Trophic cascades 2

Question 1

When did fishing landings and power increase and decrease?

Answer 1

Thurstan RH et al. (2010) Nature Communications -

Relates to the North Sea – landings of demersal fish. Record going back to the 1880s. Dips correspond to world wars.

Change in the fishing effort is complicated by sail vessels up to the 1900s. From 1890’s steam, 1940 on surge in diesel power.

Fishery landings

• Monitored since 1880s
• Dropped in both world wars when fishing declined
• Overall growth 1889-1940s
• Decline 1950s-present

Fishing vessels

• Unit = typical sail-powered fishing boat
• Sail began with 2.5 fishing power of steam
• Steam then grew rapidly, peaked in the 1930s, declining from 1940s
• Total fishing power peaked in 1972

Question 2

How do landings per unit of fishing power change for fishing activity over the past century?

Answer 2

Landings

• Landings per unit of fishing power (LPUP): correcting for changes in fishing power
• Landings per unit effort (LPUE, t/100 h)
• Relate these different types of fishing activity to a scale of fishing power.
• LPE – only from the 1980’s do we have data that made it possible to get an idea of the number of hours spent fishing.
• Previous records were very crude.
• From the 1950’s a dramatic decline

Question 3

What phases of fishing development impacted on the industry?

Answer 3

• Phase 1 1889-1914: rapid industrialisation, intensification of fishing, fleet converting from sail to steam
• Phase 2 1919-1939: further expansion, distant grounds (Arctic, W Africa)
• Phase 3 1956-1982: rapid decline, distant waters fully exploited, contraction of grounds (e.g. Iceland EEZ 50 then 200 n miles) as a response to fishery depletion
• Phase 4 1983-present: Common Fisheries Policy etc

Question 4

What study shows the conversion of a UK fishery?

Answer 4

Another study by Ruth Thurstan for the Firth of Clythe.

Landings as a whole go down. Amount of Roundfish (pelagic – herring) show a greater decrease than Flatfish. While the roundfish and the flatfish decline – trawl bands – 3 mile limit – but the decline shows through.

1985: 40% of landings mass was shellfish
2008: Nephrops 84% (rest = other invertebrates e.g. scallops)

9kg bycatch per kg of Nephrops

Northumberland: catch currently mostly lobsters, Nephrops, crabs.

Dominated by volume and value by invertebrates. Trophic cascade thing going on - removed big things from system and invertebrates are benefitting.

Question 5

When looking for top-down or bottom-up effects on the food chain (fisheries) how may looking at grazers help?

Answer 5

Looking at the abundance of the plants relative to thew predators.

Look at trends over time, relationships between these things over time.

Top-Down

If you looked at the abundance of predators, algae and grazers you would be looking for an inverse relationship between the predators and the grazers. The more predators there are the fewer grazers there will be - generating a +ve relationship between predators and algae, and inverse between grazers and the algae.

Bottom-up, everything bis positively related. More algae, more grazers, more predators.

Question 6

How may changing the food web from predator, grazer, algae to predator, mesopredator, gazer, algae effect the relationships between trophic levels in a top-down cascade? ?

Answer 6

3 trophic levels.

• Predator levels have a positive relationship with algae.

4 trophic levels

• Predator has a +ve relationship with grazer and a -ve relationship with mesopredator and algae.
• Grazer and algae still have a -ve relationship.

Question 7

What factors are at play in populations aside from predation.

Answer 7

Populations as dynamic entities:

(1) Different sources of mortality
(2) But supply-side also important: recruitment

• We are trying to find patterns in complex ecosystems. Individual populations are very complex. Lots of things influencing the size of the population.
• A population can remain stable in the face of lots of mortality is recruitment is high but will be seriously depleted if recruitment is low.
• Making an assumption that predation is the main driver of that mortality
• Making the assumption that recruitment is not a major driver of abundance.
  *

Question 8

Explain a study on abundances that shows fluctuations over a long time period?

Answer 8

Ravier C, Fromentin J-M 2001 ICES J Marine Science

They discovered, in Sicily, longstanding fishing operations have maintained records from the 1600s. Catches vary around 100-year large cycle with smaller cycles in-between.

This study is complicated as bluefin tuna migrate along the from the Atlantic, into the Mediterranean into traps.

What actually drives the limits or average level of a catch is unknown.

~100-120 and ~20 year cycles

What are the limits, what sets them?

Question 9

What are the fundamental questions posied by fluctuations in populations?

Answer 9

Do these population sizes vary about mean values?

What determines these and why do the populations return to them?

Do the population sizes have set limits, of so what determines them?

What are the fundamental factors and processes in community dynamics?

Are the ecosystems deterministic or chaotic? If there are elements of both, to which parts of the system do they apply?

Question 10

Give some population models.

Answer 10

• If you suppose the rate of change of biomass is a function of the biomass that is present at any particular time. A limit is reached and the population crashes. Boom and bust model – not equilibrial.
• As density increases so do other effects such as predation, competition, disease atc, meaning the model would stabilise.
• Schaefer model (picture) used heavily in fishery science, relies on there being a very simple sigmoid pattern of population growth. If you relate the growth rate as indicated by the slope in relation to population (biomass), as that declines – the effect of fishing (at higher levels of population size) growth rate increases and at lower levels some kind of +ve effect. Term carrying capacity.

Question 11

Why may fish populations not support the high level of recruitment from spawning you Nextmight expect?

Answer 11

Myers RA, Worm B 2005 Proc Royal Society

Calculated the lifetime maximum reproductive rate. 144 different fish populations if different habitats.

In spite of mindset we have of animals having many eggs, their lifetime reproductive rate is extremely low.

Data on 114 fish populations, all habitats, tropical to polar, coastal to oceanic

Reproductive rates of fish are in the range of 2-40 recruits per spawner - not too far from those of other vertebrates!

Typical fishing mortality of 0.4 would render 42 species extinct

Question 12

What determines population size and community structure?

Lottery hypothesis

Answer 12

Compare this with ‘Competition hypothesis’ (deterministic model) where presence of a species and community structure determined by habitat acting like a template.

Lottery hypothesis: characteristics

• Species may be ecologically similar to each other
• Randomness of larvae - community structure depends on who arrives at the scene first, described as chaotic
• Whilst individual populations would not be in equilibrium with resources, the guilds of species would. A particular patch of habitat could only fit in so many individuals of competing species, and that total size of guild would be in equilibrium with resources
• Species composition changes stochastically (chaotic)

Lottery hypothesis: weaknesses

• No easy way at looking at whole population scale, data from small scale studies. Small areas will have greater variability.
• Strong associations occur between particular species, when studies have taken place on a larger scale, suggesting something more orderly is going on.

Question 13

What determines community structure?

Answer 13

• Recruitment limitation
  
  • High species turnover on artificial reefs (ie. identical habitat chunks)
  • High mortality of fish eggs and larvae
• Importance of larval recruitment?
  
  • Wind direction influences damselfish settler density sometimes
• Do egg and larval mortality govern recruitment?
• What governs large-scale patterns?
• Are populations ‘open’ or ‘closed’ in recruitment terms?
• Larval starvation, dispersal and predation hypotheses
• Does food concentration/type influence larval survival?
• What role does dispersal play?
• What evidence is there for predation being important?

Question 14

Starvation hypotheses

Answer 14

Match-mismatch hypothesis: spawning time is fixed but plankton production cycle is not, the larval pulse may not coincide with conditions conducive to successful first feeding (DH Cushing)

Studied: Barents Sea cod larvae and Calanus Stage 1 nauplii densities in 1960, 1980-81, 1983-85

Results Variations in copepod spawning, such that mismatch in 1960 (warm, early) and 1981 (cold, late)
Temperature affects year-class: warm years (e.g., 3-4C) weak and strong, but cold years (e.g., 1-2C) only weak.

1960 and 1981 weak year-classes, 1980, 1983-1985 stronger on average
1980 salinity anomaly, 1983-85 adverse oceanographic events after first feeding?

Conclusion: timing of food supply is important for cod, but other factors also affect larval survivorship

Factors include -

food and temperature affect growth rate, in turn affecting the duration of vulnerable stages.

More predation increases the food per larva creating larger larvae. This may reduce predation later increasing the numbers of larvae.

If the limiting factor is food, increasing food would increase the size of the larvae (and decrease predation).

If size is the limiting factor - less predation would increase larvae numbers

Question 15

Predation hypotheses

What evidence that predation on larvae is influential in survivorship?
What level of predation and what are the predators?
What is their impact

Answer 15

e. g. Hewitt et al.: Californian jack mackerel
(1) egg and yolk-sac larva mortality very high, must have been largely predation because yolk provides food after first feeding

(2) histological evidence of starvation was then very high over a week or so
3) then declined to zero again, when predation again important

e. g. HW van der Veer: Wadden Sea
(1) jellyfish and ctenophore predation only in May

(2) low predation on plaice larvae because
(3) high on flounder larvae which abundant from April
(4) 2.5% of freshly-caught ctenophores have flounder in guts, this predation equivalent to 50% of starting density in a few hours!

Conclusion: predation mortality on larvae may be very important, but most evidence for role of predation highly circumstantial,

Question 16

Dispersal hypotheses

Answer 16

Observed Emergence of capelin larvae in July off Newfoundland coincides with water masses favourable to them - onshore winds, which replace cold water with warmer water.

Offshore winds: greater larval mortality in cooler water (abundant ctenophores, jellyfish and chaetognaths)

Studied In situ enclosures: larval growth correlated positively with food concentration
Food of the right size (<250um) more abundant with the onshore wind

Conclusion: ‘safe site hypothesis’- both feeding and predation conditions favourable

Question 17

The Spawning of the Capelin

STUDY

Background

Answer 17

​The Spawning of the Capelin
Leggett and Frank 1990
How starvation hypothesis and predation hypothesis can affect recruitment

• Concentrated on early life stages from spawning to metamorphosis into juvenile form
• Early period last approx. 10 months and accounts for 99% of mortality
• Capelin larvae found Newfoundland beaches – wait for onshore winds to spawn
• Used large containers in the sea (Mesocosm) containing 4000L of water – prevents unnatural proximity of prey and predator
• Larvae were fed zooplankton typically found in warm surface waters and cold surface waters alternatively
• Lecithotrophic larvae

Question 18

The Spawning of the Capelin

Results:

Answer 18

The Spawning of the Capelin
Leggett and Frank 1990
How starvation hypothesis and the predation hypothesis can affect recruitment.

Results:

• Greater the egg density the lower the survival rate
• Mass release of and hatching of larvae coincides with onshore winds which cause warm surface waters to displace cooler waters below
• Why is it beneficial to spawn in the warmer water?
  
  • Warm water harboured small zooplankton which was suitable prey for first-time feeders whose jaws were not capable of consuming larger prey
  • Coldwater also had larger zooplankton in which first time feeders could not consume with ease and also elevated numbers of jellyfish which prey upon fish larvae
  • Therefore, synchronizing spawning time with warm waters increases survivorship by both predation hypothesis and starvation hypothesis
• Larvae which consumed warm water zooplankton grew twice as fast and died at 1/3rd of the rate as those fed cold water zooplankton
• Supports the view that predation may be an important factor in larval survival
• However, if the larvae spend to much time on the beach – due to late arrival of onshore winds - deplete yolk reserve and forced to hatch and leave the beach in unfavourable conditions where there is less food and they are unable to evade predators
• Yolk reserves last 5.5 days – correlated nearly with a change of winds from offshore to onshore every 6.3 days

Question 19

Bottom up controll across trophic levels

STUDY

Background

Answer 19

From plankton to top predators: Bottom-up control of a marine food web across four trophic levels
Frederiksen et al., 2006
Control by food availability

Background

• lesser sandeel (Ammodytes marinus) – Mid trophic level fish that supports fisheries and mammals and birds alike
• Feed on small copepods
• In the North Sea
• Need to know sandeel population is controlled by bottom-up (Food availability) or top-down approaches (Predation and fisheries) – used for fish oil and meal
• Analysed 4 trophic levels phytoplankton, zooplankton, sandeel larvae, seabirds over a 30 year period in the NW north sea
• Used 5 species of seabird
• common guillemot carries single prey to the colony and relies on sandeels >1years old

Question 20

Bottom up controll across trophic levels

STUDY

Results

Answer 20

Results:

• A strong link between diatom abundance and copepod abundance – though no major linear trends over time
• Failing of sandeel populations in 2002-04 and consequential crash in the breeding of seabirds in 2004 – as usually 1 y/o sandeels are eaten – demonstrate the bottom-up control
• Biomass of sandeel larvae was positively associated with phytoplankton during their growth period and seabird breeding productivity was higher when sandeel larval biomass had been high in the previous year – Suggesting bottom up cascade – OTHER PROCESS OTHER THAN PREDATION
• Under this scenario, seabirds can be reliable, and often cost‐effective, indicators of marine environmental conditions – Therefore useful in generating quotas for sandeel fishery management
• However record herring populations in 2004 could have caused increased predation on sandeels, in addition to copepods varying in lipid quality – so not as much food after metamorphosis could both because for the sandeel crash – Top-down and bottom-up
• Overall in the sandeel populations – bottom-up controls the cascading of the food webs

Question 21

Ecological Meltdown in the Firth of Clyde, Scotland: Two Centuries of Change in a Coastal Marine Ecosystem

How has fishing pressure been shifting in the last 100 years?

Answer 21

Ruth H. Thurstan, Callum M. Roberts

PLoS ONE

• Throughout the North Atlantic, many once productive fisheries have exhibited severe declines as fishing activities have intensified during the last two hundred years [1], [2].
• To compensate for decreasing catches, fishers have shifted onto other species, gradually targeting species at lower trophic levels [3].
• Improvements in technology also enable vessels to exploit fishing grounds further offshore and in deeper waters, often using gear that is invasive and damaging to seabed habitats [4].
• It is now recognised that marine biodiversity is rapidly declining as communities and habitats are degraded through fishing [5], and the recent decline of global fish landings has begun to reflect this [6].
• It is becoming increasingly important to understand how much humans have degraded ocean ecosystems in order to set appropriate management targets for their recovery.

Question 22

Chnage in fishery species in the firth of Clyde

Answer 22

Ecological Meltdown in the Firth of Clyde, Scotland: Two Centuries of Change in a Coastal Marine Ecosystem

(Thurstan and Roberts, 2010)

• During the 19th century, diverse fisheries existed in the Firth of Clyde for herring (Clupea harengus, Clupeidae), cod (Gadus morhua, Gadidae), haddock (Melanogrammus aeglefinus, Gadidae), turbot (Psetta maxima, Scophthalmidae) and a variety of other species.
• Today these fisheries no longer exist; instead, the main targets are invertebrates such as Norway lobster (Nephrops norvegicus, Nephropidae; hereafter called Nephrops) and scallops (Pecten maximus, Pectinidae). These are usually fished using otter trawls and dredges, both of which are dragged along the seabed, which disturbs bottom-living species and destroys fragile habitats
• This is a far cry from the less invasive static nets and hooks that predominated 130 years ago. Whilst it is likely the gear then in use also had some adverse effects upon fish populations, the industrialisation of fisheries from the 1880s has intensified exploitation beyond sustainable levels for many marine species.

Question 23

Current fisheries in the Firth of Clyde

Answer 23

​Ecological Meltdown in the Firth of Clyde, Scotland: Two Centuries of Change in a Coastal Marine Ecosystem

(Thurstan and Roberts, 2008)

• In 2008, finfish such as cod, plaice and herring constituted less than 2% of landings by weight (Figure 13a).
• Eighty-four percent of landings by weight were Nephrops (Figure 13b), representing nearly 87% of the value of Clyde fisheries (Figure 14).
• The remainder was composed of other invertebrates such as scallops, crabs and lobsters.
• Discard ratios are very high in the Clyde Nephrops fishery, with 9kg of bycatch produced for every 1kg of Nephrops caught [69]. 25,000 tonnes of discards are generated every year in the Firth of Clyde from Nephrops trawling alone [70], and it is likely that many of these organisms die when returned to the sea [71].
• The other fishery that remains is for scallops which are caught using heavy steel dredges with teeth that tear up the sea bottom. This method of fishing has been shown to destroy biologically sensitive habitats and alter marine ecosystem functioning [72], and will therefore exacerbate impacts of Nephrops trawling.
• Over the course of two decades, there has been a remarkable shift in the groups of species landed from the Clyde.
• In 1985, finfish made up over 60% of the landings by weight and 37% by value, compared to just 2% by weight and 0.5% by value in 2008 (Figure 15).
• Towards the end of the 20th century, species landed at Clyde ports shifted from predatory fish to invertebrate species of a lower trophic level. Nephrops landings into Clyde ports have plateaued in recent years (Figure 13b).