48 MB. J. C. MOTTiUM ON THE 



always take a mixed diet. It is nire on opening a fish to find the remains of 

 only one kind of food : in one part of the intestine will lie found, for instance, 

 snail-shells ; in another fish-bone- 1 , and in the stomach insect-remains. The 

 arrangement of the food in the alimentary tract indicates that the fish take 

 their food in hatches, the remains of the various foods are more often than 

 not unmixed and occupy different parts of the gut. 



Observations in every respect confirm this : a fish picking up snails from 

 the river-bed will never stop to take a floating insect ; fish taking floating 

 insects will entirely ignore a shoal of minnows in the neighbourhood ; on the 

 other hand, when a fish is taking minnows other foods are neglected, This 

 hunting of one prey at a time is most clearly seen when the fish are taking 

 floating insects. It is the rule in chalk-streams to observe fish taking only 

 one species of insect when there are two or more on the water. 



These observations indicate that fish are subject to what may be called a 

 special appetite, as distinguished from a general one. When a fish is observed 

 to be feeding on one insect and neglecting another, it cannot be concluded 

 that there is more than a temporary difference in palatability between these 

 two insects ; it is not unlikely that another fish may be seen feeding on the 

 second insect and neglecting the first. There is some evidence that these 

 individual and temporary preferences depend upon what food the fish has 

 had in the immediate past ; if, for instance, the fish has recently fed upon 

 insects, it will be more likely to take some other food when next hungry. 

 In many rivers during early June Ephemera danica, Mull, (the May-fly), 

 hatches out in immense quantities and the fish at once gorge themselves with 

 it ; nevertheless, towards the close of the May-fly season, the fish may often 

 be seen taking other food, whilst M. danica is hatching out in abundance the 

 fish may even be seen taking smaller species of the Ephemeridse or some 

 species of Diptera. 



Obs. — 15.6.1tl3, E. Kennet, "Found two trout taking ' Olive Dim' (sub-imngo 

 of Baetis vermis, Curtis) and neglecting the May flies which were hatching out at 

 the same time. Also saw several dace (Leuciscus leuciscus, White) feeding upon 

 ' Reed Smuts ' (SimuUum, Latr.) and neglecting the May fly." 



It is well known to fly-fishermen that for some weeks after the Mav-fly 

 season the fish more or less neglect insect diet: " The 'May-fly carnival 

 means a long interval, during which the rod may as well be put by, for the 

 glutted fish take a ' cure ' of at least three weeks, during which they abstain 

 from insect-food of all descriptions" (from 'Happy Hunting Grounds,' by 

 A. E. Gathorne-Hardy). 



In some streams Brachycentrus subnubilus, Curtis, (Grannom) hatches out 

 in immense quantities during April, and, in a similar way, the trout quickly 

 satiate themselves with this insect. 



