EFFECTS OF TURBIDITY ON THE AEROBIC PHYSIOLOGY OF JUVENILE SNAPPER (PAGRUS AURATUS)

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Impacts of turbidity on fish visual performance

Direct impacts of turbidity on fish visual performance is perhaps rather obvious, where light attenuation by suspended matter in the water column effectively reduces the visual range of fish (Utne-Palm, 2002). The visibility of a prey item requires the predator to detect a difference in contrast between the background and the prey item (Utne-Palm, 2002). Therefore the ability of fish to detect prey can be limited by the optical environment. As such, suspended particles in the water column scatter and absorb light and decrease contrast and light penetration, thereby interfering with the visual acuity of fish (Utne-Palm, 2002). The visual range of fish has been measured in various ways, but in most cases, visual range can be determined by fish behaviour. A common measure is reactive distance, which is the distance at which the test subject may react to a visual stimulus in its environment (Utne‐Palm, 1999). This distance is commonly influenced by light levels, i.e. turbidity (Meager et al., 2005, Utne‐Palm, 1999) and is expressed in their behaviour as outlined below.
Indirect impacts of turbidity on fish visual performance include changes in ability to detect and capture prey items with observed reductions in attack rate and feeding success. Many studies demonstrate detrimental effects of suspended sediment on the foraging rate of fish, including increases in reaction distance (Utne‐Palm, 1999), reductions in number of feed items consumed (Hasenbein et al., 2013, Wenger et al., 2012) and increased time taken to find food (Wenger et al., 2012). For example, turbidity levels of just 4 NTU decreased foraging rates, and average attack success was reduced by up to 56 % in planktivorous coral reef fish (Johansen and Jones, 2013). Similar trends are seen in freshwater fish species where suspended sediments were found to have a negative impact on feeding behaviour (feeding rate and reaction distance) of both turbidity tolerant and intolerant freshwater fish species (review by Chapman et al., 2014). For example, coho salmon demonstrated a significant decrease in reaction distance to prey, capture success and the percentage of prey ingested in turbid water conditions (30 and 60 NTU) (Berg and Northcote, 1985). Nevertheless, not all fish respond negatively to suspended sediment. Many fish thrive in turbid environments as low levels of suspended sediment can enhance visual contrast of prey items, effectively increasing overall feeding rates (Morrison et al., 2009) as well as reducing risk of predation for some species resulting in increased foraging rates as seen in juvenile chinook salmon (Gregory and Northcote, 1993).
Indirect impacts of changes in feeding ability are perhaps more difficult to study than direct impacts. However, it is commonly thought that reductions in prey acquisition through a decrease in visual acuity can affect the growth, survival and fitness of fish. For example, coral reef fish exhibited prolonged larval development under turbid conditions (Wenger et al., 2014) and increased suspended sediment reduced the growth and condition of planktivourous damselfish at relatively low levels (Wenger et al., 2012). Unfortunately, there is limited literature that explores the direct cause of growth deficits in fish exposed to turbidity.

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AIMS AND HYPOTHESES

The effects of turbidity on fished species is a growing field of research. Recent research efforts describe effects on coral reef fish species and freshwater species, but the research by Lowe et al. (2015) is the only study to address the effects of turbidity in a New Zealand marine fish of commercial and recreational importance. Due to increased levels of suspended sediment in the New Zealand marine environment, (and predictions for this to deteriorate (Morrison et al., 2009, Willis et al., 2007)), surprisingly little is known regarding the effects on ecologically and commercially important demersal fish species. It is understood that suspended sediments affect the gill structure of fish but it has only been speculated that changes in gill structure affect the efficiency of oxygen transfer from water to blood across the gills, leading to respiratory stress and reductions in performance (Hess et al., 2015, Lowe et al., 2015, Wong et al., 2013). This has yet to be tested directly. Although decreased oxygen consumption has been logically assumed to be associated with poor growth performance in a number of fish species, it is possible that these growth deficits observed across multiple studies may also be at least partly, explained by a reduction in visual feeding performance with increasing turbidity. Clearly, further investigations are required.

1 INTRODUCTION
1.1 Environmental change
1.2 What is turbidity? .
1.3 Turbidity worldwide and in New Zealand
1.4 Biological effects of turbidity
1.5 General effects of turbidity on fish
1.6 Aims and hypotheses
1.7 Study species
2 EFFECTS OF TURBIDITY ON THE AEROBIC PHYSIOLOGY OF JUVENILE SNAPPER (PAGRUS AURATUS)
2.1 Introduction
2.2 Materials and methods
2.3 Results
2.4 Discussion
3 EFFECTS OF TURBIDITY ON THE FEEDING BEHAVIOUR OF JUVENILE SNAPPER (PAGRUS AURATUS)
3.1 Introduction
3.2 Methods
3.3 Results
3.4 Discussion
4 GENERAL DISCUSSION
4.1 Impact of turbidity on gill structural change
4.2 Impact of turbidity on oxygen consumption
4.3 Impact of turbidity on fish growth
4.3.1 What are the potential factors that contribute to poor growth in turbid waters?
4.4 The future of P. auratus in New Zealand
4.5 Conclusion
5 REFERENCES