Wednesday, July 28, 2010

Fisheries Collapse: When Predator Becomes Prey

In marine ecosystems overfishing of top predators has led to major changes in ecosystem properties at the most basic level. This is likely to be because a change in the food web directly changes the feedback mechanisms that are inherent within any ecosystem. In marine ecosystems a typical pattern occurs after overfishing, which includes a low abundance of predatory fish and a high abundance of small, pelagic, forage fish. These small fish are themselves predators of the eggs and larvae of marine organisms including the large predatory fish. Thus it is hypothesized that by controlling the numbers of these small forage fish the large predators are ensuring their recruitment for the next generation. 

In this study they analysed a 44 year long time series of cod recruitment and herring abundance in the North Sea. In this ecosystem, atlantic cod are the top predators feeding on the small and pelagic herring. Herring have been shown to feed extensively on cod eggs, and therefore may negatively effect cod recruitment when they are found in high numbers. This study showed that abundance of herring in the North Sea was negatively correlated to the recruitment of cod. Other studies have found that this may be due to a reduction in prey for both groups of fish as was found in the Baltic Sea where overall zooplankton biomass was reduced. But this study controlled for larval food and still found a negative relationship. Thus, it would seem that a reduction in atlantic cod leads to an increase in herring. This increase in herring then reduces the recruitment of atlantic cod as their larvae and eggs are consumed by the herring. 

Catastrophic shifts in ecosystems has been referred to as 'ecosystem hysteresis'. Ecosystem hysteresis occurs when changes in the state of an ecosystem are path dependent. Ecosystem hysteresis is generated by various forms of reinforcement that keep the ecosystem in a given state. Often there are two ecosystem states that occur under different critical conditions. This predator-prey reversal may keep the North Sea in a herring dominated state. It begs the question of whether it is important to reduce herring numbers in order to restore cod populations. Something that if true will bring a smile to any fisherman's face.
Fauchald, P. (2010). Predator–prey reversal: A possible mechanism for ecosystem hysteresis in the North Sea? Ecology, 91 (8), 2191-2197 DOI: 10.1890/09-1500.1


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