Monday, April 26, 2010

ResearchBlogCast: Fewer Big Fish In The Sea

I am quite honoured to have the ResearchBlogCast #4 pick an article that I have covered here on Fish Schooled.

For the post follow this link.
For the podcast follow this link.


Tuesday, April 20, 2010

Fish of the Day: Flying Fish

Flying fish are a group of marine species that belong to the family Exocoetidae (order Beloniformes, Class Actinopterygii). There are approximately 64 species from seven to nine genera. Flying fish are found in all the worlds major oceans, particularly the warmer subtropical and tropical regions. Of course their most noticeable feature are their oversized pectoral fins, which they spread out like wings as they glide through the air. The enlarged lower lobe of their tail acts like an outboard motor, the speedy sideways motion of the tail allows the fish to gain height from the surface of the water. This morphology allows the fish to 'fly' out of water for extended periods of time with glides usually lasting for 50m, but upto 100m . This proves to be an effective predator avoidance strategy. Below is an example of a 'flying' flying fish.


Thursday, April 15, 2010

Prey populations explode as predators get smaller.

When top predators are removed from ecosystems their prey and/or competitors increase due to decreased predation and competitive release. However, can changes in behaviour, or body size, of the predators also cause this effect? If true, this would be most evident in heavily exploited marine ecosystems where size selective fishing has lead to rapid reductions in the size of top predators. The authors in this study used a 38 year time series to examine the relationship between predator size and prey biomass within such an ecosystem, the Western Scotian Shelf.

Their analysis showed that since the mid 1990's predator biomass has remained relatively constant. If one species of predatory fish was overfished it tended to be replaced by another species of predatory fish. Yet, despite no changes in predator biomass, prey biomass has increased by a huge 300%. Statistically, what matched this increase most closely was a decrease in the size and body mass of fish at higher trophic levels. The mean lengths of benthivores decreased by 21%, piscivores by 8%, and planktivores by 16%. When translated into body mass large benthivores decreased by 59%, medium benthivores by 48%, piscivores by 45%, and planktivores by 34%. For example, a haddock in the 1970's weighed, on average, 2 kg, but now weighs approximately 0.8 kg.

The empirical results from this study support the hypothesis that reduction of predatory fish size is the dominant factor in the underlying explosion of prey biomass. Why would this occur? Larger predators have been shown to be more successful at capturing prey due to their faster swimming speeds, and greater visual acuity. Thus, larger predators can consume more prey per unit time than smaller predators, and as a result larger predators can regulate their prey populations more effectively. As predators get smaller, a reduction in predation pressure results, leading to large increases in prey populations such as the pattern observed in this study.

Shackell, N., Frank, K., Fisher, J., Petrie, B., & Leggett, W. (2009). Decline in top predator body size and changing climate alter trophic structure in an oceanic ecosystem Proceedings of the Royal Society B: Biological Sciences, 277 (1686), 1353-1360 DOI: 10.1098/rspb.2009.1020

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

Sunday, April 11, 2010

How Swimming Can Change The Way You Forage

This study really excites me as it shows how functional morphology and swimming mode can be reflected in the ecology and evolution of animals. The authors in this study used digital particle image velocimetry (DPIV) to measure the wakes produced by swimming jellyfish. DPIV is a technique where you place neutrally buoyant beads into the water that fluoresce under light. You then shine a plane of light through the water and measure the movement of those beads, which is typically caused by some biological organism such as a moving or feeding animal. For example here is a DPIV of fish suction feeding with shrimp as prey.

So back to the science. In this study two distinct types of wake patterns were formed behind the jellyfish. All jellyfish form vortex rings in their wakes. As the wake vortices grow during the formation process, which occurs during the contraction phase of swimming, the vortices approach a maximal limit for their size. In some jellyfish, within this study, this limit is not met and so their wake consists of a single vortex ring in their wake. In others this limit is passed, and as such can no longer incorporate or entrain the additional fluid, and thus a trailing jet is produced behind the vortex ring.

What they found was that those species that formed vortex rings had less effective propulsion compared to those that produced a trailing jet. However, producing vortex rings for locomotion was more energetically efficient than producing a trailing jet. In short, you either move quickly but for short periods, or you move more slowly but continuously. The cool part was that they found that those species that forage by continuously cruising are the species that produce vortex rings without trailing jets (energy efficient mode), whereas those species that ambush their prey produced trailing jets (speed mode). This of course makes perfect functional sense. This is a trade off and there was no species that could move both quickly and for long periods.

Dabiri, J., Colin, S., Katija, K., & Costello, J. (2010). A wake-based correlate of swimming performance and foraging behavior in seven co-occurring jellyfish species Journal of Experimental Biology, 213 (8), 1217-1225 DOI: 10.1242/jeb.034660

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