Abstract. — The diel vertical 

 distribution patterns of several 

 abundant ichthyoplankton taxa 

 were examined from depth-strati- 

 fied tows off Kodiak Island in the 

 western Gulf of Alaska during 

 1986 and 1987. Most larvae were 

 found in the upper 45 m of the 

 water column throughout the diel 

 period but were concentrated in 

 higher densities near the surface 

 (0-15 m) in daylight hours and at 

 greater depths at night. Four of 

 the five dominant taxa examined 

 in detail showed significantly 

 greater weighted mean depths 

 during the night than during the 

 day. This pattern was the opposite 

 to that previously reported for the 

 numerically dominant taxa (Ther- 

 agra chalcogramma) in this area. 

 Since there was no clear relation 

 between the diel vertical distribu- 

 tion of these taxa and the vertical 

 distribution of water temperature 

 and density or copepod nauplii 

 prey, we hypothesize that this re- 

 verse migration is either a strat- 

 egy to minimize spatial overlap 

 with predators that follow a nor- 

 mal diel migration pattern or one 

 to optimize light levels for feeding. 



Diel vertical distribution of 

 ichthyoplankton in the northern 

 Gulf of Alaska* 



Richard D. Brodeur 

 William C. Rugen 



Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA 

 7600 Sand Point Way NE, Seattle, WA 98 1 1 5 



Planktonic eggs and larvae of ma- 

 rine fishes exist in three dimen- 

 sions in the open ocean. Unfortu- 

 nately, traditional ichthyoplankton 

 surveys, which use non-closing 

 sampling gear, provide information 

 only on two dimensions, integrat- 

 ing the vertical indirectly into the 

 horizontal dimensions. It is well 

 known that vertical current shear 

 can be substantial over short dis- 

 tances and that light, temperature, 

 hydrostatic pressure and food show 

 much stronger gradients in the ver- 

 tical relative to the horizontal di- 

 mensions in the water column 

 (Laprise and Dodson, 1993). Thus, 

 a larva can often change not only 

 its geographic position, but also its 

 immediate environment by altering 

 its vertical position in the water 

 column. 



Diel vertical migration is well 

 documented for larval, juvenile, 

 and adult life history stages of ma- 

 rine fishes (see review by Neilson 

 and Perry, 1990). The adaptive sig- 

 nificance of these migrations is 

 presently in dispute, but it has 

 been attributed to position mainte- 

 nance, bioenergetic optimization, 

 thermoregulation, and predator 

 avoidance (Kerfoot, 1985; Lampert, 

 1989). In addition, the degree of 

 migration and amplitude of depths 

 over which a species vertically mi- 

 grates often changes during ontoge- 

 netic development (Brewer and 



Kleppel, 1986; de Lafontaine and 

 Gascon, 1989). 



Knowledge of vertical distribu- 

 tion patterns of marine fish larvae 

 is crucial not only in understand- 

 ing ecological processes but also 

 has practical implications in the 

 assessment of abundance. Sam- 

 pling just the upper depths of a 

 species range can lead to substan- 

 tial underestimates of abundance, 

 whereas sampling the entire water 

 column for surface-dwelling taxa 

 may waste limited ship time. De- 

 spite the importance of the larval 

 phase in recruitment of marine 

 fishes, relatively little is known 

 about larval vertical distribution 

 patterns off the continental shelf in 

 the North Pacific Ocean. With the 

 exception of walleye pollock, Ther- 

 agra chalcogramma, which has 

 been fairly well studied through 

 much of its geographic range 

 (Kamba, 1977; Kendall et al., 1987; 

 Pritchett and Haldorson, 1989; 

 Kendall et al. 1 ), the only compre- 

 hensive studies on vertical distri- 

 bution of coastal ichthyoplankton 

 in the northeast Pacific Ocean are 

 from the California Current region 

 (Ahlstrom, 1959; Boehlert et al., 

 1985; Brewer and Kleppel, 1986; 

 Lenarz et al., 1991). This paper 

 presents information on the verti- 

 cal distribution of five abundant 

 ichthyoplankton taxa (other than 

 walleye pollock) collected in the 



Manuscript accepted 18 October 1993 

 Fishery Bulletin 92:223-235 (1994) 



"Contribution No. 0181 of the Fisheries Oceanography Coordinated Investigations. 



223 



