Lowe et a I.: Geographic variation in genetic and growth patterns of Pleurogrammus monopterygius 
513 
We interpret the areal growth differences exhib- 
ited by Atka mackerel as indications of localized as- 
semblages made up of groupings from a single mix- 
ing stock that aggregates during the adult portion of 
its life history. We suggest that the local specific 
growth variability seen in adults is a reflection of 
environmental effects. It is unknown which environ- 
mental characteristics might be most influential on 
growth of Atka mackerel; however, temperature and 
salinity are noted to be the most important hydro- 
logical factors affecting the distribution of hexagram- 
mids, and food, predators, and parasites the most 
important biological factors (Rutenberg, 1962). Seeb 
and Gunderson (1988) noted that local growth and 
age-at-maturity differences could also reflect histori- 
cal influences and fishing pressure. A large and sus- 
tained Atka mackerel fishery has been conducted 
throughout the Aleutian Islands since the early 
1970s. Catches have fluctuated with the demise of 
the foreign fishery and the development of the do- 
mestic fishery, and in recent years the fishery has 
been concentrated in the eastern Aleutian Islands 
where the largest fish reside. The fish in the west- 
ern Aleutian Islands were not heavily exploited from 
1980 to 1994, but they have historically been the 
smallest fish. The geographic size cline noted in the 
growth data seems to run counter to what we might 
expect given the differential fishing pressure. 
Stock assessment and 
management implications 
The stock structure of Atka mackerel has stock assess- 
ment and fishery management implications. From a 
stock assessment perspective, we are interested in elu- 
cidating the underlying population processes that would 
result in stock separation, or lack of, and in evaluating 
the impacts of the fishery on the genetic stock(s), par- 
ticularly on the spawning concentrations. From a fish- 
ery management perspective, the practical recognition 
of fishery-targeted assemblages (not necessarily geneti- 
cally distinct) and the spatial and temporal affects of 
harvesting these assemblages are of interest. 
Historically, small-scale Alaskan subsistence fish- 
eries targeted spawning concentrations of Atka mack- 
erel, but the very shallow and presumably rough 
habitat of the spawning grounds are not accessible 
to the current large-scale commercial fisheries. 
Analysis of commercial fishery data indicates that 
bottom trawling is probably not disturbing the nest- 
ing sites. 6 Thus, the unique reproductive life history 
features of Atka mackerel may provide for some pro- 
tection of the spawning stock. 
Booke ( 1981) distinguished between inherited (ge- 
netic) versus “acquired” or “environmentally induced” 
characters; the latter including morphological and 
phenotypic characteristics. Because acquired pheno- 
typic markers by definition are appropriated through 
contact with the environment, they may not reflect 
the population genetic characteristics (Avise, 1994). 
However, acquired markers can serve an important 
role in population analysis because they can reveal 
where individuals have spent various portions of 
their lives (Avise, 1994). From both a stock assess- 
ment and fishery management perspective, the lo- 
calized aggregations of adult Atka mackerel, al- 
though not genetically distinct, are important to our 
understanding of the population dynamics of this 
species and the impact of the fishery. 
Indications of localized populations of Atka mack- 
erel raise the issue of potential localized depletion 
by the fishery. Another fundamental fishery manage- 
ment and assessment question has been the relation 
of Gulf of Alaska Atka mackerel to Aleutian Islands 
Atka mackerel. Atka mackerel are currently man- 
aged by two major areas, the Gulf of Alaska and the 
Aleutian Islands, and by three subareas within the 
Aleutian Islands. For management purposes, there 
are at least two catch quotas that must be set given 
the management area boundary at 170°W which di- 
vides the Gulf of Alaska and the Aleutian Islands. 
Any further subdivisions of the quota are generally 
based on stock separation rationale or the desire to 
spread out fishing effort over large geographic areas 
(or both). The Gulf of Alaska Atka mackerel are as- 
sessed separately from Aleutian Islands Atka mack- 
erel, mainly because of the different sources of data 
(two different survey time series), and to a lesser 
extent on the presumption that Gulf of Alaska Atka 
mackerel showed some level of separation from Aleu- 
tian Islands Atka mackerel. The results of this study 
show no evidence of Atka mackerel genetic stock 
separation between the western Gulf of Alaska and 
throughout the Aleutian Islands chain. However, 
because adults show evidence of localized aggrega- 
tions, it seems appropriate to set at least two sepa- 
rate quotas in Alaskan waters (Gulf of Alaska and 
Aleutian Islands), with further subdivisions to dis- 
tribute fishing effort. 
The North Pacific Fishery Management Council 
(NPFMC) implemented Amendment 28 to the “Fish- 
ery Management Plan for the Groundfish of the 
Bering Sea and Aleutian Islands,” creating three 
subareas within the large Aleutian Islands manage- 
ment area (Fig. 1). The impetus for the implementa- 
6 Fritz, L. W., and S. A. Lowe. 1997. Seasonal distributions of 
Atka mackerel (Pleurogrammus monopterygius) in commer- 
cially-fished areas of the Aleutian Islands and Gulf of Alaska. 
NMFS. Alaska Fisheries Science Center, 7600 Sand Point Way 
NE, Seattle, WA 98115-0070. Unpubl. manuscript. 
