AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS
Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 175–189 (2008)
Published online 6 September 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/aqc.863
Condition assessment of lamprey populations in the
Yorkshire Ouse catchment, north-east England, and the
potential influence of physical migration barriers
A.D. NUNN*, J.P. HARVEY, R.A.A. NOBLE and I.G. COWX
Hull International Fisheries Institute, University of Hull, Hull, East Yorkshire, UK
ABSTRACT
1. River lamprey (Lampetra fluviatilis) and sea lamprey (Petromyzon marinus) are designated
features of the River Derwent Special Area of Conservation (SAC) and the Humber Estuary (a
possible SAC). This study determined the condition of lamprey populations in the Yorkshire Ouse
catchment by assessing the species composition, distribution, abundance and size-structure of larval
(ammocoete) populations in five major tributary rivers (Derwent, Swale, Ure, Nidd and Wharfe).
2. According to EU condition assessment criteria, Lampetra populations (assumed to be mostly
river lamprey) are at present in favourable condition, with site mean ðSEÞ densities ranging from
2.7 ð1:2Þ to 160.3 ð50:5Þ individuals m2 (all river means >2 individuals m2), and at least two
size ( age) classes present in optimal microhabitats. By contrast, no sea lamprey larvae were
recorded, suggesting that populations of this species are in unfavourable condition.
3. Actions to protect and enhance nationally or internationally important stocks must be
implemented from at least a catchment perspective, because many of the issues affecting such species
are not localized. With respect to lampreys, particular attention should be given to protecting spawning
and nursery habitats, improving water quality, reducing impingement at abstraction points, preventing
exploitation at spawning grounds and increasing passage at potential physical obstructions.
Copyright # 2007 John Wiley & Sons, Ltd.
Received 29 May 2006; Accepted 10 March 2007
KEY WORDS: ammocoetes; density; larvae; Petromyzontidae; reproduction; river discharge; size-structure;
Special Areas of Conservation
INTRODUCTION
The majority of lowland rivers in Europe have been modified to some extent (Cowx and Welcomme, 1998).
Typical examples include impoundment and embankment of stretches of river for flood defence or
*Correspondence to: A.D. Nunn, Hull International Fisheries Institute, University of Hull, Hull, East Yorkshire, HU6 7RX, UK.
E-mail: [email protected]
Copyright # 2007 John Wiley & Sons, Ltd.
176
A.D. NUNN ET AL.
hydroelectric power generation. Such modifications may cause significant losses to spawning and nursery
habitats, either by direct removal of bed material or by reduced connectivity, with detrimental impacts
upon fish and lamprey populations universal (e.g. Peňáz et al., 1995; Renaud, 1997; Penczak, 2004).
Anadromous and rheophilic species, for example, may be unable to migrate to their spawning grounds
through loss of longitudinal connectivity, while loss of lateral (horizontal) connectivity may prevent
limnophilic species from gaining access to their spawning habitats in the floodplain. Accordingly, a
common symptom of river modification is the loss or reduction of specialized (stenotopic) species in favour
of a smaller number of eurytopic species. In the River Danube, for example, 30 of the 52 fish species are
considered vulnerable or endangered owing to the impacts of river modification (Schiemer and
Waidbacher, 1992), and similar shifts in community structure have been reported in a number of rivers
in mainland Europe and the UK (e.g. Copp, 1990; Jurajda, 1995; Grift et al., 2001; Jurajda et al., 2001;
Penczak, 2004).
The EC Habitats Directive (92/43/EEC) on Conservation of Natural Habitats and of Wild Fauna and
Flora stipulates that Member States maintain or restore habitats and species in a condition that ensures
their favourable conservation status in the community. To comply with this Directive, a number of rivers
have been designated as Special Areas of Conservation (SACs) because they support important populations
of designated species. Management of species of conservation interest in SAC rivers requires a monitoring
programme that establishes the status of the species against a predetermined set of conservation objectives,
a process known in the UK as ‘condition assessment’. Condition assessments are recorded using one of
seven categories, namely: (1) favourable } maintained; (2) favourable } recovered; (3) unfavourable }
recovering; (4) unfavourable } no change; (5) unfavourable } declining; (6) partially destroyed; and (7)
destroyed. The condition assessment provides information on the status of the species, with at least a broad
indication of trends. Sampling strategies must therefore be able to detect both temporal (over a period of
years) and spatial (between site) variations.
River lamprey (Lampetra fluviatilis (L.)) and sea lamprey (Petromyzon marinus L.) are designated
features of both the River Derwent SAC and the Humber Estuary (a possible SAC). However, there have
been concerns regarding the status of the populations because of habitat degradation, barriers to migration,
commercial exploitation, impingement on power station intake screens, and water quality. Indeed, prior to
significant improvements in the tidal Yorkshire Ouse in the last two decades, water quality was probably
the principal factor limiting anadromous lamprey populations in the catchment. Continued improvements
in water quality have allowed the populations to recover, with the Ouse now believed to support one of the
most important river lamprey populations in the UK (Jang and Lucas, 2005). Nevertheless, there is a
possibility that poor-quality inputs to the tidal Ouse from the rivers Aire (and Calder) and Don may pose a
barrier to lamprey migration under some circumstances, especially during low flow conditions. The aim of
the study was to assess the condition of lamprey populations in the Yorkshire Ouse catchment, north-east
England, and the potential influence of physical migration barriers. This was achieved by assessment of the
species composition, distribution, abundance and size composition of larval (ammocoete) populations in
five major tributary rivers (Derwent, Swale, Ure, Nidd and Wharfe).
MATERIALS AND METHODS
Study area, sampling strategy and data collection
The Yorkshire Ouse (Figure 1) is approximately 200 km long from its source in the Pennines to its
confluence with the River Trent, draining an area of more than 10 000 km2. The Ouse has seven major
tributaries, namely the rivers Derwent, Swale, Ure, Nidd, Wharfe, Aire and Don. All but the last two were
Copyright # 2007 John Wiley & Sons, Ltd.
Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 175–189 (2008)
DOI: 10.1002/aqc
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Figure 1. The Yorkshire Ouse catchment, showing locations of study sites (&) and potential physical barriers to upstream migration of
adult lamprey (m).
Copyright # 2007 John Wiley & Sons, Ltd.
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A.D. NUNN ET AL.
surveyed in the study, since they support impoverished lamprey populations, partly due to poor water
quality.
In total, 140 points were sampled for lamprey larvae at 16 sites in October and November 2004 (Table 1).
Sites were selected close to, and downstream of, important river lamprey and sea lamprey spawning areas,
while points were selected in areas of suitable lamprey larvae habitat. All sampling was carried out in water
51.2 m deep. The sampling strategy followed LIFE in UK Rivers protocol (Harvey and Cowx, 2003), with
a minimum of three (usually more, depending upon lamprey numbers and habitat availability) quantitative
or semi-quantitative samples taken at each site.
Lamprey larvae were sampled by electric fishing (2 kVA generator, 220 V, 50 Hz pulsed DC). For
quantitative surveys, a delimiting framework (equivalent to a quadrat base area 1 m2) was used (Harvey
and Cowx, 2003). The framework was placed at the selected sampling point and left to allow any disturbed
sediment to settle. A single anode (40 cm diameter) was immersed approximately 10–15 cm above the
substratum, energized for 20 s, then turned off for 5 s. This process was repeated for approximately 2 min.
This technique draws lamprey out of the sediment and into the water column. Immobilized lamprey were
removed using a fine-meshed (280 mm) net, and transferred to a water-filled container. The sampling process
was repeated twice (i.e. three samples in total), with a resting period of 5 min between each sample. Samples
were kept separate for analysis.
Where deployment of the framework was not possible (e.g. narrow marginal areas, near overhanging
trees, and deep or fast-flowing areas), a semi-quantitative sampling approach was used, with sampling
points fished only once, rather than three times. In areas that were too deep or turbid for clear observation
of emerging lamprey, it was sometimes useful to agitate the sediment with the anode during the electric
fishing operation. This procedure had the effect of encouraging the liberated lamprey to swim to the water
Table 1. Mean ðSEÞ density estimates of Lampetra ammocoetes captured from optimal and sub-optimal microhabitats from 16 sites
on the rivers Derwent, Swale, Ure, Nidd and Wharfe
River
Site
National Grid Reference Mean ( SE) density of Lampetra ammocoetes (no. m2)
Optimal microhabitat
Sub-optimal microhabitat
Derwent Kirkham
Howsham
Buttercrambe
Stamford Bridge
Elvington
SE
SE
SE
SE
SE
735657
729628
731587
713557
705475
5.1 (1.1)
19.2 (5.7)
18.0 (4.0)
24.7 (10.2)
8.0
*
0.8 (0.8)
1.9 (1.2)
1.0
*
Swale
Langton
Thornton
Myton
SE 439677
SE 296771
SE 356670
111.8 (9.7)
45.5 (9.6)
15.7 (3.0)
22.0
*
*
Ure
Bellflask
Westwick
Boroughbridge
SE 395670
SE 368559
SE 469546
160.3 (50.5)
68.7 (23.5)
49.5 (20.0)
*
0.0
8.0 (4.8)
Nidd
Goldsborough
SE 291964
Kirk Hammerton SE 433714
4.2 (3.0)
2.7 (1.2)
0.2 (0.2)
1.5 (0.9)
Wharfe
Wetherby
Boston Spa
Tadcaster
9.3 (8.3)
36.8 (19.0)
34.6 (11.2)
1.5 (0.8)
19.7 (8.6)
*
SE 404480
SE 431459
SE 486436
Asterisks indicate sub-optimal habitat absent or not sampled. Site names/numbers in bold indicate populations in optimal habitat
comply with favourable condition status (density > 10 m2 ; Harvey and Cowx, 2003).
Copyright # 2007 John Wiley & Sons, Ltd.
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surface, facilitating their capture by a netsman positioned downstream. For both quantitative and semiquantitative surveys, the microhabitat at each sampling point was classified as either optimal (stable, fine
sediment with organic matter, 515 cm sediment depth, low water velocity) or sub-optimal (515 cm
sediment depth, often interspersed among coarser substrata) (APEM, 2002), irrespective of whether
lamprey were captured. Sub-optimal microhabitats also include organic matter overlying coarse
substratum, submerged tree roots, and emergent vegetation rooted in silt (Harvey and Cowx, 2003).
Lamprey were identified, measured (total length, millimetres) and returned to their point of capture.
River lamprey and brook lamprey (Lampetra planeri (Bloch)) larvae cannot be reliably differentiated in the
field (Gardiner, 2003), so lamprey were identified to the following categories: (1) Lampetra (river/brook
lamprey) ammocoete; (2) Petromyzon (sea lamprey) ammocoete; (3) Lampetra transformer; and (4)
Petromyzon transformer. Ammocoetes are lamprey larvae, and transformers are metamorphosing larvae.
Lampetra transformers were not identified to species owing to time restrictions and the difficulty associated
with taking the necessary measurements (e.g. disc, eye or preorbital length; Gardiner, 2003) from
unanaethetized lamprey. Moreover, such measurements do not guarantee conclusive identification to
species during the time of survey (October–November) (Gardiner, 2003).
Data analysis
Lamprey ammocoete and transformer densities (number m2) were calculated for each sampling point. For
quantitative sampling points, absolute density estimates were calculated using depletion methodology
(Zippin, 1956; Carle and Strub, 1978), while gear calibration was used for semi-quantitative sampling
points. This involved calculating the efficiency of sampling effort or probability of capture (p) from the
quantitative samples at each site. The derived probability of capture was used to calibrate the gear for
sampling points where only one sample was taken. From this, a measure of relative density was derived:
ðC=pÞ
N¼
A
where C is the total number of ammocoetes or transformers caught in one sample at each sampling point,
and A is the sampling area (Cowx, 1996). Mean density estimates were calculated for optimal and suboptimal microhabitats at each site by summing the individual sample densities (quantitative and semiquantitative samples combined) and dividing by the number of samples. For the purpose of condition
assessment, the criterion to achieve favourable status is a mean density of >10 individuals m2 in optimal
microhabitats (Harvey and Cowx, 2003).
Length frequency distributions of lamprey ammocoetes were derived for each site to facilitate
interpretation of age-structure of the populations. Where possible, modal groups ( age classes) were
separated using the method of Bhattacharya (1967) in the FiSAT (FAO-ICLARM Stock Assessment
Tools) computer program. This method involved decomposition of the length distributions by visual
identification of frequencies perceived to belong to one cohort (Gayanilo et al., 1997). A separation index
(SI) was calculated for each pair of contiguous cohorts:
DL% j
SI ¼
Dsj
% j is the difference between the means of a pair of contiguous cohorts, and Dsj is the difference
where DL
between their estimated standard deviations; separation indices of greater than 2 indicate significant
separation of modal groups (Gayanilo et al., 1997). This methodology has been applied by Garcı́a-Berthou
and Moreno-Amich (1992), Nunn et al. (2002, 2007) and Przybylski and Garcı́a-Berthou (2004). The
analysis was restricted to the 0þ (2004 year class) and 1þ (2003 year class) age classes, which had the best
separation of modal groups; conclusive identification of older age classes was difficult because of the
amount of overlap between modes. Where data were unsuitable for analysis in FiSAT, approximate length
Copyright # 2007 John Wiley & Sons, Ltd.
Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 175–189 (2008)
DOI: 10.1002/aqc
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ranges of the assumed 0þ and 1þ age classes were derived from the mean observed values for sites with
FiSAT-valid data. For the purpose of condition assessment, the criterion to achieve favourable status is the
presence of a minimum of two size ( age) classes in optimal microhabitats (Harvey and Cowx, 2003).
Mean daily river discharge data (2000–2006 inclusive) were obtained from the Environment Agency
gauging stations at Buttercrambe (Derwent), Topcliffe (Swale), Westwick (Ure), Hunsingore (Nidd) and
Tadcaster (Wharfe) (Figure 1). An attempt was made to investigate whether river lamprey recruitment
success is in some way linked to river discharge (i.e. movement past physical migration barriers) during the
spawning migration period (October–April). The number of days that discharge during the spawning
migration was above the long-term October–April mean was calculated for each river. In addition, the
cumulative number of discharge-days above the basal discharge rate (i.e. the long-term mean daily
discharge during the lamprey spawning migration, calculated using a 7-yr data set) was calculated for each
river (Nunn et al., 2003).
RESULTS
A total of 4943 Lampetra ammocoetes and 91 Lampetra transformers were captured during the study
period. No sea lamprey ammocoetes and transformers were recorded. River lamprey were assumed to be
substantially more abundant than brook lamprey in the study areas, based upon the known spawning
distributions and numbers of adults (Bellflask Ecological Survey Team; BEST, 2003, 2004; Jang and Lucas,
2005) and the holding of ammocoetes in aquaria until transformation } Gaudron and Lucas (2006)
reported that river lamprey contributed >98% to the Lampetra ammocoete population at Boroughbridge
on the River Ure (one of the sites in the current study).
Density estimates
Mean ðSEÞ densities of Lampetra ammocoetes ranged from 2.7 ð1:2Þ to 160.3 ð50:5Þ individuals m2
in optimal microhabitats (Table 1). The criterion for compliance with favourable condition (a mean density
of >10 individuals m2 in optimal microhabitats; Harvey and Cowx, 2003) indicates that 11 of the 16 sites
supported Lampetra ammocoete populations in favourable condition. Of these, Langton and Bellflask had
the greatest Lampetra ammocoete densities (Table 1). By contrast, the Lampetra ammocoete populations at
Kirkham, Elvington, Goldsborough, Kirk Hammerton and Wetherby were in unfavourable condition (i.e.
410 individuals m2) (Table 1). Mean densities of Lampetra ammocoetes in sub-optimal microhabitats
were generally low (Table 1). Similarly, mean densities of Lampetra transformers were 51.5 individuals
m2 in both optimal and sub-optimal microhabitats. From a river perspective, the criterion for compliance
with favourable condition is tentatively set at a mean density of >2 individuals m2 (Harvey and Cowx,
2003). Thus, the Lampetra ammocoete populations ðmean SEÞ of the rivers Derwent ð9:8 3:3Þ; Swale
ð48:7 22:0Þ; Ure ð57:3 28:7Þ; Nidd ð2:1 0:9Þ and Wharfe ð20:4 6:9Þ were all in favourable
condition.
Size structure
On the River Derwent at Kirkham, Howsham and Buttercrambe, Lampetra catches were dominated by
ammocoetes in the 70–120 mm size range, with relatively few 0þ and 1þ individuals captured (Figure 2). By
contrast, at Stamford Bridge and Elvington, ammocoetes were captured in the 23–146 mm size range, with
evidence of some recruitment in 2003 and 2004 (560 mm ammocoetes) (Figure 2). A minimum of two age
classes were assumed to be present at all sites, based upon the approximate length ranges of the 0þ and 1þ
age classes derived from the mean observed values for sites with FiSAT-valid data, and the presence of
larger (> 1þ) ammocoetes (Table 2; Figure 2).
Copyright # 2007 John Wiley & Sons, Ltd.
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Figure 2. Length–frequency distributions of Lampetra ammocoetes captured from the River Derwent. Where possible, modal
distribution curves have been fitted to the 0þ (2004 year class) and 1þ (2003 year class) age classes; where this was not possible,
approximate length ranges of the 0þ and 1þ age classes are illustrated. SI¼ separation index from the method of Bhattacharya (1967).
On the River Swale at Langton, Thornton and Myton, two clear modes between approximately 20–
40 mm (0þ age class) and 40–70 mm (1þ age class) were identified by FiSAT, although ammocoetes in the
70–120 mm size range were most abundant (Figure 3). Thus, a minimum of three age classes were assumed
to be present at all sites (Table 2; Figure 3). On the River Ure at Bellflask and Westwick, a definitive mode
was apparent between approximately 20 and 45 mm (0þ age class), with evidence of poor recruitment in
2003 (few ammocoetes in the 45–70 mm size range) (Figure 4). Catches at Boroughbridge were dominated
by ammocoetes in the 85–110 mm size range (Figure 4). A minimum of two age classes were assumed to be
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Table 2. Lampetra ammocoete modal group characteristics determined by the method of Bhattacharya (1967)
River
Site
Year class (age class)
2003 (1þ)
Mean ðSEÞ TL (mm)
2004 (0þ)
Mean ðSEÞ TL (mm)
Separation index
Derwent
Kirkham
Howsham
Buttercrambe
Stamford Bridge
Elvington
n/a
n/a
n/a
59.8 (0.8)
n/a
n/a
n/a
n/a
30.0 (1.8)
n/a
n/a
n/a
n/a
3.1
n/a
Swale
Langton
Thornton
Myton
53.5 (0.5)
55.5 (0.5)
60.3 (0.5)
29.0 (0.3)
28.7 (0.5)
30.4 (0.7)
5.7
5.8
5.2
Ure
Bellflask
Westwick
Boroughbridge
n/a
n/a
n/a
32.4 (3.7)
32.0 (4.7)
n/a
n/a
n/a
n/a
Nidd
Goldsborough
Kirk Hammerton
n/a
n/a
n/a
n/a
n/a
n/a
Wharfe
Wetherby
Boston Spa
Tadcaster
65.4 (1.6)
n/a
60.0 (0.6)
31.7 (1.4)
31.1 (0.7)
27.0 (0.6)
3.4
n/a
3.6
n/a indicates data unsuitable for the method of Bhattacharya (1967).
present at all sites, based upon the approximate length ranges of the 0þ and 1þ age classes derived from the
mean observed values for sites with FiSAT-valid data, and the presence of larger (> 1þ) ammocoetes
(Table 2; Figure 4).
On the River Nidd at Goldsborough, Lampetra ammocoetes were captured in the 91–125 mm size range,
with a complete absence of 0þ and 1þ individuals (Figure 5). By contrast, there was evidence of
recruitment in 2003 and 2004 at Kirk Hammerton (565 mm ammocoetes), with a minimum of two age
classes assumed to be present, based upon the approximate length ranges of the 0þ and 1þ age classes
derived from the mean observed values for sites with FiSAT-valid data, and the presence of larger (> 1þ)
ammocoetes (Table 2; Figure 5). On the River Wharfe at Wetherby, Boston Spa and Tadcaster, Lampetra
ammocoetes were captured in the 22–128 mm size range, with >70 mm individuals dominant (Figure 6). A
minimum of three age classes were assumed to be present at all sites, based upon the modal groups
identified by FiSAT and the presence of larger (> 1þ) ammocoetes (Table 2; Figure 6).
River discharge
The River Ure is an example of a river in which lamprey recruitment was potentially restricted by physical
barriers and river levels in 2003 (almost complete absence of the 2003 year class; Figure 4), while the Swale
provides a good example of a relatively unregulated river. Analysis of discharge data from the rivers Swale
and Ure revealed that both rivers had the same number of days (69) above the long-term October–April
mean during the 2002/03 spawning migration. By contrast, the cumulative number of discharge-days above
the basal discharge rate during the lamprey spawning migration (expressed as a percentage of total
cumulative discharge) was 37% for the Swale and 30% for the Ure. Thus, although discharge in the Swale
and Ure was above the long-term October–April mean for the same number of days, the magnitude of the
Copyright # 2007 John Wiley & Sons, Ltd.
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Figure 3. Length–frequency distributions of Lampetra ammocoetes captured from the River Swale. Modal distribution
curves have been fitted to the 0þ (2004 year class) and 1þ (2003 year class) age classes. SI¼ separation index from the
method of Bhattacharya (1967).
increases in discharge was greater in the Swale. The discharge of the rivers Nidd and Wharfe was above the
long-term October–April mean for 73 days (31% of total cumulative discharge) and 62 days (31%),
respectively. The discharge regime of the Derwent contrasted with those of the other rivers; discharge was
above the long-term October–April mean for 99 days, but the cumulative number of discharge-days was
still only 31% of total cumulative discharge (i.e. the discharge of the Derwent was above the long-term
October–April mean for longer than the other rivers, but by smaller amounts) (Figure 7).
DISCUSSION
The Yorkshire Ouse catchment is believed to support one of the most important river lamprey populations
in the UK (Jang and Lucas, 2005), and brook lamprey are widespread in the upper reaches of the rivers
Derwent, Swale, Ure, Nidd and Wharfe (Whitton and Lucas, 1997). The current study has demonstrated
that each study river supports healthy populations of Lampetra ammocoetes, with mean densities in
optimal microhabitats exceeding those indicative of favourable condition, and with two or more age classes
usually present (Harvey and Cowx, 2003). The densities recorded here compare favourably with those
reported elsewhere (see Maitland, 2003). It was not possible to distinguish brook lamprey from river
lamprey larvae in the field (Gardiner, 2003) but, as mentioned previously, river lamprey were assumed to be
substantially more abundant than brook lamprey, on the basis of information from previous work in the
study areas (i.e. BEST, 2003, 2004; Jang and Lucas, 2005; Gaudron and Lucas, 2006).
Compared with river lamprey, the condition of the sea lamprey populations, based upon larval catches,
in the study rivers is unfavourable. This contrasts markedly with the rivers Wye and Usk (Wales), where
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A.D. NUNN ET AL.
Figure 4. Length–frequency distributions of Lampetra ammocoetes captured from the River Ure. Where possible, modal
distribution curves have been fitted to the 0þ (2004 year class) age class. The approximate length ranges of the 1þ (2003
year class) age class are illustrated.
Figure 5. Length–frequency distributions of Lampetra ammocoetes captured from the River Nidd. The approximate length ranges of
the 0þ (2004 year class) and 1þ (2003 year class) age classes are illustrated.
mean site densities of up to 74 sea lamprey ammocoetes m2 have been recorded by the authors (Harvey
et al., unpublished). Sea lamprey do spawn in all of the study rivers (Frear and Shannon, 1994; Whitton and
Lucas, 1997; BEST, 2003, 2004), but the long-term viability and conservation status of their populations is
a concern owing to their low abundance and the apparent irregularity of successful spawning. Adult sea
Copyright # 2007 John Wiley & Sons, Ltd.
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5
4
Wetherby
n = 53
SI = 3.4
1+ (2003)
0 + (2004)
3
2
1
0
Frequency
30
20
Boston Spa
n = 230
1+ (2003)
0 + (2004)
10
0
40
30
Tadcaster
n = 479
SI = 3.6
1+ (2003)
20
0 + (2004)
10
0
0
20
40
60
80
Total length (mm)
100
120
Figure 6. Length–frequency distributions of Lampetra ammocoetes captured from the River Wharfe. Modal
distribution curves have been fitted to the 0þ (2004 year class) and 1þ (2003 year class) age classes. SI¼ separation
index from the method of Bhattacharya (1967).
lamprey are known to locate spawning rivers using a migratory pheromone released by resident larval
conspecifics (Sorensen et al., 2003). Similarly, female sea lamprey locate males via a pheromone released by
males on the spawning grounds (Li et al., 2003). Thus, there is a potential risk that the sea lamprey
populations may become too small for chemical communication to be effective, particularly where age
classes are missing in the larval populations or where spawning stocks are lowest. In addition, there is a
possibility that the use of common spawning grounds may increase potential for competition between sea
lamprey and river lamprey larvae. For example, in the Ouse catchment, river lamprey spawn approximately
2 months earlier than sea lamprey, and 0+ river lamprey may compete with smaller 0+ sea lamprey for
resources. However, the return of river lamprey to the Yorkshire Ouse system after an absence of more than
40 years suggests that recovery of sea lamprey populations may be possible, perhaps by straying of adults
from other rivers or attraction by other mechanisms (e.g. river lamprey or brook lamprey pheromones).
The majority of Lampetra ammocoete populations were dominated by large (80–100 mm) individuals,
which may be indicative of a strong 2001 and/or 2002 year class. More importantly perhaps, a number of
size ( age) classes were poorly represented in the length distributions at some sites, which may suggest
poor recruitment in particular years. For example, there was an almost complete absence of 1þ (2003 year
class) ammocoetes in the River Ure, and comparatively small numbers of 0þ (2004 year class) ammocoetes
in the rivers Derwent and Nidd. One issue influencing recruitment success of some lamprey populations in
mainland Europe is physical barriers to migration, such as weirs and sluices (Waterstraat and Krappe,
1998; Almeida and Quintella, 2002; Almeida et al., 2002; Oliveria et al., 2004), and it is possible that similar
impacts periodically occur in the Yorkshire Ouse catchment. For example, very few river lamprey migrated
upstream of Boroughbridge Weir in 2003 (BEST, 2003), which was reflected in the current study by the
small numbers of 1+ ammocoetes captured at Westwick and Bellflask. The explanation for the failure to
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100
Derwent
80
60
40
20
0
200
Swale
150
100
50
River discharge (m3s-1)
0
150
Ure
100
50
0
80
Nidd
60
40
20
0
150
Wharfe
100
50
01/05/03
01/04/03
01/03/03
01/02/03
01/01/03
01/12/02
01/11/02
01/10/02
0
Figure 7. Mean daily discharge (m3 s1) in the rivers Derwent, Swale, Ure, Nidd and Wharfe during the main river lamprey spawning
migration period (October–April) in 2002/03, compared with their respective long-term mean October–April discharges (dashed line).
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CONDITION ASSESSMENT OF LAMPREYS
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migrate beyond Boroughbridge is uncertain, but it may be due partly to a combination of the hydrological
regime of the Ure, the large size of potential barriers (e.g. 2.5 m head-loss at Boroughbridge Weir) and a
mismatch in the timing of the main spawning run and high river levels in 2002/03. For example, although
discharge was above average for the first part of the migration period (October–January), it was below
average for the majority of February, March and April, which may have limited passage past potential
barriers if the spawning migration was delayed in that year.
By contrast, the River Swale supported good numbers of ammocoetes in each size/age class, perhaps
reflecting easier access to spawning grounds in this river. The Swale is the least impounded of the study
rivers and, as such, lamprey may be less dependent upon high flows for access to their spawning grounds. In
addition, the Swale is prone to large and rapid fluctuations in river level, so opportunities to pass barriers
may arise more frequently than in rivers with more stable discharge. Similarly, the discharge regime of the
Derwent (i.e. relatively high number of days above the long-term October–April mean during the spawning
migration) may be more amenable to the upstream migration of lamprey than the regimes of many of the
other study rivers, in spite of the larger number of potential migration barriers (Jang and Lucas, 2005).
Notwithstanding, there are likely to be a number of other factors influencing recruitment success. For
example, ammocoete catches at Tadcaster on the River Wharfe were dominated by large (>70 mm)
individuals, perhaps suggesting poor recruitment in 2003 and 2004, despite adult lamprey being able to reach
the site without physical obstruction. Similarly, catches at Boroughbridge on the Ure were also dominated
by large ammocoetes, despite large numbers of adults spawning there in 2003 and 2004 (BEST, 2003, 2004).
Between-year differences in water temperature, river discharge and water quality may all influence survival
of eggs or larvae. Slight differences in habitat may also be important, with some areas potentially more
favourable to larger than smaller ammocoetes, and it is also possible that smaller ammocoetes had dispersed
elsewhere. Although the River Nidd appeared to provide an abundance of optimal lamprey ammocoete
habitat, this was not supported by the catches in this study; it may be that the sand was too coarse and
unstable to support high densities of ammocoetes. In addition, many of the areas of habitat were ephemeral.
Indeed, water levels were above average during the Nidd surveys, and it is possible that the comparatively
low numbers may have resulted from loss of lamprey and/or habitat due to wash-out.
Power stations and other significant water abstractors may pose a threat to migrating lamprey. For
example, impingement of adult lamprey occurs at a power station on the Humber Estuary, sometimes in
large numbers. Similarly, impingement of lamprey ammocoetes is also known to occur, such as at a number
of water treatment works on the Derwent. Although water quality in the study area is generally good (Neal
and Robson, 2000), there is also a possibility that poor quality inputs to the tidal Ouse, particularly from
the rivers Aire (and Calder) and Don, may pose a barrier to lamprey migration under some circumstances,
especially during low flow conditions. Another factor that warrants attention is the impact of exploitation
on the sustainability of the lamprey stocks. It is known, for example, that large numbers of river lamprey
are caught commercially from the Yorkshire Ouse (Masters et al., 2006). It is critical that the impacts of
such exploitation pressures, including those on stocks elsewhere in the UK and mainland Europe, are
assessed to determine their role in the overall conservation of the species.
There is a potential, tangible link between flow characteristics and lamprey spawning success
(Waterstraat and Krappe, 1998; Oliveria et al., 2004). In some rivers, particularly those with large
barriers and/or relatively stable discharge, access to spawning grounds may depend partly upon
synchronization of the lamprey spawning migration with elevated river levels. This may be particularly
the case for sea lamprey, which tend to migrate in spring and early summer, when river levels are lower and
more stable. Large spawning aggregations in discrete localities are extremely susceptible to interference,
habitat degradation or environmental perturbations (Jang and Lucas, 2005). There is, therefore, a need to
facilitate upstream passage at potential physical obstructions, to improve access of migrating lamprey to
under-exploited spawning and nursery areas. Actions to protect and enhance nationally or internationally
important stocks must be implemented from at least a catchment perspective, because many of the issues
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affecting such species are not localized. With respect to lampreys, particular attention should be given to
protecting spawning and nursery habitats, improving water quality, reducing impingement at abstraction
points, preventing exploitation at spawning grounds, and increasing passage at potential physical
obstructions.
ACKNOWLEDGEMENTS
The authors acknowledge the Environment Agency for provision of funds to carry out the survey work, and David
Hopkins and Environment Agency Technical Fisheries Officers for organizing access to the survey sites. Many thanks
are extended to the various landowners and fishery owners, especially Brian and Sue Morland, for permission to
undertake the surveys. The authors gratefully acknowledge the assistance of Jon Bolland and Darren Rollins in electric
fishing surveys. The manuscript benefited greatly from the constructive criticism of two anonymous referees.
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