KJELSON ET AL.: FEEDING ECOLOGY OF POSTLARVAL FISHES 



the latter possibility, we performed a variety of 

 experiments to determine if handling and capture 

 technique influences the quantities of food 

 observed in the larval gut. 



EFFECTS OF SAMPLING TECHNIQUE 

 ON GUT CONTENT 



Larvae of all three species were first collected 

 by a 3-m channel net with an attached live box. 

 Captured fish were counted, identified, divided 

 into two groups, and transferred (underwater) 

 into separate containers. One group of fish was 

 anesthetized with 0.12 g/liter MS-222 (tricaine 

 methanesulfonate) and then dissected, while the 

 other group was transferred carefully into the 

 posterior end of a 20-cm bongo net (keeping them 

 underwater throughout transfer). The net was 

 towed for 5 min, and after retrieval the larvae 

 were removed, identified, and counted to assure 

 that none were lost and that no new larvae 

 were captured. The fish were then dissected to 

 determine the number of copepods present in 

 their guts. Menhaden lost 68% of their gut 

 contents when exposed to the stress of bongo 

 tows, whereas gut contents of larval spot and 

 pinfish before and after the bongo tow did not 

 differ statistically (Table 2). The amounts of food 

 present in all three species of larvae collected 

 at the Beaufort Inlet in the 24-h bongo samples 

 were lower than the food quantities observed in 

 larvae collected by the other techniques inside 

 the estuary (Figures 1-3). Thus, factors other than 

 the stress of capture may be responsible for 

 the low gut contents in larvae collected in the 

 bongo nets: 1) copepod abundance may have been 

 lower at the Beaufort Inlet sampling site than 

 further inside the estuary, 2) the use of Formalin 

 (restricted to bongo samples) to kill and preserve 

 the larvae may have caused defecation of the 

 copepods prior to analysis (June and Carlson 

 ( 1971) showed that larval menhaden when placed 

 in Formalin had violent spasms accompanied by 



Table 2. — The effect of bongo net tow stress upon the amount of 

 food observed in larval menhaden, pinfish, and spot. 



Capture technique 



Menhaden' Pinfish^ 



Spot^ 



Channel net 



Channel net + bongo net tow 



Mean number copepods/fish ± one SE 



7.4 ±2 1.3 ±0.6 6.4 ±4.4 



2.4 ±0.7 0,8 ±0.3 7.0 ±2 



defecation), and 3) larvae collected in midchannel 

 by bongo nets may not be feeding as actively 

 since they are exposed to a greater tidal current 

 (perhaps the protected inshore waters of the 

 estuary may allow the larvae to feed more effi- 

 ciently and result in fish with greater numbers 

 of prey in their digestive tracts). 



No significant differences were observed in the 

 food contents of spot larvae collected by routine 

 seining and those collected by seining with a 

 more gentle sampling technique (Table 3). In 

 routine seining, the larvae were picked out of 

 the seine as it lay on the shore and placed in a 

 bucket of ice water. The gentle sampling tech- 

 nique consisted of surrounding a body of water 

 with the seine and then concentrating the larvae, 

 taking care that fish were not forced against 

 the net. Once concentrated, the larvae were 

 dipped out of the water in a bucket and anesthe- 

 tized with MS-222. The results of our sampling 

 experiments indicate that routine field sampling 

 techniques used to collect spot and pinfish larvae 

 probably caused little loss of food from the 

 digestive tracts. 



EFFECTS OF HANDLING 

 TECHNIQUE ON GUT CONTENT 



In the laboratory, handling stress did not reduce 

 the food quantities present in larval spot and 

 pinfish, but did reduce the amount of food remain- 

 ing in larval menhaden (Table 4). Two groups 



Table 3. — Comparison of food quantities, mean number of 

 copepods per fish ± one SE, present in larval spot (22-33 mm, 

 X = 27 mm) collected by haul seine using rough and gentle 

 handling techniques. Ten fish were collected per sample. 



Date 



Gentle 



Rough 



April 2 

 April 3 



84.5 ± 7.4 

 69.7 ± 5.9 



78.6: 

 66.9: 



5.3 



5.5 



Table 4. — The effects of handling on the retention of Artemia 

 nauplii in digestive tracts of larval Atlantic menhaden, pin- 

 fish, and spot. Rough handling is approximately equivalent 

 to field capture by dip net and haul seine. 



22 larvae. 



18 larvae. 



5 larvae. 



141 



