Assessment of native brown trout swim up fry stocking on ...

Assessment of native brown trout
swim up fry stocking on the
Candover Brook, Hampshire

Dylan Roberts & Dr Dominic Stubbing

Format of our presentation

• Setting the scene – Dylan Roberts

1. Background
2. National Trout and Grayling Fisheries Strategy 2003
3. Why is this work important?

• Evaluation of native brown trout swim up fry stocking in
the Candover Brook – Dr Dominic Stubbing

1. Objectives
2. Techniques
3. Preliminary year 1 data

Brown trout stocking in England and Wales

• Trout have been reared on farms to
enhance angling for over 100 years

• Estimated 430,000 Trout anglers in England
and Wales

• Stocking is necessary due to insufficient
numbers of wild trout to support viable
fisheries

• Estimated 650,000 adult brown trout are
stocked annually into rivers in England and
Wales

• Some 260,000 brown trout were stocked as
eggs or juveniles (swim up or fed fry)

Legal requirements of stocking in England
and Wales

• Most stocked brown trout are reared on fish farms

• There are strict legal controls on their introduction
into rivers and lakes

• Salmon and Freshwater Fisheries Act 1975,
Section 30 in England and Wales

• Environment Agency FR1 Application to introduce
fish, fry or ova

Concerns regarding the effects of stocking
domesticated strain farmed trout on native
trout

1. Stimulating an influx of predators
2. Stimulating fishing effort hence excessive

exploitation of wild stocks
3. Introduction of disease
4. Competition for food and space
5. Predation by stocked fish
6. Change in the genetic composition of wild

stocks through interbreeding

Farmed diploid and triploid brown trout

• Two types of brown trout currently used for stocking - diploid and triploid.

• Triploid trout are produced by subjecting the ova after fertilisation to a
temperature, pressure or chemical shock (pressure shock is most
successful)

• Triploids have three sets of chromosomes, as opposed to the usual two in
normal diploid trout

• Triploids are infertile and hence removes the risk of interbreeding with
native trout

Our research on trout stocking

1. Performance of stocked adult diploid brown trout in
UK rivers 2002 - 2005

2. Effects of stocking diploid adult brown trout on wild
trout in UK rivers 2002 - 2005

3. Effects of stocking diploid brown trout fed fry on wild
trout fry in UK rivers 2002-2005

4. Comparisons of the performance of diploid and
triploid brown trout in UK rivers 2005 - 2007

5. Interactions between stocked diploid, triploid and
wild brown trout during spawning 2005- 2007

6. Diets of stocked and wild trout 2005 - 2007
7. Comparison of the fishery performance of diploid

and triploid stocked brown trout 2005 -2006
8. Comparisons of the performance of triploid fed,

unfed, winter and spring fry stocked via
incubator boxes and from a fish farm

Environment Agency trout stocking policy

• By 2015 all trout stocked into rivers in England and Wales must be
triploid or of native origin reared in a suitable rearing system e.g.
incubator boxes

• We have done the work on: -
1. Adult diploid brown trout
2. Diploid brown trout fry
3. Adult triploid trout
4. Doing the work on triploid fry
5. Via this project we are

beginning to investigate the performance of stocked native
strain trout fry reared under a suitable rearing regime

Candover Brook

Stripping salmon for hatchery boxes

Stripping wild trout for hatchery boxes

Counting eggs

Hatchery boxes ready for eggs

Marking swim-up fry

Unfenced river

Fenced river

Electric fishing

Mark detector

Looking for marks in the dark

Calcein mark

Elastomer marking

Calcein retention

• 100% of trout were marked at 12 months, after which some became
questionable.

• Some fish still have marks at 18 months and have reached 1lb.

Length, weight and calcien retention in brown trout fry.

Recognizable mark

100 Questionable mark 500

90 No mark 450

Length Length (mm) and Weight (g)
80 400

Weight
70 350

Retention (%) 60 300

50 250

40 200

30 150

20 100

10 50

00

12026038FFFJeeeabbbnrrruuuuaaaarrrrJMyyyyAMaupa2222yrrn----00000000000033333333
SAAJJFMDNAOMJJuueeeauaocpauutyrgpgcbrllnnv--------------0000000000000034434334344443

Numbers of surviving brown trout
fry in raceways with adult brown
trout.

12

10

Number of fry 8
Unmarked

6
Marked

4

2 234 5 P = 0.294
Raceway number t = 1.206
0
1

Hatchery box survival

% Survival 100
90
80 Salmon Salmon Salmon Salmon
70
60
50
40
30
20
10
0
Trout

Site retention (%)Site retention of marked trout

5
4.5

83% survival in

4

hatchery.

3.5
3

2.5
2

1.5
1

0.5
0
1 2 3 4 5 6 7 8 9 10 11 12 13
Site number (1=downstream)

Densities of trout and salmon

Density (Nos./m2) 0.4 Marked Trout
0.35 Salmon
0+ trout
0.3
0.25

0.2
0.15

0.1
0.05

0
1 2 3 4 5 6 7 8 9 10 11 12 13
Site number (1=downstream)

Densities of trout in fenced and
unfenced sites.

Density (Nos./m2) 0.12 Fenced
0.1 Unfenced

0.08 Marked Trout
0.06
0.04
0.02

0
0+ Trout

Acknowledgements

• The Vitacress Conservation Trust
• Environment Agency
• The Grange
• Watergates Fish Farm
• Jerre Mohler