OVERHOLTZ and TYLER: DEMERSAL FISH ASSEMBLAGES 

 1.0 r 



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Sr/J TION 



1.0 r 

 .8 



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STATION 



Figure 11.— Time sequence cluster analyses for autumn 1963-78 

 for the Slope and Canyon (top panel) and Shallow (bottom panel) 

 assemblages on Georges Bank. 



gen. The first three canonical axes accounted for 

 73.9% of this total, a cumulative redundancy of 

 19.0% (Ikble 3). The first canonical variable (CV) 

 reflects the importance of depth and to a lesser 

 degree bottom salinity, in determining the distri- 

 bution of these species. Although none of the corre- 

 lation loadings for CVl are particularly high, the 

 gadoids, as a group, show a positive trend. Many of 

 the shallow-water species, such as little skate, winter 

 skate, and most of the flounders, showed negative 

 correlations with this canonical variable The other 

 two CV's reflect the location variables of latitude and 

 longitude as well as bottom oxygen and salinity. 



Since the gadoids and flounders appeared to show 

 a group response to these distribution variables, we 

 decided to use them in another analysis, excluding 



the other species (Ikble 3, Fall 1976 II). This data 

 set explained 28.3% of the total variation in distri- 

 bution for a selected set of 14 species (Tkble 3). The 

 first canonical variable had a high correlation with 

 latitude (r = 0.904) and the gadoids, as a group, were 

 highly positively correlated with this CV (Ikble 3). 

 It appears that although Georges Bank spans only 

 about two degrees in total north-south latitudinal 

 variation, this variable is useful for defining centers 

 of gadoid biomass. 



The third analysis did not reveal any new trends, 

 accounting for 32.6% of the variation in species 

 distribution. In general, then, although significant 

 orthogonal canonical axes were defined in each of 

 three data sets, the amount of variation that was ac- 

 tually explained was relatively small. There appear 

 to have been trends in the distribution of some 

 gadoid and flounder species, but the strength of 

 these relationships was hardly firm. Most of the 

 variation in species distribution was related to 

 latitudinal, salinity, and depth differences. 



DISCUSSION 



Questions of community resilience (Pimm 1984) 

 are meaningful because resource managers are 

 faced with the dilemma of making decisions that may 

 alter future community structure Fishery managers, 

 in particular, are unable to deal with the long-term 

 consequences of their management decisions 

 because they lack specific knowledge of ecosystem 

 reponses. This idea may apply particularly to areas 

 such as Georges Bank, where landings of each 

 species are part of a multispecies otter trawl fishery. 

 In this case the application of single species manage- 

 ment to assemblages of fishes may result in simpli- 

 fication of the community such that less productive 

 fish populations or those more vulnerable to fishing 

 are reduced dramatically (lyier et al. 1982). If this 

 occurs, important trophic linkages may be precluded, 

 economic viability may suffer, and management op- 

 tions may be removed indefinitely. At the present 

 time the argument of these central issues is pro- 

 ceeding slowly in the literature and few, if any, 

 management agencies are considering these types 

 of questions in their decisions. We need, therefore, 

 to begin to investigate the long-term temporal scale 

 of communities so that ecologists and managers can 

 begin to function in terms of ecological time instead 

 of just a framework for short-term reaction. 



Declines in total finfish abundance on the conti- 

 nental shelf of the northeastern United States reach- 

 ed unprecedented levels over the period 1965-74 

 (Brown et al. 1976). Not only had biomass declined. 



517 



