Fish Schooled: turbidity

Showing posts with label turbidity. Show all posts

Tuesday, November 23, 2010

Postdoctoral Research Begins - Finding Sites

In the last week I have begun to work on my new postdoctoral research that focuses on how elevated turbidity levels can impact on exotic and native fish populations. This work forms part of a grant that I was awarded earlier in the year, and will continue to fund me for the next few years.

I arrived back in New Zealand this last Saturday and have been looking at sites to initiate field sampling. I will be working in the Waikato River in New Zealand. The Waikato River is the longest river in New Zealand running for 425 km in the North Island. It has a strong turbidity gradient that I aim to sample fish biodiversity along. Some site searching managed to turn up some turbid sites and some clear water sites. Here are some pics.

Whangaparino River Turbid Site

Waikato River Turbid Site

Waikato River Clear Site

Koi Carp Feeding In Turbid Waikato Tributary

Tuesday, May 4, 2010

Turbidity: A Safe Haven For Prey?

Turbidity is well known for its negative impact on fish feeding ability. As turbidity increases the visual range of the predator decreases, which leads to a reduction in the area searched, and therefore a lowered encounter rate. But what is one fish's garbage maybe another's treasure. The authors of this paper investigated whether turbidity can also provide a cover, or safe haven, for prey fish making them harder to detect.

In the experiment two predators were used, including the yellow perch (Perca flavescens), and the black bullhead (Ameiurus melas). These predators differ significantly in their sensory modes of feeding with the yellow perch relying on vision, and the black bullhead relying on chemosenses. Their unfortunate prey for this experiment were fathead minnows (Pimephales promelas). Fathead minnows were placed into a three chambered aquarium, and in order to feed from artificial feeders, they had to make a choice between two chambers. The authors manipulated the choice chambers with clear or turbid water, and with the predators. The variable measured was either the number of minnows feeding in the treatment chamber, or the number of mortalities.

The predators were active in the apparatus and mortalities did occur. There was no effect of increased turbidity on the mortality of fathead minnow when yellow perch were present. However, higher mortality was observed for the black bullhead, the non-visual predator, when in turbid water. Of course this makes sense since, within turbid water, the non-visual hunter would have the dice loaded in its favour against a prey that is strictly visual. Unfortunately, mortality events were not high enough to show any statistical significance. But the trend was there, and if a greater sample was taken Im sure this would have been found.

The fathead minnow showed significant avoidance behaviour of the clear water chamber when it contained a predator. When no predator was present, in either chambers, a strong preference for the turbid habitat was observed. When a predator was placed into the turbid habitat, fathead minnows preference for turbid water was suppressed, but they nevertheless still showed a slight preference for the turbid water over clear water. This despite the fact that a predator was only present in that chamber!

Thus, fathead minnows still maintained a preference for turbid water even when their was a real risk of been consumed by a predator. There are two possible explanations for this behaviour. The first is that increased turbidity makes it difficult for the prey to know that a predator is in that chamber. However, this is unlikely to be the case. Both predator and prey were constrained to such small areas it would have been highly unlikely the prey did not detect the presence of the predator. Also, their were many instances where the predator chased the prey. Fathead minnows are also known to possess chemical alarm signals, and therefore this also should have alerted other conspecifics of the predators presence. It would seem likely then that the second possibility is true: that increased turbidity makes it so hard for the predator to detect the prey that it becomes worthwhile for the prey to feed under the cover of turbidity where it perceives less threat.

This has rather large ecological implications. Turbidity has, thus far, been considered as a negative abiotic factor for fish. However, this experiment shows that in certain circumstances turbidity could be beneficial for many prey species. The ecological impacts of these safe havens on prey fish communities still needs to be investigated. What I would be interested in is if the benefit of having a reduced predation risk is outweighed, or eliminated, by a reduction in the ability of the prey to also find food. Im sure certain trade offs would exist with the perceived predation threat, and the fish's own ability to find food, driving a fish to choose certain habitats that would have large ramifications on their population dynamics.

Chiu, S., & Abrahams, M. (2010). Effects of turbidity and risk of predation on habitat selection decisions by Fathead Minnow (Pimephales promelas) Environmental Biology of Fishes, 87, 309-316 : 10.1007/s10641-010-9599-8

Tuesday, April 13, 2010

Sensory Plasticity in Changing Environments

Can environmental conditions during early development shape individuals phenotypes so they become more adaptive to the conditions they are likely to encounter later in life? Such phenotypic plasticity could provide organisms with the potential to respond effectively to environmental change. One area where such plasticity would be important would be in an animals sensory capabilities. Animals extract information from the environment using a number of sensory systems, and this information guides the animal as it locates food and mates, while also avoiding predators. Thus, the ability to compensate for a deficit in one sense, by increasing the acuity in another, is likely to be of critical importance within sensory disparate habitats. This is what is named the 'compensatory plasticity hypotheses'.

In this experiment the authors raised newly born guppies at low and high light intensities, and then tested their ability to locate food using both chemosensory and visual cues. Guppies, Poecilia reticulata, reared at high light intensities responded best to visual cues, while those guppies reared under low light intensity responded the strongest to olfactory cues. These results confirm the 'compensatory plasticity hypothesis' and shows that these fish have remarkable sensory plasticity. They are able to switch from vision to olfaction in environments where light is limiting.

How this switch occurs is unknown. It may be due to increased attention to sensory signals through learning, neurophysiological changes in the hard wiring of the sensory circuits, or structural changes in the morphology of the sensory unit (i.e olfactory epithelium such as increased lamellae folding) or in the brain itself. In rats that have undergone early visual deprivation you find a reduction in the grey matter within the visual cortex, and an increase in neuron density in the auditory cortex.

The ability to switch sensory modes is likely to be of upmost importance in aquatic ecosystems, which are among the most heavily impacted in the world due to human induced changes. These changes can often result in decreased visibility due to increases in turbidity, or change the olfactory environment through the release of pollutants. How fish species can respond to these changes through sensory plasticity is still largely unknown. Research on larvae of the marine striped trumpeter, Latris lineata, showed that individuals reared in clear water had reduced foraging efficiencies in turbid water. In contrast, larvae reared in turbid water were able to maintain their foraging capability. This suggests that fish are capable of doing so, but to what degree is an area that definitely requires further investigation.

Chapman, B., Morrell, L., Tosh, C., & Krause, J. (2010). Behavioural consequences of sensory plasticity in guppies Proceedings of the Royal Society B: Biological Sciences, 277 (1686), 1395-1401 DOI: 10.1098/rspb.2009.2055

Thursday, April 8, 2010

Do fish have six second memories?

The ability to find food is one of the most important behaviours an animal can undertake, and one of the best advantages an animal can have is to remember where food can commonly be found. Laboratory studies have shown that fish are able to use learning-based strategies to locate food with most studies focusing on the aquatic equivalent of the lab rat - the goldfish. This fish has been shown to typically use visual landmarks to remember a food source within laboratory arenas. In this study the authors used radio tagged common carp to investigate the ability of free ranging fish to undertake similar behaviours. Carp are very closely related to goldfish and share similar feeding habits and sensory cues while feeding. This was undertaken in a highly turbid lake with a water clarity of <>

This study showed that carp could quickly learn and find the location of a food reward in the natural environment. It typically took the carp six days to learn and remember where the food reward was. This matches that found in laboratory trials. Carp were highly nocturnal in their feeding habits and would leave their home range during the night consistently visiting the food reward once the location was known.

What impresses me the most is that they did this at night, in highly turbid conditions, and in a featureless environment. This precludes the use of visual landmarks which is what is typically used by fish in laboratory arenas. It is likely that carp were using olfactory cues to locate the food source rather than vision since carp are known to have an extremely well developed sense of smell. The authors also suggest that the speed with which carp learned to find the food may have been facilitated by social learning as carp and goldfish both learn from shoaling conspecifics in laboratory trials.

This study has some big implications. Firstly, carp are known to undergo extensive movements of 100 km or more. They will often enter areas to spawn that are unstable but predator free, and the ability to remember such locations would significantly improve the survival of their offspring. The ability to remember important sites would therefore be highly adaptive. Another implication is to use this knowledge to reduce the numbers of this fish, which is one of the most invasive species worldwide. By setting up feeding stations you may be able to attract many carp to a specific location within a short time frame and then undertake selective removal.

Bajer, P., Lim, H., Travaline, M., Miller, B., & Sorensen, P. (2010). Cognitive aspects of food searching behavior in free-ranging wild Common Carp Environmental Biology of Fishes DOI: 10.1007/s10641-010-9643-8

Tuesday, April 6, 2010

The Top Down Effect Of Turbidity Within Marine Ecosystems

Most studies on turbidity investigate freshwater ecosystems and few studies have focused on the impacts of turbidity on marine ecosystems. Eianne et al. (1999) showed that invertebrate planktivores (jellyfish) replaced planktivorous fish within Norwegian turbid fiords. This was likely to be because increased turbidity levels reduced the possibility of foraging in visually oriented fish, while tactile feeding in jellyfish allowed them to continue to feed under light-limited conditions. A reduction in fish populations was unlikely to be a result of a reduction in plankton abundance. In fiords where fish populations were reduced zooplankton were more numerous and grew to larger sizes. This confirms modeling and experimental studies which show that turbidity is likely to have a top down effect within marine ecosystems by reducing the ability of fish to feed visually and this in turn leads to changes in prey composition.

Eiane, K., Aksnes, D.L., Bagoien, E., & Kaartvedt, S. (1999). Fish or jellies - a question of visibility? Limnology and Oceangraphy, 44 (5), 1352-1357