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KillieTalk Digest V1 #2
Barry Cooper wrote:
>As you all know, Brian, Ruud and myself collected in the southern part of
>Tanzania this year. The fish I brought back are doing well, and I'm coming
>to the time when some of the eggs I've collected will be hatchable. In
>fact, I've already hatched some N. ocellatus from location 36. I'm raising
>about 10, which is about all I can manage. I hope to distribute a couple of
>pairs some time soon. They grow incredibly fast. I was away for a couple of
>days this weekend, and I swear they doubled in size on the worms I left in
>the tank when I left. They also have a surprisingly short incubation
>period. In this case, I hatched the first eggs after only 5 weeks, as they
>were eyed up. In the case of eggs I got from Brian Watters from some of the
>95 collections, I got the best hatches after only 6 weeks. I wonder whether
>this account for the relatively low numbers one finds in most locations? In
>my experience, if you incubate them for a long time, you lose a lot of the
My observations of N. ocellatus have led me to believe that they will be the first Nothos to hatch among a group of sympatric Notho species existing in the same pool. This allows the N. ocellatus to grow a bit before the other species hatch, making it easier for them to predate on the others. What do they eat until the "other" Notho species hatch ? They eat each other and in that way the numbers are reduced to a level that the system can support.
Barry, you will remember that, with the exception of location TAN 97/36, all the pools where N. ocellatus occurred, there were very small numbers of them. The same was true for for our Kikonkono TAN 95/9 locality. Of course, this is always the case with predators of any sort in that they are always present in relatively small numbers compared to the numbers of the prey.
While young N. ocellatus are capable of consuming prey that are not much smaller than themselves, this is not true for larger specimens. In relative terms, as they get bigger the size of the prey that they can ingest decreases quite dramatically. What this implies is that the N. ocellatus would stand the best chance of survival if they could hatch out a bit before the sympatric species (= the prey) and stay nicely ahead of them in terms of size. The rapid rate of growth of this species is not surprising when one considers that it has to attain a much larger size than other Nothos in order to be able to mature and reproduce in the time that the habitat has water.
If something upsets this balance then a situation will arise such as we encountered at location TAN 97/36. At this locality, there were large numbers of N. ocellatus (we were catching up to 8 specimens at a time with each short sweep of a seine) and all except the very largest specimens were basically starving and very thin. This was in spite of the fact that the pool was also loaded with N. melanospilus (plus relatively small numbers of N. janpapi and an undescribed Notho). The majority of N. melanospilus were, in my opinion, too large for all but the largest N. ocellatus to be able to eat. The large (actually enormous !) N. ocellatus were in superb condition, no doubt because they could eat just about eveything else in the pool (except for the 20 inch Clarius catfish !). What could upset the balance leading to this situation ? A hatch of N. ocellatus that is too successful leading to overpopulation ? This seems most likely. Late hatching of the N. ocellatus (or early hatching of the other species) might also be a factor.
This is, of course, all speculation and comments would be welcome.
Brian R. Watters
University of Regina
Regina, Saskatchewan S4S 0A2, Canada
Ph: (306) 584-9161 (home); (306) 585-4663 (work)
Fax: (306) 585-5433
E-mail: bwatters at sk_sympatico.ca