Shima el al Spatial distiibution of Thciagia cha/cogramma in the Gulf of Alaska 



317 



((.5 

 11,4 



(1.2 



II I 







5 

 DA 

 0,3 



o: 



II I 

 



a 1984 m\w7 



D1990 a 1991 

 H 1996 



0-100 100-200 



200+ 



B 



s^« 



0-100 100-200 200+ 



0-100 100-200 200+ 



Bottom depth (m) 



0,5 

 0,4 

 0,3 



n: 



11 1 



0.5 



0,4 

 03 

 I) 2 



60+ 



0-20 



20-40 40-60 60+ 



0-20 20-40 40-60 60+ 



Distance from land (nmi) 



Siluinagm 



Kodiak 



Shumagin Ctiinkof Kodiak 



Shumagin Chinkof Kodiak 



Geographic region 



Figure 9 



Proportion of stations with low-density concentrations (<1000 fisli/lim-) of juvenile pollock by bottom depth (first column), distance 

 from land (second column), and geographic region (third column) for age (A), age 1 (B), and age 2 (C) juveniles. 



the same main species in common, as listed in Table 9, 

 differences in dominance distinguished the various clus- 

 ters. Adult pollock were most dominant in groups 1, 3, 

 and 6, which were either made up of nearshore or deep 

 shelf stations (Table 9). Mean values of several environ- 

 mental variables in these station groups in which pollock 

 were found indicate that bottom depth and temperature 

 varied (Table 9). Shallow water stations (groups 1 and 2) 

 were grouped into warm (6.4°C) and average (5.7°C) bot- 

 tom temperature clusters (Table 9). As expected, deeper 

 water stations tended to be characterized by cooler tem- 

 peratures. Juvenile (ages 0-2) pollock were found mostly 

 in the nearshore stations. 



Diversity 



Separate diversity indices were calculated for habitat 

 between 0-100 m bottom depth and 100-200 m bottom 



depth. In the 0-100 m habitat, there was a slight decrease 

 in richness from 9-10 in the 1980s to 7-9 effective number 

 of species in the 1990s (Fig. lOA). Calculations of even- 

 ness for the same data indicated that there had been little 

 change in the evenness component of species diversity in 

 the 0-100 m habitat (Fig. IOC). Histograms of the propor- 

 tion (in terms of number offish) contributed by each of the 

 top species indicated that the decrease in richness might 

 partially be explained by an increase in the predominance 

 of pollock and a decrease in the presence of rockfish spe- 

 cies (Fig. 11,A-E). 



Richness in the 100-200 m category increased from .5 

 to 10 effective numbers of species (Fig. lOB). The predomi- 

 nance of pollock and arrowtooth flounder in the 1980s had 

 shifted to include eulachon iThaleicht/iys pacificiis (Gi- 

 rard, 1858)) and Pacific ocean perch iSebastes alutus (Gil- 

 bert, 1890); Fig. 11, F-J). Evenness was relatively consis- 

 tent throughout the survey years (Fig. lOD). 



