Relação entre regime de fluxo e abundância de peixes em um rio de cascalho, Nova Zelândia Journal of Fish Biology 1419 a 1436

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Relationship between flow regime and fish abundances
in a gravel-bed river, New Zealand
I. G. JOWETT*†, J. RICHARDSON* AND M. L. BONNETT‡
*National Institute of Water and Atmospheric Research, P. O. Box 11 115, Hamilton,
New Zealand and ‡National Institute of Water and Atmospheric Research, P. O. Box
8602, Christchurch, New Zealand
(Received 25 September 2003, Accepted 9 January 2005)
The key elements of the flow regime of the Waipara River on the east coast of New Zealand,
that affected fish abundances were the timing of floods and the magnitude and duration of
low flows. Generally, fish abundances were highest in early summer, and lowest at the begin-
ning of winter. Spring floods opened the river mouth, allowing recruitment of diadromous
fish species, and non-diadromous fish species spawned after the floods in spring or early
summer. Reductions in fish abundances over summer and autumn were consistent with the
magnitude and duration of low flows, with significant reductions in the year of lowest flow and
little change in abundance in the year when low flows were highest. Variations in
fish abundances during periods of low flow were consistent with the amount of instream habitat
available, such that abundances of species with high velocity preferences decreased
during periods of low flow, whereas abundances of species with low velocity preferences
increased. # 2005 The Fisheries Society of the British Isles
Key words: flood; flow regime; habitat; low flow; river mouth closure.
INTRODUCTION
The quantity and timing of river flow is critical to the ecological integrity of
river systems (Poff et al., 1997). Although human activities can alter the natural
flow regime to varying degrees, it may be possible to mitigate effects of flow
regime changes on biota by retaining the key elements of the flow regime, if
these are known. In the U.S.A., Richter et al. (1997) suggested a method for
setting ‘streamflow-based river ecosystem targets’ that they called the ‘range of
variability approach’ (RVA), based on the assumption that ‘the full range of
inter- and intra-annual variations of hydrological regimes, and associated char-
acteristics of timing, duration, frequency and rate of change, are critical in
sustaining the full native biodiversity and integrity of aquatic ecosystems’. It
is unlikely, however, that all the hydrological characteristics of flow regimes will
be important to stream ecosystems given the wide variation in flow regime from
year to year in any river, and the variation in flow regimes between rivers that
†Author to whom correspondence should be addressed. Tel.: þ64 7856 1791; fax: þ64 7856 0151; email:
i.jowett@niwa.co.nz
Journal of Fish Biology (2005) 66, 1419–1436
doi:10.1111/j.1095-8649.2005.00693.x,availableonlineathttp://www.blackwell-synergy.com
1419
# 2005TheFisheries Society of theBritish Isles
contain similar aquatic communities. Identification of the key or controlling
elements of a flow regime is essential if flow management is to retain the
ecological integrity of rivers. Understanding the relationships between flow
regime, primary and secondary production, life cycle requirements of fishes,
in-stream habitat, bed sediment mechanics and biological interactions is neces-
sary if the effects of flow changes on fish communities are to be predicted and
‘artificial’ flow regimes made up of seasonally varied base flows, freshes to
disturb deposited fine sediments, and channel maintenance flows to preserve
channel and flood plain morphology (Reiser et al., 1989; Milhous, 1998) are to
be recommended.
Floods and droughts are often considered to be major regulators of aquatic
communities, especially in gravel-bed rivers where floods mobilize bed sediments
and droughts can result in a dry riverbed with no surface water. The direct, short-
term effects of floods are largely a result of high water velocities and sediment
movement that cause displacement or death. In the long-term, the bed movement
that floods cause can alter the structure of a river, moving, creating or destroying
the habitat utilized by aquatic communities. Major floods can substantially
reduce taxonomic richness and total density of benthic invertebrates (Sagar,
1986; Quinn & Hickey, 1990; Giller et al., 1991; Angradi, 1997), and their
devastating effects on salmonid populations have been well documented (Seegrist
& Gard, 1972; Hoopes, 1975; Jowett & Richardson, 1989). In contrast to floods,
biotic changes that occur during droughts are slow and may favour communities
adapted to low water velocities, low dissolved oxygen and high water temper-
atures. Although stable flows are advantageous to some fish and invertebrate
communities (Sagar, 1986; Jowett, 1990), prolonged periods of low flows
(droughts) can reduce abundances (Canton et al., 1984; Closs & Lake, 1996;
Bell et al., 2000). In extreme cases, communities may be limited to those that
can survive without flow or even without water.
Gravel-bed rivers are common in New Zealand (Mosley, 1992) and floods that
disturb the gravel and cobble beds usually occur several times a year (Duncan et al.,
1999). Little is known, however, about the relative roles of flow regime attributes in
maintaining ecosystem function. Despite regular disturbance, lowland gravel-bed
rivers contain high densities of native fishes (Jowett & Richardson, 1996), and it
appears that these benthic species may be particularly well adapted to surviving in
unstable environments.
Recruitment of New Zealand diadromous fish species occurs mainly in spring
and early summer (June to December) (McDowall, 1995). There is no evidence
that recruits return to natal waters. Diadromy is believed to allow fish species to
recolonize rivers after severe disturbances, and access to the sea is an essential part
of this process (McDowall, 1995). Thus, if river flows are insufficient to maintain
an open river mouth at appropriate times of the year, recruitment of diadromous
species is prevented, and there will be a gradual decline in their abundance.
The aims of the present study were to identify the key elements of the flow
regime and their effect on the fish community in a small gravel-bed river by
examining changes in fish abundances over a 3 year period and to associate
those changes with species life cycles and flow events. In particular, the effects
of the timing of floods on recruitment and the effects of the magnitude and
duration of high and low flows on fish species abundances were examined.
1420 I . G . JOWETT ET AL .
# 2005TheFisheries Society of the British Isles, Journal of FishBiology 2005, 66, 1419–1436
MATERIALS AND METHODS
STUDY SITES
The Waipara River is a small gravel-bed river that originates in the North Canterbury
foothills and flows eastwards to the sea (Fig. 1). The geology is sedimentary and the
annual rainfall is c. 1m over a catchment area of 370 km2. Sheep and cattle farming are
the predominant land uses, although a number of vineyards have recently been esta-
blished in the lower catchment.
A preliminary fish survey showed that there were strong longitudinal gradients in the
distribution and abundance of fish species in the river. To take the longitudinal gradient
into account, eight sampling sites were established along the lower 40 km of the river
(Fig. 1). Except during floods, there is no significant flow contribution from tributaries
between the study sites. The river is confined by foothills at the upper sites (sites 1 and 2),
before flowing across a large alluvial plain (sites 3 and 4), through a wide canyon (sites 5
and 6), and finally over another alluvial plain (sites 7 and 8) to the sea. The river is
occasionally braided and contains roughly equal amounts of run and riffle habitat and
few pools (Jowett, 1994). The average river gradient between site 1 and the sea is 0�0041,
with little variation alongits length. No potential barriers to fish passage were identified
along the length of the river. Because of the alluvial nature of the river, its banks are
generally gently shelving gravel, although there are a few sections where banks are steep
and lined by willow trees.
Sampling sites were selected so that they contained at least 10–15m each of run and
riffle habitat. Sites were marked with pegs so that they could be located on subsequent
visits. Floods and associated gravel movement disturbed sites and, occasionally, the
degree of disturbance was so great that the run and riffle sequence no longer existed at
the same location. Bed disturbance resulted in the relocation of two sampling sites after
floods in July 1999, two after a flood in November 1999, and four sites after a flood in
June 2000. After the largest flood in August 2000 (peak daily mean flow of 280m3 s�1),
sites had been disturbed to such an extent that all, except site 7, required relocation. Sites
were relocated in run and riffle habitats as near as possible to the original site.
8
7
6
5
4
32
1
Amberley
Waipara River
45° S
170° E 175° E
Waipara
River
10 km
FIG. 1. Location of sampling sites (1–8) on the Waipara River.
FLOW REGIME AND FISH ABUNDANCE 1421
# 2005TheFisheries Society of theBritish Isles, Journal of FishBiology 2005, 66, 1419–1436
RIVER FLOW
Flow records were available from a water level recorder located c. 30 km from the sea
between sites 1 and 2. The mean flow over the period of the study (September 1998 to
June 2001) was 2�5m3 s�1, with a median flow of 0�7m3 s�1. Flows over the study period
were slightly lower than the long-term (17 years) mean and median flows of 2�9 and
0�9m3 s�1, respectively. The flow hydrograph was characterized by high average flows in
winter (c. 6m3 s�1) and spring (c. 3m3 s�1), and low average flows in the autumn (c.
0�9m3 s�1) and summer (c. 0�5m3 s�1). This seasonal flow variation is normal in the
Waipara River and is typical of the highly variable flow regimes experienced in east coast
rivers draining the foothills of the Southern Alps (Jowett & Duncan, 1990). Water
exchange occurs along the Waipara River; net surface loss occurs across the alluvial
plains, and net surface gain occurs in the canyon, especially where bedrock is close to the
surface. Near the mouth, the river is dry in some summers.
A gravel bar closes the mouth of the Waipara River except when the river flow exceeds
the seepage flow through the gravel bar. This is a feature of small gravel-bed rivers on the
east coast of the South Island. A water level recorder was installed in the lagoon at the
river mouth in September 1999 and these levels, in conjunction with river flow records,
were used to determine whether the river mouth was open or closed. When the mouth
was closed, the lagoon water level was high and only weakly influenced by tidal fluctua-
tions (<0�2m). When the mouth opened to the sea, there was an initial sharp drop in
water level followed by large daily fluctuations (0�6m) in accordance with the tide. Over
the period September 1999 to January 2001, the river mouth opened when floods
were> c. 8m3 s�1 and closed again when the flow fell to< c. 4m3 s�1. These flows were
used to estimate times when the mouth was open prior to the installation of the recorder.
HABITAT MEASUREMENTS
Water depth, velocity and wetted width were measured at cross sections in each run
and riffle on each quarterly sampling occasion. In the first 2 years, three cross sections in
each riffle and run were measured. During the last year of sampling, however, only a
single cross section was surveyed because the previous measurements had shown a high
degree of uniformity through each habitat type. Substratum size in each run and riffle
was measured by the Wolman walk method (Wolman, 1954) in September 1998, but was
reassessed after floods or when the site was re-located. The median particle size (D50) was
determined graphically from cumulative particle size distributions. These data were used
to describe the physical habitat over a range of river flows. The amount of instream
habitat (weighted usable area) on each sampling occasion was calculated for the most
common fish species using habitat suitability criteria described by Jowett & Richardson
(1995).
SAMPLING AND ANALYSIS
The fish population was assessed at 3 monthly intervals over 3 years (September 1998
to June 2001) by electrofishing with a double anode bank-based 350W generator-
powered machine operated at 220–310V depending on water conductivity. Equal lengths
(15m) of run and riffle habitat were fished at each site, except at site 1 where the
sampling lengths were reduced to 10m. Stop-nets were placed at the bottom of each
reach to prevent escapement. The fish species present in the river were benthic and when
disturbed, fishes tended to dart into the substratum rather than fleeing so that an
upstream stop-net was considered unnecessary. Fishing was carried out in an upstream
direction with fishes caught in scoop-nets and a hand-held seine. All fish caught were
identified, their total length (LT) measured, and then released. During the first year of
sampling, multiple pass fishing was undertaken to determine whether there were signifi-
cant seasonal or flow differences in the sampling efficiency of single pass electrofishing.
The downstream stop-net was not used after the first year of sampling because few fishes
(<1%) were caught in the net and no fishes were observed actively attempting to escape
1422 I . G . JOWETT ET AL .
# 2005TheFisheries Society of the British Isles, Journal of FishBiology 2005, 66, 1419–1436
from the electrofishing reach. Population estimates were made by the removal method
(Otis et al., 1978) and the proportion of fishes caught on the first pass was calculated for
the most common species (>300 individuals, >2�5% of total catch) (Table I). One-way
ANOVA, followed by a Tukey test, showed that the proportion of fishes caught on the
first pass was not significantly different between seasons and flows, except for upland
bullies Gobiomorphus breviceps (Stokell); a significantly higher proportion of upland
bullies was caught on the first pass in winter (0�708 in July) compared to spring (0�506
in September) and summer (0�519 in December) (Tukey test, P< 0�05). The proportion of
bluegill bullies Gobiomorphus hubbsi (Stokell) in the fish population was probably under-
estimated by single pass electrofishing because of the relatively low proportion of the
population (<0�4) caught on the first pass. Over the next 2 years, single pass electro-
fishing was carried out at all sites, recognizing that upland bully numbers were probably
underestimated in spring and summer. Single pass counts were used as an index of
population estimate, with fish abundance at each site calculated as the number of fishes
caught in a single pass per 100m length of river.
Length frequency data were examined to determine, firstly, the size of juvenile fishes
that indicated recruitment had occurred recently and, secondly, the timing of successful
recruitment episodes with respect to the flow conditions and time of year.
Abundances of common fish species (>2�5% of total catch) were compared with the
magnitude and duration of river flows and changes in habitat to determine the effect of
key flow events. Specifically, changes in abundance of fish species were compared
between each of the winter and spring (June to December) sampling periods to determine
effects of floods on recruitment and survival, and over each summer and autumn
(December to June) period to determine effects of low flows on long-term survival.
RESULTS
SPATIAL VARIATION
The physical characteristics of the river were similar along the length of river
that was studied, as would be expected given the invariant gradient, geology and
flow. There were no consistentlongitudinal variations in depth, velocity or
width at low flow among the study sites (r< 0�17, P> 0�68), although there
were differences between the most upstream and downstream sites, with the
upstream site (site 1) being more confined at high flows and with coarser
substratum than the downstream site (site 8) (Table II). There was an obvious
longitudinal distribution of fishes, however, with diadromous species markedly
more abundant at the lower three sites (Table III). In contrast,>50% of upland
bully and Canterbury galaxias Galaxias vulgaris Stokell occurred in the top two
sites, with relatively few of these two species at site 8. Because upland bullies
dominated the fish fauna, total fish numbers were highest at the top two sites.
Site 4 had low densities of all species; this site was very unstable and had to be
re-located on most sampling visits. Site 6 was the only site with good riparian
cover and a relatively high proportion (26%) of eels [longfin Anguilla dieffen-
bachii (Gray) and shortfin Anguilla australis Richardson] were caught there; c.
30% were large (>300mm LT).
On average,>80% of Canterbury galaxias, torrentfish Cheimarrichthys fosteri
(Haast) and bluegill bully and 60% of upland bully were found in riffles,
whereas shortfin eel were equally common in run and riffle habitats. When
flows were high, fishes were found in both riffles and runs, but when flows were
low, most fishes were found in riffles (Fig. 2). There were significant correlations
FLOW REGIME AND FISH ABUNDANCE 1423
# 2005TheFisheries Society of theBritish Isles, Journal of FishBiology 2005, 66, 1419–1436
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1424 I . G . JOWETT ET AL .
# 2005TheFisheries Society of the British Isles, Journal of FishBiology 2005, 66, 1419–1436
between flow at the time of sampling and the percentage of torrentfish, Canter-
bury galaxias, and bluegill bullies found in riffles (r2> 0�52, P< 0�008), but not
for upland bullies or shortfin eels (P> 0�12). There was no significant difference
in the percentage of fishes in riffles between sampling seasons (i.e. spring,
summer, autumn and winter) for any species (Kruskal–Wallis, P> 0�20), sug-
gesting that fish movements were influenced by water velocity rather than
season. Velocity and depth measurements indicated substantial differences
between runs and riffles (Fig. 3). For example, the average depth and velocity
in runs at a flow of 2m3 s�1 were equivalent to the average depth and velocity in
riffles at a flow 0�7m3 s�1.
Changes in the longitudinal distribution of diadromous species indicated
upstream movement. In December, after spring recruitment from the sea,
TABLE II. Physical characteristics of the Waipara River sampling sites at the lowest
(0�12m3 s�1 in March 2001) and highest (2�62m3 s�1 in June 2000) flows encountered
on the 12 sampling occasions between September 1998 and June 2001
Site
Distance
inland (km)
Width (m) Depth (m) Velocity (m s�1)
Average D50
in rifflesLow High Low High Low High
1 36�8 6�2 8�7 0�07 0�29 0�08 0�78 57�2
2 24�5 8�2 13�9 0�09 0�25 0�12 0�66 40�5
3 16�1 6�5 12�9 0�07 0�23 0�18 0�81 44�7
4 12�8 8�4 13�5 0�11 0�27 0�21 0�73 35�8
5 11�2 5�9 14�9 0�11 0�26 0�36 0�76 39�4
6 7�2 10�1 12�4 0�22 0�31 0�1 0�82 40�4
7 4�2 9�3 12�4 0�09 0�33 0�22 0�78 45�3
8 0�7 0* 23�7 0* 0�19 0* 0�62 29�2
*Dry in March 2001
D50, median substratum size in riffles.
TABLE III. Per cent of average abundance of each diadromous and non-diadromous fish
species (accounting for >0�5% of total catch, Table I) at each of the eight Waipara River
sites from September 1998 to June 2001
Non-diadromous Diadromous
Site
Upland
bully
Canterbury
galaxias Torrentfish
Bluegill
bully
Longfin
eel
Shortfin
eel
Common
bully
1 22�8 34�5 4�5 0 8�8 1�4 0
2 27�6 15�1 6�5 0�7 5�9 3�8 0
3 12�2 11�8 10�1 3�2 20�6 11�3 4�7
4 6�7 8�3 5 2�8 2�9 8�9 1�7
5 11�8 11�1 5�8 7�1 14�7 14�5 1�1
6 6�7 11�8 16�5 22�1 32�4 22�1 9�2
7 11�1 5�7 21 38�2 8�8 23 14�8
8 1�1 1�7 30�6 25�9 5�9 15 68�5
FLOW REGIME AND FISH ABUNDANCE 1425
# 2005TheFisheries Society of theBritish Isles, Journal of FishBiology 2005, 66, 1419–1436
numbers of torrentfish and bluegill bully at the most downstream site (site 8)
were higher than at sites further up stream, whereas by March and June,
numbers had increased at upstream sites and decreased at site 8.
FLOW REGIME
The flow regime showed a characteristic pattern of high flows over winter and
spring and low flows through summer and autumn (Fig. 4). The minimum daily
mean flow of 0�032m3 s�1 in the summer of the first year was the second lowest
recorded in the 17 years of record, and was preceded by a year when the
minimum daily mean flow was lowest on record. The minimumdaily mean
flows in the second and third years of the study were lower than the median
minimum daily mean flow over the 17 years of flow record.
40
60
80
100
0·0 1·0 2·0 3·0
40
50
60
70
80
40
60
80
100
70
80
90
100
0
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40
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0·0 1·0 2·0 3·0
River flow (m3 s–1)
River flow (m3 s–1)
P
er
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n
ta
ge
 o
f 
fi
sh
 in
 r
if
fl
es
(a) (b)
(c)
(e)
(d)
FIG. 2. Relationship between river flow and percentage of fish (adults and juveniles) (a) Gobiomorphus
breviceps, (b) Galaxias vulgaris, (c) Cheimarrichthys fosteri, (d) Gobiomorphus hubbsi and (e)
Anguilla australis in riffles (averaged over all eight Waipara River sites) between September 1998
and June 2001. The curves were fitted by: (a) y¼ 69�2�6�3x; (b) y¼ 103�9�17�8x; (c) y¼ 99�9�7�3x;
(d) y¼ 111�1�18�2x; (e) y¼ 48�0�13�2x.
1426 I . G . JOWETT ET AL .
# 2005TheFisheries Society of the British Isles, Journal of FishBiology 2005, 66, 1419–1436
At the lowest flow surveyed during the study (0�12m3 s�1), the river was
shallow (average at seven sites¼ 0�11m) with low water velocities (average at
D
ec
-9
8
Se
p-
99
Ju
l-
99
River flow (m3 s–1)
D
ep
th
 (m
)
V
el
oc
it
y 
(m
 s
–1
)
M
ar
-0
1
Ju
n
-0
1
D
ec
-0
0
Se
p-
98
M
ar
-0
0
M
ar
-9
9
N
ov
-0
0
D
ec
-9
9
Ju
n
-0
0
0
0·5 1·5 2·5
0·2
0·1
0·3
0
0·2
0·4
0·6
0·8
1·0
0 1·0 2·0
(a)
(b)
FIG. 3. Relationship between measured river flow and (a) velocity and (b) depth in riffles (�, —) and
runs (~, ---) averaged over all eight Waipara River sites between September 1998 and June 2001.
The curves were fitted by (a) riffles: y¼ 0�43þ 0�42lnx; runs: y¼ 0�65þ 0�43lnx and (b) riffles:
y¼ 0�23þ 0�12lnx; runs: y¼ 0�19þ 0�13lnx.
0·01
0·1
1
10
100
0
5
10
15
20
Jul-98 Dec-98 May-99 Oct-99 Mar-00 Aug-00 Jan-01 Jun-01
R
el
at
iv
e 
de
n
si
ty
D
ai
ly
 m
ea
n
 f
lo
w
 (
m
3 
s–
1 )
FIG. 4. Relative abundance (proportion of average abundance of each common species) of fishes (adults
plus juveniles; ^, Gobiomorphus breviceps; ~, Galaxias vulgaris; þ, Gobiomorphus hubbsi; &,
Cheimarrichthys fosteri; �, Anguilla australis) and river flow (—) in the Waipara River between
July 1998 and June 2001.
FLOW REGIME AND FISH ABUNDANCE 1427
# 2005TheFisheries Society of theBritish Isles, Journal of FishBiology 2005, 66, 1419–1436
seven sites¼ 0�18ms�1) and site 8 was dry (Table II). When the flow increased
20-fold to 2�62m3 s�1, average river width increased to 14m (179%), average
depth increased to 0�27m, and average velocity increased by a factor of four to
0�75ms�1.
Fish abundances showed seasonal variations of 0�1–10 times the study aver-
age that were related to their life cycles and the flow regime (Fig. 4). The
numbers of upland bully, Canterbury galaxias, torrentfish, bluegill bully and
shortfin eel generally decreased during the low flow summer to winter period
and then increased in the high flows through spring and early summer. For
example, four of five species showed summer reductions between December
1998 and March 1999 and again between March 1999 and June 1999, and all
species reduced in abundance between March 2000 and June 2000. Over the
winter and spring period June 2000 and November 2000 all species increased in
abundance, and four of five species increased in abundance throughout the
periods between June 1999 and March 2000.
Fish recruitment
New Zealand diadromous fishes usually migrate from the sea to fresh water
in spring (McDowall, 1995), and in the Waipara River, mouth closure prevents
immigration for much of the time (Fig. 5). Highest fish numbers were recorded
in summer 2001 following large winter and spring floods in 2000 that opened
the river mouth for several months and allowed recruitment of diadromous fish
species (Figs 4 and 5). In contrast, when sampling began in September 1998, the
river mouth had been open for only a short time in the previous 9 months, and
there was no evidence of successful recruitment of any of the diadromous
species in that year (Fig. 5).
Recruitment of torrentfish occurred following mouth openings in July and
August 1999, December 1999, and August and September 2000. The presence
of juvenile torrentfish in March 2000 indicated that there was late recruitment,
possibly in December. Juvenile torrentfish were not present either in March
1999 or March 2001 when the mouth was closed between December and
March. Juvenile bluegill bullies were found only subsequent to mouth openings
in October and November 1999 and August and September 2000, suggesting
that bluegill recruitment commenced later than that of torrentfish. Juvenile
shortfin eels were found in the river following mouth openings that occurred in
July and early August 1999 and October and November 1999, with highest
numbers towards the end of spring, and after openings in August and
September 2000. Glass eels (Anguilla spp.) found in November 2000 could
not be identified to species in the field, but were probably mostly shortfin
(Jellyman et al., 2002).
The non-diadromous Canterbury galaxias had a definite seasonal breeding
period each spring, with juveniles present each December (Fig. 5). The presence
of small Canterbury galaxias (<40mm LT) in March 2001, however, indicated
that there was some late spawning following the prolonged floods in 2000.
Recruitment of non-diadromous upland bully was less seasonal than for the
other fish species in the river (Fig. 5). Upland bully recruitment generally
occurred during summer low flow periods (March 1999, March 2000 and
March 2001).
1428 I . G . JOWETT ET AL .
# 2005TheFisheries Society of the British Isles, Journal of FishBiology 2005, 66, 1419–1436
Effect of floods and droughts
There were no significant negative correlations between adult fish abundances
and either the magnitude or frequency of high flows (Table IV); indeed all the
correlations were positive (Spearman, n¼ 5, P> 0�05). The lack of any flood
effect on fish abundances was particularly evident in 2000 when there were no
Ju
ve
n
ile
 fi
sh
 a
bu
n
da
n
ce
 (n
um
be
r 
pe
r 
10
0 
m
)
A
du
lt
 fi
sh
 a
bu
n
da
n
ce
 (n
um
be
r 
pe
r 
10
0 
m
)
0
0·5
1·0
1·5
2·0
2·5 (a)
(b)
(c)
(d)
(e)
0
20
40
60
80
0
20
40
60
80
0
100
300
400
0
20
40
60
1-
A
pr
-9
8
1-
Ju
l-
98
1-
O
ct
-9
8
1-
Ja
n
-9
9
1-
A
pr
-9
9
1-
Ju
l-
99
1-
O
ct
-9
9
1-
Ja
n
-0
0
1-
A
pr
-0
0
1-
Ju
l-
00
1-
O
ct
-0
0
1-
Ja
n
-0
1
1-
A
pr
-0
1
1-
Ju
l-
01
Mouth open
200
0
400
800
1200
0
100
200
300
0
20
40
60
0
50
100
150
200
0
20
40
60
FIG. 5. Periods when the Waipara River mouth was open (&) and average abundance of common
juvenile (&) and adult (&) diadromous, (a) Cheimarrichthys fosteri, (b) Gobiomorphus hubbsi
and (c) Anguilla australis, and non-diadromous, (d) Gobiomorphus breviceps and (e) Galaxias
vulgaris, fishes at all sites on each sampling occasion (~) between September 1998 and June 2001.
FLOW REGIME AND FISH ABUNDANCE 1429
# 2005TheFisheries Society of theBritish Isles, Journal of FishBiology 2005, 66, 1419–1436
reductions in abundances associated with severe flooding between June and
November 2000 (Table IV). The reductions in upland bully abundance occurred
mainly in late spring and may have been due to spawning mortality rather than
floods, which usually occurred earlierin the year.
The lowest numbers of fishes, both adults and juveniles, were recorded in autumn
1999 following a winter and spring with few floods and a summer with an extended
period of low flow when numbers of all species, except upland bully, were reduced
(Fig. 4). The reductions of 90–70% were significant for Canterbury galaxias, tor-
rentfish and shortfin eel (TableV). The following year, the low flow period was
considerably shorter and the low flow higher than the previous year; there was a
significant increase in numbers of upland bully, but no significant changes in
abundance of any other species except shortfin eel. In the third year (1 December
2000 to 1 June 2001), flows were intermediate between the previous two years and
numbers of upland bully increased significantly, with no significant changes for any
other fish species (TableV). The duration and magnitude of low flow appeared to
increase the detrimental effect of low flows, with significant reductions in abun-
dances of Canterbury galaxias, torrentfish and shortfin eel occurring in the period
when the minimum flow was lowest and the length of time<0�05m3 s�1 (the
average of the annual minimum flows over the study period, TableV) was greatest.
This was also the period when there was no significant increase in the abundance of
upland bully.
The effect of low flows on each of the fish species was consistent with the
variation in fish habitat measured on each sampling occasion (Fig. 6). The
amount of habitat (weighted usable area) for torrentfish and bluegill bully
reduced sharply as flow decreased, and the abundance of these two species
declined during periods of low flow, with torrentfish abundance declining
each year of the study and bluegill bully abundance declining in 2 of 3 years.
TABLE IV. Change (%) in abundance of common (>2�5% of total catch) adult fishes
(�total length given in Table I) and magnitude and duration of high flows between
sampling periods in winter and early summer. A flow of 8m3 s�1 has been used as the
high flow because it is the flow that opens the river mouth
Period
Upland
bully
Canterbury
galaxias Torrentfish
Bluegill
bully
Shortfin
eel
Peak daily
flow (m3 s�1)
Per cent
time >8m3 s�1
Sep 1998–Dec
1998
�13 15 �48 84 105 9�5 1
Jun 1999–Sep
1999
�9 44 55 �30 �9 43�4 20
Sep 1999–Dec
1999
�38 53* 121 187 123 13�9 7
Jun 2000–Nov
2000
17 87 157 102 742* 280�0 20
Nov 2000–Dec
2000
�34 �20 9 26 �16 2�2 0
*Significant difference between fish abundances at sampling sites before and after high flow
period, Kruskal–Wallis, n¼ 8, P< 0�05.
1430 I . G . JOWETT ET AL .
# 2005TheFisheries Society of the British Isles, Journal of FishBiology 2005, 66, 1419–1436
T
A
B
L
E
V
.
C
h
a
n
g
e
(%
)
in
a
b
u
n
d
a
n
ce
o
f
co
m
m
o
n
(>
2
� 5%
o
f
to
ta
l
ca
tc
h
)
fi
sh
sp
ec
ie
s
(a
d
u
lt
s
a
n
d
ju
v
en
il
es
)
a
n
d
m
a
g
n
it
u
d
e
a
n
d
d
u
ra
ti
o
n
o
f
lo
w
fl
o
w
s
b
et
w
ee
n
D
ec
em
b
er
a
n
d
Ju
n
e
(s
u
m
m
er
to
ea
rl
y
w
in
te
r)
.
A
fl
o
w
o
f
0
� 05
m
3
s�
1
w
a
s
th
e
a
v
er
a
g
e
lo
w
fl
o
w
d
u
ri
n
g
th
e
st
u
d
y
p
er
io
d
a
n
d
0
� 12
m
3
s�
1
w
a
s
th
e
re
co
m
m
en
d
ed
m
in
im
u
m
fl
o
w
re
q
u
ir
em
en
t
P
er
ce
n
t
ch
a
n
g
e
in
a
b
u
n
d
a
n
ce
P
er
io
d
U
p
la
n
d
b
u
ll
y
C
a
n
te
rb
u
ry
g
a
la
x
ia
s
T
o
rr
en
tf
is
h
B
lu
eg
il
l
b
u
ll
y
S
h
o
rt
fi
n
ee
l
P
er
ce
n
t
ti
m
e
<
0
� 12
m
3
s�
1
P
er
ce
n
t
ti
m
e
<
0
� 05
m
3
s�
1
M
in
im
u
m
d
a
il
y
m
ea
n
fl
o
w
(m
3
s�
1
)
D
ec
1
9
9
8
–
Ju
n
1
9
9
9
5
�9
1
*
�8
9
*
�5
9
�7
3
*
3
1
2
3
0
� 03
2
D
ec
1
9
9
9
–
Ju
n
2
0
0
0
1
1
8
*
1
4
�2
0
3
�8
7
*
1
0
0
0
� 06
9
D
ec
2
0
0
0
–
Ju
n
2
0
0
1
2
2
3
*
2
9
�6
9
�1
6
�8
4
6
1
2
0
� 04
7
*
S
ig
n
if
ic
a
n
t
d
if
fe
re
n
ce
in
fi
sh
a
b
u
n
d
a
n
ce
s
a
t
sa
m
p
li
n
g
si
te
s
b
et
w
ee
n
D
ec
em
b
er
a
n
d
Ju
n
e,
K
ru
sk
a
l–
W
a
ll
is
,
n
¼
8
,
P
<
0
� 05
.
FLOW REGIME AND FISH ABUNDANCE 1431
# 2005TheFisheries Society of theBritish Isles, Journal of FishBiology 2005, 66, 1419–1436
The amount of habitat for the Canterbury galaxias also decreased as flow
decreased, but not to the same extent as for torrentfish and bluegill bully, and
their abundance declined only in the year of lowest flow. In contrast, the
amount of habitat for upland bully decreased linearly as flow increased
(r¼�0�73, P¼ 0�007) and this was the only fish species that did not show a
decline in abundance during low flows.
DISCUSSION
The spatial distribution of fishes in the Waipara River was strongly
influenced by their life histories and habitat preferences. The abundance and
diversity of diadromous species decreased with elevation and distance inland,
whereas the opposite occurred for the non-diadromous species. This is a wide-
spread pattern in New Zealand fish communities (McDowall, 1993; Jowett &
0
2
4
6
8
10
12
(a)
(c)
(b)
(d)
(e)
0
2
4
6
8
10
12
0
0 1 2 3
2
4
6
8
10
12
Flow (m3 s–1)
0 1 2 3
Flow (m3 s–1)
W
U
A
 (
m
2 
m
–1
)
FIG. 6. Variation in weighted usable area (WUA) with flow measured on each sampling occasion for
(a) Galaxias vulgaris, (b) Gobiomorphus breviceps, (c) Gobiomorphus hubbsi, (d) Cheimarrichthys
fosteri and (e) Anguilla australis. The curves were fitted by (a) y¼ –0�46x2þ 1�78xþ 6�94,
(b) y¼ –0�01x2� 0�75xþ 4�85, (c) y¼�0�83x2þ 4�10xþ 0�37, (d) y¼ –0�48x2þ 3�84xþ 0�31 and
(e) y¼�0�75x2þ 3�33xþ 3�76.
1432 I . G . JOWETT ET AL .
# 2005TheFisheries Society of the British Isles, Journal of FishBiology 2005, 66, 1419–1436
Richardson, 1996; Joy & Death, 2000). On a local scale, the distribution of
fishes reflected their habitat preferences (Jowett & Richardson, 1995), with fast-
water species, torrentfish and bluegill bully, most abundant in riffles over all
flows, and upland bully equally abundant in runs and riffles at high flows, but
more abundant in riffles than runs when flows were low.
The estimation of fish numbers by stratified sampling of<1% of the total
river area was probably influenced by movement of fishes between runs and
riffles as the flow changed. This uncertainty, as well as the relocation of sites
because of disturbance, meant that interpretation of population responses to the
flow regime were based on trends and the consistency of responses over the 3
years of the study. It was clear, however, that floods and droughts were the key
elements of the flow regime that affected fish abundances. The floods in winter
and spring allowed recruitment of diadromous species and had no detrimental
effect on adult fishes, whereas summer and autumn droughts reduced habitat
for species with a preference for high water velocities and had detrimental
effects on fish abundances that were proportional to the magnitude and dura-
tion of low flows. The combination of spring recruitment and summer low flows
resulted in annual variations in fish abundances,with highest abundances in
summer and lowest at the beginning of winter.
Recruitment generally occurred within the periods described by McDowall
(1995), even though the Waipara River mouth was often closed. Bluegill bully
recruitment occurred between August and November and shortfin eel immig-
ration between July and November. Torrentfish entered the river slightly earlier
than bluegill bully, which is consistent with the extended period of recruitment
(April to November) reported by McDowall (1995). Although there did not
appear to be any torrentfish recruitment in October and November, new
recruits were present in March and probably resulted from mouth openings in
December.
Although the timing of floods allowed recruitment of a number of diadromous
species, the lack of access to the sea at other times may stop downstream migra-
tions of adult eels and larvae of other fish species. The downstream migration of
bluegill bully larvae occurs between August and February (McDowall, 1995),
during which period the mouth of the Waipara River is often open. Downstream
migration of adult eels and torrentfish larvae, however, usually occurs between
February and May (McDowall, 1995), when the mouth of the Waipara River is
often closed. Common bully Gobiomorphus cotidianusMcDowall were rare in the
Waipara River; since immigration of common bully is believed to occur between
December and February (McDowall, 1995), mouth closure at this time could be
limiting common bully recruitment to the Waipara River.
The timing of floods and lack of adult habitat at low flow may also limit
brown trout Salmo trutta L. populations. Hayes (1995) showed that floods
during spring severely impaired brown trout recruitment in the Kakanui
River, a New Zealand east coast river with a flow regime similar to the
Waipara. Given the well-known effect of floods on salmonids (Seegrist &
Gard, 1972; Hoopes, 1975; Jowett & Richardson, 1989; Cattane´o et al.,
2002), it is surprising that floods had no detrimental effect on native fishes.
Matthews (1986) suggested that during floods, fishes might move toward the
water’s edge on the floodplain or take refuge in sheltered locations such as
FLOW REGIME AND FISH ABUNDANCE 1433
# 2005TheFisheries Society of theBritish Isles, Journal of FishBiology 2005, 66, 1419–1436
deep pools, stable substrata or permanent features. The morphology of the
Waipara is such that sheltered locations are rare, and during floods, there
can be considerable substratum movement. Although, adult fishes were able
to survive in this environment, presumably by moving to stream margins, as
described by Jowett & Richardson (1994), floods may cause mortality to the
larvae of non-diadromous fishes. For example, the abundance of juvenile
upland bully in March 2001 after a period of prolonged low flow was more
than four times higher than on the previous March sampling occasions
(Figs 4 and 5).
The magnitude and duration of low flows in summer and autumn appeared
to have a negative influence on fish abundances. It is difficult to obtain statis-
tically robust data on the effects of flows on fish populations because of the
spatial variability in fish abundances. There was, however, an overall pattern in
the trends that were observed. In the year when flows were lowest (<0�05m3 s�1
for 23% of the time), the abundances of four out of five fish species were
reduced; in the year with the highest minimum flow, only two out of five species
were reduced in abundance, and in the intermediate year, three out of five were
reduced in abundance. The duration of low flow appeared to be a critical factor.
Canterbury galaxias declined in abundance when low flows (<0�05m3 s�1) were
sustained and, although upland bully increased in numbers over the low flow
periods, sustained low flows appeared to limit the population increase.
The effects of low flows on fish abundances were consistent with habitat
requirements and indicate that minimum flows based on habitat analyses
could safeguard the fish community. Jowett (1994) assessed minimum flow
requirements for the Waipara River using relationships between in-stream
habitat and flow. He considered that the amount of in-stream habitat for the
fish species in the river began to decline rapidly as flows fell below
0�12m3 s�1, and consequently recommended that abstraction of water should
cease when flows were less than this. During the period of the study, flows
were unusually low and fell to<0�12m3 s�1 every year, with the greatest
effects on fish populations when flows were lowest. The relatively small
differences in fish abundances in the year of high flow, when flows
were<0�12m3 s�1 for only 10% of the time, suggests that setting a minimum
flow based on the flow below which in-stream habitat begins to decline
rapidly would be successful in maintaining most of the fish species in the
river, with the possible exception of torrentfish.
The present study demonstrates the resilience of the native fish community in
New Zealand gravel-bed rivers to floods and droughts, with floods having little
effect on fish numbers, and riffle habitat providing a refuge during periods of
low flow. The magnitude and duration of low flows and the timing of floods
were the key elements of the flow regime.
The authors thank Environment Canterbury for use of the Waipara River flow
records. Many people assisted during the extensive field programme, in particular G.
Kelly, J. Sykes, P. Mason, F. Richards and F. Lucas, and the authors are grateful for
their help. K. Collier and B. McDowall reviewed early drafts of this paper. This study
was funded by contract C01X0023 of the Foundation for Research, Science and Tech-
nology (New Zealand).
1434 I . G . JOWETT ET AL .
# 2005TheFisheries Society of the British Isles, Journal of FishBiology 2005, 66, 1419–1436
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