Xenotoca, a note on cannibalism
I bred Xenotoca San Marcos without problems untill now, but things have changed. I separated a half-grown group for future breeding, these are now adult and start breeding. There is a single Characodon male in the aquarium as well, but no other fish. All Xenotoca's are young adults, there are about 20 of them. Very different from before, they fanatically chase and eat all young! The food is the same, but the social environment is not. Before there were always small fish present, if not Xenotoca's, then at least some Ameca's. I think that the fish were used to see small fish swimming around, then this was part of their everyday environment and it did not trigger any reaction. Now any young fish is something "new" and triggers an instant chasing reaction. Once one fish starts chasing, others joing in and the young are eaten in no time. I planted some dense plants and as experiment I put a young Ameca in the aquarium, which is faster and slightly bigger than a young Xenotoca. Like this I hope to gradually get the fish used to young ones in the aquarium. I think that this "being used to something" might explain why sometimes Goodeids become fin-biters and sometimes they don't. To be continued ...
"Solved" in the following
"Solved" in the following way:
- put a highly pregnant female in a breeding box and raised 6 ofspring to about twice the birthsize, then put these young back with the adults (too big to be eaten; adult fish got used to somewhat smaller fish)
- reduced the number of adults to 4.4 (reducing number of "pack-chasing fish" and making place for the growing up juveniles)
- now one juvenile survived but is hiding almost constantly
- let 2 females give birth in another aquarium (lots of "spare" young so I can keep the strain going regardless what happens)
- next litter: 4 juveniles survived in the presence of adults, and after about 5 days swimm freely between the adults (but mostly prefer to be higher in the water, what they would also do in case no adults are present at all).
Food and aquarium decoration has stayed the same throughout all this.
Conclusion: the social environment and the experience of individual Goodeids can have a big influence on their behaviour. I believe that this also explains the reported discrepancies between reported fin-eating and peacefullness of Xenotoca.
Second conclusion: when only one aquarium is available to breed Goodeids in, it might be best to keep different age-classes together all the time rather than keeping "one cohort" of similar aged fish. For the second aproach, 2 or more aquariums are more appropriate, so it is possible to separate young/pregnant females.
Third conclusion: the above cannot applied in the same way to all species (nor probably to all individuals of the same species), for instance I had no problems with a group of adult Ameca splendens when their first young were born.
Question: is this forum note usefull? Since there are no reactions on my original note, it could mean that everyone knows this already and thus ignores my beginners-babbling ...
I added 2 more juveniles of
I added 2 more juveniles of the same age/size. The first one swam calmly to the other juveniles and there were no problems. The second one fleed very fast downwards, to were the adults were. This triggered an immediate group-chase. It escaped to some plants and hid there, afterwards it swam toi the other juveniles and there were no further problems. To filosofyze a bit on this:
- I presume that in nature, adult and juvenile Xenotoca are in different habitats. Newborn fry is in very shallow water (at most a few cm deep) or in very dense plants. Once they are swimming well, juveniles move to open water, but stay near the surface (upper 10-15 cm) and near plants. Once they grow to 1,5 x birthsize, they gradually move downwards towards the larger fish, which are bottom-oriented. As long as they swim in a relaxed way, juveniles can do this without harm.
I suspect that juveniles can stay near the surface because they are too small to be prey for fish-eating birds, their most important predators may be larger fish of either their own or different species. Adults stay in deeper water because of the risk to be eaten by birds. This segregation also means that there is less competition for food (in addition juveniles eat different things from adults - adults are much more vegetarian and can ingest larger particles), and juveniles are in warmer water were they can grow faster.
In "normal" aquariums there is simply not enough space for this segregation. This goes OK as long as the juveniles do not panick. Any fast swimming may trigger chasing and if the adults catch juveniles, they learn to hunt juveniles even when these juveniles swim normally. Even in small aquaria some of the segregation of juvenile and adult fish is visible, because juveniles swim "under the surface" while adults swim "above the substrate".
The most risky are the first days after birth, when juveniles cannot yet swim adequately. Immediately after birth they aslo have a very strong fleeing reaction. Exactly at this stage I suspect that they are the furthest away from adult fish in nature.