Global decline of large predatory fishes: causes, consequences, and solutions

Boris Worm Dalhousie University, Halifax, Canada Our ocean planet

¾ 90% of the biosphere

¾ 50% of global net primary production

¾ 80% of

¾ Less 0.1% protected Questions

I. What has changed in the ocean?

II. What are the consequences?

III. What can we do? Tuna: Atlantic Bluefin : swordfish and marlins

Common patterns of global depletion

Source: Myers and Worm 2003. Nature 423: 280-283 …seen in all available data sets

Source: Myers and Worm 2005. Phil. Trans. R. Soc. B 360:13-20 What has changed? 800 Codfishes

Redfishes

¾ 90% decline in Skates 600 numbers Flatfishes

Wolffishes )

¾ Approx. 50% t

K (

400 s

decline in size s

a

m o

¾ Large changes in i B

species 200 composition

0 1950 1960 1970 1980 1990 Year Hawaiian survey data: the brave new ocean

1950s (6232 kg) ¾ 87% decline in numbers Blue shark

¾ Approx. 50% decline in size Yellowfin tuna 1990s ¾ Large changes (828 kg)

in species Silky shark Abundance (no./1000 hooks) Bigeye tuna

composition 0 20 40 60 80 100 120

100 50 0 50 100 50050

Mean mass (kg)

Source: Ward and Myers 2005. Ecology 86:835–847 What are the causes?

¾ Industrialized is the driving cause – Increasing effort – Increasing efficiency – Increasing global coverage

¾ Problem is exacerbated by habitat destruction, eutrophication, and climate change Global problem #2: Habitat destruction

Before

After trawling Problem #3: Climate change and eutrophication Coral bleaching Seagrass die-off

Source: Reusch et al. 2005. PNAS: 102:2826–2831

Algal blooms

Source: Cerrano et al. 2000. Ecol. Lett. 3: 284-293

Source: Worm and Lotze. Source:2005. Limnol. NASA Oceanogr.: in press Source: NASA Question II: What does this mean? bound to become a luxury item

Source: Normile 2002. Science 298:1154 Loss of biodiversity and economic opportunities

Overfishing ¾ Loss of consumer and habitat species

¾ collapses

¾ Algal blooms

¾ Decline of water quality

¾ Loss of resilience

Climate change

Habitat destruction Eutrophication Source: Pauly 2003. Island Press Question III. What can we do? Reduce fishing mortality for sensitive species

Dusky shark. Source: NMFS

Source: Myers and Worm 2005. Phil. Trans. R. Soc. B 360:13-20 Modify or ban use of unselective fishing gears

¾ Driftnets

¾ Bottom trawls

¾ Longlines

¾ Fish aggregating devices Protect key areas to recover diversity, productivity

Source: Worm et al. (in preparation) Some key areas are in national waters

Coldspot ¾ Special places where Hotspot many species aggregate

¾ Key habitats

¾ Food supply

Source: Worm et al. 2003. PNAS 100:9884-9888 Worldwide hotspots for high-seas conservation

¾ Major hotspots – U.S. east coast –Hawaiian chain –Southeast Pacific – Australian east coast – Sri Lanka

Source: Worm et al. 2005. Science: accepted manuscript Leatherback turtle habitat use from satellite tracks

Source: Ferraroli et al. 2004. Nature 429: 521-522 Conclusions

¾ The oceans have been depleted on a global scale

¾ , habitat loss, eutrophication and climate change impair services and threaten food security We must strive to – minimize destructive impacts – maintain diversity – establish networks of protected areas