Beth Scott
Research Fellow
University of Aberdeen
What happens
when you
institutionalise
science?
Using North Sea cod as
an example
Outline
• A brief look back at history of
fisheries science/management
• The essential guide to the
understanding of fisheries
management
Why it has all gone wrong
• fish as animals
• sexual maturity and genetics
• migration and mating behaviour
•climate change
What can be done?
Fisheries science
A European institution since 1902
(ICES - International council for the exploration of the seas)
Fishing records for 1000 years
and scientific investigation
from the mid 1800’s
Johan Hjort - 1914
Identified fundamentals of
biology of cod.
The good and the bad
• Hjort’s (1914) conclusions have had a most powerful influence
on fisheries research
• He argued that the number of juvenile fish surviving each year
were not due to variations in the quantities of eggs spawned
but that they were most likely due to annual fluctuations in the
amount of plankton available as food for the larvae, and to
dispersal patterns (where the larvae were ‘blown’ to).
• Good - it laid the foundations for explorations which try to
identify what influence climatic effects has on the production
of food and hence on survival of young fish.
•
Bad - it assumes there is no relationship between the number
of fish reproducing and the success of juvenile survival.
Fisheries science and the
fisherman
In the beginning the interaction was
positive - the goal was to understand why
fishing was better in some years and some
areas - to help the fishing community.
Fisheries science was at the
forefront of ecological research,
questioning animal behaviour,
population dynamics and
climatological interactions.
Technological advances
horsepower, Sonar (fish finders
Law of the Sea 1978
(200 mile limit for management)
• Recognition that the
sea’s resources were
not inexhaustible.
• The need to have tools
for agreeing
international standards
for management limits.
• Introduction of
Mathematical
modelling - the
beginning of setting in
stone the direction of
research.
The essential guide to fisheries
management
To understand why fisheries is
managed the way it is - with one
number called the
TAC (total allowable catch).
Recruitment
 Number of juveniles surviving to 1 year
 Number of animals big enough to be
caught in the fishery
Spawning stock Biomass
 The total sum in weight of the mass of
fish that are mature and capable of
spawning that year.
Maximal sustainable yield
RECRUITS
Stocks thought to be more productive at lower rather than
higher levels due to 1) a low number of adults can
successfully produce a high number of recruits and 2) there
is ‘interference’ (density dependence) of a larger stock size
with survival and growth of recruits.
Level at which stock is recommend
to be fished in order to keep surplus
production at maximum level
STOCK - BIOMASS OF SPAWNERS
Fisheries Management across the world is based on the
assumption that there is a real relationship between the
weight of the stock and subsequent recruitment.
Why the collapses ?
100 years of catch information
The fishing was reduced during the
first and second world wars
Technologically
enhanced catches
Continuously high and increasing fishing
pressure leads to loss of big fish
Number of larger fish and the percentage of fish killed each year
1.80E+07
0.75
1.60E+07
1.40E+07
1.20E+07
0.65
1.00E+07
0.6
8.00E+06
6.00E+06
0.55
4.00E+06
0.5
2.00E+06
0.00E+00
0.45
1963 1966 1969 1972 1975 1978 1981 1984 1987 1990 1993 1996 1999
Year
Number of large fish
Percentage of fish killed by fishing
Percentage of fish killed
Number of larger fish (>90cm)
0.7
Fisheries Resource Conservation
Council (FRCC) in 1997
“One factor in the collapse of Atlantic
Canada’s groundfish fisheries was a lack of
attention to the logical connection between
the spawning and future recruitment of
young fish.”
“The reproductive capacity of the stock
appears not to be properly measured by the
absolute volume of spawning biomass, as
generally assumed.”
Stock (SSB) / Recruitment
Relationship
Assumes constant number
of eggs / per gram of
biomass.
Do 10 small cod really have
the same reproductive
output as one of these
mother cod?
Critical Factors ignored
once the methodology for assessing and predicting
population size is institutionalised
• Fish are individual animals not biomass!
• At lower levels of stock sizes there is also a
lack of older/larger individuals.
• What does heavy fishing pressure do to
other aspects of a fish’s behaviour and what
are the effects in the longer term?
Decrease in Field
Work:
It’s expensive.
Biology is time
consuming.
Computer
modelling is cheap
and fast.
Photo by John Dunn
IBM: From individuals to
the Population
Reproductive output of COD
over the spawning season
• Comparison of 6 populations with same
spawning stock biomass.
• Decrease in reproductive output in more
heavily fished populations
• Differences in output due to assumptions
about the relationship between mother
and egg quality.
• Shift in peak date of 3 weeks - such that
the majority of production is later
Day of the year
What happens when you only have young fish left in the spawning population?
First time vs second time
spawners
• First time spawn
fewer and smaller
eggs than second and
subsequent spawners
• A very low percentage
of first time spawners
eggs that are fertilised
or hatch successfully.
Condition / Population structure - effect on SRP
Sexual Maturation
• The age or size of a fish
when it becomes sexually
mature is very flexible and
may be density dependent
via food limitations and
/or genetic.
• Function of growth history
(food and temperature)
• Fish that are heavier (i.e.
more surplus energy) at a
given length have a higher
probability to be sexually
mature.
Better condition
50%
Maturity
Length
length
Sexual Maturity: Effects of
fishing?
• Fishing pressure may have
caused shifts in sexual
maturity and fecundity.
• Cod on Canadian East
coast - maturity at smaller
size and age (shift from 4
to 2 years of age)
• If density driven - i.e.
more food per fish - why
are they no larger at age
and yet higher fecundity?
Life time reproductive output/
size at age
Mature later
Mature early
Age
now
Genetic Selection?
• Most fish are caught before they can spawn therefore the only ones getting to spawn are those fish
that mature early.
•If that is a trait with a degree of heritability than
fishing pressure is acting as a very large selective force
to kill all fish that don’t mature early.
•Early maturity comes with a cost in length of life and
smaller size at maturity and slower growth rates.
•If timing of and size at maturation are heritable genetic models predict it will take natural section at
least 250 years before we can hope to see the reappearance of genotypes for longer lived, bigger
bodied, later maturing cod.
Resilience because so fecund?
Doing
worse
• Jeff Hutchings points out that high
fecundity does not
mean high resilience
(think of tree seeds?)
• Most fished
populations do not
spring back
Not recovered
7
49
34
Recovered
Fish Migrate
• Fish - just like many other
animals - migrate.
• They migrate for the same
reasons - making use of
seasonal and spatial changes
in resources
• They have constraints for best routes and where to
have babies (for fish the
need is to be ‘upsteam’ - even
if attaching your eggs)
Do fish learn?
• What physical clues /
constraints can there
be?
• - currents
• - bottom features
• - watermass type
• How much is from
watching what others
do?
A. Corton - Herring
not coming back
Before
1960’s
Now
Cod Highways
• George Ross
• constraints for cod
migration - water can’t
be too cool (> 20C)
• Found 80% of the
stock in one gully following large scouts
Migrating cod
scouts out in advance
Cod spawning behaviour
along the migration route
Pairs rise above concentrated group
Spawning locations , timing and size
groups
• The interaction of
spawning location ,
timing and size/age
of fish has also
been largely
ignored.
Big Fish
Medium
•Does it matter
where and when
you kill fish during
spawning?
Small Fish
Eggs to Larvae
• Location of where the
larvae end up are
heavily dependant on
the effects of weather
and on where eggs
were released.
• Will they be
transported farther or
kept near area of
spawning?
Mortality during spawning
• Fish are more
concentrated
• Male and females are
more likely to be in
different locations at
different times
• Do some locations
contain - ‘better’
spawners than others?
Different quality of spawning areas
Percent increases from changing from High fishing pressure
of both Large and medium sized fish
(assumptiong that large fish in good quaiity site, medium in mean, small in low quality)
60.00%
med fish - low F
large fish - low F
50.00%
Percentage increase
med & large - low F
40.00%
30.00%
20.00%
10.00%
0.00%
1
2
Lots of big fish
3 populations 4
fewer big fish
5
6
very few big fish
Climate = Food
but it is the factor we can’t really
control or manage
• Condition of adults -which leads to the
number and quality of eggs
• Feeding opportunities for larvae - which
leads to the survival changes for
juveniles
North Atlantic Oscillation
Recruitment
Spawning liver condition
Look at stratified vs mixed regions
in the North Sea
• Spatial and seasonal
pattern of water mass
characteristics in the
North Sea
A day in the life of a fish
• We must understand fish
as animals.
• Ask questions about
population structure,
genetic selection,
condition, spawning
behaviour and climate
effects before we can
begin to imagine that we
understand a species well
enough to manage it.
New relationships?
• Including age/size specific fecundity and viability
should decrease uncertainty in stock-recruitment
relationships .
• A new index for stock characteristics should
produce a clearer relationship between survival
and the spawning stock
North Sea cod
Reproductive Output 1963-1999
(using mean length at age)
Egg numbers by size
6E+13
5E+13
4E+13
3E+13
2E+13
large eggs
medium eggs
small eggs
Picture of years vs # eggs
vs mean egg size
1E+13
Ye
ar
19
65
19
68
19
71
19
74
19
77
19
80
19
83
19
86
19
89
19
92
19
95
19
98
0
recruitment vs egg size
8.00E+08
7.00E+08
recruitment
6.00E+08
5.00E+08
R2 = 0.66
4.00E+08
3.00E+08
2.00E+08
1.00E+08
0.00E+00
1.37
1.375
1.38
1.385
1.39
1.395
mean egg size
1.4
1.405
1.41
What to do?
Decrease Fishing pressure
•If you want reliable, sustainable
fisheries you must accept that they
can only be fished at low levels of
fishing effort to keep around a
needed number of older, wiser,
bigger, better breeding fish.
•North Sea cod - that is 4-5 times
lower than the current level.
• The longer lived the species - the
lower the level of fishing allowed.
Wait for the news of the next one
Monk fish!