Spot the shrimp

MANUEL T SILVA divulges how biology explains why the shrimps we tie with orange in theirbodies work so effectively.

Imitations of the small crustacean Gammarus (a shrimp also known as ‘scud’) are widely used by fly fishermen to catch trout and grayling, both in the British Isles and elsewhere, because that crustacean is abundant throughout the year in most waters, and fish love them. Many fly tyers like the imitations in orange or pink colour, or with tags or central spots in orange or red, although the live healthy naturals are not of this colour. Living freshwater Gammarus are grey, olive-green, yellow-green or brownish-green, depending on their food and type of water.

John Goddard’s article in the June 2007 issue of Fly Fishing and Fly Tying, ‘Temperamental summer grayling’, extolling the virtues of nymphs with red/orange/pink colours states: “If you look back over the history of grayling patterns many of the very old and established patterns have either orange or red colours in the dressing which definitely seems to be for some strange reason very attractive to them”. This statement led me to present the following considerations based on biological facts that may explain the generally recognised success in grayling (and trout) fishing of that kind of flies.
As shown by the photos of nymphs with red/orange/pink colours selected for John Goddard’s article, those colours can be incorporated in the whole imitation or in spots. We’ll deal separately with these two alternatives.

Imitations with red/orange spots
One very successful Gammarus imitation with a red central spot, the Red-Spot Shrimp, was devised several years ago by Neil Patterson, the great fly tyer and fly fisherman living on the banks of the wonderful river Kennet. This imitation includes a small spot of fluorescent red wool in the centre of the body. Neil Patterson created this imitation when he discovered that a significant proportion of the Gammarus in the rivers he fished contained an orange-red bright central spot. In his superb book, Chalkstream Chronicle, he tells us that at first he thought that such an orange spot was a parasite but later, he assumed it to be an egg sac carried in a brood pouch by females after mating. And he wrote: “Against the dull olive colour of the shrimp, this sac glows as brightly as a farmhouse window on a dark hillside. Nature, I decided, had provided the fly tyer with a wonderful way of illuminating his shrimps in the bleak dungeon of a trout hole.” Neil Patterson therefore believed that the central bright red spot represented a conspicuous trigger that increased the catching abilities of the imitation.

Other examples of Gammarus imitations incorporating red/orange/pink coloured spots are the two devised by David Coomber, the Tag Shrimp and the Orange Spot Shrimp. The first has a tail tag, the second a spot in the middle of the body, both of Glo-Brite fluorescent orange floss. The imitation created by John Roberts, also called Orange Spot Shrimp, incorporates a fluorescent orange plastic bead at the middle of the body. The article by John Goddard showed some other examples.

Several years ago, John Roberts called my attention to a research reported in the journal, Ecology, about the effects of parasitisation of Gammarus by an orange larva (called Pomphorhynchus), effects that could have relevance regarding the effectiveness of red/orange spotted imitations for fly fishing for trout and grayling. In his book, Flyfishing for Grayling, John Roberts also makes reference to that research. John’s comments prompted me to search published scientific information regarding Gammarus. I identified two situations where these crustaceans can have a red/orange spot in the middle of their bodies, both of which confirm that the two alternatives Neil Patterson considered for the red/orange spot he found in some Gammarus he collected in British streams were correct. These situations are: presence of the orange/red parasite Pomphorhynchus or of an egg sac (in gravid females).

As we’ll see now, the research has led to interesting observations that are likely to have relevance to the success in fly fishing of orange/red tagged or spotted Gammarus imitations.

To read the fascinating and revealing detail behind how the parasite affects the way trout see a shrimp – not only in looks but also in its behaviour – read the rest of Manuel Silva's article in the August issue of Fly Fishing and Fly Tying