CHAPTER III
Fresh-water Shrimp (Gammarus pulex)[4]
[4] Rewritten from an article in The Field, April 16, 1898, under the heading of “The Fresh-water Shrimp as a Wet Fly.”
Of all the forms of food partaken of by the trout the Crustacea are the best. When I say the best, I mean that trout fed upon Crustacea seem to thrive better than trout fed on anything else. In this case, at any rate, the most wholesome form of food seems also to be the most welcome; for though I have tried feeding trout with almost every form of food, I have never come across another form which they have taken with anything approaching the voracity with which they take Crustacea.
Fortunately, I can bring forward a case to show how trout thrive when fed upon Crustacea. In April, 1897, Colonel Walker presented some trout to the Brighton Aquarium. I myself caught some of these trout, which were put in a rearing pond to await their being transferred by rail to the Aquarium. As I also assisted in the operation of taking them from the rearing pond and putting them into the tanks in which they were to travel, I can vouch for their size at that time. They were all in rather bad condition, and, even had the largest been in good condition, it could not have weighed more than three-quarters of a pound. These trout have been fed entirely on Crustacea since they were introduced into the tank they now occupy; and at the time I am writing (January, 1898), the largest of these trout must be quite two pounds or more in weight, and there are others which are nearly as large.
The voracity with which these trout seize the Sandhoppers and Shrimps upon which they are fed is a perfect revelation. I have seen them leap out of the water to catch the Shrimps thrown to them before they reached the surface.
I have also found that young trout in rearing ponds take Fresh-water Shrimps with the same greediness; and on considering these facts, I am surprised that there have not been more attempts to imitate the Fresh-water Shrimp.
The Gammarus pulex may be found in almost all streams, especially where there is much vegetation. An illustration of it is given on [Plate I]. I have however found them abundant in streams where there were no weeds. They hide under stones at the bottom of the water and among the weeds, especially among watercress and starwort. Though they will live in still water, I have found them most numerous in streams; and notwithstanding that they are generally supposed only to inhabit somewhat sluggish streams, I have found them in fairly rapid ones, with a stony bed. The Shrimp is very prolific, and if protected increase very rapidly; thus it is a most excellent plan for those who breed and rear trout to cultivate them, as they are one of the most valuable forms of food.
These animals are very similar in shape to their well-known relation, the common Sandhopper. In colour they vary very much according to the water they inhabit. I have seen them a pale yellow colour in some streams, while in others they are almost black. The commonest colour is however a reddish-yellow.
I find that the general idea is that these Shrimps travel through the water in quick leaps by bending up their bodies and straightening them out again. I have however never seen them do this, though I have kept them in an aquarium and watched them very carefully.
What I have seen is, that they use their legs to swim with, moving them as though they were walking very rapidly. They cannot, however, walk when they are taken out of the water, but lie perfectly helpless upon their sides. In a stream where the Fresh-water Shrimp swims, it seems unable to progress up stream, or at any rate, if it does it moves very slowly; when they wish to go up stream they crawl along the bottom. They can, however, as a rule, maintain the same position against the current.