Hypoxia and habitat loss

Habitat loss happens as a result of many different human activities: deforestation, for instance, or urban development. When I think about habitat loss, the habitat that I picture disappearing is usually terrestrial in nature; a forest, or a meadow or mountaintop. But aquatic habitats are easily degraded, too, with deleterious effects on their inhabitants.

Though most fish and other aquatic organisms don’t breathe air like humans do, they still depend on oxygen – in the case of fish, they absorb oxygen from water as it passes across their gills. Underwater areas low in dissolved oxygen, or ‘hypoxic zones,’ (as well as ‘anoxic zones,’ places completely free of measureable oxygen) represent habitat loss for fish.

Hypoxic zones are usually a seasonal phenomenon, caused by a chain of events that typically starts with excess nutrients entering a body of water, which leads to algal blooms that ultimately deplete underwater oxygen as they begin to die and decompose. Hypoxia can be extreme enough to cause fish-kills, massive die-offs of hundreds or even thousands of fish.

Hypoxic zones can affect fish in other ways, too, as new research recently published by a team of scientists working on Lake Erie highlights. The researchers working in Lake Erie found that the edges of hypoxic zones are more dynamic than previously thought, with some intermittent periods of normal oxygen levels; they also found “higher fish densities near the edges of hypoxia,” presumably because fish and other mobile aquatic organisms can be displaced by hypoxic zones as they seek areas where oxygen is still available.

When fish are concentrated into a small area, they may be easier to catch – and indeed, the scientists found that, depending on where trawls are taken or nets are set, “catches may actually increase in areas affected by hypoxia.” Many fish population estimates are based on how many fish commercial fishers land, so if hypoxic conditions are allowing fishers to catch more fish than they normally would, the very set of circumstances that fish are trying to escape could lead to an overestimation of their numbers. Such an overestimation could put the population at risk of overfishing if managers set higher quotas than the true number of fish can support.

Habitat loss and degradation threaten many species, both on land and in the water. By studying how fish respond to hypoxic events, hopefully we can reduce some of the risk that they face.

An algal bloom in Lake Erie, in early October 2011. Decomposing algae during and after a large bloom can result in hypoxic zones.

(Image by NASA via Flickr/Creative Commons license)