DRIFT DIVE SURVEY REPORT 2015
A summary of drift dives conducted by Fish & Game West Coast
Region in the spring and summer of 2014/2015
Rhys Adams, Fish & Game Officer, March 2015
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Drift Dive Survey Report 2015
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DRIFT DIVE SURVEYS REPORT 2015
A summary of drift dives conducted by Fish & Game West Coast in the spring and summer of
2014/2015.
Rhys Adams, Fish & Game Officer, March 2015.
Summary
Drift dives have been conducted in the West Coast Fish & Game Region since 1985. The
purpose of these surveys is to give an indication of trout abundance in a particular stretch of
river. Where dives have occurred in past years, comparisons can be made. The data and
findings of the surveys are intended for internal management purposes only. The spring dives
were conducted in December 2014 and the summer dives in February 2015 incorporating
Fish & Game staff from the Nelson Marlborough Region, North Canterbury Fish & Game
and the West Coast Regional Council. The Mawheraiti River at SH7 Bridge and Mirfins
Bridge, the Grey River at Waipuna and Hospital Flat, and the Inangahua River at Blacks
Point were dived. Small fish have increase in abundance at all sites except for the Grey River
at Hospital Flat. The only consistent and distinct seasonal change in abundance observed in
the seasonal dives in the Mawheraiti River is an increase in small fish from spring to summer
at the Mirfins Bridge site. Staff recommendations are; Continuation of drift dive programme
as a tool for gathering long term data on West Coast trout abundance and continue to assist
other neighbouring regions with their drift dive programmes on a reciprocal basis.
Introduction
Drift diving is now a commonly used method of monitoring trout abundance in clear, small to medium
sized rivers throughout the country. The West Coast Fish & Game Region’ database of drift-dive
results date back to 1985 when MAF conducted a series of dives for their “100 Rivers” survey. Since
then, Fish & Game staff have undertaken up to 10 dives per year if river conditions have been
favourable.
There is now a large West Coast data set that enables comparison at sites where multiple years have
been dived. The data provides a ‘spot’ count of trout abundance on a particular stretch of a river. The
West Coast Region performs dives in relation to specific threats or management information needs
and sites are not randomised. Therefore, results in this report should not be used to describe catchment
level or regional level observations. The Data and findings of the surveys are intended for internal
management purposes only.
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Drift Dive Survey Report 2015
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Method
The method involves a number of snorkelled divers drifting downstream, maintaining a line across
the river and counting trout that pass under or between them on one side. Trout are divided into three
size groups;
Large: Trout over 450mm in length.
Medium:Trout less than 450mm and greater than 150mm in length.
Small: Trout less than 150mm in length.
Fish numbers are recorded by each diver, with the team leader collecting the information from each
diver periodically throughout the dive and/or on completion. The Majority of West Coast Rivers are
dived specifically for brown trout (Salmo trutta) and unless specifically stated, brown trout are the
species referred to as ‘trout’ or ‘fish’ in this report.
Dive sites are between 1 and 3 Km in length. This distance is considered the longest possible to avoid
fatigue whilst being long enough to give an estimate of the actual population for that stretch of river.
Water clarity is measured at the beginning of each dive with a 200mm black disc and tape measured
horizontally through the water column. Good water clarity is required for an accurate count therefore
diving is not generally undertaken if the visibility is less than 4m. More divers are required if water
clarity is low to ensure adequate coverage.
The Mawheraiti River was selected to dive due to observations that trout abundance has been notably
low in recent years. Two dives were performed, spring and summer to assess if there is seasonal
fluctuation in trout abundance as part of 2014/15 workplan project 1115 sports fishery research. The
Inangahua River was dived in response to the potential of mining discharge in the catchment to affect
the fishery and the application for two hydro power schemes in the catchment. The Grey River at
Hospital Flat was dived to monitor the state of a highly valued section of river near the Grey River
Water Conservation Order that has a perceived high angler usage. The Grey River at Waipuna is
dived to monitor the middle reaches of one of the West Coast most fished West Coast River
catchments (Unwin, 2009).
Staff from the West Coast Regional Council, North Canterbury Fish & Game, and Nelson
Marlborough Fish & Game Region assisted with dives. Staff provided their time on a reciprocal basis
and this ensures diver numbers are adequate. Two seasonal dives were undertaken in the Mawheraiti
River in; ‘spring’ on the 16th of December. Summer dives were completed on the 19th & 20th of
February, 2015. Unless stated, historic dives were undertaken in summer during the months of
January or February.
Results
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Drift Dive Survey Report 2015
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Mawheraiti River: (Mirfins Bridge site)
The Mawheraiti River at the Mirfins Bridge site has been dived on seven occasions within three
successive sports fishing seasons. The dives are seasonal with November and December relating to
spring, January or February relating to summer and May relating to autumn. The data is presented
in the table 1 below.
Three years of spring and summer comparisons can be made. The only consistent change in trout
abundance seasonally at this site is the increase of small trout from spring to summer. This change
is distinct. Seasonal changes for medium and large fish are inconsistent.
Table 1.Mirfins Bridge dive site data
Large/km Med/km Small/km
2012 (Nov)
7
10
1
2013 (Jan)
11
38
27
2013 (May)
9
17
1
2013 (Nov)
7
15
4
2014 (Feb)
2
2
22
2014 (Dec)
15
25
12
2015 (Feb)
10
21
102
Average
9
22
24
combined
19
76
27
26
26
52
133
54
Mawheraiti River: (SH7 Bridge site)
The Mawheraiti River at the SH7 Bridge was dived in spring (December 2014) and summer (February
2015). The spring dive counts were; 8 small/km, 18 medium/km, and 8 large/km. The summer counts
were; 54 small/km, 21 medium/km, and 14 large/km (see Chart 1).
When comparing spring dives between 2014 and 2015 medium size fish increased in abundance, but
small and large fish counts were similar. Compared to summer 2014 the summer dive for 2015
revealed an increased in counts of all fish sizes, most notably small fish. All counts for summer 2015
are below the summer average but are a notable increase on record low combined counts of summer
2014.
Seasonal comparisons of abundance can be made between four spring and summer dives. The only
consistent change in seasonal abundance between spring and summer is an increase in the count of
large trout. The changes are not distinct due to the low numbers of fish counted. For medium trout,
seasonal changes in abundance are inconsistent. Three of four comparisons show a slight decrease in
abundance from spring to summer. The numbers counted are low so the changes are not distinct. For
small trout, changes in abundance are also inconsistent with three out of four comparisons showing
an increase in abundance from spring to summer.
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Drift Dive Survey Report 2015
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Grey River: (Waipuna site)
The 2015 Grey River dives at Waipuna counted 145 small/km, 54 medium/km and 14 large/km.
Small fish counts have increased from 2014 levels and are well above average. Medium fish counts
have increased from 2014 levels and are only slightly below average. Large fish counts have
decreased and sit slightly below average (see Chart 2).
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Drift Dive Survey Report 2015
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Grey River: (Hospital Flat site)
The 2015 dives at Hospital Flat reveal 8 small/km, 4 medium/km 20 large/km. Small and medium
fish are at relatively low levels of abundances, however large trout abundance is above average
compared to all ten years dived on this section of river. The counts of small and medium trout are
similar to 2014 however counts for large trout have increased (see Chart 3).
Inangahua River: (Blacks Point site)
The 2015 dives at Blacks Point reveal counts of 170 small/km and 19 medium/km. Small and medium
trout abundance has increase from record lows of 2014. Although the increase in medium trout is
evident, 2015 counts are below average. Counts of small have increased substantially from 2014
counts but are still below average. The large fish count of 7/km is a decreased from 2014 levels but
is at levels consistent with several other years (see Chart 4).
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Drift Dive Survey Report 2015
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Discussion
With the exception of the Grey River at Hospital Flat small fish counts increased from 2014 levels
and in some cases by a substantial amount. This is likely to be a reflection of successful spawning
seasons in 2013 and 2014 as small fish are considered to be approximately 6 or 18 months of age
during the summer dives. Of particular note is that small fish at the Grey River Waipuna site are well
above average abundance. When considering all trout sizes as a combined trout count, the summer
combined counts at all sites increased from 2014 levels.
The combined summer count for 2015 at the Mawheraiti SH7 site is the highest count since 2011
when the fishery declined. The main contributor to the increase is small fish which suggest an
improvement in spawning success from previous years in the catchment. The counts are still below
average and far from the highest counts.
Seasonal dives have been undertaken at the Mawheraiti SH7 Bridge site on four consecutive years,
and at the Mirfins bridge site on three consecutive years. From the two sites the only consistent and
distinct change in seasonal abundance is that small fish increase in abundance from spring to summer
at the Mirfins Bridge site.
The only other consistent change is large fish increase in numbers from spring to summer at the SH7
site. The counts of fish are low however, being well below summer averages, and the changes are not
distinct. Therefore, further dive data would be necessary in years’ of higher large fish abundance to
draw conclusions that the observed changes describes a pattern of changes in seasonal abundance in
that particular stretch of river.
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Drift Dive Survey Report 2015
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Since 1985, when the MAF initiated the drift diving data set on the West Coast it is evident that trout
populations fluctuate in abundance over the years. Climatic conditions are considered to play a major
role in governing salmoniid abundance levels in West Coast Rivers (Kelly, 2009). It is reasoned that
if large flood events occur more frequently trout abundance will be low.
Large Floods have the potential to significantly affect the abundance of trout. Two notable ways a
flood can affect abundance is; mortality from physical damage during high flow periods and
destruction of spawning redds. These factors combined have the potential to decrease small fish
abundance especially in spring during the emergence/ alevin lifecycle stage (Jensen & Johnson,
2002). Large and Medium trout populations have been shown to be negatively affected by large flood
events in the Motueka River. Food abundance, water temperatures and low flows may also have an
effect on adult trout abundance in the Motueka River (Young et al, unpublished).
Staff Recommendations

Continuation of the drift dive programme as a tool for gathering long term data on West Coast
trout abundance.

Continue to assist with neighbouring regions drift dive programmes on a reciprocal basis.

That council receives the report.
References
Jenson A.J. & Johnson B.O. (2002). The functional relationship between peak spring floods
and survival and growth of juvenile Atlantic Salmon (Salmo salar) and Brown Trout (Salmo
trutta). Functional Ecology 13, 778-785.
Unwin, M.J. (2009). Angler usage of lake and river fisheries managed by Fish & Game New
Zealand: results from the 2007/08 National Angling Survey.
Kelly, D (2009). Drift Dive Surveys. Fish & Game internal report.
Young, R.G. et al. Unpublished Cawthron Institute report. Factors controlling adult trout
abundance in the Motueka River over 20 years.
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Drift Dive Survey Report 2015
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Appendix – Raw data from 2014/2015 reported drift dive sites.
RIVER
LOCALITY
Date
GRID REF
DIST
WIDTH
Grey R
Hospital Flat
1998
2
20
Grey R
Hospital Flat
2
20
Grey R
Hospital Flat
2
Grey R
Hospital Flat
Grey R
Hospital Flat
Grey R
Hospital Flat
Grey R
Hospital Flat
16/02/2012
Grey R
Hospital Flat
30/01/2013
Grey R
Hospital Flat
10/02/2014
Grey R
Hospital Flat
19/02/2015
320/621-308/614
2431119/5861497 2429994/5860486
2431119/5861497 2429994/5860486
2431119/5861497 2429994/5860486
2431119/5861497 2429994/5860486
2431119/5861497 2429994/5860486
2431119/5861497 2429994/5860487
2431119/5861497 2429994/5860488
2431119/5861497 2429994/5860489
2431119/5861497 2429994/5860489
Grey R
Waipuna
1994
Grey R
Waipuna
1995
Grey R
Waipuna
Grey R
Waipuna
Grey R
Waipuna
Grey R
Waipuna
Grey R
Waipuna
Grey R
Waipuna
15/02/2012
Grey R
Waipuna
31/01/2013
Grey R
Waipuna
11/02/2014
Grey R
Waipuna
Inangahua R
Inangahua R
B.
Disk
L
M
S
TOTAL
10
5
4
19
6.2
34
14
21
69
20
8.4
30
32
89
151
2
20
9.5
45
25
64
134
2
20
9
56
34
155
245
2
20
4.3
46
25
127
198
2
20
11
28
6
3
37
2
20
10.6
30
10
31
71
2
20
12.8
26
6
16
48
2
20
10.8
40
8
17
65
085/735-064/748
3
40
55
557
140
752
085/735-064/748
3
40
39
129
21
189
1996
085/735-064/748
3
40
43
154
92
289
2005
3
40
6.7
20
239
185
444
3
40
7.8
51
238
604
893
3
40
7.3
24
101
65
190
3
40
8.1
31
89
181
301
3
40
10.3
65
89
190
344
3
40
9.1
38
67
56
161
3
40
8
52
44
175
271
20/02/2015
085/735-064/748
2408562/5873429 2406221/5874826
2408562/5873429 2406221/5874826
2408562/5873429 2406221/5874826
2408562/5873429 2406221/5874827
2408562/5873429 2406221/5874828
2408562/5873429 2406221/5874829
2408562/5873429 2406221/5874829
3
40
14
41
161
434
636
Blacks Point
1991
179/962-176/973
1.7
18
7
283
174
464
Blacks Point
1992
179/962-176/973
1.7
18
7
287
774
1068
Inangahua R
Blacks Point
1993
179/962-176/973
1.7
18
11
145
674
830
Inangahua R
Blacks Point
1994
179/962-176/973
1.7
18
9
127
452
588
Inangahua R
Blacks Point
1995
179/962-176/973
1.7
18
25
293
339
657
Inangahua R
Blacks Point
1996
179/962-176/973
1.7
18
10
204
396
610
Inangahua R
Blacks Point
1997
179/962-176/973
1.7
18
72
392
356
820
Inangahua R
Blacks Point
2001
179/962-176/973
1.7
18
4.5
15
93
264
372
Inangahua R
Blacks Point
2003
179/962-176/973
1.7
18
6
33
198
683
914
Inangahua R
Blacks Point
2004
179/962-176/973
1.7
18
5
26
335
1021
1382
Inangahua R
Blacks Point
2005
179/962-176/973
1.7
18
3.5
16
250
623
889
Inangahua R
Blacks Point
16/02/2012
179/962-176/974
1.7
18
5.8
13
83
205
301
Inangahua R
Blacks Point
30/01/2013
179/962-176/975
1.7
18
7.6
10
77
243
330
Inangahua R
Blacks Point
10/02/2014
179/962-176/976
1.7
18
6.5
27
18
46
91
Inangahua R
Blacks Point
19/02/2015
179/962-176/976
1.7
18
6.4
12
33
Mawheraiti
Bridge
1994
043/892-033/878
1.7
20
43
75
Mawheraiti
Bridge
1995
043/892-033/878
1.7
20
106
379
Mawheraiti
Bridge
1996
043/892-033/878
1.7
20
72
357
Mawheraiti
Bridge
1997
043/892-033/878
1.7
20
35
207
_____________________________________________________________________________________________
289
334
36
154
193
678
523
952
44
286
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Drift Dive Survey Report 2015
Page 9 of 10
Mawheraiti R
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
Mawheraiti
Bridge
043/892-033/886
1.7
20
5.6
24
27
92
Mawheraiti R
Mirfins Bridge
1/11/2012
005/810-999/798
1.4
20
5.1
10
14
2
26
Mawheraiti R
Mirfins Bridge
30/01/2013
005/810-999/799
1.4
20
5.6
15
53
38
106
Mawheraiti R
Mirfins Bridge
10/05/2013
005/810-999/800
1.4
20
6.3
13
24
1
38
Mawheraiti R
Mirfins Bridge
20/11/2013
005/810-999/801
1.3
20
4.5
9
20
5
34
Mawheraiti R
Mirfins Bridge
11/02/2014
005/810-999/801
1.3
20
4.9
2
3
29
34
Mawheraiti R
Mirfins Bridge
16/12/2014
005/810-999/801
1.3
20
4
19
33
15
67
Mawheraiti R
Mirfins Bridge
19/01/2015
005/810-999/801
1.3
20
4.4
13
27
133
173
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
Mawheraiti R
2001
043/892-033/878
1.7
20
4.6
16
39
68
123
2003
043/892-033/878
1.7
20
4.6
102
181
272
555
2004
043/892-033/878
1.7
20
4
76
364
167
607
2005
043/892-033/878
1.7
20
5.1
45
307
342
694
2009
043/892-033/878
1.7
20
4.7
44
121
322
487
11/02/2011
043/892-033/878
1.7
20
3.8
29
36
264
329
20/12/2011
043/892-033/879
1.7
20
3.9
19
20
30
69
15/02/2012
043/892-033/880
1.7
20
4.6
22
24
46
92
17/04/2012
043/892-033/881
1.7
20
5.8
31
15
6
52
1/11/2012
043/892-033/882
1.7
20
5.1
21
14
10
45
31/01/2013
043/892-033/883
1.7
20
5
22
7
30
59
10/05/2013
043/892-033/884
1.7
20
6.3
27
11
4
42
20/11/2013
043/892-033/885
1.7
20
4.2
13
10
15
38
10/02/2014
043/892-033/886
1.7
20
5
15
8
5
28
16/12/2014
043/892-033/886
1.7
20
4.2
14
30
13
57
134
20/02/2015
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Drift Dive Survey Report 2015
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