Abstract — Environmental variabil- 
ity affects the distributions of most 
marine fish species. In this analy- 
sis, assemblages of rockfish ( Sebastes 
spp.) species were defined on the basis 
of similarities in their distributions 
along environmental gradients. Data 
from 14 bottom trawl surveys of the 
Gulf of Alaska and Aleutian Islands 
(n = 6 767) were used. Five distinct 
assemblages of rockfish were defined 
by geographical position, depth, and 
temperature. The 180-m and 275-m 
depth contours were major divisions 
between assemblages inhabiting the 
shelf, shelf break, and lower conti- 
nental slope. Another noticeable divi- 
sion was between species centered in 
southeastern Alaska and those found 
in the northern Gulf of Alaska and 
Aleutian Islands. The use of envi- 
ronmental variables to define the 
species composition of assemblages 
is different from the use of tradi- 
tional methods based on clustering 
and nonparametric statistics and as 
such, environmentally based analyses 
should result in predictable assem- 
blages of species that are useful for 
ecosystem-based management. 
Manuscript submitted 7 February 2007. 
Manuscript accepted 1 August 2007. 
Fish. Bull 106:1-11 (2008). 
The views and opinions expressed 
or implied in this article are those 
of the author and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
An ecological analysis of rockfish ( Sebastes spp.) 
assemblages in the North Pacific Ocean 
along broad-scale environmental gradients 
Christopher N. Rooper 
Email address: Chns.Rooper@noaa.gov 
Alaska Fisheries Science Center 
National Marine Fisheries Service 
7600 Sand Point Way NE 
Seattle, Washington 98115 
Ecosystem-based management of 
marine fish species requires all eco- 
system components to be accounted for 
in the management framework (Liv- 
ingston et ah, 2005). This necessitates 
knowing the major environmental gra- 
dients along which important species 
are organized within marine systems. 
Rockfish ( Sebastes spp.) comprise an 
important component of marine ecosys- 
tems; they are abundant, diverse, and 
a widely dispersed group found across 
a wide range of habitats on the Pacific 
coast of North America. Rockfish 
management is problematic because 
of the different habitat requirements 
of these species at different life his- 
tory stages (Love et ah, 1991; Rooper 
et al., 2007), their episodic recruit- 
ment (Ralston and Howard, 1995; 
Field and Ralston, 2005), and their 
susceptibility to overfishing (Parker et 
al., 2000). Knowledge of the environ- 
mental gradients upon which rockfish 
organize themselves should be useful 
in predicting where different species 
and age groups co-occur and, thus, 
the best strategies for managing rock- 
fish as an important component of the 
marine ecosystem. 
Analyses used to define fish as- 
semblages typically are based on 
methods that cluster characteristics 
of the catch, such as the Bray-Curtis 
dissimilarity index (Weinberg, 1994; 
Williams and Ralston, 2002) and oth- 
er measures of co-occurrence (Wein- 
berg, 1994; Mueter and Norcross, 
2002), or that classify stations into 
categories of similar catch (Methratta 
and Link, 2006; Zimmermann, 2006). 
These analyses do not explicitly take 
into account the distribution of spe- 
cies across environmental gradients, 
even though fish species are known 
to respond to characteristics such as 
water depth, temperature, salinity, 
and sediment type (Friedlander and 
Parrish, 1998; Rooper et al., 2005). 
According to ecological theory, spe- 
cies will inhabit a preferred niche of 
environmental conditions (Hutchin- 
son, 1957). Application of this prin- 
ciple can be useful in predicting the 
biological basis for co-occurrence of 
rockfish species (Murawski and Finn, 
1988), as well as for defining groups 
of species with similar habitat re- 
quirements. Additionally, the species 
comprising assemblages defined by 
ecological relationships may be ex- 
pected to respond to environmental 
changes in a predictable fashion. 
The objective of this study was to 
analyze the distribution of rockfish 
species across two large ecosystems: 
the Gulf of Alaska and the Aleutian 
Islands. Relationships based on niche 
theory were developed for rockfish life 
history stages, sexes, and species sub- 
groups with depth, temperature, and 
geographical position in order to test 
for important overlaps and to infer 
common distributions among species. 
The co-occurrence of species in bottom 
trawl catches was compared to their 
environmental overlap to determine 
whether species with similar distribu- 
tions were likely to be captured to- 
gether. Finally, the major gradients 
along which rockfish species organize 
themselves were examined in relation 
to the varying life history stages of 
the species. These methods should al- 
low for a more robust analysis of the 
assemblage of similar species, as well 
