Mar 19, 1988 - shiner (Richardsonius balteatus Rich.) ...... Philip and Son Publishers, London. "'tpp. . :aSimov ...... PUblished D. Phil Thesis, University of Ulster.
Factors Affecting the Distribution and Movement of Fish in a Shallow Eutrophic Lake
By
Colin. W. Bean B.Sc. (Hons) M.I.F.M.
Faculty of Science and Technology of the University of Ulster
A thesis submitted for the degree of Doctor of Philosophy
September 1992
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TABLE OF CONTENTS
Page Dedication
i
Acknowledgements
ii
Declaration of Access to Contents
iii
Abstract
iv
Chapter 1. 1.1 1.2 1.3 1.4 Chapter 2.
Introduction General Introduction Conceptual Models of Fish Habitat Use Distribution and Habitat Complexity Aims of the Study
Introduction The Gravel Pits: Physical and Chemical Characteristics 2.2.1 Morphology 2.2.2 Trophic Status 2.2.2.1 Gross Chemistry 2.2.2.2 Nutrient Chemistry 2.2.3 Light, Temperature and Oxygen 2.2.3.1 Light 2.2.3.2 Temperature 2.2.3.3. Oxygen 2.3 The Gravel Pits: The Biota 2.3.1 Primary Production 2.3.1.1 Phytoplankton 2.3.1.2 Macrophytes 2.3.2 Invertebrates 2.3.2.1 Zooplankton 2.3.2.2 Macroinvertebrates 2.4 Fish 2.5 Birds
3.1 3.2
5 7
The Study Site
2.1 2.2
Chapter 3.
1 4
8 9
9 10 10 11
12 13 14 14 16 16 16 18
19 19
22 24 26
General Methods
Sample Sites Methods of Capture 3.2.1 Electrofishing 3.2.2 Gill netting 3.2.3 Fyke Nets 3.3 Sampling Protocol 3.3.1 Routine Sampling 3.3.2 Diel Sampling 3.4 Field Treatment of Fish 3.5 Marking and Tagging 3.6 Laboratory Treatment of Fish 3.6.1 Standard Measurements
27 27 27 27 28 29 29 29 29 30 30 30
3.7
Chapter 4.
3.6.2 Internal Examination Ageing 3.7.1 Scale Collection and Preparation 3.7.2 Opercular bone Collection and Preparation 3.7.3 Scale Reading 3.7.3.1 Roach 3.7.3.2 Bream, Rudd 3.7.3.3 Pike 3.7.4 Opercular Bone Readings 3.7.4.1 Perch 3.7.4.2 Pike
The Fish Community
4.1 4.2
Introduction Methods 4.2.1 Length-Weight Relationship 4.2.2 Back-Calculations of Length 4.2.2.1 Scales 4.2.2.2 Opercular Bones 4.2.2.2.1 Pike 4.2.2.2.2 Perch 4.2.3 Ford-Walford Plots 4.3 Results 4.3.1 The Community 4.3.2 Population Structure 4.3.2.1 Roach 4.3.2.2 Bream 4.3.2.3 Rudd 4.3.2.4 Perch 4.3.2.5 Pike 4.3.3 Direct Observations of Growth 4.3.3.1 Roach 4.3.3.2 Bream 4.3.3.3 Rudd 4.3.3.4 Perch 4.3.3.5 Pike 4.3.4 Back-Calculations of Growth 4.3.4.1 Roach 4.3.4.2 Bream 4.3.4.3 Rudd 4.3.4.4 Perch 4.3.4.5 Pike 4.4 Discussion
Chapter S. 5.1 5.2
30 30 31 32 32 32 33 33 34 34 35
36 38 38 38 39 39 39 39 40 42 42 42 42 43 43 44 44 46 46 47 48 49 50 51 51 52 52 52 53 54
General Distribution and Habitat Preferences
Introduction Methods 5.2.1 Field Methods 5.2.1.1 Fish 5.2.1.2 Physical Measurements 5.2.2 Computational Methods 5.2.2.1 Association and Habitat Overlap Statistics 5.3 Results 5.3.1 Habitat Preferences 5.3.1.1 Littoral/Open 5.3.1.1.1 Roach
57 60 60 60 61 61 61 63 63 63 63
5.3.1.1.2 Bream 5.3.1.1.3 Rudd 5.3.1.1.4 Perch 5.3.1.1.5 Pike 5.3.1.2 Macrophytes/Open 5.3.1.2.1 Roach 5.3.1.2.2 Bream 5.3.1.2.3 Rudd 5.3.1.2.4 Perch 5.3.1.2.5 Pike 5.3.1.2.6 Macrophyte Species Preferences 5.3.1.3 Shallow/Deep 5.3.1.3.1 Roach 5.3.1.3.2 Bream 5.3.1.3.3 Rudd , 5.3.1.3.4 Perch 5.3.1.3.5 Pike 5.3.2 Seasonal Species Associations 5.3.2.1 Spring 5.3.2.2 Summer 5.3.2.3 Autumn 5.3.2.4 Winter 5.3.3 Distribution in Relation to Abiotic Factors 5.3.3.1 Temperature 5.3 3.2 Oxygen , 5.3.3.3 Light 5.4 Discussion Chapter 6.
6.1 6.2
64 64 64 65 65 66 66 66
67 67 67 68
69 69 69 69 69 69 70 70 70 71 71 71 71 72 73
Seasonal and Diel Movement Patterns
Introduction Methods 6.2.1 Field Methods 6.2.1.1 Diel Sampling 6.2.1.2 Marking and Tagging 6.2.1.2.1 Panjetting 6.2.1.2.2 Opercular Tagging 6.2.2 Computational Methods 6.3 Results 6.3.1 The Marking and Tagging Programme 6.3.1.1 Return Rates 6.3.1.2 Effect of on Host 6.3.1.3 Patterns of Movement 6.3.2 Spawning Movements 6.3.2.1 Roach 6.3.2.2 Bream 6.3.2.3 Rudd 6.3.2.4 Perch 6.3.2.5 Pike 6.3.3 Temporal Activity Patterns 6.3.3.1 Roach 6.3.3.2 Bream 6.3.3.3 Rudd 6.3.3.4 Perch 6.3.3.5 Pike 6.3.4' Diel Horizontal Movements 6.3.4.1 Roach
81 84 84 84 84 84 85 85
87 87 87 87 87 88 88 89 89 89 89 90 90 91 91 91
92 93 93
6.3.4.2 Bream 6.3.4.3 Rudd 6.3.4.4 Perch 6.3.4.5 Pike 6.3.5 Vertical Distribution and Movement 6.3.5.1 Roach 6.3.5.2 Bream 6.3.5.3 Rudd 6.3.5.4 Perch 6.3.5.5 Pike 6.4 Discussion 6.4.1 General Movement Patterns and Tagging Effects 6.4.2 Spawning 6.4.3 Diet Activity Patterns and Movement Chapter 7.
Introduction Methods 7.2.1 The Radio-Transmitter 7.2.2 Transmitter Attachment 7.2.2.2 Pike 7.2.2.3 Bream 7.2.3 Tracking Procedure 7.2.4 Measurement of Environmental Variables 7.3 Results 7.3.1 Pike 7.3.1.1 Effect of Tags on Hosts 7.3.1.2 General Movement Patterns 7.3.1.3 Movement in Relation to Environmental Variables 7.3.1.4 Spawning Movements 7.3.1.5 Patterns of Habitat Use 7.3.1.5.1 Vegetation 7.3.1.5.2 Depth 7.3.1.6 Diel Activity Patterns 7.3.2 Bream 7.3.2.1 General Movement Patterns 7.3.2.2 Movement in Relation to Environmental Variables 7.3.2.3 Patterns of Habitat Use 7.3.2.4 Diel Activity Patterns 7.4 Discussion 7.4.1 Pike 7.4.2 Bream
8.1
95 95 95
97 97 98 99
100 100 102
106
Telemetric Observations of Pike and Bream
7.1 7.2
Chapter 8.
94 94 94
113 119 119 119 119 120 121 122 123 123 123 123 124 124
125 125 126 126 127 127 127 127 128 129 129
139
Diet
Introduction 8.1.1 The Impact of Predators on Prey Communities 8.1.2 Competitive Aspects of Predator-Prey Interactions 8.2 Methods 8.2.1 Field Methods 8.2.1.1 Fish Capture 8.2.1.2 Field Treatment of Fish 8.2.2 Laboratory Methods 8.2.2.1 Examination of Gut Contents 8.2.2.1.1 All Fish 40 mm F.L. 8.2.2.1.2.1 Quantification of Cyprinid Gut Contents
140 140 141 144 144 144 144 145 145 145 145 146
8.2.2.1.2.1.1 Cladocera 8.2.2.1.2.1.2 Copepods 8.2.2.1.2.1.3 Dipteran Larvae 8..2.2.1.2.1.4 Other Dietary Items
Perch Pike 8.2.3 Computational Methods 8.2.3.1 Electivity 8.2.3.2 Dietary Overlap 8.3 Results . 8.3.1 Seasonal Changes in Diet and Selectivity 8.3.1.1 Roach 8.3.1.1.1 Roach O.OS), revealed that the pond was generally isothermic. Summer statification episodes were Confined. to the deep section and were transient in nature.
2.2.3.3
Oxygen
Variations in the oxygen concentration of natural waters arise from a number of sources both spatially and temporally and are particularly common in shallow freshwater systems (e.g. Kramer et al., 1978; Holeton, 1980; Sand-Jensen et al., 1989a). While many benthic invertebrates have adapted to periods of low oxygen concentrations or anoxia, fish are particularly sensitive to any change, (Varley,1967; Hawkes, 1975; Holeton, 1980; Alabaster and Lloyd, 1980; Armstrong, 1987; Weiser, 1991).
14
(a)
(b)
Figure 2.7
(a) The annual temperature regime of Sections 1-3. (b) Mean annual temperature (calculated using pooled sectional data) (data are given as means ± 95%Confidence Limits).
Despite increased photosynthetic activity during the summer months, Figure 2.6 shows that oxygen concentrations were lowest in all sections during this period. Increased temperatures and elevated microbial activity are the probable causative agents of oxygen depletion in all sections. Reduction of mechanical mixing by the increased aerial foliage of macrophytes (Sculthorpe, 1967), in the shallow areas may be a lesser, albeit significant, contributory factor.
15
2.3
THE GRAVEL PITS:
2.3.1
THE BIOTA
Primary Producers
Primary production is the capacity of an ecosystem to build up primary organic compounds using energy derived from external radiant and chemical sources (Vollenweider, 1972). The relationship of primary production to nutrient content was first elucidated by the pioneering work of Pearsall (1930, 1932). He postulated that chemical variations and particularly chemical ratios in natural waters were responsible for both phytoplankton biomass and community composition. This early work has taken on a new significance with the increased importance of cultural eutrophication in recent years.
Nearly a century of observations on phytoplankton and the ease with which this Community can be sampled has led to its emphasis as the major primary producer within lakes (Westlake, 1980). Reynolds (1984) and Haslam et al. (1975) have provided extensive reviews of the habitat and chemical requirements of phytoplankton and macrophytes respecti vely.
2.2.1.1
Phytoplankton
The mechanisms involved in phytoplankton dominance within lakes have been well documented in recent years (Lynch and Shapiro, 1981; Maberly and Spence, 1983; Wetzel,1983; Elber and Schanz, 1989; Bailey-Watts et al., 1990; Reynolds, 1990). Phytoplankton are particularly important in freshwaters because of their close relationship with zooplankton, and ultimately fish, biomass (Porter, 1977; Andersson et
al., 1978; Gliwicz,1980; Anderson, 1984; Porter and McDonagh, 1984; Timms and Moss,1984; Lampert et al., 1986; Irvine et al., 1989; Balls et al., 1989; Zagareze, 1990; Gamier and Mourelatos,1991).
Phytoplankton content (measured as Chi a), was measured using the spectrophometric technique described by Vollenweider (1972). Although this method
16
yields a quick assessment of ChI a, much descriptive information on community composition is lost. Water samples were analysed for ChI a and nutrient chemical content at monthly intervals (see section 2.2.1).
Table 2.3 shows the variety of phytoplankton species present within the pond. In keeping with the dominance of Chlorophytes in waters with a low pH (Reynolds, 1984), community composition is dominated by green colonial (e.g. Volvox sp., Conium sp.) and motile (e.g. Pteromonas sp. Chlorogonium sp.) forms. The community also contains the diatom Asterionella formosa and the free-swimming chrysophyte Dinobryon divergens. Colonial pennate forms (e.g. Asterionella sp.), small colonial chrysophytes (e.g. Dinobryon sp.) and colonial chlorococcales (e.g. Pediastrum sp.), by virtue of their photosynthetic behaviour, surface area/volume ratios, cell size and ability to maintain overwintering populations, are strong candidates to dominate such assemblages (Reynolds, 1984). Those 'I
that can maintain the highest average rate of increase under prevailing environmental conditions will provide the largest initial inoculum (Reynolds op. cit.). This infers that the Observable sequence of algal dominance (Figure 2.8), can be explained by the relative
.
cOmpetitive abilities of the algal species along variable resource gradients.
The importance of Asterionella was expected, given the high Si:P molecular ratio (Tilman, 1977), and high photosynthetic efficiency (-5 mg C (mg ChI a)-l E-1m 2), (Reynolds, 1984). The prevernal, vernal and aestival co-dominance of Asterionella and
Dinobryon is particularly interesting because both species require silica for cell wall manufacture and phosphate for metabolic activity. Asterionella, despite having growth inhibited at low phosphate levels, may still attain a significant population size if silica is present (Reynolds and Butterwick, 1979). Dinobryon tends to dominate in P-rich ponds (Lehman, 1976), although Rodhe (1948) suggested that the species is traditionally aSSOciated with P-deficient lakes. It seems therefore that both Asterionella and
Dinobryon have obtained superior competitive abilities by evolving a degree of plasticity in their use of available resources. The decline in Asterionella and Dinobryon importance 17
Table 2.3
Phytoplankton species present in the pond system from 1987-1990
PHYfOPLANKTON SPECIES UST
Class Chrysophyta order Ochromandales
Dinobryon divergens (Erh.)
Class Bacillariophyceae
Asterionella formosa
Class Chlorophyta order Volvocales
Pteromonas sp. (Seligo.) Chlorogonium sp. (Ehrb.) Volvox aureus (Ehrb.) Gonium pectorale (Muller) order Chlorococcales
Pediastrum simplex (Meyen) order Chaetophorales
Stigoc1onium sp.
order Ulvales
Prasiola sp.
order Zygnematales
Closterium moniliferum (Ehrb.)
g '"
"U
C ::l
~ .~
-:a
~
J
F
M
A
M
J
J
A
5
Month
Figure 2.8
Schematic representation of the seasonal dominance of the dominant phytoplankton species in the pond 1988.
o
N
D
during the serotinal period is reflected by a coincidental increase in available silica. The Chlorophytes, although never reaching the numbers achieved by Asterionella or
Dinobryon, become relatively more important during this period. The regulating factor for Chlorophyte growth may be that much of the phosphate is unavailable and tied up in the sediment by the sinking of Asterionella and Dinobryon blooms. The appearance of ammonia (caused by increased microbial activity), after the collapse of the Asterionella and Dinobryon populations lends credence to this hypotheSiS.
2.3.1.2
Macrophytes
Macrophytes are important structural components within freshwater habitats, (Sculthorpe,1967; Marshall and Westlake, 1978). Despite being structurally important, dense growths may also have a significant effect on diel oxygen levels, (Marshall and Westlake, op.cit.; Sand-Jensen et al., 1982; 1989a; Sand-Jensen and Revsbech, 1987). In lake fish communities, increased physical structuring is known to influence both competitive interactions and predator avoidance (Johnson and Stein, 1979; Cooper and Crowder, 1982; Savino and Stein, 1982; Coull and Wells, 1983; Werner et al., 1983a; Gotceitas and Colgan, 1987, 1989a, b; Butler, 1988; Gotcietas 1990a, b; Hart and Hamrin, 1988;·Wahl and Stein, 1988; Savino and Stein,1989a, b), direct grazing (Fowler, 1985; Winfield and Townsend, 1991) and resource partitioning (Werner and Hall,1976; 1979; 1988; Werner et al., 1977; 1983b; Werner and Gilliam, 1984; Layzer and Clady, 1987; Davis, 1989; Ehlinger, 1990). A comprehensive overview is provided by de Nie (1987).
The macrophyte species present in the pond are shown in Table 2.4. The species composition indicates that the ponds are moderately enriched (using the classifications of Haslam (1978) and Harding (1981». Small water bodies show large seasonal differences
which may follow complicated cycles (Hoogers and van der Weij, 1971; Westlake, 1975). In temperate lakes, macrophyte biomass increases in the spring to reach a summer maximum and then decreases during the late summer or autumn to a winter minimum (Westlake, 1981). Growth and decline are concomitant with rises in light, temperature,
18
Table 2.4
Macrophyte species present in the pond from 1987-1990
MACROPHYrE SPECIES UST FLOATING
SPARGANIACEAE
Potamogeton natans (L.) Sparganium emersum (Rehm.)
POLYGONACEAE
Polygonum amphibium
POTAMOGETONACEAE
(L.)
SUBMERGED CALUTRICHACEAE
Callitriche stagnalis (Scop.) Callitriche obtusangula(Le Gall.)
LENTIBULARIACEAE
Utricularia vulgaris (Agg.) Myriophyllum spicatum (L.) Elodea canadensis (Michx.)
HAlDRAGACEAE HYDROCHARITACEAE EMERGENT EQUISETACEAE SPARGANIACEAE
Equisetum fluviatile (L.) Sparganium erectum (Rehm.)
AUSMATACEAE
Eleocharis palustris (L.) Sagittaria sagittifolia (L.)
TYPHACEAE
Typha laUfolia
LABIATAE
Mentha aquatica
CYPERACEAE
(L.) (L.)
MARGINAL PLANTS
GRAMINEAE
Potentilla palustris (L.) Carex acutiformis (Ehrh.) Myosotis scorpoides (L.) Phalaris aruninacea(L.)
JUNCACEAE
Juncus effusus (L.)
SCOPHULARIACEAE
Veronica beccabunga(L.)
ROSACEAE CYPERACEAE BORAGINACEAE
Site 1
Site 2
Site 4
Site 3
,
..:.
,,11:
SiteS Site 6
Site 7 SiteS
III-Potamogeton naillns (L.) (L.)
m-MyriophyUum spicatum (L.)
vulgaris (Agg.) ~Polvronium amphibium (L.)
Iatifolia (L.) H!tSparganium emersum (Rehm.) ~Sparganium erectum (Rehm.) Site 9
Figure 2.9
Site 10
Macrophyte distribution within the pond, 1987-1990
Spp
photosynthetic capacity and decreasing physical disturbance (Westlake op. cit.; SandJensen et al., 1986). An idea of the relative abundances of the main macrophyte species is given in Figure 2.9. Callitriche sp. are present throughout the year, although only reaching Significant proportions during the summer months. Overwintering emergent macrophytes such as Equisetum fluviatile, Sparganium erectum and Typha latifolia remain important as structural habitats even during the winter. Other species which are abundant during the summer period (e.g. Potamogeton natans, Myriophyllum spicatum,
Sparganium emersum and Utricularia vulgaris), disappear during the winter months.
Unlike phytoplankton, macrophytes by virtue of their simple root systems, are probably little affected by ambient aquatic nutrient concentrations, (Chambers et al., 1989). Gerloff and Krombholz (1966) and Westlake (1975) suggest that phosphate, nitrate and potassium may be the limiting nutrients in eutrophiC lakes. Gerloff and Krombholz (op. cit.) assume critical sediment concentrations of 13 mg N g-l and 1.3 mg P g-l respectively.
Macrophyte distribution is dependent on depth, sediment type and competition . (Spence and Chrystal, 1970a, b; Sheldon and Boylen, 1977; Weisner, 1991;'Wilson and Keddy, 1991). Examination of Figures 2.9 and 2.2, suggest that light is the limiting factor in the pond system. The abundance of macrophytes in the shallow areas (Sites 1 to 8), and their relative scarcity in the deep areas (Sites 9 to 10), lend credence to this hypothesis. Further evidence is provided in section 2.2.1.3, which shows that PhAR never reaches the base of the deep section at any time of year.
2.3.2 2.3.2.1
Invertebrates Zooplankton
The importance of zooplankton in the recycling of nutrients has been well documented in recent years (Andersson et al., 1978; Andersson, 1984; Post and McQueen, 1987; Vanni, 1987; Irvine et al.,1989; Evans, 1990; Gamier and Mourelatos, 1991, Lazzaro et al., 1992). This
19
modification of nutrient availability has serious implications, not only for the fish fauna, but for other components of lacustrine ecosystems.
Many of the earlier fish/zooplankton studies focussed on the effects of fish predation on particular zooplankton populations (Hrbacek
et al.,1961; Hrbacek,1962; Hillbricht-
Ilkowska, 1964; Brooks and Dodson, 1965; Wells, 1970; Lynch, 1979), or on the mechanisms by which zooplanktivores select their prey (Brooks and Dodson, 1965; Wong and Ward, 1972; Zaret and Kerfoot, 1975; Drenner et al.,1978; Kettle and O'Brien, 1978). Recent work has tended to switch attention away from size selective hypotheses to studies involving the behavioural basis for zooplanktivore prey selection (Fumass, 1979; Winfield et al.,1983; Lammens, 1985; Mills et al.,1986; Townsend et al.,1986). The prodigious volume of
publications involving the effects of fish/zooplankton interactions is reflected by the number of subject reviews in recent years (Gilyarov, 1987; Lazzaro, 1987; Northcote, 1988; Lazzaro et al., 1992).
The choice of zooplankton sampling method was relatively limited. For example, vertical net hauls were impractical due to the shallow nature of the pond and other more active methods (such as the Oarke-Bumpus sampler), were precluded as a consequence of lUXUriant macrophyte growth in these areas. It was with these constraints in mind that a 6 litre capacity Patalas trap was used (Patalas, 1954). The trap has a relatively rapid clOSing system which may reduce the possibility of zooplankton avoidance reactions (Patalas, op.cit.), and can be easily handled in small boats by individual workers (de Bernardi, 1984).
Sampling intervals are extremely important in studies of zooplankton population dynamics. Widely-spaced intervals may lead to significant distortions of zooplankton community composition and abundance over time. This effect may be greater for species
Which Occur sporadically or have short life cycles (Spodniewska, 1964). The suggested ideal sampling period of six days (Hillbricht-Ilkowska and Weglenska 1970) is beyond
20
the scope of this study,therefore this work must be considered nothing more than a general guide to the zooplankton dynamics of the pond. Samples were obtained at monthly intervals between January to December 1988. At least two replicates were taken at each of the eight shallow sites. Three replicate samples were taken at each of the two deeper sites. In addition to the routine monthly samples, two transect series were obtained from site ten during the months of February and July 1989. Three replicates were obtained from the extreme littoral edge, fringing macrophytes (principally Equisetum fluviatile) and open limnetic zone on each occasion.
Attempts to monitor zooplankton vertical distribution throughout the 24-hour cycle were unsuccessful because the Patalas trap extended from the water surface to within two or three centimetres of the bottom in most sites. Samples were obtained by plunging the trap into the water and carefully filtering the resultant material through a SO J.lIIl mesh )
plankton net. When sampling within heavily vegetated zones the water was first agitated to disperse phytophyllic animals (sensu Cryer, 1983).
After collection the live animals were killed via immersion in a soh~tion of 90% ethyl-alcohol. The alcohol kills the organisms quickly and avoids the distortive effects of other commonly used preservatives (e.g. 5% formaldehyde) (de Bernardi, 1984). The preserved specimens were examined using an Olympus SZ binocular microscope with a 0.74 ZOom lens (x40 magnification) and identified using current F. B. A. taxonomic keys. All species were counted and, depending on the numbers present, were measured. In cases were the numbers of a particular species were relatively large, a sub-sample of at least 20 organisms was selected at random. Cladocerans were measured from the anterior end, including the headshields, to the posterior end, excluding the tail spines. Copepods were measured from the top of the cephalothorax to the base of the furcal rami.
The zooplankton fauna of the pond was found to be relatively simple. A list of those Species present is given in Table 2.5. Only two truly limnetic cladoceran species are present
21
Table 25
Zooplankton species present in the pond from 1987-1990
ZOOPLANKTON SPECIES LIST
Family
Species
Branchionidae
Keratella valga (Ehrb.) Notholca labis (Gosse)
Asplanchnidae
Asplanchna priodonta (Gosse)
Synchaetidae
Synchaeta stylata (Gosse)
Philodinidae
Rotaria neptuna (Ehrb.)
Daphnidae
Daphnia hyalina (Leydig) Ceriodaphnia pulchella (Sars)
Bosminidae
B. longirostris (O.F.Muller)
Chydoridae
Alonopsis elongata (Sars)
Daphnidae
Simocephalus vetulus (O.F.Muller)
Sididae
Sida crystallina (O.F.Muller)
Chydoridae
Eurycerus lammellatus (O.F.Muller) Alona affinis (Leydig) A. quadrangularis (O.F.Muller) Chydorus sphaericus (O.F.Muller) Peracantha truncata (O.F.Muller)
Planktonic Rotifera
Planktonic Cladocera
Non-Planktonic
Oadocera
Copepoda Cyc1opoida
Cyclops vicinus (Uljanin) C. vernalis (Fischer)
(D. hyalina and B.longirostris), the others being strongly associated with benthic, littoral or vegetated habitats. Figure 2.10 shows the monthly percentage abundances of the major zooplankton groups. Rotifers dominate the deep site at all times of the year. Rotifers only dominated numerically in the shallow areas during the autumn and winter periods. This is poSSibly a function of reduced primary production during this time, although Gilbert (1980) and Williams and Gilbert (1980) found that the copepod C.
vidnus (a numerically important component of this zooplankton community), can be an important predator of Asplanchna (the dominant rotifer in this study).
Seasonality within c1adoceran assemblages is well understood (Vanni, 1986; Lampert, 1988; Tessier and Weiser, 1991). The seasonal succession of the major zooplankton Species in the pond is shown in Figure 2.10. Cladocera tend to increase in tandem with both
macrophyte density and temperature, although each species had its own particular peaks of abundance between the spring and autumn months. Most species (60%), attai~ed annual maxima during the month of June. D. hyalina and C. pulchella dominated the Cladocera in the shallow structured areas, reaching maximum densities of 2.38 individuals 1-1 in June and 1.67 individuals 1-1 in July respectively. The deep limnetic areas wer~ dominated by small or cryptic Cladoceran species (B. longirostris, A. quadrangularis and C. sphaericus). The presence of A. quadrangularis and C. sphaericus in this unlikely habitat can best be explained by Cryer (1983) when he stated that: although each zone has its own characteristic fauna a significant degree of overlap can be shown in small habitats. Figure 2.12 highlights the differences in habitat preference and spatial distributions of resident zooplankters.
2.3.2.2
Macroinvertebrates
In comparison to zooplankton, little information is available for larger Inacroinvertebrates. Macroinvertebrates are an important component of freshwater ecosystems, affecting both zooplankton (Eriksson et al., 1980; Nilssen et al., 1984; Peckarsky, 1984) and fish (Fox, 1978; Diamond, 1985; Diamond and Wakefield, 1986)
22
(a)
Abundance (%)
(b)
Abundance (%)
Figure 2.10
Seasonality of the major zooplankton groups: (a) Shallow area, (b) Deep area. Monthly values are given as proportions of the total number of zooplankters recovered.
1
Keratella valga
30
(a) !...
~ 20 :g
,.~~~~~~~~~~~~~~~-=~~
]100+ ___ 1WC;1..t'iI'.. OI...... IJJI..nI ..J'\....nI&.:.1 J
FMAMJ
JAS
=1 ...... 1
OND
Notholca labis
""
D
Month
(b)
Daphnia hyalina !... .!!l
~
:g .~ ."
oS
:L,. IllltLli II Ii Iii ,I tl • ,I ,,:1 1 ,..n,~ 11 J
F
M
A
M
J
J
A
S
0
N
I-=-I D
Ceriodaphnia pulchdla
!... .!!l
~
:g .~ ."
oS
J
F
M
,i ,i A
M
J
J
A
, ,I S
0
N
,
D
B05mina longiT05tris
:....
1:g .~ ."
oS
J
1...0 1
F
M
I""" A
M
J
1 J 1/l1iz".ri Ii ,.... , A SON D
Month
Figure 2.11a
Monthly abundances of Rotifera and Planktonic Cladocera (figures are expressed as means ± S.E.). Shallow si tes are represented by the solid bars (.) and the deep area by open bars ( 0 ).
Figure 2.11b
Monthly abundances Non-Planktonic Cladocera and Copepods, (figures are ex~ressed as means ± S.E.). Shallow sites are represented by the solid bars ( . ) ana the deep areas by open bars ( D ).
Summer 20
...
~
."
~ 10
..s
Littora l Edge 20
Open Water
Macrophytes
Winter
15
...
~
."
"> '6
10
..s 5
0 Littoral Edge
Figure 2.12
Open Water
Macrophytes
• D. hyalina
o e. puJchel/a
III B. longirostris
[) S. crystallina
IJ A. affinis
EI A. quadrangularis IDe. sphaericus
~C . vlcmus ..
~ e. vetulus
~ Rotifers
IllS. vetulus
Horizontal distribution and habitat preference of zooplankton during summer and winter 1989. Data are given as means ± S.E.
populations, as well as contributing to the diet of most fish species (Mittelbach, 1981b; GiIinsky, 1984; Butler, 1989).
Samples were taken at three monthly intervals throughout 1988. Benthos samples were originally obtained using a cable operated 0.2 m2 Ekman grab (Duncan and Associates, Cumbria, England). Although this method provided useful qualitative information regarding benthic invertebrate biomass, it became apparent that substratemediated differences in sampling efficiency limited its use as a quantitative technique. All subsequent samples were obtained using a 30 cm diameter cylinder sampler. This allowed a fixed area to be sampled, regardless of substrate type and degree of structural complexity. Macroinvertebrates were sampled from three different habitat types (open water, littoral and offshore macrophytes). Five replicate samples were taken from each of the three habitats. Vegetative dieback during the winter months precluded the taking of samples from the offshore structured habitat at this time. When sampling'vegetated areas, samples were taken from a range of different structure types to avoid error caused by SPecies-specific macroinvertebrate-macrophyte associations. All animal and vegetative material was removed from the fixed cylinder area during each samplini event. All samples were sorted in the laboratory, preserved in 5% formaldehyde solution and later identified. to species level (where possible), using current F.B.A. taxonomic keys.
The macroinvertebrate species found in the pond during 1988-89 are shown in Table 2.6. The pond is relatively species-rich, with 34 species of macroinvertebrates from 26 families and spanning six groups. The study area is host to a number of moderately tolerant forms (e.g. Asellus aquaticus,Sialis lutaria, Limnephilus vittatus and Limnaea
pereger) (Hellawell, 1986), suggesting that the ponds are slightly enriched. Thienemann (1922) based his theory of lake typology on the presence or absence of certain larval chironomid species. He suggested that Procladius sp., Polypedilum sp. and Chironomus
plumosus (all of which were observed during this study), were indicative of enriched lakes. Conversely, Hamilton (1971) and Mossberg and Nyberg (1979) concluded that
23
Table 2.6 Macroinvertebrate species present in the pond system during 1987-1989
PLATyHELMINTHES Planaridae Polycelis tenuis (ljima) OLIGOCHAEIA Tubificidae
COLEOPTERA Dytiscidae
Dysticus marginalis (L.) Acilius sulcatus (L.) Gyridae
Gyrinus natator (L.)
Limnodrilus hoffmeisteri (Oaparede) Naididae
Chaetogaster diaphanus (Gruithuisen) Nais barbata (Muller) ARACHNIDA Hydracarina
Hydrodroma despiciens (Muller) CRUSTACEA
DIPTERA Chironomidae
Chironomus plumosus (L.) C. riparius (Meigen) Polypedilum nebeculosum (Meigen) Tanypodinae
Macropelopia nebulosa (Meigen) Procladius sp. (Skuse) Tanipodinae sp. (Meigen) Ceratopogonidae
MALOCOSTRACA lsopoda
Asellus aquaticus (L.)
Bezziasp. Culicidae
Chaoborus crystallinus (Degeer)
Branchiura
Argulus foliaceus (Wagler) INSECTA
OOONATA Zygoptera
Ischnura elegans (van der Unden) Anisoptera
EPHEMOPTERA Caenidae
Cloeon dipterum (L.)
Aeshna juncea (L. ) MEGALOPTERA
Sialis lutaria (L.) HEMIPTERA Gerridae
Gerris lacustris (L.) G. odontogaster (L.)
TRICHOPTERA Limnephilidae
Limnephilus vittatus (Fabricius)
Hydrometridae
. Hydrometra stagnorum (L.) Notonectidae
Notonecta glauca (t.) Corixidae
Corixa punctata (IlHy) Pleidae
Sigara dorsalis (Leach) S. falleni (Fieb)
MOLLUSCA LAMELLIBRANCHIATA
Anodonta cygnea (L.) GASTROPODA Umnaeidae
Limnaea pereger (Muller) Planorbidae
Planorbis planorbis (L.) P. contortus (L.)
Procladius sp., Chironomus sp. and Tanytarsus sp. are typical of weakly acid humic lakes, although Walker et al. (1985) found only a slight change in the composition of chironomid communities inhabiting lakes over a broad range of lake types and pH values. Table 2.7 shows the seasonal importance (by wet weight), of each macroinvertebrate species in each of the three habitats. Limnephilus is the largest contributor by weight in most instances, perhaps reflecting the success of case structure in averting potential predatory effects (Johansson, 1991). The importance of macroinvertebrates in the diet of the the resident fish species will be discussed fully in Chapter 8. In lakes generally, the littoral has the highest rates of primary and secondary production (Winberg, 1972; Kajak et al., 1972; Wetzel and Allen, 1972; Ramussen and KoIff, 1987; Ramussen, 1988) and the highest diversity, biomass and turnover of benthos (Brinkhurst, 1974; Johnson, 1974; Ramussen, 1988; Plante and Downing, 1989). The distribution of macroinvertebrate biomass in this study lends support to these findings, open water macroinvertebrate values constituting Only 1.21%, 15.18%,5.25% and 25.28% of those found in structured habitats during spring, summer, autumn and winter respectively. The highest open water value (25.28% during the winter period) may be attributed to the absence of an offshore macrophyte sample during this time rather than an increase in open water invertebrate prod~ction. This Suggests that structured environments are important habitats for macro invertebrates, both as a prey-rich foraging site (Dermott, 1988; Ramussen, 1988) and by reducing mortality fates by inhibiting predator success (Stein and Magnusson, 1976; Nelson, 1979; Cooper and Crowdef, 1982; Gilinsky, 1984; Diehl, 1988).
2.3.3
Fish
The geographical isolation of Ireland from both Britain and the Western European mainland during the last ice age c. 8000 years ago, has meant that many of the coarse fish fOund here today are the direct result of individual species introductions (Went, 1950; ,Cragg-Hine, 1973; Fitzmaurice, 1981). The introduction and subsequent spread of roach
(R.Ufilus rutilus (1.» and perch (Perea fluviatilis 1.) in Irish waters have been well dOcumented (Tate-Regan, 1911), although historical data regarding the origin of bream
24
Table 2.7
Seasonal abundance (by wet weight), of the macroinvertebrate fauna of the pond in three habitats. Data are given as means ± S.E. for five replicate cylinder samples.
Species
Polyedis tenwis Hdobdelill stllg7llliis Limnodrilws hoffmeisteri Chlletogllster dillphllnllS Nilis bilrbiltll Hydrodromll despieiens AsdlllS IlqwztiellS ArgwlllS foliaee\lS Cloeon dipterwm Gerris IIlcwstris G. odontogllSter Hydromdrll stllgnorwm Notoneetll glllwCll Coriu pWnctlltll Sigllrll dOTSiliis S. flllleni Dysticws mllrgi7llllis Gyrinws 7IIltlltor Chironomws plwmosis C. ripllTius Polypedilwm nebecllloswm MIlCTO~lopill neblllosa Procllldiws sp. Tilnipodi7llle sp. Bezzill sp. ChIloborws crystllllinllS COenllgrion sp. Aesh7lll jwnctll SiIllis lwtllrill Limnephilws tlittlltws Limnlltll peregu Plilnorbis plilnorbis P. contorllls Habitat Total (mg/ml) Seasonal Total (gms/m')
Spring Structured
OpeD
Littor.u
Smnme!" Structured
OpeD O.04iD.04
Auiumn
Littoral
OpeD
Littoral
Structured
Winter Structured
OpeD
Littoral
31.20±31.10 0.17±(l.()9
0.D6:!D1l6
0.04iD.04
0.06±0.cl6 O.03±O.03 238±I.89
1.70±0.34 95.1±25.15
O.03±O.03 O.17±O.12
O.OI±O.OI 0.06±0.06 2.D4±2.04 142.66±5937 4.76±4.76 3.4O±1.70 4.29±2.14
85.58±49.42 952±952 1.76±1.76 653±3.77 1.97±1.97
O.68±O.34 76.06±25.14
O.34±O.34 104.62±25.14
58.81±11.76
2.63±2.63 56.33±2831 35.23±35.23
2.38±2.38 53.0±26.0 11752±23.47
129.28±4237
1.70±0.34 85.58±16.44 454±454
0.77±O39 104.62B8.03
2654±2654 141.16±107.81
29.81±29.81 2352±2352
8.16iB.16
7.88±7.88 13.66±13.66 8.16iB.16
199.7:5±58.76 75+.-33.79 3.67±3.65 5135±13.23
4.69±4.69
35.29±2037 37.4B±20.43 11.01±635 13.67±13.67
7.95±4.D8
1.42±1.42 1732±l.2.48 183.45±52.87 73.44±1836 133.99±133.99 69.49±34.74 49.16±28.38 9.78+....2.82
16.38±16.38 32.88
30.6±30.6 68.68±68.68 54.77±O36 11971l2±609.19
I
1092.48 2.73
11.76±11.76
12.12±12.12 9.38±9.38 22.01±6.34 5.84±2.92
8.77±5.06 3.40±3.40 2.85±1.42 13.86±6.93 31.78±31.78
265.7±132.85 35.12±35.12
59.69±38.62 65l±2.93
I
1610.22
393.94
7052±40.7 18.76±4.69 18.34±9.7
I
4.97±4.97
105.75±2032 3751±24.80
1038±7.53 836±439
11.01±635 26.07±9.03 3.67±3.67
3O.0l±3O.01
1836±1836 799 .8O±609.19 36.75+...36.75 81.93±16.38 8.14±2.15
36.01±18.04 531.65+...351.47 28.97±28.97 65.47±43.28 5.70±5.70
105.96±61.67
940.98
136.81
2.98
1
108.75±1951
14.68±7.34
7.14±357 275.25±140.07
1652.64
46.99±31.06
3.46±3.46 91.80±53.00
793.80±609.19 32.10±32.10 66.17±43.94 3.25±2.15
7.04±3.60
I
1338.45 2.80
14.60±7.72
8.77±5.06
354±354 31.20±31.20 73.10±73.10 56.66±31.44 797.80+...229.93
3.67±3.67
6.99±351 32.0±32.0
263.26±131.65 24.01±24.01
1063.3±351.47 17.73±17.73 98.24±49.08 4.07±1.63
66.29±44.06 5.70±2.15
I
1322.92
346.90
I
0 1.83
I
1489.80
!
(Abram is brama (L.», rudd (Scardinius erythrophthalmus (L.» and pike (Esox lucius L.), despite being widespread in Irish waters, are virtually absent.
The spread of roach into the Lough Neagh catchment area during the early 1970's (Cragg·Hine, 1973), has led to dramatic changes within the resident fish community. Community composition data for the years preceding the roach invasion are scarce. Much of the available data are only available in the form of catch statistics for commercially important species such as eels (Anguilla anguilla (L.», perch, pollan (Coregonus
autumnalis pollan Thompson), bream and resident brown trout (Saimo trotta L.). Recent studies, (Wilson, 1979; Marrion, 1986; Montgomery, 1990; Tobin, 1990), have elucidated the basic biology of the main commercial species and the newly arrived roach population. Both rudd and pike, once important components of the Lough Neagh fish community, are now rare. Pike, the most important piscivore in Irish fish assemblages, owe much of their demise to habitat modification mediated by increased eutrophication and suc~essive lough lowerings. The disappearance of rudd however, is coincident with the appearance of roach. Cragg·Hine (1973) suggested that introgressive hybridization rather than competition is the only cause of such a rapid elimination. This is an unlikey concept although loss of habitat heterogeneity and strong competitive interactions, particularly during the juvenile stage, may have had a significant impact on rudd population dynamics.
In comparison with Lough Neagh, the coarse fish composition of the pond is relatively poor. A complete list of species present in the pond during this study is shown in Table (2.8). The presence of rudd suggests that the pond was colonised by this species prior to its near extinction in Lough Neagh during the mid to lat~ 1970's. Reasons for the SUrvival of rudd in the pond system will be discussed in Chapter 5. Isolation of the pond system from Lough Neagh infers that much of the pond fish community were also the product of species introductions rather than a function of natural colonisation. The ponds are frequently visited by both anglers and commercial fishermen, both of whom are likely vectors for such introductions.
25
· Table 2.8
The fish community of the pond 1987 - 1990
Roach
Rutilus rutilus
Bream
Abramis brama
Rudd
Seardinius erythrophthalmus
Perch
Perea /luviatilis . L.
Pike
Esox lucius L.
European Eel
Anguilla anguilla
(L.) (L.)
(L.)
(L.)
A common feature of cyprinids is their ability to hybridize with other related species (Cowx, 1983; Wood and Jordan, 1987; Economidis and Wheeler, 1989; Thompson and lliadou; 1990). Limited numbers of both roach x bream and bream x rudd hybrids were found during the study. Rudd x roach hybrids were also present, although similarities in the gross morphology of the parent species make their detection difficult (Wheeler, 1976).
2.3.4
Birds
The importance of birds, particularly wildfowl, in lake ecosystems has only recently been addressed (Andersson, 1978; 1981). Wildfowl may interact directly with fish by competing for invertebrate prey items (Eriksson, 1979; Eadie and Keast, 1982; Pehrsson, 1984; Hill et
Cll., 1987; Giles et Cll., 1990; Winfield et al., 1990; Winfield, 1991).
Under the terms of the Ramsar Convention (1971), Lough Neagh was designated as a 'Wetland of International Importance". This reflects the important role of the lough as a habitat for overwintering wildfowl species (Salmon et al., 1989). Given the large aggregations of wildfowl in this area, a transient spread of individuals onto the pond system was expected. A list of visiting and resident bird species present ~n the study site is given in Table 2.9.
Despite the presence of large populations of piscivorous birds (principally Cormorants (Phalacrocorax carbo L.) and great crested grebes (Podiceps cristatus
L.» that
are known to frequent the inshore areas of Lough Neagh (Winfield et al., 1990), only two piSCivorous species were observed foraging within the study area. Kingfishers (Alcedo
Qtthis L.) were present only during the spring and summer periods. Grey herons (Ardea cinerea L.) by contrast, were resident in the area and appeared intermittently throughout the year.
26
Table 2.9 Bird species present on the pond during 1988-1989. Data is based on unpublished observations (I.J.Winfield pers. comm.). Resident Species
Moorhen
Gallinula chloropus L.
Coot
Fulica atra L.
Mute Swan
Cygnus olor (Gmelin)
Grey Heron
Ardea cinerea L.
Yisitin~
or Occasional Species
Tufted Duck
Aythya fuligula L.
Pochard
A. ferina L.
Scaup
A. marila L.
Pintail
Anas acuta L.
Mallard
A. platyrhynchos L.
Teal
A. clypeata L.
Shoveler
A. crecca L.
Goldeneye
Bucephala clangula L.
Kingfisher
Alcedo atthis L.
Chapter 3
General Methods
3.1
Sample Sites
The fish sampling area was divided into three sections and further sub-divided into ten discrete sites (see Figure 2.3).
3.2 3.2.1
Methods of Capture Electrofishing
Electrofishing was carried out using a Millstream FB3E apparatus (Millstream Engineering Ltd., England), powered by a Honda E300E portable generator. A pulsed DC current (max 10 amps) was preferred because of the ability of this mode to attract fish towards the cathode. This is particularly important in turbid or heavily vegetated conditions (Hickley, 1985). The pond is particularly well suited to this method of capture given its shallow and relatively uniform depth. Despite the fact that larger fish are particularly SUsceptible to capture (Junge and Libosvarsky, 1965; Hickley, 1985), it is generally accepted that electrofishing is the least selective of all fish sampling methods (Boccardy and COOper, 1963; Libosvarsky and Lelek, 1965; Lagler, 1978; Cowx and Lamarque, 1990). Low selectivity and capture-related mortality mean that this method is particularly useful for POpulation studies in small ponds. Inaccessibility due to excessive growths of vegetation arOund the pond fringe meant that electro fishing had to be carried out from a boat (Figure 3.1). Two samplers were required on each sampling occasion, although the need to employ constant effort dictated that the same person operated the electrofishing equipment at all times.
3.2.2
Gill Netting
Gill nets are perhaps the most widely used fish sampling tool in use today. They have the advantage of being easy to use and require the minimum of operational expertise. . Individual gill nets are, however particularly selective (Bagenal, 1972a; Woghlemuth, 1979; Hamley, 1980; Jensen, 1986; 1990; Craig et al., 1986; van Densen, 1987; Helser et al., 1991; Henderson and Wong, 1991), although Holt (1963) in a largely mathematical analysis of SOckeye salmon (Oncorhynchus nerka Walbaum) catch data in the Fraser River, found
27
Oarsman
Figure 3.1
Electrofishing Protocol. The arrow shows the direction of movement.
1.2 or 2 m (Dependent on Site Depth)
-
SUbstrate
Figure 3.2
Gill netting Protocol
that selectivity can be eliminated by using series of nets with overlapping catch curves. It is also worth noting that species morphology and behaviour as well as size is an important ; determinant of net efficiency (McCombie and Berst, 1969).
Monofilament gill net gangs of 6.25, 10 and 15 mm bar mesh were obtained from . Lundgrens Fiskredskaps-Fabrik (Stockholm, Sweden). Nylon gillnets are virtually . invisible in water, eliminating bias due to fish avoidance reactions (Craig and Fletcher, 1972). Each net gang was custom made to extend over the entire water column of all sites ,. (Figure 3.2). The gill net size range was selected on the basis of data obtained in previous , studies in nearby Lough Neagh (Bean and Winfield, 1989j Montgomery, 1990; Tobin, 1990). .• EVidence of the efficiency of the mesh sizes chosen can be observed in Figure 3.3. This figure demonstrates the catch efficiency of the gill net gang system used against the apparently unselective electrofishing technique, and indicates that only fish up to one-year old were i
selected against by gill netting. In an attempt to counter selective bias against both
· extremely small (i.e. 0+ cyprinids) and large (i.e. pike and bream) fish, both 4 mm mesh • size nylon gillnets and 30 m long survey nets, incorporating mesh sizes of 6, 10, 12,15, 18 and ,: 45 mm, were also used. As a. function of their passive employment, gill n~ts were used in this , stUdy to ascertain fish activity levels and vertical distribution patterns.
· 3.2.3
Fyke Nets
The success of static capture methods is extremely variable, depending on a number of factors including size, sex and activity levels (Bagenal, 1972bj Craig, 1975). Fyke nets 3 m in
I'
.
ength with 1.5 m leaders were obtained from Collins Nets (Dorset, England). Although
these nets were used extensively in all sites throughout 1988, they proved to be relatively ineffective and accounted for only 0.16% of the total project catch. Fish avoidance reactions to the heavy 4 mm braided nylon mesh or chemical preservative (Cuprinol) may have been caUsal factors.
28
100
Pike
Roach n=943
80
n=124
80
60
60
40
40
20
20
0
0 2
3
4
5
7
6
100
8
9
Bream
4
5
6
7
5
6
7
Perch
100
n=l60
n=201
]80
3
2
10 11 12 13 14
80
d
'0 ..... ~
60
60
40
40
20
20
'-'
r QJ
~ ~
0
0 2
3
4
5
100
6
7
8
10
9
11
2
12
3
Rudd n=l11
80
•
60
0
40
Electrofishing Gillnetting
20 0 2
3
4
5
6
7
8
9
10
Age (Years) Figure 3.3
4
Comparative "Catch at Age" percentage figures for electrofishing and gillnet gangs of 6.25, 10 and 15mm.
3.3 3.3.1
Sampling Protocol Routine Sampling
A monthly electrofishing sampling regime was carried out during 1988. This provided essential baseline community composition data (Chapter 4) and also yielded essential longterm distributional data for all species (Chapter 5).
3.3.2
Diel Sampling
Gill net gangs were used on a seasonal basis throughout 1988 and 1989. During 1988, all sites were sampled and nets were emptied at three-hourly intervals. Vertical height of all fish from the bottom lead line was recorded on all occasions (Chapter 6).
Further twenty-four hour gill net studies during 1989 focussed on small scale distributional studies of habitat preference amongst open and structured environments (Chapter 6). During this time nets were emptied of all fish at dawn and dusk.
3.4
Field treatment of Fish
Upon capture, fish taken for diet analyses were immediately immersed in a solution of 5% formaldehyde. This killed the fish quickly and minimised error caused by regurgitation of stomach contents (Windell and Bowen, 1978; Treasurer, 1988a; Hayward et ai., 1989). All returned fish had scale samples removed from the appropriate region (see section 3.7). Fork length (defined as the distance from the tip of the premaxillae to the tip of the median rays of the caudal fin) and weight (in grammes) were also measured. Fork length values Were obtained from a standard fish measuring board to an accuracy of ± 1 mm. Field eStimations of weight were obtained using a spring balance/polythene bag regime. A series of spring balances of varying sensitivity were used and comparative laboratory/field studies revealed that they provided satisfactory measurements of all but the smallest of Specimens. An evaluation of the use of spring balances for weighing live fish in the field is prOvided by Jennings (1989).
29
3.5
Marking and Tagging
Details of all marking and tagging methods are given in section 6.2
3.6 3.6.1
Laboratory Treatment of Fish Standard Measurements.
On return to the laboratory all fish were measured to fork length as in section 3.4. Wet weight was recorded on a Precisa 2200c electronic balance (Pag Oerlikon AG., Zurich, SWitzerland), to an accuracy of ±O.Olg. At least five readable scales were removed from the preferred region of each fish and placed in individually labelled scale packets. Operculars were also removed from all pike and perch specimens (see section 3.8).
3.6.2
Internal Examination
Prior to preservation in 5% formaldehyde solution, a longitudinal ventral inscision extending from the anus to a line level with the pectoral fin was made. This fulfilled two functions. Firstly it allowed a detailed examination of the body cavity and associated viscera for both state of gonadal maturity and parasite load to be asses~. Secondly, the inscision allowed the preservative to permeate the gut and halt ongoing digestive processes. Both gonads and parasites (if present), were weighed as in section 3.6.1 to an accuracy of ± 0.01 g
3.7
Ageing
There are several structures used in the ageing of fish, Bagenal & Tesch (1978), Summerfelt and Hall (1990) and Steinmetz and Muller (1991) provide extensive reviews. The most Widely used structures in contemporary European freshwater fish studies are scales and opercular bones. These methods are popular because their preparation is simple and requires no specialised equipment whilst offering the investigator an accurate means of detennining age.
30
In cyprinids, scales are usually the most easily read structure whereas both percid and esocid scales, although useful to a limited degree, are less tractable than opercular bones for age and growth studies (Le Cren, 1947; Williams, 1955; Frost &: Kipling, 1959; Banks, 1968; Craig, 1978; Hansen, 1978; Mann and Steinmetz, 1985; Mann and Beaumont, 1990).
3.7.1
Scale Collection and Preparation
All cyprinid scale samples were obtained from the 'preferred scale' region above the lateral line and below the anterior ray of the dorsal fin (Cragg-Hine &t Jones, 1969; Steinmetz, 1974; Hofstede, 1974). Steinmetz (1974) reported that cyprinids reach the length of a full year's growth when sampled during the cold season. The observations of Mann (1973) lend credence to this hypothesis by adding that roach only grow between the months of May to September, when the water temperature exceeds 12 °C. The cyprinids used in this study were captured between the months of September to April when the water temperature ranged between 8-10 0c. Age data are therefore given in complete years.
Pike scales were removed from the region above the lateral line and mid-way along the
.
body length. These scales are more typical in form and larger in size than those obtained from other parts of the body (Frost &t Kipling, 1959; Wydallis, 1960). Pike scales were only taken from those fish which were tagged and returned to the water. On any subsequent recapture a further patch of scales were removed from an area adjacent to the previous removals.
Perch age and growth data were obtained solely from opercular bones (after Le Cren, 1947).
Approximately five scales were removed from each fish using forceps and stored in individual scale packets. In the laboratory, the scales were cleansed of residual epithelial tissue via immersion in a solution of 5% sodium peroxide and rubbing between thumb and forefinger. Replacement scales were easily identifiable at this stage and were discarded. The scales were then placed on a microfiche projector and viewed at a magnification of x22. 31
3.7.2
Opercular Bone Collection and Preparation
Scales have the obvious advantage over opercular bones in that they can be easily obtained without fish mortality. All pike and perch data were derived from fish taken for diet analysis. Operculars were occasionally taken from cyprinids for purposes of cross-reference with regard to age determination and back-calculation technique.
The opercular bones of pike, perch and cyprinids were removed according to the methods of Frost &: Kipling (1959), Le Cren (1947) and Banks (1968) respectively. After removal, the operculars were soaked in boiling water for a period of ten minutes. This facilitated the removal of attatched muscular and epidermal tissue from the bone. The bones were then stored in labelled envelopes for several months. The storage period allowed the bones to dry out, this is known to enhance the visibility of the annuli (Frost &: Kipling, 1959; CraggHine, 1965). Banks (1968) found that heating operculars for several hours in an oven at 70°C I
produced the same effect. The opercular annuli can be seen with the naked eye but reliability was improved by viewing under a low power binocular microscope at x10 magnification.
3.7.3 3.7.3.1
Scale Reading Roach
Roach are the most common cyprinid in European fresh waters and this dominance is reflected by a polarisation of research effort at the expense of less numerically important CYPrinid species. Roach scales were first described by Masterman (1923), since then several Workers have defined and re-defined roach scale reading methods (Hartley, 1947; Jones, 1953; Kempe, 1962; Cragg-Hine &: Jones, 1969; Mann,1973; Hofstede, 1974; Linfield, 1974; 1979).
Several of these workers (Hartley, 1947; Jones, 1953; Mann, 1973; Linfield, 1979) have Validated the use of roach scales by observing changes in scale structure over the course of an entire year. They affirmed that annuli are laid down only once per year, usually in late
32
spring/early summer. Others such as Kempe (1962) and Hofstede (1974) corroborated these findings by studying the growth of roach of known age in artificial ponds. Hofstede (op. cit.) concluded that the ageing of roach by use of scales was 100% reliable.
As a result of the slow growth of roach in this study, annuli were particularly close together making scale reading difficult. The appearance of false annuli further compounded the situation. Annuli were rejected as false if they could not be traced all the way around the scale (Cragg-Hine, 1965; Linfield, 1974). Cross-referencing age data gathered from the scales with opercular data corroborated the scale reading technique. It is perhaps noteworthy that cross reference of roach scale data with length-frequency distributions (Petersen Method) proved unsatisfactory due to the high degree of size overlap between age classes. Linfield (1974) also found this to be the case when reviewing likely sources of error when ageing the stunted roach population of Grey Mist Mere.
3.7.3.2
Bream, Rudd
Both bream and rudd presented similar scale reading difficulties to those of roach. In older
.
fish, annuli were so close that they were sometimes almost indiscernible. This problem was compounded by the appearance of false checks during periods of summer growth. As with roach scales this problem was negated by following the path of the annuli around the entire SCale. False annuli were particularly prevalent in older fish (particularly those greater than 7 years old). Occasionally the first annulus of both species was the most difficult to lOcate. It was discovered that by placing the scale of a one or two year old fish adjacent to the problem scale the location of the first annulus usually became apparent.
3.7.3.3
Pike
Detailed morphological descriptions of pike scales have been given by several authors
