Presentation on theme: "Fishing down marine food webs: an index of fisheries impact on ecosystems by Daniel Pauly Fisheries Centre, University of British Columbia Vancouver, Canada."— Presentation transcript:
Fishing down marine food webs: an index of fisheries impact on ecosystems by Daniel Pauly Fisheries Centre, University of British Columbia Vancouver, Canada International Symposium on Marine Fisheries, Ecosystems and Societies in West Africa: Half a Century of Change Dakar, June 24-28, 2002
Biomass of table fish in 1900 (t/km 2 ) (map: V. Christensen, SAUP)
Biomass of table fish in 1999 (map: V. Christensen, SAUP)
We have talked a lot about overfishing. Let’s now talk briefly about ecosystems… wherein each organism has its own trophic level …
Thus, one index of ecosystem impact of human impacts on ecosystems is the mean trophic levels of fisheries landings, computed using: The TL for species of species groups in FishBase (www.fishbase.org); andwww.fishbase.org The global FAO database of fisheries catches (www.fao.org).www.fao.org The equation used: TL i =1+ DC ij TL j for (groups of) species i with prey j; and TL k = Y ik TL i / Y i given catches (Y) by year (k) – simple stuff...
Pauly et al. Science March 1998 Marine Freshwater This led to time series that are going down globally…
Pauly et al. Science (1998)… This process, wherein fisheries catches are increasingly based on organisms low in the food web, is now known as ‘fishing down marine food webs,’ and is particularly strong in the North Atlantic… Northeast Atlantic Northwest Atlantic
In a critique some colleagues (1998. Science 282:183a.) suggested we overlooked that: 1.The composition of landings does not necessarily reflect relative abundance on underlying ecosystem; 2.Trophic levels change with size or age; 3.Over-aggregated catch statistics may bias results; 4.‘Bottom up’ efforts are not accounted for and ‘fishing down’ may be a deliberate policy to get more food.
Ad (1) - landing trends vs. ecosystem trends. Three counter-arguments: 1.Fish are nowadays exploited everywhere they are abundant; 2.All trawl survey data so far tested for this (e.g., Gulf of Thailand, Cantabrian Shelf, Guinean Shelf, etc.) show trophic level trends similar to those of the landings; 3.Work by Pinnegar et al. (2002) for the Celtic Sea shows that decline of trophic level in landings is less pronounced than in survey data (i.e., fishers try to maintain catches of high-trophic level fishes).
Ad (2) – ontogenic changes in TL. Trophic level tends to increase with size/age; as ‘fishing down’ is associated with high F, TL decline is faster when ontogenic changes in TL are considered. From Pauly et al. (2001; Can. J. Fish and Aquatic Sci.)
Ad (3) – over-aggregated catch statistics. It turns out that over-aggregating catch statistics has the effect of masking the fishing down effect...
… because much of the world’s catch is reported in very coarse categories.
Here is an example: there seemed to be no ‘fishing down’ effect in Area 31 (West Central Atlantic), which we first attributed to the crude statistics from many countries of that region.
However, after separating the USA (‘South Atlantic’ and Gulf of Mexico) from the rest of the region (Mexico, Caribbean, NE South America), we get:
Another example: deleting the anchoveta from the other fisheries of FAO Area 87 (SE Pacific, i.e., Chile and Peru) …
Last example: we extracted all tuna and billfishes (ISSCAAP group 36) from the FAO global catch database and analyzed them separately:
Ad (4) – ‘bottom up’ effects and deliberate fishing down. Thermodynamics require that biological production, within an ecosystem, be higher at lower than at higher TL. Therefore, fish catches, as part of production should increase as trophic level declines, and not ‘bend back’ as shown here for FAO area 27 (NE Atlantic).
The fishing-in-balance (FiB) index for year (i) is: FiB = log(Y i · (1/TE) TL i ) – log(Y 0 (1/TE) TL 0 ) where Y is the catch, TL the mean trophic level in the catch, TE the mean transfer efficiency (specific to an ecosystem, often set at 0.1), i refers to species (groups) in the catch and 0 refers to any year used as a baseline to normalize the index. Thus, FiB = 0 when catches remain as expected, given their TL.
Properties of the FiB index: Remain on ‘zero-line’ if TL changes are matched by ‘ecologically correct’ changes in catch; Will go up (>0) if: –Bottom up effect occurs, e.g., increase in primary production in Mediterranean (which triggered Caddy et al.’s concern); –If geographic expansion occurs, and ‘system’ definition has in fact changed. Will go down (<0) if: –Withdrawals impair functioning of ecosystem; –Discarding occurs that is not considered in the ‘catches.’
An application example of the FiB index … FAO area 21 (NW Atlantic)
Conclusions: ‘fishing down’ is everywhere, and strong, including in West Africa. The End