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

Tracking the wakes of prey

Image: http://www.flower-horn.de

I received a comment yesterday asking about the mechanosensory lateral line. So I thought I would write a post today about one of the really cool behaviours that is mediated by this sensory system. I know this is not a well known sensory system but it is a very important one for fish and aquatic amphibians. The lateral line is a hair cell based sensory system that detects the water movement surrounding the fish. Normally this is to within one or two body lengths. However a recent study has shown that in the European catfish (see picture above) it is involved in the detection of wakes left by potential prey.

Here is the abstract:

Swimming fish leave wakes containing hydrodynamic and chemical traces. These traces mark their swim paths and could guide predators. We now show that nocturnal European catfish (Silurus glanis) locate a piscine prey (guppy, Poecilia reticulata) by accurately tracking its three-dimensional swim path before an attack in the absence of visible light. Wakes that were up to 10 s old were followed over distances up to 55 prey-body lengths in our setup. These results demonstrate that prey wakes remain sufficiently identifiable to guide predators, and to extend considerably the area in which prey is detectable. Moreover, wakes elicit rear attacks, which may be more difficult to detect by prey. Wake tracking may be a common strategy among aquatic predators.

In a later paper the lateral line was ablated and once this was done the fish could no longer track the prey. Thus the lateral line was essential in the tracking behaviour. Although this would have limited functional value in coastal water ecosystems where there is a large degree of water motion, and thus background noise breaking up the wake, this may become more important in still water environments. The catfish in this study is obviously a case in point inhabiting slow flowing or still waters such as lakes.

Image: http://oceanexplorer.noaa.gov

This predation strategy may be even more important in deep sea fish where below 1000m, when vision becomes useless, the lateral line is likely to be the dominant sensory system. In such a hydrodynamically 'noiseless' environment many deep sea fish would be capable of detecting the wakes of prey for up to three minutes since they had passed by. Although this next idea is purely conjecture this may also explain why so many deep sea fishes have rat tails (see pic above). Such a tail would likely result in a much reduced wake!

Hanke W, Brucker C, & Bleckmann H (2000). The ageing of the low-frequency water disturbances caused by swimming goldfish and its possible relevance to prey detection. Journal of Experimental Biology 203(7), 1193-1200
Pohlmann K, Grasso FW, & Breithaupt T (2001). Tracking wakes: the nocturnal predatory strategy of piscivorous catfish. Proceedings of the National Academy of Sciences of the United States of America, 98 (13), 7371-4 PMID: 11390962

Pohlmann K, Atema J, & Breithaupt T (2004). The importance of the lateral line in nocturnal predation of piscivorous catfish. The Journal of Experimental Biology 207, 2971-2978