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I have a 2' x 1' x 15" high aquarium which contained four large bristlenose, 2 males & 2 females, despite the fairly cramped quarters all 4 seemed to co-exist quite happily. Every 4-6 weeks the smaller female would lay eggs in the larger male's cave. This was the only time when I saw any aggression between the two males, as the smaller male would try to muscle in on the proceedings. Within an hour or so, this first batch of eggs were ejected from the cave and the second larger female would be inside spawning. The male always looked after this second batch very well. This happened on several occasions, as a consequence, the tank has been steadily filling up with youngsters of varying size. About 3 - 4 months ago I decided that the degree of overcrowding was excessive, so I removed both the smaller male and smaller female along with a fair number of the babies. Since then the remaining pair haven't spawned. I have maintained the same maintainance & feeding routine.
So my questions are.
1. are bristlenose seasonal in their spawning cycles - these are wild caught fish, possibly Ancistrus pirareta?
2. Are bristlenose more likely to spawn when kept in groups, rather than in pairs?

Thanks for any help

I have a 24x15x12 which at one time held a pair of large individuals of common bristlenose. They would spawn regularly along the lines you indicate without problem without any seasonal influences.

I had cause to remove a quantity of the fry (from several spawnings) & tidied up the decor after catching a couple of hundred youngsters. This seemed to act as a disruptor as the parents didn't spawn again for nearly 5 months despite everything else remaining the same.

The best ratios of sexes in tanks for ancistrus may vary between species but ultimately they should resume sometime as 1:1

To speculate on an answer to question 2 (Are bristlenose more likely to spawn when kept in groups, rather than in pairs?), some believe that spawning fish release pheremones that induce spawning in other fish (Eric Bodrock had a talk about how he gets Cories to spawn by having drilled tanks on a central system; his proposition is that the pheromones released by spawning in one tank trigger spawns in other tanks). So a spawning bn may emit pheremones to induce other pairs to spawn. Or maybe it could be a "greedy" alpha male that coerces all the females to breed.

Thanks for the feedback guys,

It all depends on whether you want to be overrun with fry again

At the moment I'm happy that they're not spawning as I'm running out of tanks I can keep the babies in, but once the majority have grown and been moved on, I'd like them to start again!

some believe that spawning fish release pheremones that induce spawning in other fish .... So a spawning bn may emit pheremones to induce other pairs to spawn.

from my limited experience I find this very plausible, which is why I asked the question. When they were in together both males seemed happy to stay in their respective caves, except when the one pair started spawning, the second male got very excited and was hanging around the alpha's cave, trying to get in; and for both females to spawn within a couple of hours of each other, to me strongly suggests a hormonal trigger.

Pterophyllum wrote:

some believe that spawning fish release pheremones that induce spawning in other fish .... So a spawning bn may emit pheremones to induce other pairs to spawn.

from my limited experience I find this very plausible, which is why I asked the question. When they were in together both males seemed happy to stay in their respective caves, except when the one pair started spawning, the second male got very excited and was hanging around the alpha's cave, trying to get in; and for both females to spawn within a couple of hours of each other, to me strongly suggests a hormonal trigger.

Pheremones could also have a "negative" effect. Some people think that the largest male will produce pheremones inhibiting the growth of other plecos; not a big problem in a river, can be a problem in a small tank with low water changes.