Spatialised ecosystem indicators in the Southern Benguela

1
Spatialised ecosystem indicators in the Southern
Benguela

• Pierre Fréon, L. Drapeau, J. David, R. Crawford,
  A. Fernández Moreno, D. Grémillet, R. Leslie, M.
  Meyer, H. Oosthuizen, L. Shannon and C.D. van der
  Lingen
• Outlook
• Introduction on EAF in the Southern Benguela
  ecosystem
• Data
• Methodology for mapping the species distribution
  or foraging ranges
• Methodology and results on 7 spatialised
  indicators
• Comparison with other indices

2
Introduction on EAF in the Southern Benguela

• An upwelling ecosystem bounded by the Lüderitz
  permanent cell at its northern boundary and by
  warm waters at its southern boundary
• Includes the Agulhas Bank where most species
  migrate to spawn
• High biodiversity

3
Data (main species and fleets)
Cephalopod Chokka squid Seabirds Cape
Gannet African Penguin Marine mammal Cape
Fur Seal Major fisheries (96 catches)
Pelagic purse-seine fishery Demersal trawl
fisheries Long-line fishery SOURCE OF DATA MCM
Scientific survey (demersal and pelagic), four
commercial fisheries, GPS and satellite tracking
of top predators
Small pelagic Anchovy Sardine Round
herring Mackerel Horse-mackerel Large
pelagic Albacore Yellowfin Bigeye Snoek Demers
al Hakes Kingklip Sylver kob
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Data (sampling efforts)
Demersal surveys (1985-2001)
Acoustic surveys (1988-2001)
Pelagic fishery (1987-2001)
34 cruises
102 974 sets
54 cruises
Hake longline (1994-2001)
Tuna longline (1996-2001) 19 904 sets
Trawl fisheries (1985-2001)
11 404 sets
102 974 trawls
5
Methodology for mapping the species distribution
or foraging ranges

• Mapping the fish species distribution
• Mapping the foraging ranges of top predators

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Mapping the species distribution (e.g. hake)
Trawl survey
Trawl fishery
Longline fishery
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Catching the seals
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Fitting the transmitter
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Release
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Mapping the foraging ranges (e.g. Seal Island,
False Bay)
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Foraging ranges of seals by colony
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Foraging ranges of penguins by colony
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Foraging ranges of gannets by colony
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Foraging ranges of the 3 top predators
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Methodology and results on 7 spatialised
ecosystem indicators

• Two categories of ecosystem indicators presented
  here
• Ecosystem characterisation indicators
• - Connectivity indicator
• - Spatial biodiversity
• Pressure indicator
• - Mean ratio between the exploited area the
  distribution
• area per species
• - Exploited fraction of the ecosystem
• - Catch per exploited area
• - Mean bottom depth of the catches
• - Mean distance of catches from the coast
• Terminology provided by Grieve et al. (2003)

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Matrix of interaction between items (species and
fleets) in the ecosystem
Medium competition
Strong competition
medium predation
Heavy predation
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Connectivity index (CI )

• CI is the average of Relative Overlapping Areas
  (ROAi/i',p) between any pairs of items in the
  ecosystem that has a substantial trophic
  relationship with the other. This applies here to
  the distribution areas of exploited species
  (DistAi,p), top predator, and fishing effort for
  any time period p  
• ROAii',p (DistAi,p?DistAi,p)/DistAi,p ?DAi,p
  

X X X
or or
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Connectivity index (continued )

• (CIp) 100 S i1 to n (ROAi/i',p) / n
• CI tells us the intensity of trophic links
  within the ecosystem.
• In the Southern Benguela, CI 22.5
• Limitations Depends on the number of species
  difficult to compute on an annual base spatial
  information difficult to access for all species

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Spatial index of biodiversity (SIB )

• SIBp is the average number of species per grid
  cell (sk,p), per period p divided by the total
  number of species (Sp)
• SIBp
• 100 (S k1 to n sk,p / np) / S p
•  It tells us how spread is the biodiversity
  within the ecosystem
• SIB7.5 (1985-1989) 374 sp.
• 6.1 (1990-2001) 602 sp.
• Limitations same as CI

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Spatial index of biodiversity (SIB ) cont.
Note benthic species not identified prior to 1993
21
Mean ratio between the exploited area and the
distribution per species (MRED)

• (MREDp) is a subset of the previous indicator CI
• MREDp 100 S i1 to n (DistAi,p?ExplAi,p)/Ex
  plAi,p)/n
• Tells us what is the spatial pressure of
  exploitation on the commercial species and the
  need of implementing or not MPAs or closure of
  fishing areas.
• Limitations same as CI and SIB except for
  dependence on the number of species difficult to
  interpret for migratory species

MRED values are 31 for the pelagic fleet, 78
for the demersal fleet and 58 when combining the
two fleets
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Exploited fraction of the ecosystem (EFE)

• EFE is the relative overlapping area between the
  exploited area (ExplAi,j) and the ecosystem area
  (EA) considered as invariant
• EFEi ,j 100 (ExplAi ,j ? EA) / EA
  
• A global EFE can be computed by considering all
  fisheries (here only two) exploiting the
  ecosystem
• EFEj 100 (ExplA1,j ?ExplA2 ,j) ? EA/EA
• Limitations no major ones (can easily be
  computed by year)  

EFE values varied from lt1 to 20 for pelagic
fleet More stable for demersal fleet (80).
Combined value for the two fleets is up to 92.
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Total catch per exploited area (CPEA)

• CPEAi,j is the ratio of the total annual catches
  (Ci,j) by the annual exploited area during year j
  for fishery i
• CPEAi ,j Ci ,j / ExplAi ,j

CPEA for the pelagic fisheries dropped from 100 t
km-2) in 1951 to 20 km-2 in 1956 and then
decreased more progressively until the end of the
80s. In the recent period it tend to stabilise
around of 10 t km-2. No trend in the demersal
fishery.
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Mean annual bottom depth of catches (MBDC) and
mean distance of catches from the coast (MDCC )

• (MBDCj) is a weighted average of BDC
• MBDC,j S i1to n BDC / n
• (MDCCj) is a weighted average of the DCC
• MDCC,j S i1to n DCC / n where j is the
  year and i the grid cell.
• The three indices of exploitation (EFE, MCDC,
  MDCC) tells us the same (pelagic fleet) or
  different (demersal fleet) stories

Pelagic fishery Demersal fishery
DIFFERENTES DATES DE DEPART?
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Comparison of spatialised ecosystem indices to
conventional indices (cont.)
Pelagic fishery Demersal fishery
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Conclusion

• Pressure indicators are richer, easier to
  interpret, quicker to compute and less prone to
  bias (i.e. increase in fishing power) than the
  global effort or CPUE. Nonetheless they are not
  aimed to replacing them, but rather complement
  them.
• Ecosystem characterisation indicators are more
  difficult to compute yearly because they require
  too many data, except the exploited fraction of
  the ecosystem which is a promising indicator.
• Rather than weighing up different ecosystem
  indicators one against another, one should
  compare them and try to gain knowledge on the
  status of the ecosystem by interpreting the
  similarities/discrepancies between them.
• Despite some limitations, the seven indicators
  proposed here met the three main criteria usually
  assign to indicators, that is simplification,
  quantification and communication.

Fréon, P., Drapeau, L., David, J., Fernández
Moreno, A., Leslie, R., Oosthuizen, H., Shannon,
L.J., and van der Lingen, C.D. 2005. Spatialised
ecosystem indicators in the Southern Benguela.
ICES Journal of marine Science 62 459-468.