Transcript 04- Oceanography - Secretariat of the Pacific Community

Stock Assessment Workshop
19th June -25th June 2008
SPC Headquarters
Noumea
New Caledonia
Day 1 Session 3
How do oceanographic and climatic
processes impact upon tuna fisheries
(and stock assessment)
Overview
1.
Introduction
2.
Basic Principles of Physical Oceanography
•
3.
4.
Oceanography of the Pacific
•
Ocean structure, Currents, Warm pool/cool tongue system
•
ENSO
Relationship between oceanographic processes and fish populations
•
5.
6.
Ocean properties, structure, movement, productivity
Survival, Growth, Movement, Recruitment
Oceanographic impacts on fisheries
•
Catchability and catch rates
•
Movement and distribution
Scales of impact
•
WCPO v. EEZ v. eddies, current lines etc
7.
Climate change
8.
Implications for stock assessments
Introduction
•
Oceanographic and climatic factors influence the distribution
and abundance of pelagic fish (through impacts on recruitment,
growth and mortality), and subsequently, the distribution and
activity of the fisheries that target them
•
Understanding the relationship between fish
abundance/distribution and oceanographic and climatic factors
can provide fishers, managers and scientists with an
understanding of fishery variability.
•
This may allow management and development plans to consider
fluctuations in fish biomass and availability.
•
With climate change, understanding these relationships may
become even more important.
Basic principles
•
There are four key features of the ocean about which we need to be
aware if we are to understand how fish populations and fisheries are
influenced by the ocean:
•
The oceans are not uniform water masses but have a physical
structure, both vertically and horizontally
•
This structure results from spatial differences in the properties (e.g.
temperature, salinity, water pressure and other factors) of the oceans water
•
Oceanic waters are constantly moving and this movement can be
horizontal (e.g. wind driven surface currents) or vertical (e.g. upwellings
or downwellings).
•
The properties, structure and movement of oceanic waters are
strongly influenced by climate and atmospheric processes, and
conversely have a strong influence upon these…ocean and atmosphere are
a coupled dynamic system
Basic principles
•
Subsequently…..
•
The structure of the water column is constantly
changing depending on prevailing climatic conditions,
and hence the ocean provides a heterogeneous and
dynamically changing environment within which fish
must survive.
•
We are going to now briefly look in more detail at:
• Oceanic water properties,
• Ocean structure, and
• Movement of water within oceans.
• Climatic and atmospheric influences
•
Each has significant implications for fish population
ecology, abundance, distribution and catchability
What are the key properties and
structure of oceanic waters?
Key properties of sea water - Temperature
•
The temperature of the oceans varies by depth and by latitude
•
Ocean surface temperature strongly correlates with latitude because
insolation, the amount of sunlight striking Earth’s surface, is
directly related to latitude, and is highest in the tropics, hence
tropical waters are warmer.
Key properties of sea water - Temperature
Warmer waters are generally less dense than cooler water and
therefore “sit on top” of the cooler waters (i.e. so temperature
decreases with depth). In tropical and subtropical waters, there is a
thermocline (depth at which rapid temperature change)
Surface Mixed Layer
Deep Layer
Why are surface
waters typically
warmer, less salty
and less dense?
Key properties of sea water - Temperature
•
In the tropical Pacific Ocean,
warmer waters are typically
distributed westward, but can
shift with climatic conditions
(discussed further later).
The mean, upper-ocean, thermal
structure ALONG the equator in
the Pacific from 140E to 100W for
November 1997
The mean, upper-ocean, thermal
structure ACROSS the equator in
the Pacific from 8N to 8S for
November 1997
Images from NOAA Pacific Marine Environmental Laboratory
Key properties of sea water - Temperature
•
Tropical/subtropical oceans are permanently layered with warm, less dense
surface water separated from the cold, dense deep water by a thermocline.
•
Temperate regions have a seasonal thermocline, polar regions have none.
Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/
Key properties of sea water - Salinity
The salinity of the oceans waters
also varies by depth. More
saline waters are typically
denser than less saline
waters and therefore tend to
sink beneath less saline
water (i.e. salinity increases
with depth) and by latitude
(relating to precipitation and
evaporation)
Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/
Key properties of sea water - Density





Density of sea water is a function of (related to) temperature, salinity and water pressure.
Density increases as temperature decreases and salinity increases as pressure increases.
Pressure increases regularly with depth, but temperature and salinity are more variable.
Higher salinity water can rest above lower salinity water if the higher salinity water is
sufficiently warm and the lower salinity water sufficiently cold.
Pycnocline is a layer within the water column where water density changes rapidly with
depth.
Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/
Key properties of sea water - Density




The density of oceanic waters varies by depth
Low density waters (due to heating or low salinity) lye at the surface, denser waters
below.
Pycnocline is a rapid change in density with depth (similar principle to thermocline)
The pycnocline is transitional between the surface and deep layers
 In the low latitudes (tropics), the pycnocline coincides with the thermocline.
 Surface water in high latitudes cools, becomes dense, sinks (convects) to the
sea floor and flows outward (advects) across the ocean basin.
Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/
Why do oceanic waters move?
Ocean water movement
•
The waters of the ocean are constantly in motion….why is this?
•
There are different types of ocean water movement, including:
• horizontal currents, gyres and eddies, and
• vertical upwellings and downwellings.
•
These movements are caused by two main factors:
• Wind
• Gravity
•
The following section will discuss how these factors drive the
movement of water in the ocean
Ocean movement – Currents
Eddies
Surface
Mixed Layer
Surface currents
Gyres
Coastal
Upwelling/
Downwelling
Seamount/
Ridge
Upwelling
Subsurface
currents
Divergence based
upwelling
Convergence/
Gravity based
downwelling
Ocean movement – Horizontal currents
High pressure
(Cooler air)
Solar radiation and wind
creation
Low pressure
(Warmer air)
High pressure
(Cooler air)
Ocean movement – Horizontal currents


Wind-driven currents - As wind moves across the water, it drags on the
water. Water moves at about 3-4% of the wind speed.
Zonal wind flow is wind moving nearly parallel to latitude as a result of
Coriolis deflection.
Currents carry warm water poleward on the
western side of basins and cooler water
equatorward on the eastern side. Westerlydriven ocean currents in the trade winds,
easterly-driven ocean currents in the
Westerlies and deflection of the ocean
currents by the continents results in a
circular current, called a gyre, which
occupies most of the ocean basin in each
hemisphere.
Source:
http://www.tulane.ed
u/~bianchi/Courses/
Oceanography/
Ocean movement – Vertical currents

Ocean water can also move vertically, with the two most common processes
called upwelling (movement of deep water to the surface) and downwelling
(movement of surface waters to the deep).
Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/
Ocean movement
Thermohaline circulation is a density
driven flow of water between ocean
basins that appears largely driven by
waters of the North Atlantic, where,
poleward of 45 degrees (north), the
density of water increases because of
declining temperature and increased
salinity because of evaporation or ice
formation.
The water sinks to a density-appropriate
level and then slowly flows outward in
all directions across the basin until they
are blocked by a continent.
Ocean basins interconnect and exchange
water with each other and with the
surface.
Source:
http://www.tulane.ed
u/~bianchi/Courses/
Oceanography/
Oceanographic structure and movement summary
Differences in solar heating between areas causes differences in air
pressure which results in the air moving, i.e. wind.
Winds at the surface of the ocean drags on the water, causing the
water to move also, i.e. creating surface currents and gyres.
Some wind patterns can cause divergence of surface waters which
can result in upwelling of deeper waters. Subsurface topography
can also divert deep currents to the surface, as can the
convergence of currents.
Evaporation leading to increased salinity and water density, or
cooling leading to increased density, can result in downwelling
(sinking) of surface waters.
The location, strength and duration of all these water movement
features varies dynamically in time and space, being driven by
climatic and atmospheric factors.
Oceanography and Primary Production
Oceanographic and Primary production
Primary production refers to the amount of inorganic C (mainly carbon
dioxide) converted to organic C (e.g. simple sugars) by microscopic
algae in a process known as photosynthesis (photo – (sun) light;
synthesis = to make something). Land plants also do a similar thing
Primary producers represent the base of the oceanic food chain and their
abundance is critical to the abundance of animals higher in the chain
Primary production
Plants require sunlight, nutrients, water and carbon
dioxide for photosynthesis. Phytoplankton
blooms occur when light and nutrients are
abundant.
Other factors that influence phytoplankton growth
include upwelling, turbulence, grazing intensity
and turbidity.
Tropics/subtropics - sunlight is abundant, but strong
thermocline restricts upwelling of nutrients and
results in lower productivity. High productivity
can occur in areas of coastal upwelling, in the
tropical waters between the gyres and at coral
reefs.
In temperate regions productivity is distinctly
seasonal.
Polar waters are nutrient-rich all year but productivity
is only high in the summer when light is
abundant.
Oceanography and Primary
production
Low productivity in the gyres, due to
downwelling (opposite of process
needed to bring nutrients to surface)
Source:
http://www.tulane.edu/~bianchi/Cours
es/Oceanography/
Why is oceanography important to fisheries?
“Habitat – the place where an organism (or a community of organisms) lives,
including all living (biological) and nonliving (environmental) factors or conditions
of the surrounding environment.”
The ocean is not a homogenous uniform water mass. It is structured, it moves, and it
has different properties (of temperature, salinity, density) in different areas and
at different times. As a result, some areas are productive and some aren't.
What does this mean for fish?
The ocean also has habitats within it. An area or mass of water within the ocean
which is warm, shallow, with low salinity, low nutrients and productivity,
constitutes a very different set of living conditions to one which is shallow, cold,
high salinity and high productivity. Deep waters constitute a very different
habitat to shallow waters.
Fish species have evolved different physiologies and morphologies to exploit different
habitats in the ocean
Tuna species are no different.
e.g. Skipjack - exploit warm, shallow tropical waters.
Bluefin tuna - exploit deeper, colder more temperate habitats.
Oceanography and fish populations
……the following sections are going to discuss:
1. Some of the key features of oceanographic and climate
processes (and ocean habitats) of the Pacific Ocean in more
detail.
2. What we currently know about the relationship between
oceanographic processes and the key species targeted by
pelagic fisheries in the Pacific
3. The implications of these relationships for the fisheries targeting
them, and,
4. For assessments of their status and subsequent management of
the fisheries.
Oceanography and climate of the
Pacific
Pacific Ocean - Surface currents
Three major current features are the North Pacific Subtropical Gyre, the South
Pacific Subtropical Gyre, and the equatorial currents
The strength and direction of the equatorial and subequatorial currents is
dependant on the prevailing winds and climatic conditions
60o
Major shifts occur in currents
due to changes between
South East Trade Wind
and North West
Monsoon seasons
warm pool
cold tongue
Subarctic Gyre
convergence
40o
KUR
divergence
Subtropical Gyre
20o
NEC
NECC
0o
SEC
SECC
20o
EAC
Subtropical Gyre
HBT
40o
60o
120o
140 o
160 o
180o
160 o
140o
120 o
100 o
80o
The strength and direction of
the wind driven
equatorial and subequatorial currents play
a major role in the
location and size of
another major
oceanographic feature
of the Pacific, the warmpool/cold tongue
convergence zone.
Pacific Ocean – Warm Pool / Cold Tongue
In the eastern and central Pacific, wind driven movement of currents along the equator
creates an upwelling that extends westward from South America…this feature is called
the “cold tongue”
Western equatorial Pacific - low primary production, extreme uniformity of high sea surface
temperatures (SST) (up to 28° C year-round). This water mass is referred to as the
“warm pool”. These two water masses meet at the “convergence zone”.
Convergence zone
Warm pool
Cold tongue
Pacific Ocean – Warm Pool / Cold Tongue
Cooling in the WCPO is
absent due to high
rainfall causing haline
stratification that
prevents mixing of
surface and deeper cold
nutrient rich waters.
However, the low nutrient,
low salinity surface
waters from the warm
pool can move eastward
on a seasonal basis
under the influence of
westerly wind events.
The eastward movement of
warm pool meets the
western movement of
the cold tongue,
creating a convergence
zone where they meet.
Pacific Ocean - El Nino Southern
Oscillation Phenomena (ENSO)
La Nina
Warm
pool
c)
ENSO
•
Results from complex
interaction between
surface layers of
equatorial Pacific and
overlying atmosphere
•
Has a very significant
influence on the fisheries
in the Pacific Ocean
•
Irregular cycle of 3-7 yr
•
Has 3 states – El Nino,
Neutral, La Nina
Cold tongue
a)
El Nino
Warm pool
•
Pacific Ocean - ENSO
El Nino conditions – expansion of the warm pool eastwards, resulting in warmer than
average waters in the central and eastern Pacific, higher rainfall in that region, a deepening
of the thermocline in the east and rising of thermocline in the western region, and cooler
than average waters in the western Pacific.
Neutral conditions, and
La Nina conditions –characterised by stronger Pacific Trade winds, the contraction of the
warm pool into the equatorial western Pacific, higher rainfall in the western Pacific and lower
rainfall in the eastern Pacific, a deepening of the thermocline in the west and rising of
thermocline in the eastern region.
Pacific Ocean – ENSO and PDO
0
-2
-4
Measured by difference in air pressure
between EPO and WPO – negative
values (<-2) signify El Nino and
positive values (>2) La Nina.
SOI
2
El Nino magnitude/duration varies
1910
1920
0
SOI
-2
1930
1940
1950
1960
1970
0
-2
SOI
2
1920
-4
PDO operates on a scale of 20-30
years, is characterised by long term
shifts in average SST between the
EPO and WCPO. ENSO is a pattern
that can be thought of as lying on top
of the large scale temperature
distribution determined by the Pacific
Decadal Oscillation.
1900
-4
Another climate phenomena exists for
the Pacific – the Pacific Decadal
Oscillation (PDO) - characterized by
abrupt, but infrequent changes that
are described as “climate regime
shifts”
1890
2
1880
1970
1975
1980
1985
1990
1995
2000
2005
(*dotted line is the standard deviation calculated over the entire time series)
Pacific Ocean – Primary Production
The convergence zone between the warm
pool and cold tongue in equatorial Pacific is
the meeting point of cool nutrient rich
upwelled waters with warm surface waters
from the west.
Under the influence of sunlight, these
conditions promote the rapid growth and
reproduction of phytoplankton (i.e.; blooms).
The explosion in phytoplankton population
has a cascading effect through the food
chain…zooplankton>>small fish>>big fish
(e.g. tunas)
This process takes weeks to months, during
which time equatorial waters are diverging
away from the equator
The regions of high tuna density and catch
rates can therefore be displaced somewhat
from where primary productivity was initiated
Pacific Ocean – Inter-annual Primary Production
During La Nina, upwelling induced primary production is enhanced in the equatorial Eastern
pacific and brought by wind driven surface waters across to central and Western Pacific, which
at the same time diverge north and south of the equator. During El Nino conditions, the
Eastern upwelling is suppressed, but upwelling and productivity in the far western area (PNG,
Phillipines, Palau) can be enhanced.
Sea Surface
Temperature
El Nino
(Jan 98)
La Nina
(Jan 99)
Chlorophyll a
El Nino (example year 1997)
1st Quarter
ENSO –
EEZ
impacts
There are very clear
differences in seasonal
surface current directions
and strength that result
from interannual climatic
state variations, and these
cause large changes in the
oceanography associated
with any given EEZ in the
WCPO
For example, current
speed and direction in the
FSM region during El Nino
and La Nina events
La Nina (example year 1999)
Surface
currents –
1st Quarter
Seasonal and interannual
variation
2nd Quarter
2nd Quarter
3rd Quarter
3rd Quarter
4th Quarter
4th Quarter
Source data: www.oscar.noaa.gov
ENSO – EEZ impacts
1997
There are very clear
differences in seasonal sea
surface temperatures that
occur in Pacific EEZs, that
result from interannual
climatic state variations,
For example, SST in the
FSM region during El Nino
and La Nina events
January
March
May
July
August
October
1998
1999
ENSO – EEZ
impacts
2006
28
29
30
31
2001
2000
2002
2004
2006
1998
2000
2002
2004
2006
1998
2000
2002
2004
2006
100
140
180
20
60
100
140
1998
60
For example, SST, Depth
of 27C and 22C
thermoclines in 5
subregions within the FSM
EEZ
Depth (m) of 22C thermocline
Thermal structure of the
water column also changes
dramatically with changes
in climate state.
Depth (m) of 27C thermocline
27
Thermal
structure
1996
Sea surface temp.
1991
Summary – Oceanography of the Pacific
There are three key and interacting features of the Pacific
ocean-climate system that have a large influence on the
distribution and abundance of the target tuna species.
These are:
1. The direction and strength of the major surface
currents,
2. The size and location of the warm-pool-cold tongue
interaction, and
3. The overall influence/interaction of prevailing climatic
conditions (in particular ENSO and PDO) on these.
These are play a large role in tuna distribution, abundance
and fishing success, as will be see in following sections
How are oceanographic and climatic
processes relevant to fish population
dynamics and stock assessment in
the Pacific?
WCPO tuna stock assessments take account of:
1. Growth
2. Recruitment
3. Survival
4. Movement
5. Catch rates (Abundance indices)
Oceanographic/environmental impacts on
fish populations – Introduction
What are the key processes that determine population
biomass?
1. Growth
2. Reproduction (Recruitment)
3. Survival (or mortality)
Oceanographic/environmental impacts on
fish populations – Introduction
What environmental factors impact GROWTH?
1. Food availability
2. Water temperature
3. Energy expenditure (holding position in water
column, so currents and turbulence etc)
4. Other
Oceanographic/environmental impacts on
fish populations – Recruitment
What environmental factors impact RECRUITMENT?
1.
2.
3.
4.
5.
6.
7.
8.
Parental condition (Nutritional status>>Food availability)
Food availability
Water temperature
Predation/cannibalism (of eggs, larvae)
Disease
Current speed (turbulence) and advection
Salinity
Oxygen
Oceanographic/environmental impacts on fish populations –
Recruitment
Source: Adam Langley, Karine Briand, David Seán Kirby, Raghu Murtugudde (In press)
Influence of oceanographic variability on recruitment of yellowfin tuna Thunnus albacares in the western and central Pacific Ocean
Oceanographic/environmental impacts on fish populations –
Recruitment
YFT
BET
Primary Productivity
SKJ
Secretariat of the
Pacific Community
Oceanography impacts on fish populations - Survival
•
Few fish species can survive in all oceanographic conditions.
Instead, fish species have evolved physiological and
behavioural characteristics which allow them to exploit
specific conditions.
•
Typically, if they move outside these “habitats” then
condition and survival decline.
•
For example, bigeye tuna have specialised circulatory
features that allow them to exploit deeper colder waters than
some other species, such as skipjack tuna, are capable of.
Oceanographic/environmental impacts on
fish populations – Survival
What environmental factors impact SURVIVAL?
1.
2.
3.
4.
5.
6.
7.
Predation
Disease
Food availability
Water temperature
Current speed and advection
Salinity
Oxygen
Impacts vary depending on stage of development
Oceanography impacts on fish populations - Survival
•
Numerous oceanographic factors have a significant influence
on the survival of pelagic fish species, and the degree of
influence often varies between different life history stages
Adults
Spawning and
fertilisation
Eggs
Maturation
Hatching
Larvae
Juvenile stages
Vertical Movement
Climate related changes in habitat volume in the Pacific
There are implications of habitat volume and variation in species vertical
movements for interpretation of CPUE data and its use in stock
assessments. The effect of increasing/decreasing thermocline depth on
catch rates, for example, is very much species specific, and also
dependant on the gear type being used.
Bigeye and yellowfin tuna movement
1.
2.
Bigeye tuna show strong
diurnal vertical movement
(deep during day, shallower at
night) due to tracking of
mesopelagic prey. Bigeye
physiology enables exploitation
of deeper colder lower oxygen
waters.
FAD associated
Yellowfin show diurnal pattern,
but mostly within mixed layer
above the thermocline. YFT
lack key physiological
adaptations of BET that enable
BET to exploit deeper waters
3.
Movements differ on/off FADs
for both species
4.
Complicates use of CPUE data
in stock assessments
Source: Bruno Leroy, SPC, 2007
BIGEYE TUNA
FAD associated
YELLOWFIN TUNA
Bigeye tuna vertical
movements and purse
seine susceptibility
Implications for CPUE analyses
– catch rates will vary
according to depth/volume of
habitat, prey movement, time
of day and depending on
whether tuna are associated to
a “feature” (seamount, FAD,
log etc)
Implications for catchability by
purse seine….bigeye more
vulnerable when associated to
floating objects
From: MICHAEL K. MUSYL,1,* RICHARD W. BRILL,2 CHRISTOFER H. BOGGS,2 DANIEL S.
CURRAN,1 THOMAS K. KAZAMA2 AND MICHAEL P. SEKI2 Vertical movements of bigeye tuna
(Thunnus obesus) associated with islands, buoys, and seamounts near the main Hawaiian Islands from
archival tagging data. Fish. Oceanogr. 12:3, 152–169, 2003
Bigeye and yellowfin vertical movements and longline
susceptibility
BET CPUE higher on
deep sets (in WCPO)
5S
50
YFT CPUE higher on
shallow sets
5N
150
Catch rates depend
on depth of longline
gear relative to the
thermocline depth.
15N
15S
250
Yellowfin
predominantly above
thermocline, bigeye
predominantly below
(during the day)
140E
5
10
160E
15
20
25
180
30
160W
140W
120W
Bigeye tuna movement
Bigeye tuna follow the vertical
migration of mesopelagic prey,
which move up into shallow
waters above the thermocline
during the night, and return to
deeper waters at dawn.
Figure shows movement of a
bigeye tuna in relation to prey
(sound scattering layer) over the
course of an hour during the night
Source: Erwan Josse, Pascal Bach & Laurent Dagorn. Simultaneous
observations of tuna movements and their prey by sonic
tracking and acoustic surveys Hydrobiologia 371/372: 61–69,
1998.
Bigeye tuna movement is closely related to their
physiological capacity to cope with different
environmental conditions
From: Richard W. Brill1, Keith A. Bigelow, Michael K. Musyl, Kerstin A. Fritsches, Eric J.
Warrant BIGEYE TUNA (THUNNUS OBESUS) BEHAVIOR AND PHYSIOLOGY AND
THEIR RELEVANCE TO STOCK ASSESSMENTS AND FISHERY BIOLOGY Col. Vol.
Sci. Pap. ICCAT, 57(2): 142-161 (2005)
Recent findings on yellowfin tuna movement
SOURCE: Laurent Dagorn1,a, Kim N. Holland2, Jean-Pierre Hallier3, Marc Taquet4, Gala Moreno5, Gorka Sancho6,
David G. Itano7, Riaz Aumeeruddy8, Charlotte Girard1, Julien Million9 and Alain Fonteneau10
Deep diving behavior observed in yellowfin tuna (Thunnus albacares) Aquat. Living Resour. 19, 85–88 (2006)
Oceanography, gear impacts and depth of capture
Temperature-depth timer study of species caught by longline off New Caledonia
Above thermocline
Below thermocline
Beverly, S (2005) Notes on a longline
trip in the New Caledonia EEZ using
TDRs in combination with remote
sensing data (SSH and SST) ,
WCPFC–SC1 FT IP–4
Problem for interpretation of CPUE data
(where hooks per basket is assumed an
indicator of fishing depth) – hook depth can
vary very significantly during the course of
a single fishing operation – this variation
will depend on prevailing local
oceanographic conditions, current shear etc
Scientists need to be careful regarding
assumptions pertaining to fishing depth
when incorporating CPUE data into
assessments
William Sokimi, SPC,
2007
Depth and catch rates
Japanese study indicates that catch rates vary by depth, reflecting
the differing movements and depth habitats of different species
BET
YFT
BSH
Marlins
1.0
Depth layer (m)
Figures courteousy of Kotaro Yokawa (2007) – National Institute of
Far Seas Fisheries
350-375
325-350
300-325
275-300
250-275
225-250
200-225
175-200
150-175
125-150
100-125
75-100
50-75
25-50
0.0
0-25
Scaled index
2.0
Depth and catch rates
0
0.2
Ratio to the total
0.4
0.6
Absolute depth layer (m)
0-25
25-50
50-75
75-100
100-125
125-150
150-175
175-200
Vertical CPUE
Vertical distribution
(PAT tag)
200-250
Striped marlin caught deeper layers than expected by tag data
Figures courteousy of Kotaro Yokawa (2007) – National Institute of
Far Seas Fisheries
0.8
Influence of thermocline (and hypoxic layer) on movement
Recent study compared vertical movement of
marlin and sailfish in Eastern Pacific with that
of same species in the western Atlantic
Oceans.
Both marlin and sailfish in the Atlantic study
(Red lines) area had a significantly different
mean depth distribution to the same species
tagged in the Eastern Pacific (Blue lines).
Evidence presented suggested that this
difference in depths was due to the differing
depth of the cold hypoxic layer between the
two areas.
In the Atlantic this layer was considerably
deeper than in the Pacific, allowing surface
layer dwelling species greater vertical habitat
range.
Source: ERIC D. PRINCE1,* AND C. PHILLIP
GOODYEAR2 Hypoxia-based habitat compression of
tropical pelagic fishes. Fish. Oceanogr. 15:6, 451–464,
2006
Influence of thermocline (and hypoxic layer) on movement
Australia
3/29
0
4/3
4/8
4/13
4/18
4/23
4/28
5/3
Similarly, research into striped
marlin has revealed that the average
dive by striped marlin appears
closely related to the depth of the
mixed layer in the region inhabited
by the fish
5/8
50
100
150
200
250
mixed layer
deepest dive
average dive
300
350
400
Costa rica
2/7
2/12
2/17
2/22
2/27
3/4
0
50
3/9
e.g. Average dive is much shallower
in the eastern Pacific (off Costa
Rica) than in the Western Pacific
(off Australia)
Implications for habitat volume and
catch rates within stock assessments
100
150
200
Mixed layer
250
Deepest dive
Average dive
300
Figures courteousy of Michael Domeier (2006) –
Pfleger Institute.
Horizontal Movements
Horizontal movements and oceanographic shifts in the
Pacific
Horizontal shifts in the warm pool correspond to changing thermocline
depth and together may influence the movement of tropical tuna.
Bigeye and Yellowfin tuna
movements in the Pacific
in relation to
Oceanographic/ Climatic
factors. There are a
number of theories
regarding these
mechanisms.
Typically, a decreasing in
thermocline depth is
associated with higher LL
CPUE for bigeye
Source: Lu et al. (2001)
Oceanography impacts on fish populations - SKIPJACK
Horizontal movement
Displacements of tagged skipjack
tuna during representative El Nino
(top) and La Nina (bottom) periods.
Thick arrows indicate the direction
and magnitude of displacement of the
skipjack CPUE gravity centre during
the tag recapture periods.
El Nino period
From (Lehodey et al, 1997)
These ENSO related changes in
movement pattern are not yet
captured in the movement
parameterisation for MFCL
assessments
La Nina period
Skipjack tuna and climate/oceanographic processes–
Basin wide and EEZ impacts
2000 (+)
DFADs: allow fishing in east in non-El Niño periods
AFADs: allow fishing in west in all years
2002 (-)
Changes in the depth of the thermocline also impacts
on catchability of fish by longline fisheries
Skipjack tuna – horizontal movement
and association with the Warm Pool
a) Skipjack tuna CPUE for US purse seine fleet (88-95)
b) Monthly mean CPUE;
pink line is the longitudinal gravity centre of
CPUE (G)
blue line is the 29.8C sea surface temperature
(SST) isotherm
broken red line is the southern oscillation
index (SOI)
(Source: Lehodey et. al , 1997 - NATURE |VOL 389 | 16
OCTOBER 1997)
***Implications for understanding
environmental effects on movement
estimation in population models
Bigeye and yellowfin tuna
Bigeye tuna
Yellowfin tuna
0
10 20 30 40 50
Albacore CPUE (kg/hhhooks)
35 S
Latitude of isotherm
25 S
15 S
0
10 20 30 40 50
Albacore CPUE (kg/hhhooks)
35 S
Latitude of isotherm
30 S 25 S 20 S 15 S
Albacore tuna movement and environmental forcing
1998
1998
2000
2000
2002
25 C SST isotherm
2002
2004
2006
2004
2006
Summary – Impacts on tuna fisheries
Four likely key impacts of oceanographic and climatic processes on WCPO stock
assessments:
1.
Calculation of CPUE index – as an index of abundance to which the
model is fitted, is a critical component of stock assessments. Variation in
oceanographic processes effect catch rates of pelagic tuna due to:
a. Changing the volume of the species habitat (effects their density)
b. Effecting the ability of fishers to target their gear appropriately
c. Movements of tuna into/out of stock assessment regions in
response to oceanographic/climatic shifts
2.
Incorporation of movement – Fish move in response to changes in their
physical environment (one reason). Stock assessment models need to
incorporate variation in fish movement and migration that occurs due large
scale oceanographic and climatic changes
3.
Recruitment Estimation – the least easily estimated but most important
biological factor in SAs. Variations in recruitment can be very substantial and
are predominantly due to environmental processes. Current efforts are
attempting to use oceanographic analyses to predict recruitment to feed into
stock assessment models.
4.
Growth rate impacts – variation in productivity of oceanic waters may relate
to variation in growth rates by cohorts.
Climate Change
Climate Change
In considering the relationship between tuna, tuna fisheries
and climate-ocean system, we must also consider the
possibility that this relationship will change (or is
changing) as a result of climate change.
How will it change?
There is currently significant uncertainty about the
implications of climate change for tuna fisheries.
However, research is underway, and the following
observations and hypotheses have been made………
Climate Change
Fact: Increases in global average sea temperature have been observed. Sea
surface temperatures affect the patterns in atmospheric pressure, which in
turn are responsible for wind generation.
Hypothesis: Changes in wind generated surface currents would not only
modify the weather conditions but also alter the timing location and extent
of the upwelling processes upon which much oceanic primary productivity
is reliant. Some studies suggest that primary productivity in tropical
oceans would decline due to increased stratification between warmer
surface waters and colder deeper water (and consequent reduction in
upwelling) (Bopp et al., 2001), but further research is required.
Implication: Decline in the upwelling system of the central and eastern
equatorial Pacific may lead to reduced productivity that is normally
advected westwards and upon which pelagic fish stocks depend. This
decreasing production could lead to a decline in tuna abundance,
particularly in the bigeye and adult yellowfin population (the species
targeted by the longline fleet).
Climate Change
Fact: El Nino events appear to have become more frequent in recent decades,
(possibly linked to climate change) and are associated with an eastward
shift of major tuna resource (e.g. skipjack) in the WCPO. Total catch does
not appear effected by ENSO related shifts in the stocks.
Hypothesis: Climate change may, however, imply more permanent El Niños,
which are likely to increase the annual fluctuations of the spatial
distribution and abundance of tuna. First simulations of global warming on
skipjack suggest a global improvement of its habitat conditions east of the
date line and a spatial redistribution of this species to higher latitudes
(Loukos et al., 2003).
Distant water fishing fleets should be able to adapt to changes in the spatial
distribution and abundance in tuna stocks. But domestic fleets would be
vulnerable to fluctuations of tuna fisheries in their Exclusive Economic
Zones.
Climate Change
Uncertainty remains on the change in the productivity
of the western equatorial Pacific. The impact of
climate change on tuna recruitment and spawning
migration is also poorly known.
Current thinking is that climate change may lead to a
shift in the spatial tuna distribution, as well as
possible changes in productivity, total abundance
and total catch in different regions.
Session Summary and Conclusions
The ocean is not a homogenous uniform water mass. We have seen that
the water in the ocean is structured, it moves in a complex manner,
and that it has different properties (of temperature, salinity, density)
in different areas and at different times. Furthermore, some areas
possess properties and conditions that are more productive than
other areas.
Effectively these sets of conditions constitute mobile, sometimes
transient, but differing habitats within the ocean
Few if any fish species, including tuna species, can thrive and survive
across all the sets of conditions (habitats) in the ocean. Each
species has evolved to exploit specific sets of conditions (or specific
ocean habitats), and if placed for long periods outside these
conditions, will do poorly or die.
Target tuna species have different habitat preferences (which vary over
life history), and different behaviours evolved to exploit these
habitats.
Session Summary and Conclusions
Finding and exploiting the ideal habitat has very large implications for
species recruitment, growth and mortality, and population dynamics.
Because each species differs in its relationship to the ocean environment,
each species will be impacted differently by large scale changes in
climate and oceanography.
For humans and our fisheries, the impacts of oceanograhic phenomena on
species recruitment, survival and growth have very significant
implications for population dynamics and status, irrespective of our own
impacts.
Furthermore, the movement of habitats and therefore the tuna that
associate to them, as a result of changes in oceanography, have
significant implications for catchability of these species.
It is imperative that we understand the relationship between oceanographic
and climatic phenomena and fisheries catches for ensuring more
reliable assessments, and for ensuring management decision making
processes that take account of this.