Agree likely conditions became unsuitable. I've seen another cyprinid, European carp,  exhibiting similar behaviour on the Lower Murrumbidgee River, S.E. Australia; massing around the outlet of a regulator that separated a creek and wetland from the main channel. Flow was rapid through the gap in the regulator, and the water in the creek and wetland contained lots of coarse macrophyte organic matter and had low dissolved oxygen content (ca. 2-3 mg per litre).

Completely agree with previous opinions.

This behavior reminds me to other cyprinids (in anoxic conditions) trying to find more oxygenated waters. The flow improved oxygenation conditions although it is appreciated that many fish are "touched" (dying).

We see something like that when summer mortality appears by anoxia. Fish look for more oxigenated waters.

This sounds like a good opportunity to at least reduce the number of goldfish in that system, if not totally eradicate them.

Hi

there are other factors possible that may induce such migratory behaviour, like food availability or, as we assumed in relation to these two papers for juvenile fish schools in winter, predator avoidance behaviour (though we could not test this experimentally).

https://www.researchgate.net/publication/229709913_Winter_shortdistance_migration_of_juvenile_fish_between_two_floodplain_water_bodies_of_the_Lower_River_Rhine

https://www.researchgate.net/publication/227841557_Lateral_migrations_of_fishes_between_floodplain_lakes_and_their_drainage_channels_at_the_Lower_Rhine_diel_and_seasonal_aspects

Greetings

Jost

Article Winter short-distance migration of juvenile fish between two...

Article Lateral migrations of fishes between floodplain lakes and th...

I also agree with the previous options! I actually just saw something very similar when I cycled to work - the regulated outflow, which has been stagnant for a while, had a fish kill last night. The source lake is very eutrophic, has a bloom, and the outflow itself has smelled of decomposition for a while - so I was expecting this to happen especially with the warm weather depressing DO further. I was curious to see if I could find any surviving fish and went to the dam between the lake and the outflow, and there was a pipe with running (more oxygenated) water feeding into the outflow. Next to a horrible smell of putrefaction of dozens of dead fish, survivors were desperately flapping around at the pipe mouth. I would certainly measure oxygen at the goldfish hangout and the rest of the lake to see if that could be a possible explanation.

Very useful responses everyone, thank you.

Jost - Interesting work. I do wonder about the role of temperature with goldfish and koi becoming very sluggish at winter temperatures here. Growth must stop and predator avoidance become weak.

I saw similar juvenile migrations for goldfish and koi carp between a large shallow lake and the outlet canal (-37.405529, 175.210524). We did not look at the diurnal timing, with fish caught in traps set overnight on their way into the lake. That was a summer migration numbering up to several thousand per day. We did not monitor through winter.

Perhaps the movement of adults in this latest case is more indicative of water quality avoidance.

One more voice of support for already-proposed hypotheses: e.g., large biomasses of photosynthesizers (like a cyanobacterial bloom) can prompt large diel fluctuations of dissolved oxygen.

One more voice of support for already-proposed hypotheses: e.g., large biomasses of photosynthesizers (like a cyanobacterial bloom) can prompt large diel fluctuations of dissolved oxygen.

Do and Nutrient load are the regulatory factors

DO and Nutrient load are the regulatory factors

Without specific water quality information of the prevalent conditions in both -lake and outlet channel- this info will be merely speculative. However keep in mind that fish tend to avoid unfavorable environmental conditions such as low dissolved oxygen concentrations, water temperatures out of their optimum physiological range, and/or presence of toxic compounds/byproducts. It seems that any of these conditions could have forced those fish to migrate to "better waters", such as the outlet channel on that particular period of the day.

Certainly, hypoxia is commonly produced by contaminants or over-fertilization.

in general, accumulations of fish near outlet or inlet of small lakes at certain times (for example spring or late autumn) are not unusual, but this is species specific and remains often undetected. goldfish are easily to bee seen. 

I apologize for my English, and I hope that you at least understand something

In my practice, I have seen several different cumulation gibel carp Сarassius auratus s. lato. I work in the waters of the river basins of the Volga and Don near the city of Volgograd (within 200 km). Almost all reservoirs are so many gibel carp. Some reservoirs have a population of carassius, where part of the fish painted as a goldfish.

Examples carassius painted like a goldfish.

https://www.researchgate.net/publication/226522731_Population_of_silver_crucian_carp_Carassius_auratus%28Cypriniformes_Cyprinidae%29_with_golden_individuals_in_the_pond_of_the_city_of_Volgograd?ev=prf_pub

https://www.researchgate.net/publication/237049147_________-__

In water body where I work during the spawning gibel collected in dense cumulation of single-species and actively moves along the littoral. At this time, it comes in a lot of small rivers, streams, creeks, and as he goes along with the flow in the ducts, which watered pond-fish farm. He is looking for suitable spawning grounds. His active movement along the littoral, gibel carp is different from all of our fish. Nothing like other fish are not observed.

The stream of water flowing from the spillway of the Rostov NPP since the beginning of its operation in 2010 of Tsimlyansk reservoir come in gibel carp and almost exclusively. This is a very striking sight. It is my article, but in Russian

https://www.researchgate.net/publication/259558918_____-__

I placed in the same video applications in researchgate

I hope, will soon be here, but also in Russian, Vekhov D.A. Gibel carp in flowing water bodies of the Don River basin and the lower Volga basin // Povolzhskiy Journal of Ecology

Data on occurrence of gibel carp Сarassius auratus s. lato in flowing water bodies of the Don River basin and the lower Volga basin are discussed. It is shown that gibel carp actively seeks habitats with flowing water and it is capable to remain prolonged periods of time in streams with current velocity of 1.0-1.3 m/s. Furthermore gibel carp is able to climb fast flowing (2.6 m/s) and shallow (depth less than its body height) rapids. On this basis is proposed that gibel carp eurytopic fish. The massive appearance of this species in streams is associated with its spawning period, when he moved in search of spawning grounds. It is assumed that these movements can contribute to settling up the rivers.

Due to possible spawning migration I have a question.

When your local spawning goldfish? On the basis of which it was concluded that the state of the gonads? How many fish examined, as it has been opened? As the development of secondary sexual characteristics - breeding tubercles and shape of the belly?

To test the hypothesis of adverse factors need to have more information.

What share in% of goldfish, including fish of this reservoir. What happened to the other species fish (especially wild carp). How often carried out a survey of the lake and out of the lake, to find out whether this before, and if it was, what preceded this event.

Our fish gibel carp is one of the most sustainable fish to oxygen deficiency, high temperatures and toxic water pollution. I have not had time to see the ponds, which killed a lot of fish. At the same time over the pond it was often the stench, but the victim was not gibel carp. In some cases, the gibel carp was so active that even caught on fishrod.

In Tsimlyansk reservoir mass «flowering» of cyanobacteria are common, when they are concentrations of 0.5 or even 1.0 g / l and a water temperature of 25-30 C in the littoral zone is often a lot of fingerling gibel carp. Although other species of these places go, or even die (neogobius).

One time (15.12.2010) was in a place where the pipe that discharges stormwater runoff from the territory of the city Volzhsky chemical plants fell into the canal. At the exit of the pipe on the surface of the water was thick oil film, which was a lot of dead carp, and as recently deceased, and already half decomposed. Then the water from the pipe is on the many kilometers of canal to the pond, and as self-cleaning movement. I know that lives in the pond gibel carp, wild carp and crayfish. I examined the duct 0.5 and 1 km below the water discharge. The 0.5 km below the pipe fish are not noticed the oil slick was not. 1 km below the pipe was seen for many hold on the flowing gibel carp, but the bodies and the oil slick was not. Thus, the carp could choose a better environment, but it is concentrated in the most polluted place and died.

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