The ecological effects of fishing and implications for coastal management: the case of San Miguel Ba

1
The ecological effects of fishing and
implications for coastal management the case of
San Miguel Bay, the Philippines

• Alida Bundy,
• Department of Fisheries and Oceans,
• Canada.

2
Objective

• To develop tractable, sustainable management
  strategies for multispecies, multigear fisheries.

3
Method

• Ecological Modelling explore the biological and
  ecological impacts of a multigear fishery on a
  multispecies resource using Ecopath with Ecosim.
• Develop fisheries management policies robust to
  uncertainties using an adaptive management
  approach.

4
San Miguel Bay

• Important Fishery in the Philippines
• Multispecies - gt100 species in the catch
• Multigear - gt 29 types of gear
• Over-fished
• Little known about the fishery or the ecosystem.
• ICLARM conducted 2 intensive studies of San
  Miguel Bay, in 1979-1982 and in 1992-1994.
• The Ecopath model presented here represents the
  early 1990s.

5
Comparison of catch by large and small scale
gears in early 1980s and 1990s.

• Small Scale Gear
• Gillnets
• Liftnets
• Fish Corrals
• Seines
• Traps
• Scissor nets

• Large Scale Gear
• large, medium and small trawlers
• (mini trawlers)

6
Ecopath/Ecosim Model and Uncertainties

• energy flow
• immigration
• model structure

7
Bottom Up Top Down

• Each trophic level is food limited (Power
  1992) and production and variation at lower
  trophic levels determine, directly or indirectly,
  the abundance at higher trophic levels.

• All trophic levels below the top level are
  dependent on that top level, directly or
  indirectly (Menge 1992). Trophic levels below the
  top predator are alternatively predator limited
  and food limited (Power 1992).

8
Changes to biomass under top down and bottom up
energy control when effort by trawlers is
decreased to zero.
9
Ecopath/Ecosim Model and Uncertainties

• energy flow
• immigration
• model structure

10
Adaptive Management

• Adaptive management explicitly addresses
  uncertainty about resource dynamics
• Its a feedback system where empirical
  information generated from the resource in
  response to management is used to increase our
  knowledge and shape future management.
• 2 forms adaptive/active policies
  non-adaptive/passive policies
• First Step determine which form..
• Adaptive management does not have to be active to
  be adaptive.

11
Adaptive Management

• Identify alternative models
• Develop Baseline Policies
• Estimate EVPI - the importance of learning
• Develop Adaptive, Probing Policy Options
• Develop Performance Criteria
• Formal Comparison of Options.

12
Identify Alternative Models

• Model 1 Mixed control
• Model 2 Top-down control
• Model 3 Mixed control and immigration
• Model 4 Top-down control and
  immigration
• Model 5
• Assume that all models are equally likely

13
Develop Baseline Policies

• 1 Ban all trawling by large, medium and baby
  trawlers
• 2 Reduce trawling by baby trawlers from 50
  vessels to 11 vessels and increase the number of
  fish corrals from 123 to 767.
• 3 Ban all trawling by large, medium and baby
  trawlers and increase the number of crab
  gillnets by 100.
• 4 Ban all trawling by large, medium and baby
  trawlers and ban the use of the hunting gillnet.
• 5 Ban all trawling by large, medium and baby
  trawlers and ban the use of the filter net.
• 6 Ban all trawling by large, medium and baby
  trawlers and ban the use of the hunting gillnet
  and the filter net.
• 7 Ban all trawling by large, medium and baby
  trawlers and reduce effort by other gears by 25
  (except the crab gillnet, crab liftnet, set
  longline, handline, fish corral, scissor net,
  fish trap and "others" gear).
• 8 Ban all trawling by large, medium and baby
  trawlers and reduce effort by all other gears by
  50.
• 9 Reduce effort of all types of gillnet by 50.
• 10 Increase effort of all gears by 33

14
Decision Tables
Change in fished biomass after 30 years

• EVPI ?i Pt (Mi) V(UMi) - (V(UMi)
• where, Mi model (i), Pt probability of Mi,
  and V(UMi ) expected value of policy U, given
  model Mi (Walters 1986).

15
Performance Criteria
16
Biomass Diversity and Catch Criteria
Change in biomass diversity after 30 years.
EVPI1.25
Change in catch after 30 years
17
Revenue and Profit Criteria
Change in revenue after 30 years
Change in profit per small scale gear after 30
years
18
Comparison of best non-adaptive policy for each
performance criteria

• Two main policies robust to all models
• Policies reflect opposite values

19
Conclusions

• Robust policies over all models for most
  performance criteria
• No need for active experimental management
• Division between catch and revenue and ecological
  criteria
• Policy of banning all trawling not best policy or
  best non-adaptive policy
• Adaptive management, using feedback information
  should be pursued.

20
More specific performance criteria give different
results
Change in Sciaenid Biomass after 30 years.
EVPI9.5

• Here it may be worth developing probing policies
  to determine which model best represents the
  ecosystem and the effects of fishing.

21
Changes to biomass under top down and bottom up
energy control when effort by all gillnets is
decreased to zero.
22
Changes in effort from early 1980s to 1990s

• Large Scale Gear (Trawlers)
• Decrease in Effort
• Decrease in Catch
• Small Scale Gear (Gillnets, Fixed,
  Lines,.....etc.)
• Increase in overall effort
• Increase diversity of effort
• Increase competition within the small-scale
  sector
• No increase in catch