2. Fishing effects on
populations and communities
Fishing effects on populations and
communities
• Vulnerability to fishing:
– Behaviour – Catchability, susceptibility
• Intraspecific effects:
– Age and size structure
– Reproduction
– Genetic structure
• Community effects:
– Diversity
– Community structure
– Size structure
Bottom trawling in North Sea
Average annual trawling
frequency of the sea bed
by the total Dutch beam
trawl fleet in the 4-year
period between 1993 and
1996 as estimated on a
1x1 nautical mile scale.
• 30% of the seabed is
trawled 1-2 x per year
• 10% of the seabed is
trawled more than 5 x per
year
From Rijnsdorp et al.
1998
Ghost Fishing
Cyanide fishing
Muroami fishing
Dynamite fishing
Vulnerability to fishing:
Catchability
C
C
 q
B
F q f → CpUE
f
B
F = fishing mortality, C = catches, B = average biomass,
f = nominal effort (fishing power), and q = catchability
(fishing efficiency)
Catchability (q) is defined as the relationship between the
catch rate (CpUE) and the true population size (B).
So the unit of catchability is fish caught per fish available
per effort unit and per time unit.
Vulnerability to fishing:
Catchability
The probability of a fish being caught at any time
depends on several factors, which broadly can be
grouped into biological and technological factors:
The biological factors include:
• fish availability on the fishing ground,
• fish behaviour (incl. towards the fishing gear),
• the size, shape, and external features of the fish,
• where some of these factors again are depending on
season, age, environment and other species.
Vulnerability to fishing:
Catchability
Technological factors include:
-
gear type, design, size, colour, and material,
gear position, duration, and handling,
experience of the fisherman
As both the unit and the different notations epitomise, the
catchability coefficient (alias efficiency, or fishing power, or
probability of a fish being caught), is therefore a composite and
very complicated factor.
‘Fish catchability’ normally refer to changes in fish behaviour.
‘Fishing efficiency’ refers to fishing practises or relative fishing
power.
Fishing mortality (F)
C
F   q  f  catchability  effort
B
Better methods
Increasing these is
development
So while we ‘manage’
and ‘develop’ the
fishing mortality stays
the same.
catchability (q)
Fishing mortality (F)
Effort (f)
More of the same
Decreasing these is management
Behaviour: Many fish shoals
Shoaling protects against predators
But not all!
Behaviour:
Fish Aggregation Devices (FADs)
‘Home made’ FAD used on the Nippon Maru
Fish Aggregation Devices (FADs)
Small pelagic
fish
Algae fixed to FAD
Juvenile fish
Large predators
Effects of FADs - catchability
Effects of FADs in La Reunion Island
http://www.spc.int/coastfish/News/FAD/FAD3.pdf
FAD??
Life history and vulnerability
Susceptible
Resilient
Most resilient
Long-lived
species
Unspecialised
ecologically
flexible species,
adapted to
fluctuating
environments
Small species with
high population
turnover rates
(P/B)
Species with
spawning
migrations
Highly specialised
endemic and
territorial species
Predation vs fishing mortality..
.. is almost exactly opposite
Predation
mortality
Fishing mortality
Age (years)
From ICES (1997).
..and this is what happens:
Median age-at-maturation (sexes combined) of Northeast Arctic cod based
on spawning zones in otoliths (from Jørgensen, 1990).
But we know that – we even use it
as a sign of fishing
CPUE  q  B
Age and size structure changes
under selective fishing to younger
and smaller individuals.
effort
Intraspecific effects:
age and size structure
As age and size structure changes
under selective fishing to younger
and smaller individuals there will be
a decrease in:
• size (age) of maturity
• fecundity,
• egg quality
• egg volume,
• larval size at hatch,
• larval viability,
• food consumption rate,
• conversion efficiency,
• growth rate.
Intraspecific effects: age and size
structure – density dependence
Abesamis and Russ 2005
MPA in the Philippines
on Apo Island
The r-K selection principle:
genetic changes?
Abundance (Log N)
Increased juvenile mortality
= K-selection
Slope = total mortality rate Z = r
Increased adult mortality
= r-selection
Age (size)
Kolding (1993)
K-selection: Stable environment, biotic mortality (predation) – predictive, size selective
r-selection: Unstable environment, abiotic mortality – non-predictive, non-selective
Size selection = genetic changes
Increased mortality on:
Small
Random
V
Large
After Conover and Munch Science 2002
Mean individual weight at age for
six harvested populations after 4
generations. Circles, squares, and
triangles represent the small-,
random-, and large-harvested
populations, respectively.
Effect of size-selective fishing
Mortality on:
Small
Random
Large
Trends in average total weight harvested (A) and mean weight of harvested
Size
selective fishing with large mesh sizes on adults
individuals (B) across multiple generations of size-selective exploitation. Closed
circlesis
represent
small harvested
lines,
open and
squares
are the random-harvested
decreasing
mean
size
lowering
yields
lines, and closed triangles are the large-harvested lines. Conover and Munch
We
are deliberately inducing r-selection on the stocks.
2002
Size selection: genetic changes?
Correlated responses to
harvest selection on
(L) Large individuals
(R) Random individuals
(S) Small individuals
(a) egg volume,
(b) size at hatch,
(c) growth efficiency
(unlimited food),
(d) growth efficiency
(restricted food).
After Walsh et al. 2006
L
R
S
L
R
S
Size selection: genetic changes?
Correlated responses to
harvest selection on
(L) Large individuals
(R) Random individuals
(S) Small individuals
(a) % survival (10 days),
(b) consumption rate,
(c) # vertebrae
(d) Forage response time
After Walsh et al. 2006
L
R
S
L
R
S
Community effects: Diversity
• Extirpation: Local loss of population
Relative abundance
– Sedentary coral reef species
– Elasmobranchs
Baum et al. 2003
Diversity
Worms et al. 2006
32
100
320
1000
3200
10000
…and the doomsday predictions
based on the extrapolation of
regression there will be 100%
collapse in the year 2048
Global loss of species from LMEs. Trajectories of collapsed fish and invertebrate
taxa over the past 50 years (diamonds, collapses by year; triangles, cumulative
collapses). Worm et al. (Science 2006).
Which is most diverse?
3 species of grass ?
2 species of grass
+ a rabbit ??
Diversity – how measured?
Order
Family
Genus
Species
Fishing and diversity
Relationship between species richness (± 95%CL) and fishing intensity for
groupers on Fijian reefs. After Jennings and Polunin (1997).
Except for perhaps first 2 obs no significant difference – same habitats?
0-hypothesis: All reefs have same number of groupers at all times ??
Diversity changes naturally !
Filling phase
19 species
Early succession phase
29 species
Late succession phase
35 species
• Number of species and relative evenness
increase during succession
Attributes of ecological succession

Odum (1969) proposed a number of ecological attributes as
indicators of the maturing process during succession in an
stable ecosystem. In Kariba we could test 8 out these.
No. in
Odum’s list
2
4
6
8
9
13
18
22
Attribute
Prediction
P/B ratio ( total mortality Z)
decreasing
Net production (= system yield)
Total organic matter (biomass )
Species diversity (variety)
Species diversity (evenness)
Size of organisms (mean weight)
Growth (population fluctuations)
Stability (resistance)
decreasing
increasing
increasing
increasing
increasing
decreasing
increasing
The intermediate disturbance
hypothesis (Connell 1978)
Here the diversity is a result of a balance
between the frequency of disturbances
that provide the opportunity for recolonization, and the rate of competitive
exclusion. Disturbances are here seen as
catastrophic and density independent
events.
What is a disturbance?
"A discrete, punctuated killing, displacement, or
damaging of one or more individuals (or
colonies) that directly or indirectly creates an
opportunity for new individuals (or colonies) to
become established". (Sousa 1984).
"Any relatively discrete event in time that removes
organisms and opens up space which can be
colonized by individuals of the same or different
species" (Begon et al. 1990).
What about human induced stress?
Any (anthropogenic) factor that has the
effect of increasing the death rate, or
decreasing the birth rate. Harvesting, as
well as pollution, falls within this definition".
(Pimm & Hyman 1987)
Is that not the same as a disturbance??
Impacts are not random
Fishing has different impact depending on the trophic level. Traits that make species
vulnerable co-vary both between and within trophic levels. Body size (size of the
circles) of top carnivore species tends to be larger than that of species at lower
trophic levels. Range of body sizes and number of species are larger at lower trophic
levels. From Raffaelli (2004)
Community structure: Under fishing
the processes are reversed
Trends in mean growth rate, maximum length, age-at-maturity, and length-atmaturity in the North Sea demersal fish community (Jennings et al. 1999)
Fishing down the food web
Pauly et al. (1998)
Trophic level
5
4
3
2
Fishing down the food web
Mean trophic level of the landings in North-east Atlantic fishing area
The biomass size spectre
Biomass
intercept
The distribution of biomass by body
size follows regular patterns
phytoplankton
slope
zooplankton
small fish
big fish
Size
Slope and intercept changes with pressures and drivers
Changes in intercept is informative about changes in biomass
While changes in slope is informative about mortality pattern
Jennings & Blanchard, 2004
Size-spectre in practise
Example of size spectre for the North Sea demersal fish community in 1977 and
1993. The loge numbers of fish per loge 10 cm size classes are shown. Note the
steeper slope and higher intercept of the fitted relationship in 1993. After Rice and
Gislason (1996).
Size-spectre in practise
Slopes (a) and intercepts (b) with 95% CL of linear regressions fitted to size
spectre for the North Sea demersal fish community from 1977 to 1993. After Rice
and Gislason (1996).
Conclusions
• Fishing has profound effect on populations and
communities:
– Density reduces (not bad to a point)
– Susceptible species are easy to overfish
– Diversity and life history traits are altered
• Man when fishing is a predator:
– But we are a highly selective predator
– In general we behave opposite of all other predators
– This is the main reason for the observed effects!
• All natural populations are adapted to predation
– Perhaps we should rethink the way we predate