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 fisheries
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 Billfish: 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 fishing 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 trawling
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? Seafood 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 Fishery 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 Overfishing, habitat loss, eutrophication and climate change impair marine ecosystem services and threaten food security We must strive to – minimize destructive impacts – maintain diversity – establish networks of protected areas