Lecture 14: Fisheries 1 & 2

Question 1

“Fish” (finfish and shellfish) are important for food production, economics and employment

Key source of animal protein in developing countries

Answer 1

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

Question 2

Fish are important to food production

Answer 2

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

Question 3

State of world fisheries resources

Answer 3

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

Question 4

Case study: Northern Cod

Answer 4

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

Question 5

How widespread is catch depletion?

Answer 5

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

Question 6

Spatial and temporal patterns of predator biomass (e.g. shark, tuna, billfish)

Answer 6

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

Question 7

Compensatory fisheries

Answer 7

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

Question 8

Long-term depletion in megafaunal fishes

Answer 8

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

Question 9

Worsening shark mortality despite conservation measures

Answer 9

*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

Question 10

“Fishing down the food web”

Answer 10

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

Question 11

Is aquaculture the saviour? It has potential

Answer 11

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

Question 12

Impacts of industrial fishing for fishmeal

Answer 12

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

Question 13

Wider ecosystem impacts

Answer 13

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

Question 14

Bottom trawling has severe impacts on benthic diversity

Answer 14

*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

Question 15

Fishing is not the only stressor on marine biodiversity – but it is an important one…

Answer 15

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)

Question 16

Types of fisheries

Answer 16

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

Question 17

Major commercial fishing methods

Answer 17

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

Question 18

Comparison between major commercial and minor artisanal fisheries

Answer 18

See figure in notes

Question 19

Aims of fisheries stakeholders

Answer 19

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

Question 20

Climate change is causing mismatch increase

Answer 20

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.

Question 21

Spawning and stock recruitment relationships ‘Ricker curves’

Answer 21

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

Question 22

Exploitation models

Answer 22

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)

Question 23

What is a stock?

Answer 23

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

Question 24

Surplus production models and “maximum sustainable yield”

Answer 24

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

Question 25

Surplus production models and “maximum sustainable yield”

Answer 25

* 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

Question 26

Concept of the “surplus yield” model (Beverton & Holt)

Answer 26

*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

Question 27

Five typical stages of evolution of a commercial fishery

Answer 27

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….

Question 28

Brief history of early fisheries science

Answer 28

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

Question 29

The growth of fisheries science

Answer 29

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).

Question 30

The Great (War) Fishing Experiment

Answer 30

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

Question 31

Development of single species, surplus-production models

Answer 31

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

Question 32

Development of single species surplus production models – too simple

Answer 32

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

Question 33

Development of single-species, economic models

Answer 33

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

Question 34

Stochastic variation in fish populations

Answer 34

…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.

Question 35

National versus international fisheries

Answer 35

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

Question 36

Information sources for stock assessment and management advice

Answer 36

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

Question 37

Information needed to assess exploited stocks

Answer 37

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)

Question 38

Size and age structure of stock

Answer 38

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

Question 39

Recruitment

Answer 39

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

Question 40

Match-Mismatch hypothesis

Answer 40

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).

Question 41

The 'Gadoid outburst'

Answer 41

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

Question 42

Variation in cod recruitment

Answer 42

Appears to have been driven by changes in the environment (climate?) Altered biological conditions, especially zooplankton (Beaugrand et al., 2003)

Question 43

Spawning stock recruitment relationships

Answer 43

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

Question 44

Catch and effort data can be used to estimate mortality

Answer 44

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

Question 45

Recording of Landings

Answer 45

(one component of Fishing mortality) by species and port – published in Annual UK Sea Fisheries Statistics

Question 46

Exploitation models

Answer 46

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)

Question 47

Fishery-Independent Methods

Answer 47

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

Question 48

Simple theory of underwater hydroacoustics for fish detection (figure)

Answer 48

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

Question 49

Movements of fish stocks

Answer 49

*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

Question 50

Stock segregation

Answer 50

*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

Question 51

Atlantic blue fin tuna stock segregation

Answer 51

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.