Lecture 14: Fisheries 1 & 2 Flashcards
“Fish” (finfish and shellfish) are important for food production, economics and employment
Key source of animal protein in developing countries
Finfish – standard fish
Shellfish – crabs and molluscs but also includes echinoderms, cephalopods sea cucumbers etc. Basically just a term for all non- finfish
Total value of fisheries production (capture + aquaculture), 2016, US$ 362 B (FAO, 2018) [aquaculture: US$ 232B, capture: US$ 130B]
see FAO figure in notes
^catcher fisheries orange – plateau due to saturation/over exploitation
^aquaculture blue – growing industry
Check FAO fishery statistics for more up to date info
Fish are important to food production
Total value of fisheries production (capture + culture), 1999, US$53.4 B (FAO, 2000)
Capture fisheries are saturated – annual increases are mainly from aquaculture
increase is mostly related to aquaculture catcher fisheries are in decline now due to over exploitation
see figure in notes
^ human consumption vs per capita food fish supply and fishmeal info for terrestrial fodder and aquaculture feed.
Total value of 2008 capture fisheries + aquaculture production = US$ 192.3 billion contribution from each, about equal
4.3 million fishing vessels around the world in 2008
Note annual increases & contributions from capture fisheries
see figure in notes
^ big increase in inland aquaculture
^ importance of asia and particularly china in aquaculture and catch contribution
Misreporting of data from large population countries e.g. china impacts global estimates
State of world fisheries resources
Production trends are upward but only because of aquaculture.
China is the most important nation for fisheries production – about 2/3 is from aquaculture, principally from freshwater.
World capture fisheries at saturation (or decline).
FAO (2010) estimates 85% (2012- 87%, 2016 – 93%) of world’s fish stocks to be fully exploited, overexploited or depleted or recovering. Many in decline
Case study: Northern Cod
Northern (East coast US) and North sea (East coast UK ocean) are 2 populations of the same species that are referred to as two separate ‘fish stocks’
see figure in notes
^ massive increase in catch post world war 2 using sonar developed for war submarines to identify locations of large shoals and capture whole shoals
^ landings is not the same as stock levels – landing will be 0 if fishing is banned but landing would also be 0 if none of that species was being captured in that area anymore due to stock relocation e.g related to el nino cycles – as you can see in the 1980s excess heat reduces plankton levels resulting in negative impacts on catch
Be careful when interpreting fishing data always check how measurements are being defined
^ this is also a huge social and economic problem as if you have cannery factories that employ many workers and suddenly there is a temporary lack of catch then this puts their income at risk
^ ofcourse it also causes potential knock on effects in market value and can outprice consumers + result in food shortages in areas that rely on fish as a dietary staple
Sablefish – deepwater rocky bottom reef fish
see photo:
Unusually large cod (Gadus morhua) for the North Sea (Age 5 years) – most do not survive to Age 2, the age at which they start to reproduce
Many cod never reach maturity, if caught undersized they are thrown back – of which most die
Cod have high fecundity so females that reach the size pictured have great potential to recover populations – adult female ~1million eggs per year – as they age they become larger and larger clutches of eggs
How widespread is catch depletion?
How widespread is catch depletion?
Relative biomass estimates from standardised fishing effort
Myers & Worm (2003) Nature 423, 280-283.
see fig.1 in notes;
^ biomass levels decline
^ gleaned from fishery log books of large commercial trawlers
^ catch per unit of effort (CPUE) used for a measure of relative abundance
Vast declines recorded :
a-i : ‘ocean’ pelagic - fishing mid-depth - Long lines targeting large predatory fish (sailfish, swordfish, tuna) measure = catch per 100 hooks
J-m ‘shelf’ benthic/ demersal trawl usually using nets – catching things near the bottom (cod, haddock,whiting) that feed on the sea floor and also fish that live on the seafloor (flatfish like plaice, sole etc. Measure = estimated stock biomass
Spatial and temporal patterns of predator biomass (e.g. shark, tuna, billfish)
See: Myers & Worm (2003)
Areas depleted of large carnivore fish experience issues associated with lack of apex predators in the same way as in terrestrial ecosystems e.g. absence of wolves in yellowstone
^colours represent CPUE red for high and blue for low
^ Asian fishing boats target more pelagic fish
Basically all oceans show the same kind of response with perhaps the exception of the atlantic due to large marine sanctuary zones – but even there some countries are harvesting large quantities of krill in pelagic fisheries
Compensatory fisheries
Start with most prized species - as these fished out & become less profitable, pass on to other (usually less desirable species).
Leads to flux of responses in community components (Myers & Worm, 2003).
Higher trophic levels targeted first (large carnivores).
see fig. 3 in notes
^CPUE goes down e.g. in Tuna then you start catching sailfish and then swordfish
Fishing from most to least valuable – sustaining catch levels by switching species caught
Dwindling fish stocks are not given time to recover
Notably sometimes a reduced population of adults of one species e.g. cod can reduce competition amongst adults of another species e.g. flatfish and reduced predation of young resulting in an increase in abundance of the non-catch target species
Long-term depletion in megafaunal fishes
Most British fishing is bottom trawl and has hugely depleted megafauna in north atlantic
Sguotti et al. (2016) Global Change Biology
E.g. Common skate (Dipturus batis complex) and angel shark (Squatina squatina) extirpated in North Sea by 1970s
^ Now both listed as critically endangered by IUCN in North Atlantic
^ These rare k strategist species unintentionally captured by fisheries and added to fish markets
*long history of continental shelf trawling: Late 1800s – Royal Commission evidence from fishermen: dragging and damaging 0.6 m wide soft corals from North Sea (UK part) with great frequency
Soft coral damage has been recorded since 1800s and nothing has been done for the north sea
Worsening shark mortality despite conservation measures
*Global shark mortality rising, despite increasing conservation measures designed to reduce shark finning & prohibit landing of threatened species.
*Estimated 80-101 million sharks killed in global fisheries in 2019 and 25% of these are threatened spp.
*Worm et al. (2024) Science: https://www.science.org/doi/10.1126/science.adf8984
“Fishing down the food web”
A term used by Daniel Pauly et al. (1998) – Science.
Dramatic reduction in mean trophic index of fisheries / fish communities over 40 years.
see food web figures from Jackson (2001) PNAS, 98, 5411-5418
Direct and indirect impacts – complex food webs get disrupted by fisheries
-fishing algal grazers causes algal blooms that cover corals shading them and preventing their photosynthesis resulting in coral decline resulting base level change in trophic and physical structure of the marine environment
-benthic trawl nets uproot seagrasses
-nutrient increase (e.g. due to aquaculture) also disrupts seagrass growth
Is aquaculture the saviour? It has potential
Ongoing research: How can we generate an aquaculture industry that benefits the environment to provide good quality produce for a growing population?
*Not a saviour for top predators farming e.g. salmon, cod, bass, sea bream. These species are typically reared in western aquaculture and this practice is not sustainable because: Typically adds a trophic link (with associated lower conversion efficiency) as they are fed on fishmeal we do not directly consume the small fish source making it inefficient in a similar way to growing soy to feed to beef cows
*Source of feed? Fishmeal (e.g. from sandeels, anchoveta) for carnivorous fish e.g. farmed salmon.
Depleting fish essential for seabirds to feed their young and disrupting the food web
*Anoxic sediments under cages – due to feed and waste buildup + eutrophication risk reducing benthic fauna
*Reduce exploitation of wild stocks?
Wild stocks are still increasingly depleted due to :
*Disease from closely grown farmed fish transfer to wild stocks
*Farm escapees breed with wild stocks causing genetic introgression reducing wild stock resilience
does provide important income and is more efficient in eastern practice of prawn fishing and herbivorous or freshwater pond aquaculture however prawn aquaculture has a huge neg. Ecosystem impact in mangrove coastal areas:
*Asian/ S. American prawn-farmed areas have ~10% of ecosystem services economic value after mangrove removal (loss of storm surge protection).
*Best option: farming herbivorous fish e.g. some carps – China. But still, manuring of ponds used to increase the quantity of microalgae for fish to feed on results in nutrient buildup which leaches into the water table and into surrounding areas causing hypereutrophication
Impacts of industrial fishing for fishmeal
Planktivorous mass-shoaling, small ‘forage fish’ such as sandeel and Peruvian anchoveta are heavily fished for producing livestock feed / fertiliser
- In UK part of North Sea, average (2015-19) of 257,000 tonnes of sandeel landed, 96% by Denmark. 2021: 446,000 tonnes (11 billion sandeel) landed in whole of North Sea. New ban in UK part (Jan 2024) – see later material…
*Impacts on food webs, including charismatic seabirds such as puffin
^ due to decline in easily digestible fish species which are collected in fishmeal fishing
Wider ecosystem impacts
Impacts on non-target species (Kaiser et al., 2005)–
- accidental capture / injury of non-target species e.g. diving seabird capture with longlines (like albatross); ghost fishing
- mortality of discarded biota, e.g. turtles, fish, cetaceans, many invertebrates – bycatch and undersize thrown back and most die
- physical impacts upon habitat during fishing processes, e.g. damage to benthic habitats during trawling; damage to fragile coral (soft coldwater corals), seagrass habitats etc
*altered foodwebs and marine communities
see photos in notes: ghostlines, trawl in florida, coral damage in sri lanka collecting fish for the ornamental trade a very lucrative livelihood for those in reef regions of developing countries
Bottom trawling has severe impacts on benthic diversity
*Not all corals are in the tropics
*Cold, deep waters have diverse soft coral, sponge, crinoid and polychaete communities giving much structural diversity & hosting large biodiversity
*Parts of the North Sea are bottom-trawled (equivalent to terrestrial ploughing or tilling) 5 times a year. Some have been for 60+ years…
this disrupts life cycles and prevents ecosystem recovery
Fishing is not the only stressor on marine biodiversity – but it is an important one…
See wheel of threats to marine organisms in notes from: Martin et al (2021) Ten new insights in climate science 2021: A horizon scan.
Global sustainability DOI:10.1017/sus.2021.25
*UK has lost ca. 90% of its seagrass habitat in 30 years (pollution, disease, physical destructive activities, coastal development
*50% coral cover (hard coral) decline from 1957 to 2007; 14% global decline in coral since 2009 (ocean warming, physical destructive activities, pollution, ocean acidification)
Types of fisheries
Artisanal, Commercial, Industrial, Recreational
^ scale causes the difference
Small scale can be highly damaging too e.g. bomb fishing and poison usage
Majority of world’s consumption fisheries are in shallow marine, but freshwater very important for landlocked countries, especially developing ones
Pair trawling is the 2 boats together
Major commercial fishing methods
see figure in notes
+ simple traps, nets, spears, baited lines, [explosives, poisons] in artisanal fisheries
Purse seine is a medieval approach to net whole shoals less commonly used today
Comparison between major commercial and minor artisanal fisheries
See figure in notes
Aims of fisheries stakeholders
Artisanal – provide enough food for family, sell surplus, low demand for intensification of fishing activity
Commercial – maximise profit, by maximising yield of harvestable (and saleable fish). Necessitates opportunistic and selfish behaviour. However, high capital investment in vessel, gear requires some sustainability of income (but, can switch)
Fish processors – sustainable profit because of very high capital investment (sustained and predictable source of fish of right type and size)
Fish marketers – relatively sustained source and magnitude of landings
Environmentalists (conservationists) – long-term sustainable use of resources, favouring local populations and minimising environmental damage & biodiversity loss
Fishery managers – sustain fish stocks for broad community and provide for all interests, while seeking to limit environmental damage
Politicians – Votes!!!!???? tradeoffs, a middle way?
Key:
*Conservation for recovery is essential
*If legislations for artisinal fishing are too strict people can starve
*Big companies are profit-focused
Climate change is causing mismatch increase
Match - mismatch hypothesis - MMH)
Many species of fish e.g. cod, plaice, herring rely on zooplankton (rotifers, copepod nauplii etc) as food in larval stage.
Cushing et al. suggested MMH in crucial first few days of life where level of mortality has massive influence on future recruitment
Matching (good survival) or mismatching (poor survival) of peak larval production with availability in space and time of peak zooplankton.
Spawning and stock recruitment relationships ‘Ricker curves’
Expect an increase in recruitment with number of eggs released (fecundity) which is directly related to number of spawners
Up to a maximum
Thereafter, recruitment levels off
or even declines because of the
effects of intraspecific competition
on survival
Where density-dependent processes are most developed
e.g. territoriality in brown trout,
stock-recruitment relationship is
most apparent
“Ricker Curves”
= poorer recruitment can occur with large no. Of adults due to competition amongst young – e.g. very stochastic with cod
^ Underexploited fisheries can be used to estimate natural mortality
Exploitation models
Exploitation models
Data split by age to assess F, M and recruitment annually and recombined on the basis of:
‘Cohort analysis” or “Virtual Population Analysis”
(do not confuse with Viable Population Analysis, VPA in Conservation)
What is a stock?
A group of fish (or other organisms) of a species, that will react independently to exploitation from other such groups and can be managed as an entity. Usually self-sustaining, though there may be immigration / emigration (which can disrupt counts and estimations of stock size).
e.g. Herring on UK coast
west stock autumn spawning east stock summer spawning but can mix
Surplus production models and “maximum sustainable yield”
Idealised exploitation of one cohort of a fish stock
see figure in notes
Ideal to harvest at peak and avoid harvest of pre-recruits (individuals that have not reached sexual maturity) replacing natural mortality with fishing mortality at peak body mass
Bigger fish preferable particularly in western markets where wasteful consumption practices like filleting are used whereas in the east the whole fish are more commonly used and size is slightly less important
Surplus production models and “maximum sustainable yield”
- Population size (number or biomass) described by logistic growth model.
*Growth fastest at half of carrying capacity.
*As population increases further towards K, intraspecific competition slows population growth (reduced individual growth, higher mortality rates).
*Cropping pop. back from K, reduces competition & keeps population near fastest growth rate. Especially effective for pops with high r (NB steady state – constant env.)
(see figure in notes)
Max. Sustainable yield is hard to estimate as the system is continually in flux and not steady as in the sigmoidal curve used in modelling see second year ecology
K/2 is point at most yield occurs in right hand graph – this is the ideal max. Sustainable yield but due to interactions and unpredictable flux max. Yield estimates are often too high
Concept of the “surplus yield” model (Beverton & Holt)
*Within-population mortality rates close to carrying capacity are higher than at moderate densities, because of increased intraspecific competition.
*“Cropping” of population (e.g. by fishery) reduces competition & enhances survival and growth of remaining individuals, including @ key life cycle ‘bottlenecks’. The degree of cropping possible is the ‘surplus yield’.
*At some point, cropping exceeds the rate at which compensatory, density-dependent processes can replace fishing mortality – driven towards extinction.
see figure in notes
^ Overexploitation catch rate decreases driving biomass down – catch is higher but size lower – biomass reaches 0 at extinction
^ When catch rate exceeds birth rate stock goes down
^ To avoid hitting the tipping point maximum sustainable yield needs to be lower than prev. Estimates
Five typical stages of evolution of a commercial fishery
Historical examples e.g. in Cushing (1988), Smith (1994), Kurlansky (1999).
1 – find the market/suitable approach,
2 – rapid exploit increase,
3 – saturation,
4- crash/decline due to overfishing,
5 -regulation and recovery – usually only partial and only a small portion of the original stock size is recovered See Patagonian Toothfish study
Linked to tragedy of the commons – international access to the same resources – race to reach quota first before complete annual exploitation is reached and fishing permit is cut off
‘Tragedy of the Commons’ - selfish behaviour in the pursuit of a freely accessible resource [or jointly managed resource with equal access] – common in fisheries….
Brief history of early fisheries science
The Great Fisheries Debate
“Probably all the great sea fisheries are inexhaustible”
^Huxley (1883) Royal Commission on state of UK fisheries
Size declines were being reported but scientists overlooked this
Late 19th Century, fisheries biology in its infancy. Most did not foresee the possibility of overfishing
New technology allowed for more efficiency and resulted in overfishing, by 1900 rapid mechanisation led to exploitation of the whole north sea in areas never prev exploited
*Rapid mechanisation
*Steam power – mechanised hooks so humans no longer had to drag in the net
*Steel hulled boats – able to function better on rough open water
*Power block
*Diesel engine
*Sonar - to capture whole shoals
The growth of fisheries science
Decreasing plaice (Pleuronectes platessa) catches in Firth of Forth, Scotland (and North Sea)
Despite Huxley’s (ultimately) incorrect views concerning likely impacts of fisheries in large environments, he insisted on an experimental approach.
Experimental fishing conducted to measure stock characteristics in areas open and closed to commercial fishing/
Careful collection of fisheries statistics/
Subsequent analysis contradicted Huxley’s assertion (Garstang, 1900).
From Grimsby, catch of fish per standardised vessel declined by 33% as fishing effort increased by 150% (Garstang, 1900).
The Great (War) Fishing Experiment
Fishing activity in the North Sea increased through late 19th and early 20th centuries (more demand, apparent depletion of larger target fish). Area of fishing widened (‘refuge’ areas reduced) with increased fishing capability
Fishing greatly reduced during WWI – marked effect on post-war catches
Identified the prospect of population recovery. Replicated in WWII; same result.
Postwar plaice size was far greater due to limitations on fishing implicated by submarine presence during war allowing them to reach maturity and thus larger body size: see fig. in notes
Size frequency distribution of captured plaice shifts to larger fish, due to increased survival through to older (larger) classes (Borley et al., 1923) following the first world war strong recovery of fish stocks was observed
Development of single species, surplus-production models
Initial single cohort, for one species
Intermediate level of fishing mortality ‘replaces’ natural mort. from intraspecific comp. Encourages improved survival of remaining individuals through reduced competition
“Optimum” amount of fishing mortality (quota) gives maximum yield (MSY) (Beverton & Holt, 1957)
BUT – TOO SIMPLISTIC? INCREASINGLY REALISED NOT TO BE ‘ REAL WORLD’ WHEN LOW ENVIRONMENTAL DAMAGE IS REQUIRED FOR CAPTURE FISHERIES
Impacts on many non-target species in trawl systems – see pics in notes of bycatch discard
Development of single species surplus production models – too simple
fisheries have low selectivity making it hard to determine quotas due to the huge amount of bycatch and discards
FISHERIES…demersal (near bottom) net fisheries catch a range of cohorts for single species, as well as a range of species – low selectivity. Difficult to set single species MSY
STOCHASTICITY IN ENVIRONMENT causes year-to-year variation in natural mortality (difficult to set right quota)
BYCATCH - non-target catch - problem for fixed quota
DISCARD – thrown back (no value, or forced return) – problem for fixed quota, high mortality of target & non-target spp but birds do use bycatch as a source of food
Development of single-species, economic models
Income dictated by catch obtained relative to the effort applied – we know that steady-state catch rate is highest at intermediate effort.
Cost of fishing increases proportionally to effort.
Fishery profit yield is maximised at a lower effort than MSY (more precautionary)
BUT – still a single-species model.
BUT – Still an equilibrium model.
How stable are natural fish populations?
see figure
effort modelling could be more beneficial than quota as it determines how cost will increase with reduced yield
Still a single species equilibrium model which does not consider flux, interactions and bycatch
Stochastic variation in fish populations
…occurs in many fish populations
See in notes an extreme example - of fish population impacted by El Nino :
Figure 15.4: Total catch for the Peruvian anchovy fishery, 1950-2005. This fishery was the largest in the world until it collapsed in 1972 during an El Niño event. In spite of reduced fishing, it took 20 years for the fishery to recover. (Data from FAO FishStat.)
Note that South American waters are exploited internationally with many commercial fishing vessels coming from china, korea, japan US etc.
National versus international fisheries
National waters around coastline (+ inland fisheries) theoretically enable autonomous fisheries management
BUT many countries share fishing rights through international treaties, requiring, international cooperation for stock assessment and management
Nationally managed Icelandic cod fishery (and “Cod Wars”) vs. North Sea cod fishery
+ BREXIT! Icelandic cod is responsibly managed – meets marine stewardship requirements
In international waters there is no national limitation, except by international treaty
Long-term cooperation in North Atlantic for surveying / management of fish stocks
ICES – International Council for Exploration of the Sea (1902)
50 x 50 km grid squares form basis for collection of fisheries data
Fish move around – fish in british waters will be in danish waters at another time of year
Information sources for stock assessment and management advice
Independent fish stock surveys e.g. Government research vessels “Scotia”
Data from fisheries – Fishing vessels & markets
Data can be inaccurate as fish vessels will try to cheat the system e.g. covering quota limited catch with a layer of un-quota reg fish
We need to collect more info on fish that are killed and stunned due to fishing technique that are not caught and the bycatch discard
Information needed to assess exploited stocks
1)Age / size composition and individual growth (including age-length and length-weight relationships)
2)“Birth rate” – actually Recruitment – incorporation of a cohort into the fishery, at the size at which they become susceptible to capture, after which they are progressively fished
3)Stock size – absolute (ideal) and relative measures (usually used as actual is hard to verify)
4)Mortality – Total mortality (Z) comprises natural mortality (M) in absence of fishery and fishing mortality (F)
5)Movements / distribution of stock
Also, to manage, need: Levels of fishing activity, types of fishing activity, locations of fishing
Size more important to fishermen age more important to management
Age and size are proportional as unlike humans, other mammals and birds many fish continue to get larger with age (some not this is genetic and environmentally impacted)
Size and age structure of stock
Measured from landed fish and independent surveys
In fish market, large samples of fish measured (gives length frequency)
Same at sea (but usually survey nets catch younger, smaller fish too)
Sub-samples for age, weight, sex, fecundity
Length-weight conversions , Age-length relations (with age validation)
Scales and otoliths – fish earbones and scales have patterns like tree rings that can be used to determine their age
Recruitment
Recruitment definition:
abundance of target fish as it becomes susceptible to the fishery (at a certain size)
Relative strength of recruited cohorts (Year Class Strength) assessed through regular sampling and ageing
Recruitment often highly variable in fish populations
Adult population may be strongly influenced or even dominated by one strong year class
– important fish fisheries, but also many poor year classes can put viability of fish stock at risk
one successful year class dominated this fishery for many years – so overfishing in one year can have longterm neg impact and will not be replenished in the same way every year
Quotas need annual adjustments – we need to understand stochasity and variation in recruitment
Match-Mismatch hypothesis
Many species of fish e.g. cod, plaice, herring rely on zooplankton (rotifers, copepod nauplii etc) as food in larval stage. Same as caterpillar emergence before arrival of spring breeding migratory birds
Cushing et al. suggested that in crucial first few days of life where level of mortality has massive influence on future recruitment
Matching (good survival) or mismatching (poor survival) of peak larval production with availability in space and time of peak zooplankton. (But match-mismatch very relevant in terrestrial envs. too).
The ‘Gadoid outburst’
A period of very good recruitment for cod (and many other gadoid fishes) between the mid 1960s and early 1980s – unusually cold period in the North Sea and much of N. Atlantic.
Beaugrand et al. (2003).
^warming period causes northward movement of plankton bloom and fish stock crash in the area
Variation in cod recruitment
Appears to have been driven by changes in the environment (climate?)
Altered biological conditions, especially zooplankton (Beaugrand et al., 2003)
Spawning stock recruitment relationships
Expect an increase in recruitment with number of eggs released (fecundity) which is directly related to number of spawners …..
Up to a maximum…
Thereafter, recruitment levels off or even declines because of the effects of intraspecific competition on survival
Where density dependent processes are most developed e.g. territoriality in brown trout, stock-recruitment relationship is most apparent
“Ricker Curves”
^ Ricker curve as prev. Mentioned
Intermediate smaller no. Of salmon strong relationship – but not strong relationship observed in trout
Catch and effort data can be used to estimate mortality
see schematics in notes
Total mortality (Z) = Natural mortality (M) + Fishing Mortality (F)
F = Official landings (L) + Illegal landings (L) + a/Discards + non-landed fishing mortality (S)
^ also can be estimated by decline in catch of older individuals
Recording of Landings
(one component of Fishing mortality) by species and port – published in Annual UK Sea Fisheries Statistics
Exploitation models
Data split by age to assess F, M and recruitment annually and recombined – basis of:
“Cohort analysis” or “Virtual Population Analysis”
(do not confuse with Viable Population Analysis, VPA in Conservation)
Fishery-Independent Methods
Standard survey transects (e.g. trawls, direct observation)
Egg / larval surveys & Hydroacoustics
see notes for figures
^ census of laid eggs
^ index of pre-recriutment for species such as cod
^ swim bladder returns response on hydroacoustics – analysis of frequency can determine shoal location and size of individuals – this can assist in identification of species type (but this is a guess/ not certain)
Can be more accurate for pelagic fish
Simple theory of underwater hydroacoustics for fish detection (figure)
Complex split-beam transducers and complex software can count and size individual targets or aggregate returns (e.g. shoals)
Main return is from material with different density
e.g. gas (swimbladder) in fish
Movements of fish stocks
*Need to be able to determine patterns of movement within stocks. Determine where they spawn, overwinter etc
*Shared use between geographically distinct fisheries / double counting
*Whether stocks are reproductively discrete (population genetics)
e.g. see notes
1.North Atlantic bluefin tuna stocks – East and West, satellite telemetry
2.North Sea plaice fishery – depth and temperature data-logging tags (Metcalfe & Arnold, 1997 and subsequent papers)
Telemetry including satellite telemetry
depth and temp log tags provide data for location per date
Stock segregation
*Are Eastern (of 45oW) and Western stocks of Atlantic bluefin tuna (Thunnus thynnus) separate? DNA studies suggest substantial segregation of spawning. But do they mix widely in the Atlantic fishing zones? Is ICCAT management effective?
*Tracking of ABT movements over 4 years using reconstructed tracks from archival (data logger) tags
See ^Block et al 2005
Tagging identified movement between two stock areas – not two discrete populations as below
Atlantic blue fin tuna stock segregation
Movements of all tracked Atlantic bluefin tuna, showing tagging locations and (triangles) and final locations (circles, either at recapture or tag loss), showing degree of separation and overlap of spawning stocks.
See ^Block et al 2005
Clearly, the nominal ICCAT management split of stocks at 45 degrees West is farcical. New approaches to ABT management being developed.
