FISHERY BULLETIN: VOL. 76. NO. 1 



al. (1959), we presume both the daily ascent and 

 descent occurred near sunset and sunrise, respec- 

 tively. 



Ahlstrom (1959) studied the vertical move- 

 ments of larvae of several fishes off the coast of 

 California. He found no evidence that larvae 

 moved through the thermocline. His collections 

 showed that they migrated vertically but the 

 movements were usually restricted to the upper 

 mixed layer. In contrast, neither the salinity gra- 

 dient at 10 to 20 m nor the temperature gradients 

 beginning at 10 and 30 m had a noticeable effect on 

 the vertical movements of yellowtail flounder lar- 

 vae in our study. Our collections indicate that the 

 small flounder that migrated between middepths 

 and the surface routinely tolerated salinity differ- 

 ences of 1 . 5%o and temperature changes of 5°C , and 

 those that moved throughout the water column 

 withstood changes of about 10°C. Such rapid 

 changes in temperature seem deleterious but our 

 survey collections indicated that larvae of most 

 flatfishes spawned in the Middle Atlantic Bight 

 are physiologically adapted to wide ranges in 

 temperature. For example, in 1966, when yellow- 

 tail flounder spawned mostly at bottom tempera- 

 tures between 4° and 9°C, we caught their larvae 

 where the surface temperature was 5°C in April 

 and 23°C in August (Smith et al. 1975). 



The amplitude of the vertical migrations by yel- 

 lowtail flounder larvae increased in proportion to 

 their size. Similar behavior was reported for larval 

 haddock, Melanogrammus aeglefinus (Miller etal. 

 1963), and larval Clupea harengus (Seliverstov 

 1974). Recently hatched yellowtail flounder re- 

 mained most abundant beneath the shallow ther- 

 mal gradient, whereas late-stage larvae exhibited 

 extensive vertical migrations that included most 

 or all of the water column. Larvae > 10 mm proba- 

 bly spend some time on the bottom. Bigelow and 

 Schroeder (1953) reported that young yellovd:ail 

 flounder descend to the bottom when 14 mm long. 

 Royce et al. (1959) concluded that they seek bot- 

 tom when 12 to 19 mm long. Judging from this 

 information and the advanced stage of develop- 

 ment of some larvae we caught near the surface 

 after dark, we concluded that the change from a 

 pelagic to a demersal life is not abrupt. Larvae 

 making the transition to a demersal life continue 

 to migrate towards the surface at night. This noc- 

 turnal behavior might reflect a gradual dietary 

 change from planktonic to benthic organisms. Al- 

 though we are unsure of how long they continue 

 the vertical migrations, the 20.7-mm SL specimen 



176 



collected during our survey (see Smith et al. 1975) 

 might represent the maximum size at which they 

 ascend toward the surface. 



In his review of the "critical period" concept, 

 May (1974) pointed out that field studies of larval 

 feeding have produced highly variable results. He 

 cited several investigations that found the feeding 

 incidence of clupeoid larvae very low, others that 

 found it very high, and discussed theories that 

 have been advanced to explain this variability. 

 They include rapid digestion; nutrition from dis- 

 solved organics; low food requirements; daily feed- 

 ing patterns; defecation upon capture and preser- 

 vation; escapement by healthy, feeding larvae; 

 and food availability. Our data on yellowtail 

 flounder larvae support at least two of these 

 theories, namely, a daily feeding pattern and 

 rapid digestion. Both the highest and lowest inci- 

 dence of feeding occurred at predictable times on 

 all 3 days and, with the exception of five speci- 

 mens, the guts of all larvae appeared to be empty 

 within hours after the period of maximum feeding. 



Several studies report that fish larvae feed most 

 actively at high light intensities, but others differ. 

 For example, Kjelson et al. ( 1975) found the diges- 

 tive tract of young Atlantic menhaden, Brevoortia 

 tyrannus; pinfish, Lagodon rhomboides; and spot, 

 Leiostomus xanthurus, fullest at midday. Ruda- 

 kova (1971) estimated that an average of 25% of 

 the Atlantic herring, Clupea harengus harengus, 

 larvae that he caught fed during the day, only 

 3.2% at night. Feeding studies by Blaxter (1965), 

 Schumann (1965), and Braum (1967) support the 

 above studies. On the other hand, Marak (1974) 

 reported that young redfish, Sebastes marinus, fed 

 during day or night and Blaxter ( 1969) found that 

 larval sole, Solea solea, feed at night. Shelbourne 

 (1953) reported that all postlarval plaice, 

 Pleuronectes platessa, that he collected between 

 1400 and 2000 h had food in their guts. The per- 

 cent of feeding larvae declined to between 70 and 

 80% in his samples collected from 2000 to 0200 h, 

 then dropped sharply until daylight when it again 

 increased to 100% for a short time. 



Our results resemble Shelbourne's (1953), ex- 

 cept that we caught fewer feeding larvae and we 

 did not find an indication of feeding at sunrise. The 

 near absence of feeding larvae during daylight 

 morning hours suggests that something other 

 than, or in addition to, light triggers feeding by 

 yellowtail flounder larvae. It appeared to us that 

 feeding intensity increased during afternoon and 

 evening hours. Larvae that had food in their guts 



