Journal of Fish Biology (2015) 86, 288–303 doi:10.1111/jfb.12585, available online at wileyonlinelibrary.com
Hypoxic refuges, predator–prey interactions and habitat selection by fishes K. J. Hedges† and M. V. Abrahams* Department of Biology, University of Manitoba, Winnipeg, Manitoba, R3T 2 N2, Canada (Received 1 May 2014, Accepted 10 October 2014) Localized hypoxic habitats were created in Delta Marsh, Manitoba, Canada to determine the potential of regions of moderate hypoxia to act as refuges for forage fishes from piscine predators. Minnow traps and giving-up density (GUD) plates (plexiglas plates covered with trout crumble and fine gravel) were used to assess habitat use and perceived habitat quality for forage fishes, respectively, while passive integrated transponder tags provided data on habitat use by predator species to assess the level of predation risk. Data were collected both before and after a hypoxia manipulation (2–3 mg l−1 dissolved oxygen, DO) to create a before–after control–effect style experiment. Fathead minnows Pimephales promelas were more abundant and consumed more food from GUD plates in hypoxic bays after the DO manipulation, indicating hypoxic locations were perceived as higher quality, lower-risk habitats. The frequency of predator visits was not consistently affected. The duration of visits, and therefore the total time spent in these habitats, however, was significantly shorter. These predator data, combined with the prey information, are consistent with the hypothesis that hypoxic regions function as predator refuges. The refuge effect is not the result of predator exclusion, however; instead predators are rendered less capable of foraging and pose less of a threat in hypoxic locations. © 2014 The Fisheries Society of the British Isles
Key words: environmental effects; field manipulation; fish community composition; refuge from predation; habitat quality; giving-up density.
INTRODUCTION Habitat use decisions involve costs and benefits that incorporate individual needs (i.e. food, shelter or mates), competitor density and the interplay between the degree of risk and resource quality among various locations. Given the dynamic nature of environments, changes in abiotic and biotic factors will occur, affecting individuals, populations and species interactions (Menge & Sutherland, 1976, 1987; Heggenes et al., 1999). Changes in habitat use by individuals will affect the degree of habitat overlap between species, and therefore predator–prey interactions (Alheit & Niguen, 2004). As two species are likely to differ, at least marginally, in their physiological tolerances *Author to whom correspondence should be addressed at present address: Department of Biology and Department of Ocean Sciences, Memorial University of Newfoundland, St John’s, NL A1B 3X7, Canada. Tel.: +1 709 864 8153; email: [email protected] †Present address: Arctic Aquatic Research Division, Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, Manitoba, R3T 2 N6, Canada
288 © 2014 The Fisheries Society of the British Isles
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to abiotic factors [e.g. temperature, dissolved oxygen (DO), pH or salinity], these factors are reasonable candidates for reducing habitat overlap and providing refuges from predation (Neuenfeldt & Beyer, 2003). Both predators and prey are likely to experience stresses in these physiologically based refuges, yet because prey experience a stronger selective pressure from being eaten than predators experience from missing a potential meal, prey can theoretically incur a greater cost to realize a reduced risk of predation (Dawkins & Krebs, 1979; Abrahams, 2006). Ecological communities incorporate numerous species, creating a web of interactions. Many species are prey for more than one predator; a species’ antipredator strategies must evolve to counter a host of predator strategies and may vary among locations (Flecker, 1992; Bernot & Whittinghill, 2003). At the same time, predators must adapt to cope with a varied suite of competitors, prey and antipredator strategies. As environmental conditions fluctuate, the intensity of interactions between a prey species and each of its individual predators changes, the relative competitive abilities of the predators are altered, and changes in community composition can result. Adaptations that allow a prey species to use a habitat that is inhospitable to even one of its predators should result in a reduced risk of predation, assuming the pressures from the other predators do not experience balancing increases. Such refuges can result from transient or permanent physical structures that filter by body size (Everett & Ruiz, 1993; Warfe & Barmuta, 2004), sensory impairments such as turbidity (Engstrom-Ost et al., 2006; Castro et al., 2007), and physiological tolerances to factors such as hypoxia (Kolar & Rahel, 1993; Robb & Abrahams, 2003), pH (Scott et al., 2005), salinity (Witman &Grange, 1998; Hampel et al., 2005) and temperature, which may also drive seasonal changes in community composition (Maes et al., 1998). The relationship between body size and hypoxia tolerance cannot be generalized; some studies have reported greater tolerance by small-bodied fishes (Yamato & Iida, 1994; Robb & Abrahams, 2003) and others have reported the opposite (Almeida-Val et al., 2000). Tolerance clearly varies among species and understanding the effects of hypoxia on a specific fish community requires assessing the relative tolerances of the species present. Giving-up densities (GUD) (Brown, 1988) have been used by terrestrial ecologists as a simple method for assessing habitat quality. Generally, the technique involves placing reducible food patches in the environment and measuring the density of food remaining after allowing sufficient time for discovery and foraging by animals. The technique relies on predictions from the marginal value theorem (Charnov, 1976) that animals will give up foraging within a specific patch once their rate of energy acquisition equals the average rate from all possible patches. When food is equally available and the risk of predation is not uniform among patches, animals are expected to give up sooner (leaving a higher density of food) in riskier patches (Brown et al., 1992; Brown, 1999). Therefore, with information on predator distributions or predation rates, GUD can be used to assess perceived habitat quality, from the perspective of the foraging animal. GUD has been used in aquatic environments ( Stenberg & Persson, 2005, 2006; Polivka, 2007; Petty & Grossman, 2010) but so far only with benthic species that can dig through food plates in a manner similar to terrestrial animals in typical GUD patches. The purpose of this research was to determine the potential of hypoxic regions to act as refuges from predation for forage fishes. Natural developments of hypoxia are often accompanied by elevated water temperatures, following a relationship described by Henry’s Law, imposing two simultaneous stresses on individuals and confounding
© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2015, 86, 288–303
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K . J . H E D G E S A N D M . V. A B R A H A M S
determination of a causal factor. To decouple the typical water temperature-hypoxia pairing, artificial bays were created in which moderate hypoxia (2–3 mg l−1 DO) could be induced without increasing water temperature, under otherwise natural conditions. By monitoring the use of manipulated and control bays by fathead minnows Pimephales promelas Rafinesque 1820 and their various predator species, the potential of hypoxia for providing refuge from predation was assessed and the relative importance of water temperature and DO in affecting seasonal changes that were previously documented in the fish community of Blind Channel, Delta Marsh, Manitoba, Canada, were determined (Suthers & Gee, 1986). The primary hypothesis was that a refuge effect would be characterized by an environment that decreases the threat imposed by predators and should therefore result in increased P. promelas abundance and lower GUD values. Lower GUD values would indicate that fish were reducing the food patches to a lower density and accepting a lower rate of energy acquisition before switching to alternative food sources in hypoxic locations.
MATERIALS AND METHODS A field manipulation was conducted in Blind Channel, Delta Marsh, Manitoba, during summer (May–August) 2005. Blind Channel [Fig. 1(a)] is a relatively isolated water body that is c. 3⋅5 km long, has a maximum width of c. 90 m and a maximum depth of c. 1⋅5 m. Blind Channel has a single connection with the rest of Delta Marsh, The Cut [see Fig. 1(a)], through which water and fishes must pass. The northern end of Blind Channel reduces to a small creek that ends in an embankment and the eastern end passes through grated culverts that bar the passage of large fishes, finally terminating in a second earthen embankment. As a model system, Blind Channel exhibits distinct seasonal changes in the composition of the piscine community (Suthers & Gee, 1986; Kiers & Hann, 1995; Goodyear, 1996). Blind Channel has connections through Delta Marsh into Lake Manitoba; species and individuals can move between Blind Channel and Lake Manitoba as environmental conditions fluctuate, allowing changes in the local fish community to occur. During May and June of most years, the Blind Channel predator community consists of walleye Sander vitreus (Mitchill 1818) (early May only), pike Esox lucius L. 1758, yellow perch Perca flavescens (Mitchill 1814), freshwater drum Aplodinotus grunniens Rafinesque 1819, black bullheads Ameiurus melas (Rafinesque 1820), brown bullheads Ameiurus nebulosus (LeSeuer 1819) and channel catfish Ictalurus punctatus (Rafinesque 1818), while the prey community is dominated by P. promelas and supplemented with various shiners Notropis spp., five-spined sticklebacks Culaea inconstans (Kirtland 1841), nine-spined sticklebacks Pungitius pungitius (L. 1758), log perch Percina caprodes (Rafinesque 1818) and darters Etheostoma spp.. Typically, the community changes around the end of June or early July when E. lucius, P. flavescens and A. grunniens all leave the channel, presumably moving to Lake Manitoba or the larger bays within Delta Marsh, leaving Ameiurus spp. and I. punctatus as the only fish predators upon a continually abundant P. promelas population. To capture fishes entering and leaving Blind Channel, two trap nets were placed back-to-back in The Cut [Fig. 1(a)] during the summers of 2004 and 2005. Predators (E. lucius, P. flavescens, A. grunniens, A. Melas, A. nebulosus and I. punctatus) captured entering Blind Channel were implanted with passive integrated transponder (PIT) tags (Texas Instruments 32 mm tags; www.ti.com) and released into Blind Channel. Only fishes > 20 cm total length (LT ) were tagged and they were anaesthetized in clove oil (0⋅15 ml l−1 ). Tags were implanted using a canula needle (5 mm exterior diameter) that created a shallow incision into the peritoneal cavity. The PIT tag was inserted by placing it inside the canula needle and pushing it into the fish with a glass rod. The canula was removed and the incision sealed with Vetbond (http://solutions.3m.com/wps/portal/3M/en_US/GovernmentSolutions/Home/Product Information/ProductCatalog/˜/3M-Vetbond-Tissue-Adhesive?N=7580910+4294929329&rt=d) and a single suture.
© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2015, 86, 288–303
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(a)
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130°
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90°
80°
70°
Lake Manitoba
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Field station
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N
Bay 1 Blind Channel, Delta Marsh, Manitoba Bay 2
Bay 5 Dead end
Data sonde Culverts No fish passage
The Cut Trap nets
Bay 4 Data sonde
Bay 3 Data sonde
(b) Antennae
10 m
Control box
Mesh bag Minnow trap
Sonde Barrier nets
Mesh bag Shore
10 m
Fig. 1. (a) Placement of sampling gear within Blind Channel, Delta Marsh, Manitoba, during summer 2005. The inset panel in the upper left indicates the position of the field station within Canada (50∘ 11′ N; 98∘ 23′ W). Two trap nets were placed back-to-back in The Cut to catch predators entering or leaving Blind Channel. (b) Five artificial bays (10 m × 10 m) were constructed within the channel by erecting two barrier nets perpendicular to shore. Data sondes (YSI 6920) were placed in bays 2, 3 and 4 to collected turbidity, temperature and dissolved oxygen (DO) data. All fishes entering or leaving the system must pass through the cut. The north end of Blind Channel is naturally blocked; the eastern end is blocked by grated culverts (5 cm gaps), which prevent large fish passage and then terminates in an earthen embankment. , control bays; , experimental bays during a hypoxia manipulation experiment.
© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2015, 86, 288–303
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Fishes were able to enter and leave Blind Channel prior to trap net placement in each year and during severe storms when water levels rose above the trap net depth because of the influence of Lake Manitoba seiching on Delta Marsh. Lake Manitoba is a large (surface area = 4617 km2 ), relatively shallow (mean depth = 4⋅5 m) lake with a long north-south fetch (c. 200 km) and experiences considerable seiching during high-wind events. During these seiches, high water velocities occurred in The Cut, because of the in or outflow of water, that prevented the trap nets from being deployed or fished. Predators captured trying to leave Blind Channel in May were also tagged and returned to the channel to maintain a large population of tagged individuals within Blind Channel; 65 fishes were tagged leaving Blind Channel in May 2005. Predators captured leaving Blind Channel after 31 May were moved past the trap nets and released outside of Blind Channel because potentially lethal water temperatures and levels of hypoxia can develop during July (Hedges, 2007). Throughout both summers, gillnets (live sampling; 15⋅25 m × 1 m gangs, 5⋅08 cm stretched mesh, set for 1⋅5 h) were used to capture additional predators within Blind Channel to increase the proportion of PIT-tagged individuals in the predator populations. Five artificial bays (maximum depth = 1⋅00 m, minimum depth = 0⋅75 m) were constructed within Blind Channel during May and June of 2005 [Fig. 1(a)]. Each bay was created by extending two 10 m long barrier nets (heavy weight minnow netting; 2 mm mesh-size) perpendicular from shore; the nets were placed 10 m apart creating a 10 m × 10 m bay [Fig. 1(b)]. The barrier nets extended from the substratum to the water surface and were held in place by wooden posts. Neither P. promelas nor predators were able to pass through the barrier nets; all fishes were forced to enter the bays through the offshore opening. The substratum in each of the bays was a uniform mud bottom. There was very little aquatic vegetation (0⋅05 (1, 121) 27⋅15
Interaction P
F
0⋅05
119 (1, 441) 0⋅22 >0⋅05 (1, 441) 127⋅71
Hypoxic refuges, predator–prey interactions and habitat selection by fishes K. J. Hedges† and M. V. Abrahams* Department of Biology, University of Manitoba, Winnipeg, Manitoba, R3T 2 N2, Canada (Received 1 May 2014, Accepted 10 October 2014) Localized hypoxic habitats were created in Delta Marsh, Manitoba, Canada to determine the potential of regions of moderate hypoxia to act as refuges for forage fishes from piscine predators. Minnow traps and giving-up density (GUD) plates (plexiglas plates covered with trout crumble and fine gravel) were used to assess habitat use and perceived habitat quality for forage fishes, respectively, while passive integrated transponder tags provided data on habitat use by predator species to assess the level of predation risk. Data were collected both before and after a hypoxia manipulation (2–3 mg l−1 dissolved oxygen, DO) to create a before–after control–effect style experiment. Fathead minnows Pimephales promelas were more abundant and consumed more food from GUD plates in hypoxic bays after the DO manipulation, indicating hypoxic locations were perceived as higher quality, lower-risk habitats. The frequency of predator visits was not consistently affected. The duration of visits, and therefore the total time spent in these habitats, however, was significantly shorter. These predator data, combined with the prey information, are consistent with the hypothesis that hypoxic regions function as predator refuges. The refuge effect is not the result of predator exclusion, however; instead predators are rendered less capable of foraging and pose less of a threat in hypoxic locations. © 2014 The Fisheries Society of the British Isles
Key words: environmental effects; field manipulation; fish community composition; refuge from predation; habitat quality; giving-up density.
INTRODUCTION Habitat use decisions involve costs and benefits that incorporate individual needs (i.e. food, shelter or mates), competitor density and the interplay between the degree of risk and resource quality among various locations. Given the dynamic nature of environments, changes in abiotic and biotic factors will occur, affecting individuals, populations and species interactions (Menge & Sutherland, 1976, 1987; Heggenes et al., 1999). Changes in habitat use by individuals will affect the degree of habitat overlap between species, and therefore predator–prey interactions (Alheit & Niguen, 2004). As two species are likely to differ, at least marginally, in their physiological tolerances *Author to whom correspondence should be addressed at present address: Department of Biology and Department of Ocean Sciences, Memorial University of Newfoundland, St John’s, NL A1B 3X7, Canada. Tel.: +1 709 864 8153; email: [email protected] †Present address: Arctic Aquatic Research Division, Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, Manitoba, R3T 2 N6, Canada
288 © 2014 The Fisheries Society of the British Isles
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to abiotic factors [e.g. temperature, dissolved oxygen (DO), pH or salinity], these factors are reasonable candidates for reducing habitat overlap and providing refuges from predation (Neuenfeldt & Beyer, 2003). Both predators and prey are likely to experience stresses in these physiologically based refuges, yet because prey experience a stronger selective pressure from being eaten than predators experience from missing a potential meal, prey can theoretically incur a greater cost to realize a reduced risk of predation (Dawkins & Krebs, 1979; Abrahams, 2006). Ecological communities incorporate numerous species, creating a web of interactions. Many species are prey for more than one predator; a species’ antipredator strategies must evolve to counter a host of predator strategies and may vary among locations (Flecker, 1992; Bernot & Whittinghill, 2003). At the same time, predators must adapt to cope with a varied suite of competitors, prey and antipredator strategies. As environmental conditions fluctuate, the intensity of interactions between a prey species and each of its individual predators changes, the relative competitive abilities of the predators are altered, and changes in community composition can result. Adaptations that allow a prey species to use a habitat that is inhospitable to even one of its predators should result in a reduced risk of predation, assuming the pressures from the other predators do not experience balancing increases. Such refuges can result from transient or permanent physical structures that filter by body size (Everett & Ruiz, 1993; Warfe & Barmuta, 2004), sensory impairments such as turbidity (Engstrom-Ost et al., 2006; Castro et al., 2007), and physiological tolerances to factors such as hypoxia (Kolar & Rahel, 1993; Robb & Abrahams, 2003), pH (Scott et al., 2005), salinity (Witman &Grange, 1998; Hampel et al., 2005) and temperature, which may also drive seasonal changes in community composition (Maes et al., 1998). The relationship between body size and hypoxia tolerance cannot be generalized; some studies have reported greater tolerance by small-bodied fishes (Yamato & Iida, 1994; Robb & Abrahams, 2003) and others have reported the opposite (Almeida-Val et al., 2000). Tolerance clearly varies among species and understanding the effects of hypoxia on a specific fish community requires assessing the relative tolerances of the species present. Giving-up densities (GUD) (Brown, 1988) have been used by terrestrial ecologists as a simple method for assessing habitat quality. Generally, the technique involves placing reducible food patches in the environment and measuring the density of food remaining after allowing sufficient time for discovery and foraging by animals. The technique relies on predictions from the marginal value theorem (Charnov, 1976) that animals will give up foraging within a specific patch once their rate of energy acquisition equals the average rate from all possible patches. When food is equally available and the risk of predation is not uniform among patches, animals are expected to give up sooner (leaving a higher density of food) in riskier patches (Brown et al., 1992; Brown, 1999). Therefore, with information on predator distributions or predation rates, GUD can be used to assess perceived habitat quality, from the perspective of the foraging animal. GUD has been used in aquatic environments ( Stenberg & Persson, 2005, 2006; Polivka, 2007; Petty & Grossman, 2010) but so far only with benthic species that can dig through food plates in a manner similar to terrestrial animals in typical GUD patches. The purpose of this research was to determine the potential of hypoxic regions to act as refuges from predation for forage fishes. Natural developments of hypoxia are often accompanied by elevated water temperatures, following a relationship described by Henry’s Law, imposing two simultaneous stresses on individuals and confounding
© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2015, 86, 288–303
290
K . J . H E D G E S A N D M . V. A B R A H A M S
determination of a causal factor. To decouple the typical water temperature-hypoxia pairing, artificial bays were created in which moderate hypoxia (2–3 mg l−1 DO) could be induced without increasing water temperature, under otherwise natural conditions. By monitoring the use of manipulated and control bays by fathead minnows Pimephales promelas Rafinesque 1820 and their various predator species, the potential of hypoxia for providing refuge from predation was assessed and the relative importance of water temperature and DO in affecting seasonal changes that were previously documented in the fish community of Blind Channel, Delta Marsh, Manitoba, Canada, were determined (Suthers & Gee, 1986). The primary hypothesis was that a refuge effect would be characterized by an environment that decreases the threat imposed by predators and should therefore result in increased P. promelas abundance and lower GUD values. Lower GUD values would indicate that fish were reducing the food patches to a lower density and accepting a lower rate of energy acquisition before switching to alternative food sources in hypoxic locations.
MATERIALS AND METHODS A field manipulation was conducted in Blind Channel, Delta Marsh, Manitoba, during summer (May–August) 2005. Blind Channel [Fig. 1(a)] is a relatively isolated water body that is c. 3⋅5 km long, has a maximum width of c. 90 m and a maximum depth of c. 1⋅5 m. Blind Channel has a single connection with the rest of Delta Marsh, The Cut [see Fig. 1(a)], through which water and fishes must pass. The northern end of Blind Channel reduces to a small creek that ends in an embankment and the eastern end passes through grated culverts that bar the passage of large fishes, finally terminating in a second earthen embankment. As a model system, Blind Channel exhibits distinct seasonal changes in the composition of the piscine community (Suthers & Gee, 1986; Kiers & Hann, 1995; Goodyear, 1996). Blind Channel has connections through Delta Marsh into Lake Manitoba; species and individuals can move between Blind Channel and Lake Manitoba as environmental conditions fluctuate, allowing changes in the local fish community to occur. During May and June of most years, the Blind Channel predator community consists of walleye Sander vitreus (Mitchill 1818) (early May only), pike Esox lucius L. 1758, yellow perch Perca flavescens (Mitchill 1814), freshwater drum Aplodinotus grunniens Rafinesque 1819, black bullheads Ameiurus melas (Rafinesque 1820), brown bullheads Ameiurus nebulosus (LeSeuer 1819) and channel catfish Ictalurus punctatus (Rafinesque 1818), while the prey community is dominated by P. promelas and supplemented with various shiners Notropis spp., five-spined sticklebacks Culaea inconstans (Kirtland 1841), nine-spined sticklebacks Pungitius pungitius (L. 1758), log perch Percina caprodes (Rafinesque 1818) and darters Etheostoma spp.. Typically, the community changes around the end of June or early July when E. lucius, P. flavescens and A. grunniens all leave the channel, presumably moving to Lake Manitoba or the larger bays within Delta Marsh, leaving Ameiurus spp. and I. punctatus as the only fish predators upon a continually abundant P. promelas population. To capture fishes entering and leaving Blind Channel, two trap nets were placed back-to-back in The Cut [Fig. 1(a)] during the summers of 2004 and 2005. Predators (E. lucius, P. flavescens, A. grunniens, A. Melas, A. nebulosus and I. punctatus) captured entering Blind Channel were implanted with passive integrated transponder (PIT) tags (Texas Instruments 32 mm tags; www.ti.com) and released into Blind Channel. Only fishes > 20 cm total length (LT ) were tagged and they were anaesthetized in clove oil (0⋅15 ml l−1 ). Tags were implanted using a canula needle (5 mm exterior diameter) that created a shallow incision into the peritoneal cavity. The PIT tag was inserted by placing it inside the canula needle and pushing it into the fish with a glass rod. The canula was removed and the incision sealed with Vetbond (http://solutions.3m.com/wps/portal/3M/en_US/GovernmentSolutions/Home/Product Information/ProductCatalog/˜/3M-Vetbond-Tissue-Adhesive?N=7580910+4294929329&rt=d) and a single suture.
© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2015, 86, 288–303
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Bay 5 Dead end
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The Cut Trap nets
Bay 4 Data sonde
Bay 3 Data sonde
(b) Antennae
10 m
Control box
Mesh bag Minnow trap
Sonde Barrier nets
Mesh bag Shore
10 m
Fig. 1. (a) Placement of sampling gear within Blind Channel, Delta Marsh, Manitoba, during summer 2005. The inset panel in the upper left indicates the position of the field station within Canada (50∘ 11′ N; 98∘ 23′ W). Two trap nets were placed back-to-back in The Cut to catch predators entering or leaving Blind Channel. (b) Five artificial bays (10 m × 10 m) were constructed within the channel by erecting two barrier nets perpendicular to shore. Data sondes (YSI 6920) were placed in bays 2, 3 and 4 to collected turbidity, temperature and dissolved oxygen (DO) data. All fishes entering or leaving the system must pass through the cut. The north end of Blind Channel is naturally blocked; the eastern end is blocked by grated culverts (5 cm gaps), which prevent large fish passage and then terminates in an earthen embankment. , control bays; , experimental bays during a hypoxia manipulation experiment.
© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2015, 86, 288–303
292
K . J . H E D G E S A N D M . V. A B R A H A M S
Fishes were able to enter and leave Blind Channel prior to trap net placement in each year and during severe storms when water levels rose above the trap net depth because of the influence of Lake Manitoba seiching on Delta Marsh. Lake Manitoba is a large (surface area = 4617 km2 ), relatively shallow (mean depth = 4⋅5 m) lake with a long north-south fetch (c. 200 km) and experiences considerable seiching during high-wind events. During these seiches, high water velocities occurred in The Cut, because of the in or outflow of water, that prevented the trap nets from being deployed or fished. Predators captured trying to leave Blind Channel in May were also tagged and returned to the channel to maintain a large population of tagged individuals within Blind Channel; 65 fishes were tagged leaving Blind Channel in May 2005. Predators captured leaving Blind Channel after 31 May were moved past the trap nets and released outside of Blind Channel because potentially lethal water temperatures and levels of hypoxia can develop during July (Hedges, 2007). Throughout both summers, gillnets (live sampling; 15⋅25 m × 1 m gangs, 5⋅08 cm stretched mesh, set for 1⋅5 h) were used to capture additional predators within Blind Channel to increase the proportion of PIT-tagged individuals in the predator populations. Five artificial bays (maximum depth = 1⋅00 m, minimum depth = 0⋅75 m) were constructed within Blind Channel during May and June of 2005 [Fig. 1(a)]. Each bay was created by extending two 10 m long barrier nets (heavy weight minnow netting; 2 mm mesh-size) perpendicular from shore; the nets were placed 10 m apart creating a 10 m × 10 m bay [Fig. 1(b)]. The barrier nets extended from the substratum to the water surface and were held in place by wooden posts. Neither P. promelas nor predators were able to pass through the barrier nets; all fishes were forced to enter the bays through the offshore opening. The substratum in each of the bays was a uniform mud bottom. There was very little aquatic vegetation (0⋅05 (1, 121) 27⋅15
Interaction P
F
0⋅05
119 (1, 441) 0⋅22 >0⋅05 (1, 441) 127⋅71
