FISHERY BULLETIN: VOL. 76, NO. 2 



The purpose of this study was to develop mor- 

 phological and histological criteria for assessing 

 the nutritional condition of jack mackerel larvae 

 and to evaluate these criteria by comparing their 

 success in identifying fed and starved larvae 

 reared in the laboratory. Ultimately, criteria 

 based on these results may be useful for estimat- 

 ing larval survival in the field by assessing the 

 condition of sea-caught larvae. 



MATERIALS AND METHODS 



Jack mackerel eggs were collected by towing a 

 1-m (mouth diameter, 0.505-mm mesh) plankton 

 net just below the sea surface at various locations 

 between 20 and 200 mi (32 and 320 km) off the 

 coast of southern California in June and July 1975 

 and in May 1976. The eggs were separated from 

 most of the plankton at sea and then sorted by 

 developmental stage at the Southwest Fisheries 

 Center, La Jolla, Calif. Temperature was main- 

 tained at 15°C during sorting and in the larval 

 rearing containers. The light cycle was 12 h light 

 and 12 h dark. Five hundred normally developing 

 eggs from a single day's spawning were transfer- 

 red into 100 1 black Kydex^ circular rearing tanks 

 containing filtered seawater {5fxm, Cuno filtered). 

 There were three experiments and two treatments 

 in each experiment; larvae in one tank were of- 

 fered food while those in the other were not. The 

 fed larvae were given a diet of a naked dinoflagel- 

 late, Gymnodinium splendens (50/ml), a rotifer, 

 Brachionus plicatilis (30-40/ml), and a copepod, 

 Tisbe sp. (1 or 2/ml). This feeding method has been 

 described (Lasker et al. 1970; Theilacker and 

 McMaster 1971; Hunter 1976b). 



Histological criteria were developed in the first 

 two experiments. The sampling procedure and the 

 number of larvae sampled differed depending on 

 the requirements of the analysis. Collectively, a 

 total of 152 larvae were examined. In the third 

 experiment, usually 15 larvae were sampled daily 

 for 5 days from the "fed" tank (n =69) and 3 days 

 in the "starved" tank (n = 48). All larvae were 

 examined both histologically and morphologi- 

 cally. No dead larvae were sampled because the 

 postmortem change which takes place in tissues of 

 fish larvae, due to digestion by their own enzymes 

 (autolysis), resembled antemortem destruction 

 caused by starvation. Standard length of each 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



404 



larva was measured on a slide; then seawater was 

 removed and replaced by Bouin's fixative. Pre- 

 serving individual larvae in this manner assured 

 that each would be straight and flat, facilitating 

 subsequent morphometric measurements. Five 

 measurements were taken after preservation to 

 monitor daily changes in larval body form and 

 determine effects of starvation: standard length 

 (SL, tip of upper jaw to tip of notochord), head 

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

 diameter (ed), body depth at the pectoral (bd-1), 

 and body depth at the anus (bd-2). Standard length 

 shrinkage in Bouin's fixative was 11.5%. Next, 

 measured larvae were prepared for histological 

 examination using standard techniques. Larvae 

 were transferred from Bouin's to 70% ethyl al- 

 cohol after 24 h, dehydrated with an ethyl n-butyl 

 alcohol series in a Fisher Tissuemation, and em- 

 bedded in Paraplast-plus. The Paraplast paraffin 

 blocks were frozen with fluorocarbon spray 

 (Cryokwick) just before the larvae were serially 

 sectioned at 5 fxm in a sagittal plane. The mounted 

 sections were stained with Harris' hematoxylin 

 and eosin and mounted in synthetic resin. 



Histological Grading System 



Recently O'Connell (1976) developed a numeri- 

 cal, histological grading system to characterize 

 the nutritional condition of individual northern 

 anchovy larvae. He examined tissues of the larvae 

 microscopically to determine the features of tissue 

 microstructure that were affected by starvation. A 

 grade was assigned to each feature based on the 

 degree of similarity or dissimilarity of the his- 

 tological microstructure between the starved lar- 

 vae and fed larvae. I followed this method and 

 modified it as necessary for the tissues of jack 

 mackerel. 



Each of the histological characteristics used to 

 assess starvation in jack mackerel larvae was 

 evaluated and assigned a grade. A grade of "3" was 

 given to a characteristic which resembled that in 

 "normal or healthy" larvae, a grade of "2" was 

 given to an intermediate condition, a grade of "1" 

 to the starved condition. Since these criteria were 

 established by comparing actively feeding, seem- 

 ingly healthy larvae with moribund larvae, it was 

 assumed that an average of the 12 graded features 

 examined for each larva classified the larva into 

 the correct nutritional group: "healthy group," 

 average grade = 2.34 to 3.00; "intermediate 

 group" = 1.67 to 2.33; and "starved" = 1.00 to 



