Over several years in the early spring in the Dniester reservoir (Ukraine, the Eastern Europe) large individuals of a silver carp (Hypophthalmichthys molitrix) and a bighead carp (Hypophthalmichthys nobilis) in large quantities perish during migration from upper reservoir to the bottom. Why do the silver carp and a bighead carp accumulate near a dam for 10-14 days in the early spring (if does not get to turbines)?
Fish bypass is absent in this reservoir. Not clearly why fish does not resist to a current
Actually it should be the upstream migration (to breed). I believe you know the biology of Asian carps. It seems that river doesn't flow on a quite long part. So probably fish is gathering at the dam because there is only the flow.
Hydropower plant on map:
http://maps.google.com.ua/maps?q=%D0%9D%D0%BE%D0%B2%D0%BE%D0%B4%D0%BD%D0%B5%D1%81%D1%82%D1%80%D0%BE%D0%B2%D1%81%D0%BA&hl=ru&ie=UTF8&ll=48.595968,27.460327&spn=0.018278,0.055017&hnear=%D0%9D%D0%BE%D0%B2%D0%BE%D0%B4%D0%BD%D0%B5%D1%81%D1%82%D1%80%D0%BE%D0%B2%D1%81%D0%BA,+%D0%A7%D0%B5%D1%80%D0%BD%D0%BE%D0%B2%D0%B8%D1%86%D0%BA%D0%B0%D1%8F+%D0%BE%D0%B1%D0%BB%D0%B0%D1%81%D1%82%D1%8C,+60236&t=h&z=14
That because gonads develop during migration.
Do you stock SC/BH or it is naturalized self-reproducing population? Fishes look young. Is there are any older individuals (20 kg)?
The last stocking with fish of reservoir was in 2005. Installed individuals age 1 + and 2 + (SC, BH and hybrid).
It is a photo of 2009 (the end of March). Majority of the lost individuals had weight about 15 kg.
In 2010 and 2011 of a congestion of fish near a dam it was possible to find out in time and station work was carried out in emergency operation. In 10-12 days fish has departed from HYDROELECTRIC POWER STATION. This year fish has accumulated near a dam on February, 28th. Because of an ice cover fish did not manage to be found out in time. Weight of some of the lost individuals reached 30 kg.
Authentic data on spawning of these fishes in a water basin are not present. However, occasionally in the end of June it is possible to meet individuals with developed gonads.
If such behaviour is connected with reproduction why fish does not leave against a current in the river? The reservoir's top is opened.
Migration descendants are very prejudicas the current transposition systems, but this type of migration is carried out through spillways and turbines, but survival rates are highly variable and bypasses. A comparative study with data collection and study of ichthyoplankton gonadal helped us understand these carp.
If I understand correctly, these species are not natives, they are cultured. Are there any other fish species living the system? Which are they? There might be an interspecific relationship and this might be force them migration because of the competition.
Or this could be a natural reproduction behaviour? If you have not obseved this behaviour before several years ego, there might be any other changes in stream habitat characteristics.
We need more data about these questions:
What is the geographic distribution of these species in your river system (you have to look at all catchment)?
Is these species native? If not, when did you introduce these species in these system?
What are the other native or nonnative fish species living in this system?
What are the maturation age of these species?
Are there any special spawning habitat preferences of these species?
What are the diet and food preferences of these species?
the migration is a part of the fish biological function to find feed, suitable temperature and habitat for gonads maturation and reproduction, they may be migrate in the down stream for these reasons or due to they are not native species in this locality and do not have Knowledge about the new habitat features
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