SECTION 1: Introductory Papers
C O N C E P T S USED IN T H E B IO C H E M I C A L AND S E R O L O G IC A L I D E N T I F I C A T I O N
OF FISH STOCKS
By
D ag M ø lle r1
Fisheries Research Board of Canada, Biological Station, St. Andrews, N.B. Canada
IN T R O D U C T IO N
T h e significance of the biochem ical and serological
identification o f fish stocks, a p a rt from its intrinsic
biological value, depends on the contribution this re ­
search makes to the study of population dynamics. In
the book o f B e v e r t o n and H o l t (1957) “ O n the d y­
nam ics o f exploited fish populations,” the population
is described “ as a self-m aintaining open system, ex­
changing m aterials w ith the environm ent a n d usually
tending to a steady state” . However, they do not relate
the characteristics of their system to other possible bio­
logical systems no r do they define the limits of their
m ath em atical model. T he goal in identification o f fish
stocks, therefore, is to identify the group of individuals
w hich a p p e a r to correspond m ost closely to this gene­
ralised theoretical m odel o f fish populations.
O nly a few exploited fish populations are found
w ithin the boundaries o f one country. I t is of im por­
tance, therefore, b o th for understanding betw een indi­
vidual scientists an d to establish a ration al basis for
in tern ationally agreed fishery regulations th a t scien­
tists from different countries concerned w ith the ident­
ification o f fish stocks should use the same concepts,
the sam e definitions of these concepts, an d explore the
m a tte r along the same lines.
CONCEPTS
T h e differentiation of fish stocks is n ot new. T he
classical w ork of H e i n c k e began in the 1880’s, an d he
was able to show th a t the herring (Clupea harengus L.)
in the N ortheast A tlantic could no t be treated as a
single, homogeneous unit, b u t instead has to be divided
into a n u m b er o f m ore or less well defined “ stocks” .
These stocks showed differences in m eristic-m orphom etric characters, an d w ere exposed to different de­
grees of exploitation.
Since th en volum inous d a ta of m orphological and
physiological characters have been collected to iden­
1 Present address: Institute of M arine Research, Bergen, N o r­
way.
tify intraspecific fish populations. T h e results have
rarely been conclusive, and it is now possible to give
the m ain reasons for this. T h e characters used depen­
ded bo th upon genetic an d environm ental factors, and
relatively little effort has gone into separating these
two sets of factors an d determ ining their interactions.
N or were differences ignored which were actually p re­
sent in the same stock, b u t w hich depended on yearto-year differences in environm ental factors.
A n um b er of concepts have been used to define differ­
ence an d sim ilarity in subdivisions o f species in fishery
m anagem en t ( M u z i n i c , 1960). However, a t the jo in t
IC N A F /IC E S /F A O Scientific W orkshop in Lisboa in
1957 the form ulation o f a “ basic” population subdi­
vision, the “ u n it stock” was stated so: “ . . . a un it stock
m ay be considered as a relatively homogeneous and
self-contained population, whose losses by em igration
a n d accessions by im m igration, if any, are negligible
in relation to the rates of grow th an d m ortality . . . ”
( P a r r i s h , 1964).
This definition seems to have been form ulated b e ar­
ing in m ind the need for a u n it for w hich param eters
could be conveniently determ ined for the sort of cal­
culation curren tly in use in population dynam ics. As
stated by P a r r i s h (1964), “ I t is clear th a t the range of
possible biological situations satisfying this definition
is very wide and varies greatly for different species of
fish.” For instance, it does not seem to exclude the
possibility th a t different populations of the same spe­
cies or even o f two different species, inhabiting the
same general locality and having m uch the same p o p u ­
lation param eters, could be treated together as a
single u n it stock. Before such a practical tre a tm e n t can
be justified, however, the behavioural an d param etric
sim ilarity of the two units should be established.
N or does the definition indicate possible m ethods
for the identification o f a u n it stock, or even justify a
belief th a t such a thing as a u n it stock actually does
exist as a clearly recognizable biological unit.
T h e unit stock concept has been defined m ainly on
the basis o f phenotypic characters w hich reflect the
8
D. M øller
interaction betw een the genotype of the stock an d the
environm ent. Because o f this the “ sm allest” u n it stock
m ust, logically, tend to be identified with th a t group
of individuals w hich b o th belongs to the same breeding
u n it and, as a self-contained group, lives together u nder
the action o f the same set of environm ental forces. By
contrast, the biochem ical an d serological identification
of fish stocks is concerned w ith the genotype alone, and
intra-specific differences caused by environm ental for­
ces lie outside its scope an d do not com plicate the
issue.
T h e “ u n it” concept in p opulation genetics is defined
b y D o b z h a n s k y (1950): “ A M endelian population is,
. . . , a reproductive com m unity o f individuals w hich
share in a com m on gene pool.” T he concept underlies
the most im p o rta n t factor in the m aintenance of the
“ homogeneous an d self-contained p o p u latio n ” ( P a r ­
r i s h , 1964), the com m on gene pool. I t also indicates
w here the difference betw een the populations can be
found, in the frequencies of certain genes in the pools.
T h e definition is n ot universally applicable; there
is, for instance, some d o u b t ab o u t the existence of
“ sim ple” gene pools in environm ents w ithou t definable
limits. I t is usually possible to find clear-cut differences
betw een pools, b u t groups of individuals w ithin the
same pool often seem to represent a continuous change
in gene frequency. Even so, the term expresses clearly
w h a t the biochem ical an d serological identification of
fish stocks is trying to achieve.
R esearch along these lines will, w ith out dou bt, also
identify groups of individuals a t the species level. H ow ­
ever, differences in gene frequency do not prove th a t
the stocks belong to different species. T o show this, one
has then to dem onstrate th a t the stocks are completely
reproductively isolated from each other.
T h e difference betw een the un it stock concept an d
the u n it concept in po pulation genetics lies in the
effect o f the environm ent upo n the genotypes of the
individuals. H owever, population dynam ics is concer­
ned w ith short-term periods of tim e in the history of
fish stocks, and of a m uch shorter tim e period th a n
is usually associated w ith evolutionary changes. A daptationally speaking, the term environm ent represents
the evolutionary interaction betw een the gene pool
and its surroundings so as to achieve a stability w hich
is not und uly disturbed by n orm al short-term changes.
R eally drastic changes, for exam ple, those caused by
severe overexploitation, m ay disru pt this stability and
cause such m ajor changes in the gene pool itself and
in its relation to the environm ent th a t steady state con­
ditions m ay not apply.
A single gene pool m ay cover a wide geographical
area w ith relatively great environm ental differences
over the whole range, an d this m ay result in ph en oty ­
pic differences betw een groups of individuals in diffe­
re n t localities. How ever, this seems no t to be com m on,
and w hen it does occur the differences tend to be insig­
nificant as far as p ractical p opu lation dynam ics is
concerned. U sually, therefore, th e u n it concept in
population genetics and the un it stock o f population
dynam ics can be equ ated an d the genetic u n it used as
a tool in the identification of u n it stocks.
ID E N T IF IC A T IO N O F F IS H S T O C K S
T h e biochem ical a n d serological identification of
fish stocks is a n identification o f existing gene pools
w ithin a species. In practice this is a n identification of
individual genetic differences an d the d eterm ination
of the frequencies o f the genes responsible for these
differences in different localities.
For the research to be successful, the individual dif­
ferences or characters m ust be proved to have a genetic
basis, either by breeding an d rearing individuals or by
the H ardy-W einberg law o f genotype distributions in
large ran d o m m atin g populations ( H a r d y , 1908;
W e i n b e r g , 1908). T h e genetic basis of a character can
also be inferred if the character shows the same kind
o f variation in populations as do other proven genetic
individual differences.
I t is essential th a t the ch aracter is reproducible by
o ther scientists. This m eans th a t the ch aracter one is
using should be identified, an d all m ethods reported
clearly an d in detail. T h e a m o u n t of d a ta required for
a dem onstration o f h eredity should, of course, be large
enough to show statistical significance.
Difference betw een two stocks is show n if the diffe­
rence betw een values o f gene frequencies o f the same
ch aracter in the two stocks is significant. This will be
difficult to show, of course, if the two stocks in h a b it the
same locality. How ever, w hen specimens are collected
in an area w hich is in hab ited by two or m ore in te r­
breeding populations, the observed num bers of the
different genotype characters often disagree w ith the
expected num bers calculated according to the H ardyW einberg law. By com parison, the observed d istribu­
tion will contain a n excess o f homozygotes in relation
to the distributions expected.
F or results to serve as a basis for fishery regulations
the whole fishing grounds o f the stocks should be cove­
red a t different seasons a t such a sam pling level th a t
estimations of gene frequencies an d distributions o f the
different genetic characters can be established for each
season. All differences m ust be statistically significant.
Biochemical an d serological identification of fish
stocks uses characteristics w hich prim arily belong to
the group and n o t to the individual. R arely are diffe­
rences in gene frequency betw een individuals big
enough to identify them as belonging to different pools.
O n the o ther h an d , if two different homozygotes o f a
genetic system are found each in th eir own separate
9
Biochemical a n d serological identification
stock only, this indicates strongly th a t the stocks b e­
long to two different species.
Ideally, to show differences betw een stocks, ch arac­
ters representing m ore th a n one genetic system should
be used, perhaps two or three independent systems
being a m inim um . T he num ber of genetic systems re­
quired is dependent, o f course, on the difference be­
tw een the gene frequencies an d the com plexity o f the
systems themselves. A m ultiple allelic system will have
probably a greater discrim inatory pow er th a n a simple
two-allelic system.
Sim ilarity in values o f one or m ore gene frequencies
in two stocks do not prove th a t the stocks belong to the
same gene pool. A strong indication of genetic sim ilar­
ity betw een stocks, however, can probably be said to
have been indicated w hen the gene frequencies o f four
or five different genes from different systems have the
same values in stocks, w here the values are not unity.
SUM M ARY
T h e rep o rt reviews the two m ain concepts on which
identification offish stocks is usually based. T h e a uthor
suggests necessary an d sufficient criteria to w hich d a ta
on polym orphic characters, frequencies and distribu­
tions of polym orphs should confirm in order th a t stock
identity can be established.
ACKNOW LEDGEM ENTS
I express m y sincere thanks to M r D. I l e s , Fisheries
R esearch Board, St. A ndrews, N. B., for m any discus­
sions, criticism, and help du ring the p rep aratio n of
this report.
REFERENCES
R. J . H . and S. J . H o l t , 1957 “ O n the dynamics of
exploited fish populations” . Fish. Invest., Lond. Ser. 2, 111 :
1-533.
D o b z h a n s k y , T h ., 1950. “ M endelian populations and their evo­
lution” . Am . N at., 84: 401-418.
H a r d y , G. H . , 1908. “ M endelian proportions in a mixed popula­
tion” . Science N.Y., 28 : 49-50.
M u z i n i c , R., 1960. “ Identification of populations” . Pp. 1287-1305
in: H . R o s a jr., and G. M u r p h y , ed. “Proceedings o f the world
scientific meeting on the biology o f sardines and related species”. 3:
Rom e, FAO.
P a r r i s h , B . B ., 1964. “ Notes on the identification o f sub-popula­
tions of fish by serological and biochemical methods, the status
of technique and problem s o f their future application” . F.A .O .
Fish. tech. Pap., (30) : 1-9.
W e i n b e r g , W . , 1908. “ Ü b e r den Nachweis der V ererbung beim
M enschen” . J h . V er. vaterl. N aturk. W ürtt., 64: 368—382.
Beverton,