There are some issues to keep aware of in this scientific paper (Crampton, 2008).
Just to get the taxonomy out of the way, there is no recognized species
Symphysodon haraldi. This fish was originally described as
S. aequifasciata haraldi by Schultz (1960). Kullander (1986) made it a synonym of
Symphysodon aequifasciata Pellegrin 1904. This remained in Kullander in Reis, et al (2003) and Ready et al (2006). Bleher (2007, 2011) gave it distinct species status as
S. haraldi, but this was not accepted, and Amado et al (2011) placed it in synonymy with
Symphysodon tarzoo Lyons 1959. This is the accepted name (California Academy of Sciences, Eshmeyer's Catalog of Fishes, 2022). The Amado et al. paper is online free at
https://www.hindawi.com/journals/ijeb/2011/360654/
The above has no bearing on the issue, but I like taxonomy and others may too.
There is considerably good information in this paper on the foods discus eat. We had a thread a few weeks back about this, and without question this is critical information on keeping these fish.
The group living behaviour however has to be very carefully read to avoid misunderstandings. Discus are what aquarists understand to be shoaling fish. They live in groups. From the study:
Discus form multi-cohort ‘colonies’ in galhadas, which appear to function as low-water refuges. The short dispersal distances indicated by mark-release data, coupled with the relatively small number of discus captured from newly constructed (‘artificial’) galhadas suggest that discus colonies are relatively static during the low water period. Discus were not evenly distributed among galhadas at the study sites. Instead, many apparently suitable galhadas (sheltered sites in 2 – 4 m depth), were devoid of discus or contained only a few specimens, while other, similar galhadas contained hundreds of specimens. These patterns suggest that discus actively form social aggregations, rather than simply fill available space [my emphasis].
During the rising water period, colonies in the galhadas of lago Amanã disintegrated. Discus dispersed into adjacent flooded forests where they were thereafter observed alone or in small groups. The persistence of a sizable colony at one site in the Uxi bay throughout this study, and the return of a single tagged fish to this site over an 11 month period suggest that individual discus may routinely return to the same low water refuge from one year to the next.
The formation of large, multi-cohort aggregations is unique to Symphysodon among Neo-tropical cichlids. Most South American cichlids live alone or in small groups but form breeding pairs and undertake biparental care of the eggs and young (Breder & Rosen, 1966; Lowe-McConnell, 1969; Goodwin et al., 1999). Aggregations of more than a few dozen individuals are rarely if ever seen in other South American cichlids (H. Bleher, H. López-Fernandez, R. Lowe-McConnell, pers. comms.). In the Tefé region, the only cichlid species other than discus ever seen in adult aggregations exceeding six individuals is the angelfish Pterophyllum scalare (Lichtenstein). This species forms groups of up to around 30 individuals during the low water period (Crampton, 1999c).
The behaviour to disperse during the high-water period is not at all surprising. This is when the fish spawn, and brood care is much safer when there are no other pairs present. We all (I hope) know how angelfish pairs can tear each other to shreds, and other cichlids are similar. The inference to be taken here is that discus should be housed in groups the same as angelfish and other non-cichlid shoaling fishes, but provision must be made if successful spawning is expected.
I know I came across another paper on similar different group behaviours in a species of characin but I can't find it now.
