



Fertilized eggs were placed in the mesh com- 

 partments of the floating pens and in glass or 

 polyethylene containers in the laboratory. Sam- 

 ples of the developing embrvos were "removed 

 periodically from the floating pens and placed in 

 tlie laboratory containers. Development was 



-I 1 r 1 r 



TEMPERATURE 



SALINITY 



o< 



-I 1 1 L. 



^0 



HOURS AFTER FERTILIZATION 



Figure 3.— Fluctuations of temperature and salinity 

 during the development of eggs and yolk-sac larvae o'f 

 yellowfin menhaden. 



observed with a microscope, and samples were 

 removed and preserved in 5 percent formahn. 

 The time required for developmeiTt of the 

 embryo was recorded as age-in-hours from manual 

 fertDization and for the yolk-sac larva, from the 

 time of hatching. The water temperature of 

 Indian River, immediately adjacent to the dock 

 and rearing floats, and of the culture bowls was 

 recorded at infrequent intervals during develop- 

 ment (fig. 3). The observed temperature ranged 

 from 16.4° C. to 22.7° C, with a mean of 19.6° C. 

 Salinity was determined at eacli temperature 

 observation. The observed salinity ranged from 

 20.1%o to 27.2%o, with a mean of 22.1°/oo. 

 Plankton collections of yellowfin menhaden 

 eggs and larvae developing under natural condi- 

 tions were obtained in the Indian River. Ten- 

 minute tows were made with a half-meter net in 

 the vicinity of the gill-net fishing grounds near 

 Sebastian Inlet, and at one mile intervals for a 

 distance of 6 miles north and 12 miles south of 

 the inlet. Developing eggs from the plankton 

 collections were used for the photograplis of 

 several stages not obtained during the develop- 

 ment of artificially fertilized eggs. Although tiie 

 size and appearance were similar to eggs of known 

 origin, there wore slight differences that are with- 

 out adequate explanation. However, the identity 

 of the planktonic eggs was assumed because of 

 structural similarities and the concurrence of 

 spawning yellowfin inenhadoii in tlic immodiate 

 vicinity. 



