NOTES 



REARING CONTAINER SIZE AFFECTS 



MORPHOLOGY AND NUTRITIONAL CONDITION 



OF LARVAL JACK MACKEREL, 



TRACHURUS SYMMETRICLS 



Container size may be a critical variable in the 

 rearing of marine fish larvae. Northern anchovy, 

 Engraulis mordax, grew faster in 500 1 than in 

 10 1 containers (Theilacker and McMaster 1971), 

 and Blaxter (1976) suggested that growth of lab- 

 oratory-reared fish may depend on space avail- 

 able in tanks as well as food supply and fish 

 density. On intuitive grounds, large containers 

 are preferable for rearing studies but small con- 

 tainers are often used because fewer food orga- 

 nisms are required, treatments can be replicated 

 easily, and daily mortality can be easily moni- 

 tored. Thus, information is needed on the extent 

 container size affects results of laboratory studies 

 on marine larvae. In this paper, I compare the 

 growth, morphology and nutritional condition of 

 jack mackerel, Trachurus symmetricus, larvae 

 reared in 10 1 and 100 1 containers. 



Methods 



jaw to perpendicular at end of notochord), head 

 length (HL, tip of upper jaw to cleithrum), eye 

 diameter (ED), body depth at the pectoral fin 

 (BD-1), and body depth at the anus (BD-2). 

 Shrinkage of jack mackerel body parts in Bouin's 

 solution was as follows: SL, 8%; HL, 18^; ED, 

 10%; BD-1 and BD-2, 25% (Theilacker 1980). 

 (Data in Table 1 are preserved measurements.) 

 After measurement, I used standard techniques 

 (Theilacker 1978) to prepare larvae for histolog- 

 ical examination. 



I examined the tissue microstructure of all 

 larvae in fed and starved treatments to determine 

 whether larvae were eating or starving. The onset 

 of starvation in jack mackerel larvae is charac- 

 terized by a change in the acinar arrangement of 

 pancreatic cells and a sloughing of mucosal cells 

 from the midgut into the lumen (Theilacker 1978). 

 I graded these characteristics of the pancreas 

 and gut and classified individual larval nutri- 

 tional condition as "healthy," "intermediate," or 

 "starved" (Theilacker 1978). Because histological 

 assessments of larval condition are based on 

 tissue microstructure, these assessments are in- 

 dependent of larval size. 



I collected jack mackerel eggs by towing aim 

 (0.505 mm mesh) plankton net just below the sea 

 surface 61 km off southern California in May 1976. 

 Sea surface temperature was 15.3° C. I sorted and 

 staged normally developing eggs from the plank- 

 ton and stocked eggs of the same stage at 5/1 into 

 10 1 and 100 1 black circular rearing containers 

 containing 5 pim filtered seawater at 15.0° C and a 

 light cycle of 12 h light and 12 h dark. I used two 

 treatments for each container size, one fed and the 

 other unfed. Data from the 100 1 treatments were 

 reported earlier (Theilacker 1978). Larval diet 

 consisted of a naked dinoflagellate, Gymnodinium 

 splendens (50/ml), a rotifer, Brachionus plicatilis 

 (30-40 /ml), and a copepod, Tisbe sp. (1-2/ml). This 

 feeding method has been described previously 

 (Lasker et al. 1970; Theilacker and McMaster 

 1971; Hunter 1976). 



Larvae began to eat 5 d after hatching. I sampled 

 5-30 larvae daily from fed and starved treatments 

 beginning on day 6 and preserved the larvae in 

 Bouin's solution. Four to five weeks after preserva- 

 tion, I measured standard length (SL, tip of upper 



FISHERY BULLETIN; VOL. 78, NO. .3, 1980. 



Results 



Diameter of jack mackerel eggs collected for this 

 study averaged 1.0 mm. Larvae hatched at 2.45 

 mm SL (preserved) on day and began to eat 

 at 3.35 mm SL at age 5 d when most yolk 

 was absorbed. 



Fed larvae were larger in 100 1 than in 10 1 

 containers on each day after the onset of feeding 

 (day 5), but statistically significant differences in 

 size among larvae in the large and small contain- 

 ers did not occur until the larvae had fed for 4 d. 

 age 9 d (P = 0.002; Hotelling T'^ multivariate 

 analysis; Table 1). 



Among groups receiving no food, larvae in 10 1 

 containers were larger than those starved in 100 1 

 containers at age 8 d, third day of starvation 

 (P = < 0.001; Hotelling T^ multivariate analysis; 

 Table 1.) Also, starved larvae in small containers 

 survived 2 d longer than those in large containers. 

 10 d versus 8 d. 



Nutritional condition of fed larvae reared in 10 1 

 and 100 1 containers was similar for 5 feeding days, 



789 



