So it's true...once in a Blue Moon

'Once in a Blue Moon' is a great fly fishing film that other than being wonderfully shot, talks about the phenomenon of trout eating mice!

You can buy the 'Once in a Blue Moon' DVD here

New Zealand's Year of the Mouse!

Regular anglers of the backcountry in New Zealand will know that trout, while sometimes very large, are normally long and well proportioned rather than wide and fat.

This pattern seems to change every few years and apparently over the last couple of summers there have been numerous reports of record size trout being caught throughout the country.

It has been suggested (here and here) that it could be because of an increase in the mice population and the trout’s taste switching accordingly. Let’s look at this a little further because understanding phenomenon such as this will help us to understand the behaviour of these fish.

The likely reason for the increase in mice is probably the favorable climatic conditions, which have caused the Beech trees to produce larger than usual quantities of flowers and seeds. Prior to seeding, Beech trees shed huge amounts of flowers and leaves and this abundance of food on the forest floor and the accompanying insects encourages the mice.

So how do they end up being eaten by fish? It is possible that their drive for new accommodating areas is so strong that they will brave a swim across a river. Or maybe it is just that as there are more mice about, there are simply more of them falling into the rivers.

Do fish take live mice or dead mice? Mice are primarily nocturnal and this coincides with the preferred feeding time of many large trout. Movement is a cue that can often stimulate a stubborn fish and a swimming mouse would be easier to find at night than a dead one.

What about the fish? Why is it that some fish relish mice and others don’t? James and Fox seem to both think that for larger trout, feeding on mice is a learned ability. Trout are known to specialise on one type of prey because as new prey types become available and more abundant, some individuals realise that they have been missing out on a lot of food and change their diet to in turn become skilled at dealing with a new prey.

How many mice can a trout eat? There are reports of up to 23 mice being found inside a brown trout. So one can assume… quite a few. However this follows reports of fish bleeding from the gills and anus. It seems that they can be very greedy and possibly to their detriment.

Does a fish taste differently? Samples suggest not, however I wouldn't recommend broadcasting the reason for the large size of the fish to any dinner guests.

Both images from: James & Fox, 2009. New Zealand Mouse Year. Fish and Game Magazine New Zealand. Accessed here