LONG-TERM RESPONSES OF THE DEMERSAL FISH ASSEMBLAGES 



OF GEORGES BANK 



William J. Overholtzi and Albert V. Tylers 



ABSTRACT 



The resilience of demersal fish assemblages on Georges Bank was investigated with data from seasonal 

 bottom trawl surveys conducted by the Northeast Fisheries Center, National Marine Fisheries Service, 

 Woods Hole, Massachusetts, from 1963 to 1978. Cluster analysis proved to be a useful statistical method 

 for delineating assemblage boundaries and associated species. Assemblages persisted over the long-term 

 and changed spatial configuration only slightly on a seasonal basis. Declines in biomass, numerical densi- 

 ty, and changes in relative abundance occurred ranging from mild to severe Assemblage changes were 

 probably triggered by intense fisheries as well as inherent trophic dynamics of component species. Results 

 have useful multispecies management connotations. The assemblage concept appears to be an appropriate 

 operational or conceptual framework for further management and modeling applications. 



Most community ecological studies have necessari- 

 ly concentrated on the short-term aspects or season- 

 ality of assemblages. Typically 1 to 3 yr of field 

 measurements are analyzed with information theory, 

 niche breadth procedures, or multivariate statistical 

 methods. Demersal fish assemblages in particular 

 have been investigated in a number of locations [see 

 studies by Tyler (1971), Oviat and Nixon (1973), 

 Stephenson and Dredge (1976), Hoff and Ibara 

 (1977), Gabriel and Tyler (1980), and Inglesias 

 (1981)]. The recurrent theme in most of these studies 

 centers around seasonally varying diversity because 

 of environmentally induced migration, temperature 

 usually acting as the dominant driving variable 



Unfortunately, many interesting questions cannot 

 be addressed in these studies because of their short- 

 term horizon. It is important to consider the long- 

 term ramifications of fishery system responses. The 

 temporal scale referred to here as "long-term" does 

 not refer to geologic time, but rather ecological time, 

 the span of years during which the actions of fishery 

 ecologists evoke system responses. Fishery 

 ecologists are limited in their ability to function 

 within this time frame. For instance, a plant ecologist 

 could predict with some certainty the type of forest 

 that would eventually occupy a cleared site, if left 

 undisturbed, but comparable knowledge for fishery 



'Department of Fisheries and Wildlife, Oregon State Universi- 

 ty, Corvallis, OR 97331; present address: Northeast Fisheries 

 Center Woods Hole Laboratory, National Marine Fisheries Service, 

 NOAA, Woods Hole, MA 02543. 



^Department of Fisheries and Wildlife, Oregon State Universi- 

 ty, Corvallis, OR 97331; present address: Department of Fisheries 

 and Oceans, Pacific Biological Station, Nanaimo, BC V9R 5K6, 

 Canada. 



systems is lacking, especially in the marine 

 environment. 



Are fish assemblages stable? How do they respond 

 to exploitation? Holling (1973) investigated sys- 

 tem responses to man's activities, showing that in 

 closed systems, such as freshwater lakes, the pro- 

 pensity to remain stable is high, but not infallible 

 Smith (1972) critiqued the Great Lakes experience, 

 concluding that the activities of man, notably fishing 

 and pollution, when coupled with biological inter- 

 actions, caused significant community alterations in 

 this system. Few marine studies, with the exception 

 of Soutar and Isaacs (1969), Sutherland (1980), 

 DeVries and Pearcy (1982), and some general over- 

 view papers (Brown et al. 1976; Richards et al. 1978), 

 have stressed the long-term temporal and spatial 

 aspects of marine system response 



Longer term temporal and spatial questions were 

 examined with data from research conducted at the 

 Northeast Fisheries Center (NEFC) (Grosslein 

 1969). Concentrating on Georges Bank, we used 

 cluster analysis to produce yearly fall and spring 

 dendrograms for the period 1963-78 and 1968-78, 

 respectively. Assemblages were defined, component 

 species were identified, distributional maps plotted, 

 and the information was examined to elucidate long- 

 term temporal and spatial patterns. Further 

 analyses led to trajectories of species catch-per-unit- 

 effort (CPUE), assemblage total biomass, estimates 

 of intra-assemblage diversity, and other measures of 

 community response It is suggested that fishing, 

 coupled with interspecific interactions, appeared to 

 have played a major role in determining trends in 

 the Georges Bank assemblages. 



Manuscript accepted November 1984. 

 FISHERY BULLETIN: VOL. 83, NO. 4, 1985. 



507- 5 



