The world’s oceans are becoming over-crowded with sardines and small fish. Within the last 100 years, the population of small fish has more than doubled. The rise of sardines and small fish has been caused by a major decline in large predator fish due to the overfishing of sharks, tuna, cod, and swordfish. Recently, the Monterey Bay Aquarium has even added sardines to their Seafood Watch green list. Indicating, that sardines have made a comeback as a sustainable food source.
So, what type of impact does this have on our oceans? Sardines are known to feed on the free floating organisms called zooplankton. Zooplankton in return feed on plant plankton which is the organism usually at the bottom of the oceans food chain. As the population of forage fish such as the sardine, anchovy, and capelin are on the rise. The ocean has seen a dramatic decrease in zooplankton populations. Without zooplankton, the population of plant plankton is rising sharply and may become out of control. Thus, resulting in large blooms of green algae.
If a green algae bloom becomes large enough, it can choke the sea life in the ocean by reducing the water’s dissolved oxygen concentration, thereby knocking the ocean’s natural predator and prey relationship out of balance. During a bloom, a liter of sea water may contain millions of algae. Which could be harmful to the animals that feed on that particular algae, and the animals that prey on the species that eat that specific algae. A harmful algae bloom can have adverse effects to many species of marine mammals. Some may include specific toxicity-induced reductions in the development of immunological, neurological, and reproductive capacities within species.
The negative effects of having too much plant plankton in the ocean creating algae has effected many species in the past. In Spring 2004, the death of 107 Bottlenose Dolphins occurred along the Florida panhandle. It was found that the dolphins ingested harmful algae with high levels of brevetoxin. The endangered North Atlantic Right Whale, has been exposed to neurotoxins by consuming and preying on highly contaminated zooplankton. Even the endangered Loggerhead Sea Turtle has ingested contaminated prey, leading to clinical signs of increased muscle weakness causing the turtle to wash ashore exhausted or dead.
With the number of small fish having doubled over the past 100 years and the effects of overfishing. There are just not enough natural hunters in the ocean to combat the ecological consequences. In fact, within the past 120 years the numbers of natural hunters have decreased by two thirds, due mostly to human fishing. By removing the larger natural hunters from the ocean, small forage fish are thriving and putting the ocean’s overall health at risk.
Photo credit: nlm.nih.gov