Mueter and Norcross: Spatial and temporal patterns in the demersal fish community off Alaska 



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Ophiodon e/ongafus 



Squafus acanfhras 



Sebastes brevJspinls 



Sebasfes vahegatus 



Sebasfes zacenfrus 



Sebastes hefvomacufafus 



Parophrys vefufus 



Sebastes proriger 



Sebasfes rubemmus 



Lyopseffa exilis 



Sebastes efongatus 



Hydrofagus coffiei 



Sebastes cflFafus 



Eopsetta jordani 



B 



Sebastes ateutfanus 



Hemitripterus bofmi 



A/batrossia pectora/is 



200 400 

 Depth (m) 



1000 2000 

 Alongshore distance (km) 



Figure 9 



Distribution of CPUE by depth and alongshore distance for all spe- 

 cies that had a strong positive (A) or negative (B) association with the 

 second index of species composition derived from an NMDS ordination 

 of abundances averaged by strata. For details see Figure 8. 



munities are generally higher on the continental shelf 

 than on adjacent slope regions, reflecting a strong gradient 

 with depth (Day and Pearcy, 1968; Colvocoresses and Mu- 

 sick, 1984; Farina et al., 1997). In contrast to our results 

 for the GOA, these studies did not find peaks in richness 

 or biomass at intermediate depths. This may have been 

 partly the result of aggregating data into relatively large 

 depth ranges in those studies (and the associated loss in 

 resolution), whereas our study examined patterns in rich- 

 ness and biomass along a continuous depth gradient. 



We suggest that the high biomass of demersal fishes 

 near the shelf break (in particular the biomass of flat- 

 fishes, rockfishes, and gadids) reflects favorable feeding 

 conditions at this depth range. Favorable feeding condi- 

 tions may result from enhanced benthic productivity or 

 availability of suitable prey at this depth. The region along 

 the shelf break and over the upper slope has high primary 

 productivity that may be driven by shelf-break fronts, 

 seasonal upwelling, strong alongshore currents, and tidal 

 mixing (Parsons, 1986). Shelf-break fronts have frequently 

 been observed in the GOA (Weingartner^) and can enhance 

 phytoplankton and zooplankton biomass, particularly 

 when coupled with upwelling (Mann and Lazier, 1991), 

 which is common during the summer in the western GOA 

 (Reed and Schumacher, 1986). We speculate that increased 

 production in the water column becomes available to the 

 benthos when sinking particles are consumed by larger 



2 Weingartner, T. J. 2000. Personal commun. Institute of Ma- 

 rine Sciences, University of Alaska Fairbanks, Fairbanks, Alaska 

 99775. 



organisms such as shrimp or euphausiids, which are often 

 concentrated near the shelf break and are consumed by 

 demersal fish (Mackas et al., 1997; Robinson and Gomez- 

 Gutierrez, 1998). The most abundant demersal fishes in 

 the GOA, arrowtooth flounder and walleye pollock, as well 

 as several rockfish species, have been shown to feed near or 

 off the bottom on euphausiids, mysids, copepods, shrimps, 

 and other fishes (Yang, 1993). Thus, a region of enhanced 

 benthic productivity near the shelf break is consistent with 

 the total biomass of groundfish being highest just inshore 

 of the shelf break between 150 and 200 m (Fig. 6). 



In addition to depth (or elevation), latitude is a major 

 ecological gradient in both marine and terrestrial environ- 

 ments. Demersal fishes on the continental shelf off both 

 the east coast of North America (Mahon et al.. 1998) and 

 the U.S. west coast (Jay, 1996) form loose assemblages that 

 are clearly separated by latitude. Although the latitudinal 

 extent of our study region is relatively small, we found 

 strong alongshore (roughly corresponding to longitudi- 

 nal) differences in community structure from the eastern 

 GOA to the Aleutian Islands, with the steepest gradient 

 between the Yakutat and Kodiak areas. This gradient was 

 apparent in species richness and diversity (Figs. 2 and 4), 

 total CPUE (Fig. 6), and indices of species composition 

 (Fig. 7). The alongshore gradient is perpendicular to the 

 depth gradient, with depth generally increasing in the 

 offshore direction. However, these two variables were 

 moderately correlated (-0.32, ranging from -0.20 to -0.44 

 in individual years), primarily due to the larger number of 

 shallow stations in the western part of the study area and 

 a relative lack of shallow stations in Southeast Alaska. 



