DIEL-DEPTH DISTRIBUTION OF SUMMER ICHTHYOPLANKTON IN 



THE MIDDLE ATLANTIC BIGHT 



Arthi'k W. Kendall, Jr.' and N. A. Naplin^ 



ABSTRACT 



A series of discrete depth plankton tows made every 3 h over a 72-h period off Ocean City, Maryland, in 

 July 1974 allowed analysis of the diel-depth distribution of ichthyoplankton. Overall egg and larval 

 densities averaged 5.6 eggs/m^ and 6.3 larvae m''. Seven .species of eggs made up over 90'r oftho.se 

 caught with Merluccius hilinearis eggs accounting for 45.9'? of the eggs taken. Over 16 species offish 

 larvae were identified, of which Urophycis sp., Pomatomus saltatrix, and Citharichthys arctifrons were 

 the most abundant. The fi.sh eggs were concentrated near the surface and their age distribution at 

 different times ofday provided information about diel spawning times, spawning depth, and embryonic 

 mortality. Larvae of all species moved to shallower depths at night. The actual depth distribution and 

 extent of vertical movements varied among the species. The surface and the thermocline were the 

 primary water column features to which diel movements were related. 



Studies of the diel vertical distribution of early 

 stages of fish contribute to knowledge of several 

 phases of their life history. For eggs, diel and 

 depth differences in age distribution can be used 

 to estimate time of day and depth of spawning 

 and hatching, rate of embryonic development, 

 and to some extent egg mortality. For larvae, a 

 large body of literature has confirmed the con- 

 clusions of the early work by Russell (1926) and 

 Bridger (1956) that most species exhibit diel 

 vertical migrations. Although the depth ranges 

 differ, most species move to shallower depths 

 at night. In some species, e.g., herring, Clupea 

 h. harengus, (Seliverstov 1974) and yellowtail 

 flounder, Limanda ferruginea, (Smith et al. 1978) 

 the extent of movement increases as the larvae 

 grow. Speculation about causes of vertical migra- 

 tion has centered around diel feeding behavior 

 and predator avoidance. Most larval fishes are 

 visual feeders on zooplankters, which undertake 

 vertical migrations similar to those of larval fish. 

 In addition, Zaret and Suffern (1976) concluded 

 that vertical migration patterns occur in prey 

 species that are vulnerable to visually dependent 



' Northeast Fisheries Center Sandy Hook Laboratory, National 

 Marine Fisheries Service, NOAA, Highlands, N.J.; present 

 address: Northwest and Alaska Fisheries Center, National 

 Marine Fisheries Ser\'ice, NOAA, 2725 Montlake Boulevard 

 East. Seattle, WA 98112. 



^Northeast Fisheries Center Sandy Hook Laboratory, National 

 Marine Fisheries Service, NOAA, Highlands, N.J.; present 

 address: 613 Riverbend Drive, Fort Collins, CO 80524. 



Manuscript accepted Mav 1981. 



FISHERY BULLETIN: VOL. 79, NO. 4. 1981. 



predators; thus, larvae may reduce predation by 

 moving to deeper water during the day. 



Aside from understanding the biological con- 

 sequences of vertical distribution of larval fishes, 

 the effect this distribution has on results of broad- 

 scale ichthyoplankton surveys is critical. During 

 most such surveys, samples are taken at more-or- 

 less random times during a 24-h day at stations 

 that are separated by tens of kilometers. Depend- 

 ing on the sampling procedures, the diel spawning 

 and embryonic developmental cycle and the diel 

 vertical distribution of larvae may affect inter- 

 pretation of catches from surveys ( Ahlstrom 1959: 

 Miller et al. 1963). If sampling fails to include the 

 entire depth range of the taxa sought, errors in 

 abundance estimates will be made. 



METHODS 



After making several trial tows at varying 

 distances from shore between Sandy Hook, N.J., 

 and Ocean City, Md., we located a concentration of 

 fish larvae 95 km off Ocean City (lat. 38=^32' N, 

 long. 73°52' W) in 57 m of water (Figure 1). 

 Earlier studies (Kendall and Walford 1979i indi- 

 cated that larvae of bluefish, Pomatomus salta- 

 trix, which were among those found here, occurred 

 primarily near the surface, above the thermocline. 

 To track the concentration of larvae we deployed a 

 parachute drogue with a lighted staff buoy and the 

 parachute centered at 5-10 m in the layer above 

 the thermocline which was present from 10 to 30 



705 



