Náttúrufræðingurinn - 2010, Blaðsíða 69

69
Tímarit Hins íslenska náttúrufræðifélags
dromous tegundir). Það eru meðal
annars steinsugur og styrjur. Aðrar
tegundir lifa að mestu leyti í fersku
vatni en hrygna í sjó (e. catadromo-
us), til dæmis álar.21 Kvarnir hafa
reynst vel þegar þarf að tímasetja
hvenær fiskar færa sig milli fersk-
vatns og sjávar. Það er gert með því
að taka þversneið af kvörninni og
mæla snefilefnastyrk með ákveðnu
millibili frá kjarna kvarnarinnar að
útjaðri hennar. Mikill munur er á
hlutfalli milli snefilefnanna strontí-
ums (Sr) og kalsíums (Ca) í kvörn-
um í fersku vatni og sjó, en hlutfallið
er mun lægra í fersku vatni. Með
því að mæla hlutfall Sr og Ca eftir
þversneið kvarnarinnar er hægt að
tímasetja hreyfingar fisksins milli
ferskvatns og sjávar.23
Lokaorð
Hér hefur verið farið yfir helstu
eiginleika kvarna og hvernig má
nota þær við rannsóknir á fisk-
stofnum. Þetta var þó ekki tæm-
andi úttekt því kvarnir nýtast til að
rannsaka ýmislegt fleira, til dæmis
vaxtar- og lífsferla einstaklinga og
við stofnstærðarmat. Hér við land
hafa kvarnir verið notaðar til að
rannsaka þorsk og ýsu. Þær rann-
sóknir benda til að kvarnir séu
góður kostur til að kanna lífssögu
fisktegunda sem lifa við mismun-
andi umhverfisskilyrði við Ísland.
Summary
Otoliths in fishes and research
Otoliths are calcified structures found in
all teleost fishes. There are three pairs of
otoliths (Sagitta, Lapillus and Asteriscus)
located in chambers in the inner ear be-
side and behind the brain. They play a
role in hearing and sense of equilibrium.
The major element (96%) of otoliths is
calcium carbonate (CaCO3). There are
three different polymorphs of the crys-
tallization of calcium carbonate: calcite,
aragonite and vaterite. Most sagittal and
lapilli otoliths have been found to be
built up as aragonite. Asteriscus, built
up as vaterite, has a different appear-
ance (glassy) caused by the different cal-
cium carbonate structure. Inorganic ele-
ments are absorbed via the gills and
intestines into the blood plasma, from
where they are transferred with the
blood to the endolymph and are finally
crystallized into the otolith. The otolith
elemental composition does not reflect
the composition of the surrounding wa-
ter, as there are other factors influencing
the rate of ion uptake into the otolith.
Otoliths grow continuously through-
out the lifespan of the fish and difference
can be seen in growth between seasons.
One of the visible periodicities in oto-
liths is the alternation of opaque and
translucent zones (laid down around an
opaque nucleus), which are commonly
used to determine the age of fish.
There is substantial difference in
shape and size of otoliths of different
fish species. This is relatively easy to
notice and species can be identified by
their otoliths. The relationship between
fish size and otolith size is complex as
some small fish species have large oto-
liths (e.g. blue whiting) while some
large fish species have small otoliths (e.g.
wolffish). The sagitta tend to show the
greatest diversity in shape and size
among species. More complex shaped
otoliths have been found in fish species
at great depth, which rely on their hear-
ing rather than sight, and more simple
shaped otoliths in fish species in shal-
lower waters. Difference in otolith shape
has also been detected within the same
species. Both environmental and genetic
factors influence otolith shape. Environ-
mental effects are generally thought to
be more influential.
Otoliths are used to determine age of
various fish species. Age distribution of
fish stocks is necessary for stock assess-
ment, and with the knowledge of age it
is possible to follow changes in weight,
maturity and length-at-age. In some
species, scales and vertebrae are used
for age determination. In 1999 it was es-
timated that up to 2 million individuals
were aged in the world (using otoliths,
scales and vertebrae).
With otolith shape and chemistry it
has been possible to discriminate be-
tween fish stocks. Differences in otolith
shape and chemistry indicate prolonged
separation of fish inhabiting different
environments, not necessarily a genetic
differentiation.
The movements between juvenile
and adult populations are poorly un-
derstood. Elemental composition of
otoliths has been used to study the
combination of different groups in a
mixed group, like juvenile origin of dif-
ferent spawning components or contri-
bution of different spawning compo-
nents in a mixed stock fishery. Many
fish species have various different
spawning locations but disperse to the
same feeding area after spawning. As
elemental composition varies between
individuals from the different spawn-
ing locations it is possible to estimate
their proportion in the mixed area.
To study the juvenile origin the ele-
mental composition of the core (corre-
sponding to the juvenile otolith) from
the adult otolith was compared with
the elemental composition of the juve-
nile otolith. For Icelandic cod, juvenile
origin was estimated for the 1996 and
1997 year classes. Environmental con-
ditions were found to have a large im-
pact on the ability to discriminate be-
tween juveniles. In 1997, a strong in-
flow of Atlantic water presumably
mixed juveniles from various spawn-
ing locations and it was hard to dis-
criminate between the juveniles.
Therefore, it was not possible to esti-
mate the origin of the spawners. In
1996, the inflow was not as strong and
less mixing of the juveniles.
Around 160 diadromous fish species
exist (inhabiting both fresh water and
sea water); anadromous species live
most of their life in sea water but spawn
in fresh water (i.e. lampreys and stur-
geons) and catadromous species spend
most of their life in fresh water but
spawn in sea water (i.e. eels). The ratio
between strontium (Sr) and calcium (Ca)
is lower in fresh water. By measuring
this ratio from the core to the edge of the
otolith, the timing of movements be-
tween fresh water and sea water can be
estimated.
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