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



567 



95% confidence intervals suggest a significant drop in 

 species diversity between 1993 and 1996. Area swept and 

 temperature did not enter the best model, suggesting they 

 had no strong effect on species diversity. Diagnostic plots 

 indicated approximate normality and no apparent trends 

 in the residuals (Fig. 3, C and D). 



Total biomass (CPUE) showed strong trends with depth 

 and alongshore distance (Fig. 6) but had high variability 

 around these trends (Pseudo-r-=0.14). Average CPUE in- 

 creased sharply with depth to about 150 m — a trend that 

 reflected, on average, approximately a doubling of CPUE 

 between the shallowest sampling stations and stations at 

 150-200 m depth. A similarly strong gradient exists in 

 the alongshore direction between Yakutat (km 700) and 

 the Kodiak Island area (km 1500), where total CPUE has 

 a pronounced maximum. Estimates of the spatial trend 

 suggested that highest CPUEs are found around Kodiak 

 Island, particularly in Shelikof Strait, along the shelf 

 break and upper slope, and on the banks and in the gullies 

 northeast of Kodiak Island (Fig. 5). Total CPUE appeared 

 to decrease significantly after approximately Julian day 

 240 (Aug. 28); however, Julian day was confounded with 

 alongshore distance (r =-0.38, Table 3) and the effect could 

 in part be due to lower abundance in the eastern GOA, 

 which was often sampled later than other areas. Gear 

 effects were significant and indicated a lower catch rate 

 for one of the Japanese trawls (gear 717, Fig. 6). The esti- 

 mated time trend (Fig. 6) indicated a significant increase 



in total CPUE between 1984 and 1996 (P<0.001), based 

 on a chi-square test of the difference in residual deviances 

 (Hastie and Tibshirani, 1990). We estimated that the com- 

 bined CPUE of all groundfish species included in the anal- 

 ysis increased from a gulf-wide average of 14,786 kg/km^ 

 in 1984 to 20,600 kg/km- in 1996 (Table 2), representing 

 an increase of 40%. No significant effect of temperature on 

 total CPUE was found. Diagnostic plots indicated approxi- 

 mate normality and no apparent trends in the residuals 

 (Fig. 3, E and F). 



The biomass of 72 groundfish species in 240 stratum/ 

 year combinations could effectively be summarized by an 

 NMDS ordination in three dimensions (stress=0.085). Our 

 findings suggested that the main gradients along which 

 species composition varied were depth and alongshore 

 distance, whereas temperature and temporal gradients 

 (within and between years) had relatively minor effects on 

 these indices of species composition. The first two dimen- 

 sions (axes 1 and 2) of the ordination accounted for 67% 

 and 19% of the overall variation respectively. An ordina- 

 tion plot of the first two axes indicated that the sample 

 strata were most clearly separated along the depth gradi- 

 ent (Fig. 7). The alongshore gradient was roughly perpen- 

 dicular to the depth gradient and clearly distinguished ar- 

 eas Southeast from the other areas (Fig. 7). Yakutat strata 

 occupy an intermediate position between the Southeast 

 strata and strata in other areas. Kodiak, Chiniak, and 

 Shumagin were similar in species composition, as sug- 



