Redd'n Breakfast

Last time, we proposed a quizz, with five possible answers. Which one did you pick up ? Well, it almost does not matter since all possibilities but the D one have been observed in brown trout.

In this precise observation though, the individual is a male, and he's darting towards the egg to take a good meal. Eggs are indeed a good meal, full of energy, in a period of the year where food is scarce for many organisms.

This is the kind of interesting behaviour that you may discover, when you spend some time observing the reproduction in natural rivers. In my lab, we have been doing so for several years, using hand cam recorders or subaquatic cams too. Females use to digg a nest in the gravel for several hours, a time during which competition between males can escalate. At the precise time where female lay eggs, fertilization is very quick, often achieved by the closest male, but other males can participate too (multiple mating, as in the picture below).

After fertilization, females quickly start to cover the redd with gravel, probably to protect the eggs from predators – or conspecifics. The thing is, in our observations, the cannibalistic behaviour is relatively frequent, it can even reach 41% of the redds in some populations ! What we call cannibalism here is not some picking on drifting eggs, but something that severely impacts the redd. Something like that, at 1:40 :

Because it seemed so frequent, we decided to investigate a little bit more this behaviour, its occurrence, and how it could be triggered or prevented.

First, we quantified the dynamics of digging events by the female to cover the redd after fertilization.

 

The above graph shows the cumulative frequency of these events as a function of time since fertilization: as you can see, the female is very quick at the beginning, then begins to slow down after about 120 seconds, or 2 minutes. Could they be tired ? Well, if we look at males during the same time window, on redds where cannibalism happens, we find a very interesting result:

 

The above graph shows that 90% of cannibalism events occur in the first 120 seconds after the fertilization ! The females do hurry for a good reason, and somehow, their parental care will provide an efficient protection against conspecifics and predators after these two minutes. But before that, it is not fully safe, and the moments after fertilization are critical for eggs survival, and therefore for the parents' reproductive success.

Additionally, this cannibalistic behaviour is more frequent when the male competition is intense (an elevated operational sex ratio), it can also be perpetrated by the male that fertilized the female, and in that case, the occurrence increases with paternity uncertainty, that is, when multiple mating occurs.

Now, nothing new here: Greeley, in 1932, reports such observations in the Transactions of the American Fisheries Society, although they are not quantified. But this rings a bell: if egg cannibalism is so frequent, it could be obviously a potent force to drive selection at that precise moment.

And because females are already occupied covering the redd with gravels, the doom of cannibalism remains. Could such a sin could even threaten the species ?

Answer next time !

References:

Greeley J.R. 1932. The spawning habits of brook, brown and rainbow trout, and the problem of egg predators. Transactions of the American Fisheries Society 62:239-248. 

Aymes J.C.Larrieu M, Tentelier C, Labonne J. 2010.  Occurrence and variation of egg cannibalism in brown trout Salmo trutta. Naturwissenschaften 97:435-439.