Shrimp

Killer Shrimps again.

A rather unhappy trout, taken on a killer shrimp pattern.

This entry is totally unscientific. It is based on just a few field observations by an angler; and we all know what that means: total observer bias.

On the other hand, most scientific studies are conducted in a laboratory where all the variable parameters such as temperature, rainfall and disturbance can be controlled. The main advantage of this approach is that a repeat run is likely to produce the same results but, possibly more importantly, it keeps down costs by having all your scientists tied to a desk.

For studying the reproduction rates, diet, growth rate and metabolism of the killer shrimp, you need to have them in a tank. For an insight into their ecology and behaviour you need to go down to the reservoir, which is where I happened to be last week.

I have a favourite spot where the water is quite shallow and the bottom is strewn with stones. It has always been a good place to find freshwater shrimps and caddis larvae and therefore a good spot to fish. I like shallow water because, if a trout is there, you are likely to have your lure in its sights. In deep corners you have to decide which line to choose; floating, neutral or sinking. I much prefer floating lines but use a sinking leader made of fluorocarbon to get the fly beneath the surface. Most nymph patterns sink of their own accord because they are made around a heavy hook and their dressing soaks up water, but some tend to be quite buoyant.

General purpose patterns like the pheasant tail nymph or the gold-ribbed hare's ear are quite sparsely dressed and sink well. You can control the depth you fish them by delaying the start or changing the speed of your retrieve. I find that these nymphs work like small fish fry when stripped quickly or caddis pupa when suspended. A sharp, short twitch can turn them into a shrimp or small sizes might be midges.

Killer shrimp. This one floats.

If you are fishing a shrimp imitation, you normally want to twitch it along the bottom because that's where shrimps live. They swim on their sides and are usually half curled-up in a C shape. To be a good imitation of this, a lure would need to be on a  hook with a curved shank. If the bend with the point at the end was too heavy, it would turn the shrimp upright rather than on its side. On the other hand, a light hook makes for a slow sink rate.

Shrimp are crustaceans like their marine cousins, shrimps, prawns and lobsters. They have their skeletons on the outside and they are jointed like suits of armour with legs on each section. The jointed appearance is very characteristic and the killer shrimp has zebra-stripes that accentuate this. It also has stonking great mandibles in front.

Here is what I found out on my last couple of sessions in the wild.

In early July I turned over a few stones and found almost no shrimps ; I also caught few trout. This is always a tricky time when the fish are probably eating pin-fry or daphnia. Where were the killer shrimps that are supposed to have taken over the whole lake?

I think the cold start to the year delayed the breeding cycles of all the freshwater invertebrates and so populations of shrimps and other species were low until August. On the other hand, there was a good hatch of caddis flies on the go and I found that a pheasant tail nymph caught one or two fish over the stones.

In early August the shrimps were there, but the trout were already hitting the quite large coarse fish-fry that were hiding around the stones and groynes. I tried a fry pattern but could not get a pull so put on a killer shrimp.

On my first cast with the shrimp, I struck into a large rainbow that took out all my line and most of the backing, and it leapt like a salmon. It had inhaled the fly, only inches below the surface; not on the bottom. What was going on?

This was a firm, silver, fin-perfect chunk of fish. Not a "stockie" and not part of a school; just an educated singleton that was exploiting a rich source of natural food. Back at the fish farm, they look for pellets at fixed feeding times, but in the wild they have to learn to find proper trout food. So why was he not on the bottom, nosing around the rocks?

As I waded in the shallows, I saw quite a few killer shrimps swimming about near the surface, even in a metre of water. I saw the same thing last year; these guys actively hunt for food, unlike our native shrimps that seem to be mainly detritus feeders. Judging by the number crawling up my waders, they also have a rubber fetish.

So, I'm suggesting that in late summer, educated trout (the ones we are really interested in) are used to finding killer shrimps at a variety of depths and will take them readily over the right habitat. In open water, over muddy bottoms you would probably be wasting your time to use a shrimp. In the autumn you would probably do better on a zonker or other fry imitation since, like us, the trout like the biggest, easiest meal they can get at the time.

Just for now, it's killer shrimp season.