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

FCB

q f → Bqf

CCpUE

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)

Effort (f)

catchability (q)Fishing mortality (F)

More of the same

Decreasing these is management

Better methods

Increasing these is

development

So while we ‘manage’ and ‘develop’ the fishing mortality stays the same.

FC

Bq f ca tchab ility e f fo r t

Behaviour: Many fish shoals

Shoaling protects against predatorsBut not all!

Behaviour:Fish Aggregation Devices (FADs)

‘Home made’ FAD used on the Nippon Maru

Fish Aggregation Devices (FADs)

Algae fixed to FADJuvenile fish

Small pelagic 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 resilientLong-lived species

Species with spawning migrations

Highly specialised endemic and territorial species

Unspecialised ecologically flexible species, adapted to fluctuating environments

Small species with high population turnover rates (P/B)

Predation vs fishing mortality..

Fishing mortality

Age (years)

Predation mortality

From ICES (1997).

.. is almost exactly opposite

..and this is what happens:

Median age-at-maturation (sexes combined) of Northeast Arctic cod based on spawning zones in otoliths (from Jørgensen, 1990).

Age and size structure changesunder selective fishing to younger and smaller individuals.

But we know that – we even use it as a sign of fishing

effort

CPUE q B

As age and size structure changesunder selective fishing to younger and smaller individuals there will bea 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

Intraspecific effects: age and size structure – density dependence

Abesamis and Russ 2005

MPA in the Philippineson Apo Island

The r-K selection principle:genetic changes?

Age (size)

Abu

ndan

ce (

Log

N)

Slope = total mortality rate Z = r

Increased juvenile mortality= K-selection

Increased adult mortality= r-selection

K-selection: Stable environment, biotic mortality (predation) – predictive, size selectiver-selection: Unstable environment, abiotic mortality – non-predictive, non-selective

Kolding (1993)

Size selection = genetic changes

Small

Random

Large

Increased mortality on:

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.After Conover and Munch Science 2002

V

Effect of size-selective fishing

Trends in average total weight harvested (A) and mean weight of harvested individuals (B) across multiple generations of size-selective exploitation. Closed circles represent small harvested lines, open squares are the random-harvested lines, and closed triangles are the large-harvested lines. Conover and Munch 2002

Small

Random

Large

Mortality on:

Size selective fishing with large mesh sizes on adults is decreasing mean size and lowering yields

We are deliberately inducing r-selection on the stocks.

Size selection: genetic changes?

L R S L R S

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

Size selection: genetic changes?

L R S L R S

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

Community effects: Diversity• Extirpation: Local loss of population

– Sedentary coral reef species– Elasmobranchs

Baum et al. 2003

Rel

ativ

e ab

unda

nce

Diversity

32 100 1000 10000320 3200

Worms et al. 2006

…and the doomsday predictions

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).

based on the extrapolation of regression there will be 100%

collapse in the year 2048

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 !

• Number of species and relative evenness increase during succession

Filling phase19 species

Early succession phase29 species

Late succession phase35 species

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. inOdum’s list Attribute Prediction

2 P/B ratio ( total mortality Z) decreasing

4 Net production (= system yield) decreasing

6 Total organic matter (biomass ) increasing

8 Species diversity (variety) increasing

9 Species diversity (evenness) increasing

13 Size of organisms (mean weight) increasing

18 Growth (population fluctuations) decreasing

22 Stability (resistance) 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 re-colonization, 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-at-maturity in the North Sea demersal fish community (Jennings et al. 1999)

2

3

4

5

Tro

phic

leve

lFishing down the food web

Pauly et al. (1998)

Fishing down the food web

Mean trophic level of the landings in North-east Atlantic fishing area

Bio

mas

s

Size

The biomass size spectre

The distribution of biomass by body size follows regular patterns

phytoplankton

zooplankton

small fish

big fish

Slope and intercept changes with pressures and driversChanges in intercept is informative about changes in biomass

While changes in slope is informative about mortality pattern

Jennings & Blanchard, 2004

intercept

slope

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