Population study of the Danube sterlet
(Acipenser ruthenus L.)
* Kolarevic, J., **Lenhardt, M., **Cakic, P. and *Jaric,I.
* Centre for multidisciplinary studies, Kneza
Viseslava 1, 11000 Belgrade
** Institute for Biological Research, 29 Novembra 142,
11000 Belgrade
INTRODUCTION
Sterlet (Acipenser ruthenus L.) is the
smallest species among the Danube sturgeons. It is a potamodromous resident
species. However, in spring it leaves for flooded areas in quest for food and
place where they can reproduce (Billard and Lecointre, 2001).
Sterlet is particularly intriguing due to its wide
dispersal range and high variability. In contrast to other acipenseriformes,
which belong mainly to one out of nine biogeographic provinces in which
sturgeons occur, sterlet spawns in three provinces, on two continents (Bemis
and Kynard, 1997) and has different history patterns in different river
systems.
In first half of XX century, Russian ichthyologist
and biogeographer, Leo Semenovich Berg, recognized two morphs within European
sterlet. After that, many contributions were published for and against
existence of two sterlet morphs and today these questions are still open.
Together with all other sturgeon species, sterlet is
listed in Nature 2000 list, IUCN Red List of Threatened species, Convention on
the Conservation of Migratory species of wild animals, Convention on the
conservation of European wildlife and natural habitats and CITES convention.
Unfortunately this preservation effort was not very fruitful considering that
most of the sturgeon populations are still declining.
Bearing in mind, that intensive catch, especially of
younger juvenile sterlet specimens, exists from the middle of XX century and
that it was followed by habitat destruction and water pollution in Serbian part
of the Danube River, at the beginning of 2002, we started a complex
investigation on natural sterlet population in the Danube near Belgrade.
This research is done as a part of a project
sponsored by Ministry of Science and Technology of Republic of Serbia and it is
carried out by Centre for Multidisciplinary Studies in Belgrade.
Our main goal is to learn as much as we can about
this endangered species and its habits.
MATERIAL AND LOCATION OF RESEARCH
Material, which
consisted of 500 sterlet specimens was gathered monthly in the Danube River
during years 20002 and 2003. Fish was caught using modified drag nets on three
localities: Zemun (1173km), Visnjica (1162km) and Oresac (1123km). Specimens
were than submitted to the series of different analysis, which included
morphological analysis, age and sex determination, growth rate analysis,
histological analysis, biochemical analysis, blood film analysis and
parasitological analysis.
METHODS AND
RESAULTS
Morphometrical
analysis
We have
conducted broad morphometrical analysis, in attempt to resolve question about
existence of two sterlet morphs. Beside weight and total length, 24 different
morphometric and 3 meristic variables were used for fish comparison, 15 of them
in head region. Acquired data were tested by detailed statistical analysis. As
a result, some indications in favor of existence of two sterlet forms have
already appeared, but further data analysis are expected to give more precise
answers. Genetic analysis of sterlet fin samples will be done at the University
of Ferrara, Italy.
We believe that
the use of more sophisticated statistical methods for analysis of gathered
data, together with genetic analysis of sterlet populations, will offer better
understanding of this problem.
Age structure
Altered
Stevenson and Secor method (1999) was used In order to determine age structure
of sterlet populations. Pectoral-fin spine sections approximately 2 mm width
were removed from gathered specimens, mounted on glass slides, polished and
watched under microscope. According to all 500 analyzed sterlet specimens,
almost all were in their first or second year of life cycle.
Growth rate
Length and
weight growth was monitored on monthly basis, in order to determine variations
in growth rates during different seasons. Special attention was given to the
comparation of results from different localities and to the differences between
existing age classes. Length – weight relationship and condition factor were
calculated and used as good indicator of water pollution level and food
availability.
Sex
determination
Sex was
determined using histological analysis of gonad segments. Gonads were fixed in
Bouin’s fixative, dehydrated, lightened in xylol and molded in paraffin. Tissue
sections (5 mm) were cut and stained with haematoxylin and eosin
(Lenhardt and Cakic, 2002). Results of examination showed that nearly all
gathered specimens were reproductively immature.
Histological
analysis and biochemistry
Histological
analysis of gills, liver, kidney and skin is used as an indicator of exposure
to different environmental contaminants. This kind of analysis is a useful tool
for assessment of habitat pollution levels particularly for sub-lethal and
chronic effects. Standardised method according to Bernet et al. (1999) is
applied for description and assessment of histological changes. According to
this method the extension of a pathological change is rated with a “score
value”, and the pathological importance of the alteration with an “importance
factor”.
Biochemical
analyses include determination of alanine aminotransferase (ALAT), aspartate
aminotransferase (ASAT) and alkaline phosphatase (AP) activity in sterlet blood
sera, as well as concentration of total protein, urea and creatinine.
Results of
histological and biochemical analysis will be used to determine if correlation
between certain events exist or not: histopathological changes in liver and
transaminase activity increase, histopathological changes of gills and urea
concentrations, and histopathological changes of kidney and increase of
creatinine concentrations.
Sterlet in
investigated part of the Danube River is exposed to discharge of Belgrade
industrial and domestic wastewater. Sterlet specimens were collected along the
50 km of the Danube River, the section which included localities in Belgrade
and downstream. Comparative analysis of histopathological and biochemical
parameters of sampled specimens in the vicinity of Belgrade waste water
discharge sites, and specimens collected 50 km downstream, will be performed in
order to explain possible effects of water pollution on investigated
parameters.
Blood film
analysis
Blood is one of
the most assessable tissues, which gives you the opportunity to assess
physiological and nutritional status without sacrificing the fish.
Hematological indices commonly used by fish biologist (red blood cell count,
hemoglobin concentration, blood hematocrit levels…) are adopted from human and
veterinary medicine. Houston & Murad (1991) stated that fisheries biologist
underscore a need for more detail characterization of red cell populations and
that certain caution should be taken into a consideration during interpretation
of hematological indices. The main reason for that is the fact that in healthy
humans all circulating erythrocytes are mature while in fish, erythropoetic
sites release cells to circulation at an early stage of development, which
induce morphological heterogeneity in peripheral red cell population (Houston,
1997). That was the reason why Houston (1997) suggested an alternative way of
measuring fish hematological status, called erythron profile. The description
of the erythron profile involves estimation of relative abundance of various
developmental stages found on blood smears. In our study we have applied method
suggested by Houston (1997) in analysis of the influence of different water
temperatures on morpholoogical characteristic of sterlet erythrocyte
populations. Our goal is to fallow sterlet erythron profile during whole year
cycle in attempt to determine standard values, which could be used later on as
indicators of fish health.
Parasitological
analysis
Standard
parasitological method was used to examine 260 sterlets (Acipenser ruthenus L.) so
far. From all the sterlets examined 89.8% was infested. We established presence
of 2 protozoa’s (Trichodina sp. and Ichthyophirius multiphilis)
and 2 trematodes species (Skrjabinopsolus
semiarmatus (Molin, 1956), Azygia robusta (Othner, 1911). Nematodes were present with two species (Contracaecum bidentatum (Linstow, 1899) and Contracaecum sinipercae Dogiel et Achmerov, 1959). Genus
Acantocephala appeared with two species (Pomphorhynchus laevis
(Müller, 1786) and Acanthocephalus
lucii (Müller, 1786)). The
most common parasite found was Skrjabinopsolus
semiarmatus that was present with
14-360 specimens per fish and its prevalence was 78.57%. One to six specimens
of Contracaecum sp. were found per fish as well, with the
prevalence of 6.12%.
Analysis of the skin mucous film determined 100% prevalence of
infestation with Trichodina sp. Other parasites were not
so common and they were found only at few sterlet specimens.
Our investigation has showed that the degree of sterlet parasite
infestation is very high and the further examinations are necessary to
determine the role of parasites in regulation of starlet’s population density.
This data can also be a good indicator of the extent of sterlet habitat
pollution.
CONCLUSIONS
Sturgeon
species have several aspects that make them important for protection and
preservation. Apart from the fact that they make good scientific object
(philogeneticaly very old group with different adaptation strategies) their
economical value is very high as well. However there is a lack of recent and
relevant data on natural populations of many sturgeons. One of those species is
sterlet as well. It was especially noted by Reinartz (2002) that little is
known about sterlet populations in the Lower Danube region. That is why we are
trying to update existing data made by Jankovic (1958) and Ristic (1971) and to
go further in investigation of life history aspects of this species. So far
attention was given to morphology, physiology, histology, parasitology and
ecology. In the future we would like to concentrate more on sterlet behavior in
order to complete the knowledge on this species. Especially important are its
migrations, which can excide more than 300 km (Hensel and Holcik, 1997) in the
Danube River.
Hopefully our
further knowledge, together with existing one, will provide us with a good base
for resolving “two morphs” dilemma, which will then lead to appropriate, and
adequate sterlet protection at the end.
LITERATURE
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