Trout and mice

Trout that easts ...mouse

I'm not sure that torut was that hungry however here is the story from By Gavin James and Steve Fox :
"Our recent capture of a large brown trout containing mice stimulated our scientific curiosity and prompted an investigation into what is actually known of the relationship between mice and trout. We have attempted to provide some answers here to the more commonly asked questions that perplex anglers. What causes good beech tree seeding years, or "masting" years in botanical terms, and can they be predicted? What is the link between beech seeding and "mice plagues" and for that matter why do mice apparently enter the water so readily and end up becoming food for trout? Why do only some trout seem to catch on to this new food source and does this phenomenon only happen in New Zealand?
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Mice are opportunistic feeders and are quick to exploit any increases in the abundance of seeds. Not surprisingly, analyses of mice stomach contents reveal that the huge numbers of beech seeds that can fall on to the forest floor (in extreme cases up to 10,000 per m2), form an important source of food during seed years. However, recent research on food webs within beech forests indicates other food items, especially insects, may also be important. Prior to seeding, beech trees shed surprising quantities of flowers and leaves. The resulting increase in the volume and quality of leaf litter triggers a population boom of insects, particularly caterpillars that live on the forest floor. As a consequence, during seed years mice benefit from a bountiful supply of food.

Mice, of course, have been introduced into New Zealand, along with the rats, stoats, and trout that now form parts of this beech forest food web. When well fed, mice are prolific breeders and can mature as young as two months of age, with litters being produced as frequently as ever), 20 to 30 days. In non-seed years, breeding lasts from spring through to late autumn, although this changes drastically during seed years when breeding extends through winter. Under these ideal conditions population densities continue to rise and generally peak in summer. However, it seems mice numbers started to decline in late February. We suspect this because one very experienced angler, who fishes South Island high country rivers extensively, noticed a marked decline in the condition of large mice-feeding fish in January and early February.

So how do mice find their way into rivers and the stomachs of trout? It is often thought that high population densities force mice to search for new areas, much as its overseas relative - the lemming - has often been portrayed as doing. Contrary to popular opinion, however, it is now known that lemmings do not commit suicide by jumping off cliffs into the sea. Back to the mice. It is now known that mice do not appear to defend territories when living at high densities in beech forests. In spite of this, it is still possible that their drive to find a new area is so strong that when they encounter a river or lake they make a brave, but often ill-fated, attempt to swim across. An alternative and simpler explanation is that as mice numbers build up, there are simply more mice around to fall into waterways and many mice that finish up in rivers may do so inadvertently. In the latter stages of a mouse plague, they are also likely to be very hungry and to search more widely for food.

A related question is whether trout take mice that are actively swimming, or drowned individuals as well? As already mentioned, juice are primarily nocturnal and this coincides with the preferred feeding time of many large trout. Many anglers would appreciate that movement is a powerful cue that can stimulate uncooperative trout to take a fly or lure. A swimming mouse would probably be easier to locate at night and be more likely to stimulate a predatory response. Indeed, there are a number of anecdotal reports from both New Zealand and North America of trout taking swimming mice. This, of course, doesn't exclude the possibility that dead mice are also consumed. The large fish we caught was captured in the eye of a pool, above which there was several kilometres of rapids. In that position it is hard to imagine many mice successfully navigating the rapids and making it to the trout alive. This brings us to the question of trout selectivity. Why do some fish apparently relish mice, while others do not? On a recent trip into the headwaters of a large West Coast river, we landed 11 trout over two days, but judging from their condition only one of these had been feeding on mice. Several other fish landed were large enough to have been able to feed on mice, but their very average condition suggested they had not been doing so.

It seems that for larger trout, feeding on mice is a learned ability."

Source:bestofnzflyfishing.com