Algal blooms are a phenomenon in which algal populations in a marine area proliferate rapidly, creating a water-column shield that blocks sunlight and oxygen. These blooms are usually attributed to rises in nitrogen levels from human agriculture and industrial runoff, which fertilize the algae. But a study in the current issue of Ecological Applications shows that overfishing of top fish predators can also lead to algal blooms.
After reviewing a year’s worth of data on the Baltic Sea, the authors found that areas with high algal concentrations also had large populations of small fish and small populations of large fish. Specifically, predatory perch and pike shortages resulted in a 50 percent chance of that area experiencing an algal bloom, compared to only a 10 percent chance in normal areas.
Experimental studies in which the authors excluded top predators from marine areas corroborated the findings. Lead author Britas Klemens Eriksson of the University of Groningen in the Netherlands says this pattern suggests that the problem is a trophic one: when top predators are scarce, their smaller fish prey become more abundant and eat more invertebrates. These invertebrates feed primarily on algae, so when their numbers are reduced, algae can grow freely.
Eriksson says that the key to controlling algal blooms may not be through simply controlling nitrogen, but also by controlling fishing. Said Eriksson in the Nature article:
If we want to manage algal blooms effectively, we need to start by taking an ecosystem perspective … we have to restore depleted fish communities.
Eriksson, B., Ljunggren, L., Sandström, A., Johansson, G., Mattila, J., Rubach, A., Råberg, S., & Snickars, M. (2009). Declines in predatory fish promote bloom-forming macroalgae Ecological Applications, 19 (8), 1975-1988 DOI: 10.1890/08-0964.1