FISHERY BULLETIN: VOL. 86, NO. 2 



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Sampling Date 



Figure 9. — Census estimates of Dungeness crab megalopae at 

 night from Manta net samples for each cruise in a 1 m wide x 1 

 m deep track extending from the coast to 55 km offshore (30-95 

 km offshore on 7/8). Single samples from convergence zones 

 sampled with the neuston trawl contained more megalopae 

 than the entire census estimates for the respective cruises. 



the midpoints of the distances be- 

 tween the adjacent stations 

 Z), = density of megalopae at station i. 



Although census estimates varied between 

 cruises from approximately 4,000-78,000, these 

 estimates ignored the concentrations of megalo- 

 pae in convergence zones or other aggregations 

 that were sampled by the neuston trawl. The con- 

 vergences alone contained 5-15 times more 

 megalopae than the entire census estimates for 

 their respective cruises. 



DISCUSSION 



The neustonic habitat off Oregon is a dynamic 

 environment that supports an abundant and di- 

 verse fauna including Dungeness crab larvae and 

 numerous fish species. The large mouth size of 

 the neuston trawl and minimal disturbance from 

 the vessel probably contributed to larger catches 

 of surface-dwelling ichthyofauna than have been 

 made in previous studies in the same region 

 (Boehlert et al. 1985). 



Several species of neustonic organisms were as- 

 sociated with specific oceanographic features 

 such as convergence zones and water masses like 

 the Columbia River plume. These aggregations of 

 Dungeness crab megalopae and several fish spe- 

 cies accounted for the major portion of the total 

 catch of each taxon. Recent studies by Shanks 

 (1983, 1985) and Kingsford and Choat (1985, 

 1986) further emphasize the importance of aggre- 

 gation of neustonic organisms in oceanic conver- 



gences, surface slicks, and around floating ob- 

 jects. This information clearly demonstrates that 

 randomized or grid sampling plans will often fail 

 to detect micro- or meso-scale features of the 

 environment. Surveys may thus severely under- 

 estimate the abundance of species, and impor- 

 tant data on ecological characteristics such as 

 predator-prey interactions may not be obtained. 

 This is particularly important in understanding 

 the role of spatial co-occurrence of patches of lar- 

 val fishes and their prey on larval survival and 

 growth (Lasker 1975, 1981; Grover and 011a 

 1986). 



Diel patterns in abundance were striking. Most 

 of the neustonic organisms were collected at 

 night, and their absence from daytime collections 

 may be attributed to vertical migration out of the 

 surface layer or to visual avoidance of the nets. 

 The fact that very few larval or juvenile sablefish 

 and greenlings were collected in subsurface sam- 

 ples in earlier studies (e.g., Richardson and 

 Pearcy 1977; Richardson et al. 1980; Kendall and 

 Clark 1982a; Clark 1984), suggests that these 

 fishes are obligate inhabitants of the neuston, 

 and their low abundance in daytime hauls indi- 

 cates substantial visual avoidance of the nets. 



Conversely, other species may be facultative 

 neuston that undergo diel migrations into the 

 surface layer (Hempel and Weikert 1972), where 

 they can most easily be assessed. Dungeness crab 

 megalopae, in particular, disperse to at least 50 m 

 during the day, but concentrate at the surface at 

 dawn and dusk (Booth et al. 1985). Still other 

 species (the pseudoneuston) may have depth 

 ranges that overlap with the surface layer. 



Previous studies on northwest ichthyoplankton 

 have defined basic coastal, transitional, and off- 

 shore assemblages of species during different sea- 

 sons (Richardson and Pearcy 1977; Richardson et 

 al. 1980; Kendall and Clark 1984a, b; Clark 

 1984). In general, the transitional region roughly 

 parallels the shelf break (approximately 30-40 

 km offshore in the vicinity of Newport, OR). 

 Richardson et al. (1980) ascribed the consistency 

 of these zonal assemblages to the spawning habits 

 of adults and larval transport in the alongshore 

 coastal circulation. According to Parrish et al. 

 (1981), the spatial and temporal patterns of 

 spawning, and durations of pelagic larval stages 

 of these species, should correspond with the sur- 

 face drift patterns of the region to minimize lar- 

 val advection out of suitable habitats. In the 

 Pacific Northwest, species with larvae adapted to 

 the nearshore zone should spawn from fall 



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