8108

• UCREFRP Catalog Number: 8108
• Author: Dizon, A. E., C. Lockyer, W. F. Perrin, D. P. Demaster and J. Sisson
• Title: Rethinking the Stock Concept
• USFW Year: 1992
• USFW - Doc Type: A Phylogeographic Approach
• Copyright Material: YES

Dizon et ,},

differentiated. Presumably the genetic stock is the pop-
ulation that Sinclair (1988) called the "local popula-
tion" in his essay on population regulation and specia-
tion. Recently, Gauldie (1991) published a thorough
review of stock concepts and their deficiencies as ap-
plied to exploited fish populations and argued for a
model of populations defined on the basis of degree of
interchange.
This sense of biological uniqueness via isolation is an
evolutionary one, because it acknowledges the popula-
tion's adaptation to local conditions and indicates that it
possesses a reservoir of unique genetic variability, This
is the "evolutionarily significant unit" (ESU) (Ryder
1986; see Waples 1991 for a review of its application to
northwestern salmonid stocks); it is a biological popu-
lation that is distinguished by its presumed 'evolutionary
uniqueness and significance, It is a natutal unit and
should be a better management unit than an unnatural
unit (Sinclair 1988).
Few would argue with this reasoning. In practice,
however, it is difficult to define a stock based upon
principles of adaptive genetic uniqueness. Existing mea-
sures of fitness are of little use in defining specific ad-
aptation to a local environment; experimentally this is
an intractable problem for higher animals. As a result,
stocks are typically defined using a variety of proxies
that suggest this adapted genetic uniqueness.

Stock Criteria

To study stock structure, an investigator makes obser-
vations of allopatry or isolation that imply reproductive
isolation; differential life history responses, which also
imply reproductive isolation; morphological (i.e., pro-
tein structures) differentiation, indicating drift or evo-
lution under different selective regimes; or differentia-
tion of neutral genetic characters quantifying the degree
of isolation and the time since an ancestor was shared.
We have categorized these sources of information as (a)
distributional, (b) population response, ( c) phenotypic,
and (d) genotypic.

Distributional Data

Initially the most important items of knowledge are
those of distribution and abundance. These data pertain
to all aspects of abundance, migration, pollutant and
parasite loads, zones of fishery interaction and conflict,
etc., that provide information about the population
movements relative to geographical space and time.
These data largely define whether there are major geo-
graphical barriers between putative stocks and whether
they are allopatric, parapatric, or sympatric.
At the most basic level and without additional infor-
mation, disjunct populations frequently have been des-
ignated as separate stocks for management purposes
(e.g., Perrin et al. 1985). 1bis is certainly a conservative

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Rethinking the Stock Concept' '1.7

approach. Even populations that overlap for portions of
their life cycle may be disjunct Slocks (lles & Sinclair
1982). Overlapping distribution does not necessarily
imply gene flow; natural processes favoring adaptation
to local conditions, or social selection favoring mainte-
nance of local social behaviors, especially in the case of
marine mammals, may preserve genetic differentiation
in the face of apparent overlap or movements between
populations (Ehrlich & Raven 1969; Slatkin 1987; Baker
et al. 1990).
Presence and absence data on distribution are the
most readily available sources of information on which
to base stock identification decisions, but they may be
misinterpreted when search effort is not continuous,
causing artificial discontinuities in the reported distri-
bution. Abundance information provides a better proxy
for exchange rates, Density "troughs': or large areas of
zero density indicate extrinsic barriers. Unfortunately,
the variability associated with most estimates of density
is such that the statistical power of using differences in
density gradients in asSigning stock status is often low.
Because of the importance of reproductive isolation
in speciation, information on the physical movement of
individuals (telemetry studies, mark-and-recapture,
etc.) within their ranges brings one closer to properly
assigning stock status to populations. Reeb and A vise
( 1990) and Avise and Ball ( 1990) described evidence of
concordance of genetic patterns with movements for a
variety of species in response to biogeographic barriers.
Thus, for most populations, research on movement pat-
terns should be given high priority.

Population Response Data

Although used frequently to distinguish putative popu-
lations, a population's life histories and behavioral traits
may be modified by the environment through density-
dependent control mechanisms. Data include all aspects
of demography (age at sexual maturity, fecundity,
growth rate, and mortality), other biological parame-
ters, social behavior, vocalization, and specific interac-
tions peculiar to a particular population. Behaviotal
characteristics are probably modified by exploitation
and intra- and interspecies competition, which affect
breeding site fidelity, nursing and care-giving to the
young, and feeding habits, which in turn affect the pol-
lutant and parasite loads. Still, an advantage to using
population response criteria over abundance criteria
should be recognized: Samples taken at one point in
time when pOpulation response criteria are used inte-
grate movement patterns over a much greater time scale
than do abundance criteria.
Differences in the timing of breeding provide a par-
ticularly valuable criterion, because they imply that a
barrier exists to, gene flow between populations. A stock
division of spotted dolphins (Stenella attenuata) in the

ConscrYatlon BJoIogy
Volume 6, No.1, March 1992