Entomology

Entomological study is, in my opinion a key element of watercraft for the fly angler.  The aim of this section is to provide information on these species to help the fly angler with identification and thus artificial fly selection and to explain some of the scientific method used in data collection and processing

Analysis of kick sample study results by statistical means.


After we have sampled the riverbed according to a defined method we need to make sense of  the data collected. Aquatic macro invertebrates are excellent indictors of water quality and the health of the river.  There are 2 key things we need to consider, firstly there is species diversity “how many species can i see”.  This is a qualitative descriptor of the rivers health BUT lots of species doesn’t mean good biodiversity, if the quantity of individuals for a given species is low then it follows that biodiversity is also low. The second thing we need to consider is species evenness, it is a quantitative descriptor which is a measure of the abundance of individuals in each species that is present, abundance can be measured by

  • Pecentage cover – the proportion of each quadrat occupied by the species.
  • Population density – the number of individuals per quadrat
  • Species frequency – the proportion of quadrats with the species in it.

Abundance within a species alone is also not a good indicator of biodiversity as lots of one species cannot be considered diverse.  Only when both species richness and species evenness increase, there is by definition an increase in BIODIVERSITY.

Below is data collected from 2 different stream sites:

Species observed Population density
Stream location A Stream location B
Gammarus pulex ( water shrimp) 57 38
Asellus aquaticus ( water louse) 32 16
Baetis rhodani ( mayfly nymph) 3 14
Lymnaea peregra ( snail) 3 22
Rhyacophila ( caddis-fly nymph) 1 5
Perlidae (stone fly) 4 5
Total 100 100

Both have the same ‘richness’ ( 6 species), but stream location B has greater ‘evenness’; so Field B is more diverse. In order to give this statement context we are going to use Simpsons index of diversity.  The value of this index ranges between 0 and 1 the greater the value, the greater the biodiversity. In order to calculate diversity (D) we are going to apply the formula

D = 1 – [ ∑ ( n / N)2 ]

Where

n = the total number of organisms of a particular species

N = the total number of organisms of all species

Species observed Population density
Stream location A Stream location B
Gammarus pulex ( water shrimp) 57 38
Asellus aquaticus ( water louse) 32 16
Baetis rhodani ( mayfly nymph) 3 14
Lymnaea peregra ( snail) 3 22
Rhyacophila ( caddis-fly nymph) 1 5
Perlidae (stone fly) 4 5
Total 100 100

To calculate the D value for stream location A

Species observed Population density
Stream location A (n) n/N (n/N)2
Gammarus pulex ( water shrimp) 57 0.57 0.349
Asellus aquaticus ( water louse) 32 0.32 0.1024
Baetis rhodani ( mayfly nymph) 3 0.03 0.0009
Lymnaea peregra ( snail) 3 0.03 0.0009
Rhyacophila ( caddis-fly nymph) 1 0.01 0.0001
Perlidae (stone fly) 4 0.04 0.0016
Total 100 ∑ = 0.4308

D = 1 – [ ∑ ( n / N)2 ]

n = the total number of organisms of a particular species

N = the total number of organisms of all species

∑ = sum of (n/N)2

D = 1 – [ ∑ ( n / N)2 ]

D =  1 – 0.4308

D =  0.5692

We can then apply the same formula to the data from stream locaton B

Species observed Population density
Stream location B  (n) n/N (n/N)2
Gammarus pulex ( water shrimp) 38 0.38 0.1444
Asellus aquaticus ( water louse) 16 0.16 0.0256
Baetis rhodani ( mayfly nymph) 14 0.14 0.0196
Lymnaea peregra ( snail) 22 0.22 0.0484
Rhyacophila ( caddis-fly nymph) 5 0.05 0.0025
Perlidae (stone fly) 5 0.05 0.0016
Total (N) 100 ∑ = 0.243

D = 1 – [ ∑ ( n / N)2 ]

D =  1 – 0.243

D =  0.757

Simpsons index of diversity for stream location A = 0.5692

Simpsons index of diversity for stream location B = 0.757

Stream location B has the higher diversity index, so has more species richness AND evenness. It would be more resistant to any environmental damage or change and is indicative of greater general “health” within the environment.

Simpsons index may be expressed in three ways

    1. Simpson’s index: D = sum(Pi2)
      The probability that two randomly selected individuals in the zone belong to the same species.
    2. Simpson’s index of diversity: 1 – D
      The probability that two randomly selected individuals in a zone belong to different species.
    3. Simpson’s reciprocal index: 1/D
      The number of equally common species that will produce the observed Simpson’s index.

It is important that the same index is applied to all samples to be compared to give real meaning to the results, Simpsons index will return a high number to indicate LOW biodiversity whilst Simpsons index of diversity would indicate high biodiversity. The reciprocal index starts with 1 as the lowest possible figure. This figure would represent a community containing only one species. The higher the value, the greater the diversity. The maximum value is the number of species (or other category being used) in the sample. For example if there are five species in the sample, then the maximum value is 5.

The use of statistical tests should help give meaning to the data generated from river monitoring operations, i think the Simpsons index of diversity is an excellent way of quantifying biodiversity in our river habitats and should give a clear indicator when things start to change.


PROFILE: The large dark olive Baetis rhodani

Common name: Large dark olive

Scientific name: Baetis rhodani

Adult Size:~10mm

No of tails: 2

Rear wing: small, oval with coastal projection

Nymph type: Agile darter

Nymph size: ~12mm

Habitat: fast flowing rivers and streams

Hatches: All year round, most prominent hatches in March and April

Time of day: Thorough the day

The large dark olive is a medium large fly that is resident in most rivers of the UK and the most common of the Baetis genus. It is one of the first species to hatch in any quantity on the southern chalk streams at the start of the Trout season. Hatches of the large dark olive are most prolific in March and April although a second peak smaller in numbers than the spring hatch in autumn. The LDO can be seen most of the year round hatching on mild days.

The Nymphs  of the  Baetis genus are classified as agile darters, in the case of the are dark olive,  they are slim bodied, pale green-olive coloured, they are can be found inhabiting areas of heavy weed growth. Baetis rhodani nymphs are strong swimmers the margins of their tails are heavily fringed with hairs, it is thought that they aid swimming.

Both the Male and the female dun have pale grey wings with darker veins,  the body colour is dark olive to brown though in the male the last segment of the abdomen tends to be paler in colour, legs are similar in colour to the body whilst the feet are black, tails are grey-pale olive.

The major way of differentiating between the sexes is by observation of the eyes. The female dun has green-grey coloured whilst the males are somewhat larger and a reddish brown colour.

The spinners of Baetis rhodani have transparent, brown veined wings.   The body colouration of the female is a reddish brown with olive rings, hence the common name of the red spinner, her eyes are dark brown.  The male is light olive with brown markings; the last three segments of the abdomen tend to have an orange hue to them. The male’s eyes are dark red in colour ans somewhat larger than the females.  The legs of both the male and female dark olive to grey colour with almost black feet.  The tails are olive-grey with reddy brown rings.  The spinner is generally regarded to be of little interest to the fly angler, as an early season hatching species the temperature is low, conditions that do not favour swarming behaviour seen in other ephemera. Mating occurs in the day the female spinner crawls down plant stems below the water’s surface to lay her eggs.

It is worth mentioning the similarities between Baetis rhodani and Baetis atrebatinus (the dark dun) , both flies occupy the same sort of habitat and hatching periods tend to overlap, Baetis atrebatinus is far less abundant  and for the fly angler can imitated as the same thing, the two species may be distinguished simply by observation of the hind wing, Baetis atrebatinus will not have the coastal projection found on the wing of Baetis rhodani

Artificials

Nymph: LDO nymph (Edwards), GRHE, Pheasant tailed nymph

Dun: USD dun, funnel dun, rough olive (skues), beacon beige, Kites imperial.

Spinner: Lunns particular, Red spinner, Dark bloa.


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2 responses

14 02 2011
15 02 2011
chris avery

I pop on to the forum occasionaly to see if there is anything of quality going on, but usually end up dissapointed.
So it was gratifying and refreshing to see this.
I have kind of rush-read it as i get ready for the working day, but will go back to it later. Meanwhile I am sending it to as many interested parties as I know, so we can have a good discussion about it later.

Your insect profile particuarly inspires a ”God i wish that was around when i was just learning’ moment.
Do you hope to cover all British species like that?

all the best
whinging pom

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