An article in the Vancouver Sun today discusses how scientists from the University of Calgary have found that Long Dase Minnow in Southern Alberta are feminising. In fact 90% of fish in two of the provinces rivers are female. This is attributed to man-made contaminants entering the rivers and includes chemicals such as pesticides, flame retardants, steroids, growth hormones and birth control pills. Nearly all locations studied showed that males had elevated levels of a protein that is typically only found in the girls. The two main culprits are agriculture and waste water that has not been treated sufficiently. Sigh. 

Cyprinid fishes (carps) show fright, or escape behaviour, when smelling alarm signals produced by conspecifics. These chemical alarm signals are found within special club cells and are released when these cells are ruptured. In nature, fish possessing the alarm club cells may become aware of a predator as it becomes labelled with the alarm pheromones when ingesting prey. In cyprinids it has been found that higher club cell densities exist in regions where there is a high abundance of predators. Higher club cell densities means the fish is able to produce a greater volume of alarm pheromones. Important when a predator could be lurking around any corner. The authors in this study hypothesized that club cell density is plastic in regards to predator presence.

For the study they raised crucian carp individually and in groups of four. For both rearing types, fish were exposed to the skin extracts of either conspecifics (alarm signals) or brown trout (without club cells), and provided food in either low or high food rations. Interestingly, they did not find an association with club cell density and the presence of an alarm pheromone or predator chemical cue. However, what they did find was that club cell density increased when fish were living in close quarters with conspecifics (ie group of four fish).

The data from this study suggests that group-raised fish are more chemically on guard than those raised singly. The data shows that club cell density can show a ten-fold increase through an increased feeding regime combined with group rearing. Club cell density is plastic and is likely to be controlled through internal physiological regulators such as blood androgen levels, but also through external regulators such as nutritional status and growth promoting factors via chemical sensing. But in the end it just makes sense to not spend growth energy on lots of alarm substances when there will be no one else around to heed your warning.

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