Effects of Shark Decline

Population depletion of apex predator shark species has indirect effects on commercially important shellfish species in the Northwest Atlantic Ocean.

Worldwide, overfishing has had profound effects on coastal ecosystems. Additionally, the top-down approach that humans have taken to commercial fishing (i.e. - targeting of apex predators first and then fishing down the food chain as stocks decrease) has negatively affected predator stocks in most regional seas of the world. However, while researchers may know the effect that overfishing has had on specific species, how this may influence the rest of the ecosystem is relatively unpredictable. It is hypothesized that the loss of apex predators can allow mesopredator populations to bloom, which can have negative affects on lower level prey species.

Because the worldwide catch of large sharks has increased as the demand for shark meat and shark fins has risen, researchers from the Future of Marine Animal Populations (FMAP) project of the Census of Marine Life have conducted an analysis of shark population data for the Northwest Atlantic Ocean as it relates to mesopredators (notably elasmobranchs such as rays, skates, and smaller sharks) and the decline of commercially important shellfish stocks. The aim of the study was to explain how the removal of apex predatory sharks has affected the lower trophic levels of the ecosystem. Of the apex sharks in the Northwest Atlantic, 11 species consume rays, skates, and smaller sharks. The results of the study show that populations of all 11 shark species have fallen over the past three decades. The consequence of this is that the populations of 12 of the 14 elasmobranch prey species associated with these sharks have increased.

The result of this change in the ecosystem community structure is most visible in the case of the cownose ray and the North Carolina bay scallop fishery. Cownose rays are extremely abundant in the waters along the Mid-Atlantic seacoast of the United States. A large portion of their diet is bivalves such as clams, oysters, and scallops. Researchers suggest that recent cownose population increases are due to the decline in sharks that prey on these rays. As the population of these rays increases, obviously their demand for prey has also risen. This rise in demand for prey, such as the bay scallop in North Carolina waters, has outpaced the ability of the prey species to replenish its own stock. The effect of this cascade though the food web is that bay scallop numbers fell so low that in 2004 North Carolina had to close the scallop fishery that had flourished there for over a century.

Concern has been expressed over this restructuring of the ecosystem and whether the negative effects may begin to extend to other commercially important species such as hard shell clams, soft shell clams, and oysters. A similar case in Japan has shown that intense fishing pressure on apex predators caused a population bloom of longheaded eagle rays. This increase in eagle rays, in turn, decimated both wild and maricultured stocks of various shellfish. With the realities of predator removal and its effect on oceanic food webs now coming to light, especially in terms of negative effects on commercially important species, appropriate management efforts have never been more critical. Future sustainability of food resources and conservation of biodiversity may depend on an ecosystem scale approach to management.

Research Location: The Eastern Seaboard of the Unite States in hte Northwest Atlantic Ocean.
  • What: Population depletion of apex predator shark species has indirect effects on commercially important shellfish species in the Northwest Atlantic Ocean.
  • Who: FMAP Scientists --> R.A. Myers, J.K. Baum, T.D. Shepard, S.P. Powers, and C.H. Peterson
  • When: 2007
  • Where: Atlantic coast of the United States in the Northwest Atlantic Ocean, with references to other case study areas around the globe
  • How: Analysis of time series U.S. North Atlantic longline catch data, targeted shark and elasmobranch survey data, and fisheries observer and logbook data
  • References: Myers, R.A., J.K. Baum, T.D. Shepard, S.P. Powers, and C.H. Peterson. 2007. Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science. Vol. 315. No. 5820 pp 1846-1850.