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| This page provides a brief background on turbidity and fish behaviour in marine systems, together with a description of my research at the University of Bergen in Norway on the effects of turbidity on juvenile Atlantic cod behaviour,
Turbidity is the scattering and absorption of light within water, caused mainly by suspended sediment, dissolved organic matter and plankton. It has also been termed 'water clarity'. Levels of turbidity can fluctuate with natural events such as tidal flux, rainfall, seasonal algal blooms, ice melting and meteorological forcing. Human activities leading to the suspension of sediments or eutrophication can also increase turbidity levels. Changes in turbidity can have direct and indirect effects on fish. At extremely high levels, turbidity can directly affect fish growth and survival, for example, by interfering with gill function or the quality of substrata for egg laying (see review by Bash et al. 2001). By limiting the photic zone turbidity can also reduce habitat quality, for example, by reducing macrophyte cover from predators (Goldsborough and Kemp 1998; Berger et al. 1999). Turbidity also limits fish vision, which can interfere with social behaviour (Berg and Northcote 1985), foraging (Gregory and Northcote 1993; Vogel and Beauchamp 1999) and predator avoidance (Miner and Stein 1996; Meager et al. 2006). This can have varying effects on fish growth and survival, depending on a range factors such as ambient light levels and depth; relative visual sensitivities of predators and prey, and non-visual sensory abilities (follow this link for references). Although the effects of turbidity on freshwater fishes are well known (see reviews by Henley et al. 2000; Bash et al. 2001), little is known of the effects on marine fishes. Turbidity levels in marine systems are generally not as extreme as fresh water hence, behavioural effects are considered to be more important than physiological effects (e.g. Utne-Palm 1999). The impact of changes in turbidity on fishes in marine systems is likely to depend on background turbidity levels. In systems with high levels of algal productivity, high flow and terrestrial inputs (e.g. estuaries), turbidity levels are naturally high. In such areas, turbidity can have a positive effect in reducing predation from visual predators (e.g. Johnson and Hines 1999; Meager 2003). Turbidity can also have a positive effect in areas with low background turbidity (e.g. fjords and oceanic areas), where small increases can enhance feeding of planktivores and larval fish by increasing prey contrast (Boehlert and Morgan 1985; Utne 1997). A similar visual effect is evident when we view a nearby object in thick fog. However, with larger and more conspicuous visual targets, for example, fish prey, conspecifics or predators, even small increases in turbidity rapidly limit visual range. This webpage details my experiments on the influence of turbidity on the behaviour of fish at the University of Bergen, funded by the EU Ethofish project.
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Suspension of sediments and eutrophication are increasing the turbidity of coastal waters in the north Atlantic (Frid et al. 2003) and Baltic Oceans (Bonsdorff et al. 1997). This has led to concerns for the effects of increasing turbidity on commercially exploited fish stocks, particularly for species that are found in heavily affected areas (coastal and estuarine areas). One such species is Atlantic cod (Gadus morhua), which use shallow (< 20 m) coastal habitats as nurseries (Godø et al. 1989; Grant and Brown 1998). Juvenile cod forage on highly mobile and evasive prey items (e.g. crustaceans and fish, Hawkins et al. 1985; Grant and Brown 1998) and have high predation pressure from avian, teleost and mammalian predators, including larger conspecifics (Salvanes and Nordeide, 1993; see also review in Svåsand et al. 2000). As vision has an important role in both foraging and predator avoidance in juvenile cod (e.g. Brawn 1969; Chinarina Troshicheva 1975), increased turbidity may lead to reduced growth and survival, or migration to other habitats. The following text covers experiments conducted at the University of Bergen between 2002 and 2005, looking at the effect of turbidity on foraging, predator avoidance, spontaneous activity and habitat choice of juvenile Atlantic cod. In our experiments we used turbidities ranging from clear oceanic to highly turbid estuaries (Table 1) and measured turbidity by beam attenuation in a spectrophotometer (more details of turbidity measures here).
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Juvenile cod in turbid water
Juvenile cod and mysid prey in turbid water
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We examined the effect of turbidity on prey localisation by juvenile cod. Juvenile cod have well developed vision and chemoreception, but are believed to preferentially use vision (Brawn 1969: Chinarina and Troshicheva 1975). Our experiments aimed to examine the influence of turbidity on the reactive distance and search time of juvenile cod, and to determine if turbidity affects their foraging rates. We also tested the general hypothesis that cod mainly use visual cues to locate prey in clear water (in photopic conditions) and chemoreception in turbid water. We separated the effects of chemical and visual cues by locating prey (mysids) in cylinders with clear and opaque cylinders with and without holes (prey cylinders). We also examined predation rates on free ranging mysids. Our results showed that turbidity did not affect the ability of juvenile cod to forage on free-ranging mysids over a two-hour time period. Reactive distance to chemical cues was unaffected by turbidity. However, cod used slightly longer times to locate prey in turbid water. Visual range did not decrease as rapidly with turbidity as was expected and vision is likely to be more important in turbid water than was previously thought. Our results suggested that cod use distance chemoreception to reduce their search volume and vision when they are closer. Collaborators: Dr Anne Christine Utne Palm, Turid Solbakken, Tina Oen Publication: Meager, J.J.; Solbakken, T.; Utne-Palm, A.C. and Oen, T. (2005). Effects of turbidity on the reaction distance, search time and foraging success of juvenile Atlantic cod (Gadus morhua). Canadian Journal of Fisheries and Aquatic Sciences 62, 1978-1984.
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Table 1: turbidity levels used in our research, and corresponding natural occurrences. Where unavailable, beam attenuations (c) are estimated from the downwelling attenuation coefficient (k) |
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PREDATOR EVASION AND TURBIDITY Most fish species evade predators by using escape responses involving a high-energy burst of swimming. Visual escape responses require both perception of the predator and a decision to escape (Blaxter and Fuiman, 1990; Hemmi, 2005a), based on the level of perceived risk (Ydenberg and Dill, 1986). The time available for prey to detect predators and decide on an appropriate response is limited by predator attack speed. Similarly, turbidity may limit the time prey have to evade a predator, by reducing visual distance. The combined effects of predator attack speed and turbidity are therefore likely to be complex. We examined the effect of turbidity and predator attack speed on escape responses of juvenile cod in the laboratory. We triggered escape responses with a predator model that approached at two speeds and we measured escape timing, direction and locomotor performance. We also measured responsiveness and estimated the likelihood of fish escaping the ‘predator attack’ (putative escape success, PES). We used a model predator to trigger escape responses across a range of turbidity levels (see pictures to the right). The response was filmed from above using a high-speed camera (250 frames per second). Turbidity affected both PES and the type of escape response used by the fish, but these effects depended on predator speed. PES decreased with turbidity for the fast predator speed, due to decreased responsiveness and poorly timed escapes. Intermediate turbidity enhanced PES and responsiveness to the slow predator attack. Locomotor performance was reduced by turbidity, whereas predator model speed had the opposite effect. Our results suggested that both predator attacking speed and turbidity have important roles in determining the vulnerability of fish attacked by piscivorous predators. Collaborators: Dr Paolo Domenici, Dr Alex Shingles, Dr Anne Christine Utne Palm Publication: Meager, J.J.; Domenici, P.; Shingles, A.; Utne-Palm, A.C. Escape responses of juvenile cod Escape responses in juvenile Atlantic Cod (Gadus morhua L.): the effects of turbidity and predator speed. Journal of Experimental Biology 209:4174-4184.
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In situ predator model
Predator model (15 cm diameter), as seen from the fish's perspective at a distance of approx. 8 cm, in turbidities (c) ranging from 0.5, 3, 6 and 14/m (left-right from top left).
Putative escape success (PES, %) of juvenile cod to the fast (black columns) and slow predator attacks (grey columns), with increasing turbidity. n: sample sizes
Fish escaping the model in turbid water (footage slowed down and frame rate reduced). |
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TURBIDITY AND SPONTANEOUS ACTIVITY Activity levels determine energy intake and predation risk (Werner & Anholt 1993), and are an important component of bioenergetic models predicting the effects of abiotic factors on growth and performance (Kerr 1982; Sweka & Hartman 2001). Although variations in light intensity are known to influence fish activity (see reviews by Blaxter 1970, Reebs 2002), much less is known of the influence of other aspects of the optical environment (e.g. spectral composition and turbidity) on fish activity. The experiment outlined above (Meager et al. 2005) showed that turbidity (up to 28 beam attenuation m/1) had little effect on the foraging rate of juvenile cod. Although this was attributed to cod using chemoreception in conjunction with vision to locate prey, foraging rates may also be maintained by increased activity. Higher activity, however, is energetically costly and may offset benefits from increased foraging return. We examined the effects of turbidity on prey searching and spontaneous activity of juvenile Activity increased over time when prey odour was present and decreased when absent, but the effects of prey odour were similar across all turbidity levels. Position in the tank was unaffected by turbidity or prey odour. Reduced activity at intermediate turbidities is likely to offset longer prey-search times. At high turbidity (> 17 m/1), both longer prey-search times and higher activity indicate that increased energetic costs are likely. Collaborators: Dr Robert Batty, Scottish Association for Marine Science, Dunstaffnage Marine Laboratory Oban, Argyll, UK Publication: Meager, J. J. & Batty, R. S. (2007) Effects of turbidity on the spontaneous and prey-searching activity of juvenile Atlantic cod (Gadus morhua). Philosophical Transactions of the Royal Society B, 362 (1487): 2123-2130.
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Screen shot of MotionGrab (R.S. Batty, unpublished) the spatial actograph program used to measure fish activity. Activity in the central (blue) trigger zone has been recorded as the number of pixels changed (192, to the lower right). The turbidity was 10 m/1 when this frame was taken.
Effect of turbidity (beam attenuation) on activity of juvenile cod (mean fish activity h-1 ± 1 s.e.). Also shown are the results of the Waller-Duncan test (A, homogenous subset 1; B, homogenous subset 2) and sample size (below line). .
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The laboratory experiments outlined above have shown that juvenile cod are likely to have higher energetic costs associated with foraging on evasive prey and reduced ability to escape from predators in highly turbid water. We would predict from these results that juvenile cod avoid areas of high turbidity. In this study, we tested this prediction by comparing habitat preference of three turbidity levels ranging from clear oceanic to a highly turbid estuary. We also aimed to determine if the response to turbidity is similar between two major contributors to turbidity in seawater: clay and algae. In the first experiment, we compared three turbidity levels of kaolin ( 3, 8 and 21 beam attenuation m/1). In the second experiment, we looked at the effect of turbidity media (kaolin vs algae). Although cod preferred intermediate kaolin turbidity over clear water, most combinations of turbidity did not influence habitat preference. Similarly, algae did not influence habitat preference by cod. We suggest from the results of this, and other studies, that factors such as food availability and predation risk have more important effects on habitat preference by juvenile cod than differences in turbidity per se. Collaborators: Dr Anne Christine Utne Palm Publication: Meager, J. J. & Utne-Palm, A. C. (2008) Effect of turbidity on habitat preference of juvenile Atlantic cod, Gadus morhua. Environ. Bio.l Fish. 81: 149-155. |
'Shuttle Box' - tanks connected by a funnel (right) and filmed from above. Fish have a choice of which tank they prefer
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Further reading: Follow this link for a detailed reference list of turbidity research (including references cited on this page). EU Community Research and Development Information Service University of Bergen Magazine, 2005
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This webpage is maintained by Justin Meager • Email: justin.meager@gmail.com |
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