Chapter 7

Question 1

Prokaryote – eukaryote

Answer 1

• Prokaryotes: unicellular - archaea and bacteria. Simple cellular structure, no organelles, no nucleus as DNA is in cytoplasm. They take care of most of the metabolic processes on earth (decomposition of organic material, main actors in global cycle of material/nutrients). Have no true sexual reproduction
• Eukaryotes are often multicellular, eukaryotic cells have subcompartments, organelles (e.g. mitochondria, chloroplast), DNA enclosed with a nuclear membrane

Question 2

What are Cyanobacteria

Answer 2

• Type of phytoplankton, commonly referred to as blue-green algae but name is misleading as they are prokaryotic (so bacteria and not algae).
• Cyanobacteria have photopigments (chlorophyll a) and are capable of photosynthesis under oxygenated conditions
• Occur in unicellular, filamentous or colonial forms. Most are enclosed in mucilaginous sheaths either individual or in colonies. Reproduce by binary fission
• Structurally similar to bacteria but functionally similar to plants

Question 3

Why is the phytoplankton community in nutrient rich systems often dominated by Cyanobacteria?

Answer 3

• In eutrophic lakes nitrogen can become limiting but cyanobacteria can take it up from the atmosphere. Eutrophic = lots of zooplankton grazing but they avoid being grazed. Also eutrophic = not a lot of sun but cyanobacteria have low light requirements
• They are poor food or even inedible to zooplankton due to 1) their mucilaginous sheaths surrounding their cells and colonies, 2) the large size of many colonial forms, 3) several species produce toxins
• Also have relatively high temperature optimum

Question 4

Where do you expect blooms of Cyanobacteria? Why?

Answer 4

Expected in eutrophic lakes (defenses against zooplankton grazing + nitrogen fixing), in the summer and in the tropics (high temp optimum)

Question 5

Do green algae show adaptations to reduce grazing by zooplankton?

Answer 5

Green algae includes lots of filamentous algae which form large filaments - can be too big for zooplankton to eat

Question 6

When do you expect high densities of green algae?

Answer 6

In eutrophic lakes during the summer after bloom of desmids

Question 7

Where do we expect desmids?

Answer 7

In soft waters - they are an indicator for clear water, can function optimally at lower nutrient concs

Question 8

When and where do we expect high densities of diatoms? Why?

Answer 8

Often dominate the spring bloom in freshwater lakes but are very species rich - are found in lots of different conditions, usually alkaline

Question 9

The spines of certain dinoflagellates (e.g. Ceratium) varies throughout the season. Explain.

Answer 9

They have spines to reduce sinking rate (need to spend significant time above compensation depth). The spines are longer when temperatures are higher (summer) because that is when grazing pressure of zooplankton is highest after spring maximum

Question 10

Many green algae form colonies. Why?

Answer 10

Forming colonies makes them too large to be eaten by zooplankton

Question 11

Cryptomonads are almost only found in winter. Why?

Answer 11

They are unicellular, small and motile and are adapted to live in low light and temp conditions. They are very good food for zooplankton + vulnerable to grazing but in winter can survive because zooplankton are slower and eat less

Question 12

Explain the seasonal succession of phytoplankton both in terms of biomass as well as in terms of taxonomic composition. Explain the changes.

Answer 12

• Seasonal succession in the phytoplankton community of a temperate dimictic lake often involves low phytoplankton standing stock during winter, spring bloom dominated by diatoms, a clear water phase caused by sharp decline in phytoplankton abundance at the end of spring and a second phytoplankton bloom during fall. Use chlorophyll as measure of phytoplankton biomass
• Winter: low phytoplankton numbers and biomass due to low light availability and low temps. Winter algae community below ice is dominated by small and motile alga like green algae and cryptomonas which adapt to live in low light + temp
• Spring maximum: increase in light intensity + temp and increase in nutrient availability during spring circulation = strong increase in algal biomass in spring. Nutrient-rich water from lower depths is mixed with surface water so that nutrients aren’t limiting anymore at depths where light intensity is sufficient enough for photosynthesis. Dominated by diatoms.
• Spring minimum: in late spring there is a decline in overall phytoplankton biomass and the algal community gets increasingly dominated by green algae. Late spring = clear water phase (low phytoplankton biomass and high water transparency). Caused by physical and biotic factors:
  > Reduction of nutrients in photic zone of epilimnion. Reduction of silica concentrations to limiting levels cause decline in diatom population that dominates the spring max
  > Gazing pressure by zooplankton contributes to decline in phytoplankton abundance. Zooplankton population also increases during early spring and since they are not food limited (due to high pop of phytoplankton) they grow fast.
  > Parasitism - high population densities stimulates the development of diseases + it is easier for parasites to be transmitted from one host to another
• Summer: as silica concs are reduced, in less productive lakes diatoms as dominant phytoplankton are succeeded by green algae and then later by diatoms again. In eutrophic lakes, cyanobacteria take over. In eutrophic lakes with continuous supply of P, growth during midsummer can be so intense (high temp + light intensity) that N conc becomes limiting and cyanobacteria have competitive advantage (nitrogen fixation) AND grazing pressure is high (cyanobacteria are resistant = become dominant compared to green algae or diatoms).
• Autumn bloom: increase in phytoplankton abundance during autumn turnover as mixing of water brings nutrients into photic zone. Often dominated by diatoms, green algae or cyanobacteria

Question 13

Explain the plankton paradox

Answer 13

A limited range of resources (light, nutrients) supports a wider range of planktonic organisms. Paradox stems from the competitive exclusion principle: when 2 species competing for the same resource, one will ultimately persist and other is driven to extinction. But high diversity of phytoplankton stands in contrast to limited range of resources due to different resource dependencies (light, N, P, S etc) and different spatial and temporal gradients

Question 14

Explain the changes in biomass of phytoplankton with depth in lakes with varying nutrient loading.

Answer 14

• In oligotrophic lakes chlorophyll concentrations (representative of phytoplankton biomass) are found in deep water. As lake becomes more productive and turbidity increased due to particulate organic matter of living biota, these concs move closer to the surface (light).
• High nutrient loading = increased turbidity = phytoplankton at shallower depths

Question 15

What mechanisms do algae use to remain above compensation depth?

Answer 15

Density of most planktonic organisms is slightly higher than water so they have a tendency to sink in undisturbed water. To reduce sinking rates:
- A large number of algae have flagellae with which they can move through water
- Most algae that don’t have flagellae are not spherical - they have a shape with high surface to volume ratio which reduces sinking. Some have protrusions (e.g. spines) that further reduce sinking rate (e.g. Desmids)
- Production of mucilaginous sheaths reduce sinking - present in nearly all cyanobacteria, some diatoms and green algae
- Gas vacuoles are very efficient in regulating buoyancy - they can decrease density of the cells to below that of water so they can float to the surface of the water where densities are high and counteracting shading effects
- Accumulation of fats can decrease sinking rate

Question 16

Give an example of a trade-off with respect to strategies that algae develop to cope with nutrient requirements and other challenges

Answer 16

• Many characteristics that reduce losses due to grazing by zooplankton or in reducing sinking rates (e.g. mucilaginous sheaths, increased volume, spines) result in a decreased efficiency of uptake of nutrients (through decreased diffusing rates). Small spherical and motile cells are most efficient in capturing nutrients but also most vulnerable to grazing.
• Due to this tradeoff, the phytoplankton community in oligotrophic lakes are dominated by small and motile cells (efficient nutrient uptake) whereas in eutrophic lakes they are dominated by large and filamentous forms that have mucilaginous sheaths (reduction of gazing loss is more important than efficiency of nutrient uptake). Low nutrients = desmids, high nutrient conc = cyanobacteria
• Although nutrient uptake is hindered by mucilaginous sheaths, several species are capable of nitrogen fixing (using N2 as nitrogen source), increasing their competitive strength in highly eutrophic lakes (which can sometimes be N limiting rather than P).

Question 17

Why is primary productivity in tropical lakes expected to be higher than in temperate lakes?

Answer 17

Total phytoplankton biomass and productivity are large + more constant seasonally than temperate lakes. This is because abrupt changes in abiotic factors (e.g. wind induced mixing, increased nutrient loading from high rainfall) can be more frequent leading to more frequent changes in phytoplankton succession and productivity

Question 18

Can seasonal succession of phytoplankton best be explained by biotic interactions or by responses to the abiotic environment ?

Answer 18

• Biotic (living things): zooplankton grazing, parasitism
• Abiotic (non living): light, temperature, wind (mixing = nutrients),

Question 19

How can one determine the relative importance of Top-Down or Bottom-Up control of biomass of a given trophic level?

Answer 19

• Bottom-up control: nutrient limitations. Top-down control: biotic interactions like parasitism and predation. Whether phytoplankton is mainly controlled by bottom-up or top-down factors depends on the lake and can vary with season
• Conducting experiments - grazing pressure by zooplankton is altered or adding nutrients

Question 20

Why do phytoplankton engage in diurnal vertical migrations?

Answer 20

Most migrating (with flagellae) algae migrate downwards at night and back into the photic zone during the day. By spreading out over a larger depth range during the night, phytoplankton reduce losses by zooplankton grazing (that tend to concentrate in surface layers during the dark). Also, migrating into the metalimnion enables algae to take up nutrients, enabling them to better cope with nutrient depletion in the epilimnion

Question 21

How can one quantify algal biomass?

Answer 21

Use chlorophyll as measure of phytoplankton biomass

Question 22

What is periphyton (aufwuchs). What is its ecological role?

Answer 22

Periphyton = community of algae, bacteria and small animals that form a slimy layer around underwater stems of aquatic plants. Contain both autotrophic and heterotrophic organisms that use macrophytes as a substrate or as a nutrient source (e..g on the DOC excreted by the plant). Productivity by periphyton can rival that of planktonic algae. Periphyton growth increases with nutrient load and in eutrophic lakes thick layers of periphyton can limit growth of macrophytes due to competition for light and nutrients. They are an important source of food for zooplankton, macro-invertebrates (e.g. snails) and fish

Question 23

What kind of communities do you expect in anaerobic water layers such as the deep water layer of meromictic lakes?

Answer 23

Ciliates (unicellular zooplankton) can grow very well even under very low oxygen conditions. In these habitats, their food (bacteria) is very abundant and their main predators (mesozooplankton) are absent or occur in reduced densities as they can’t cope with anaerobic conditions

Question 24

When (in which habitat) do you expect high densities of rotifers?

Answer 24

• Mostly occur in freshwater, a few marine species.
• In lakes with a well-developed littoral zone, in extremely productive systems

Question 25

What is cyclic parthenogenesis?

Answer 25

Rotifers reproduce by cyclical parthenogenesis = involves phases of parthenogenesis (development of unfertilized eggs, as long as environmental conditions are favorable) and periods of sexual reproduction (when conditions deteriorate). Results in formation of clones. Unfertilized eggs develop into males whose job is to fertilize sexual females. Fertilized eggs are dormant and remain variable for years = important for recolonization of habitats after a period of harsh conditions.

Question 26

Many aquatic organisms produce diapauzing eggs. What is the ecological significance of the production of such resting eggs?

Answer 26

Dormant eggs can survive harsh conditions (drying and freezing) for years - recolonize habitats after harsh conditions or colonize new habitats (dispersal)

Question 27

What is the reason why so many Ponto-caspian species have colonized the Great Lakes in N. Am.?

Answer 27

Ponto-caspian species were brought from Caspian sea to Great Lakes with ships - invasive species so they are not affected by parasites (none have evolved to attack them there) so they had a competitive advantage and they can withstand variation in salinity

Question 28

Cladocerans and copepods have strongly differing feeding modes. What are the consequences for their interaction (competition) and effect on the phytoplankton?

Answer 28

• Both species generate water currents to force food into their mouths. Cladoceran (e.g. Daphnia) are filter feeders and are largely unselective (eat all particles under a given size) - can detect toxic compounds but then reject all the food they almost ingested. Copepods are highly selective and can pick out edible algae even with high concs of inedible/toxic algae - can even differentiate between algae of high and low quality.
• Presence of calanoids - promotes algal species that are of low nutritional quality or toxic. Presence of cladocerans will promote fast-growing algal species and although they reject toxic species it also helps non-toxic species present since they cannot select just the edible species

Question 29

Where (in which habitats) do you expect large branchiopods (anostracans, conchostracans and notostracans)?

Answer 29

• These are type of shrimp
• Habitats where fish predation pressure is very low (they are big).
  > Semi-arid regions as they are abundant in shallow water bodies that only contain water for very short periods (e.g. puddles)
  > Saline inland waters e.g. brine shrimp, fairy shrimp

Question 30

What are the main ecological differences between the Cyclopoida, Calanoida and Harpacticoida ?

Answer 30

Distinguished by the general structure of the first antennae, shape of abdominal segments, number of egg sacs, how egg sac is attached to animal
- Calanoida (2): 2 egg sacs carried laterally, short antenna, littoral habitat
- Cyclopoida (1): 1 egg sac carried medially, long antenna, planktonic habitat
- Harpacticoida (3): 1 egg sac carried medially, very short antenna, littoral + benthic habitat

Question 31

What is the ecological role of the zooplankton in a lake food web?

Answer 31

• They are primary consumers - they introduce energy produced by producers to the trophic chain
• Protozoa/unicellular zooplankton play an important role in the microbial loop - feed on bacteria that are grow on dissolved organic nitrogen

Question 32

Make a scheme on the seasonal changes in zooplankton biomass in a temperate lake and explain which factors contribute to these changes. Explain also how this may influence seasonal changes in phytoplankton biomass and size distribution.

Answer 32

• In winter population density and mortality are low and virtually no reproduction. In early spring, population increases without an increase in birth rates due to hatching of dormant eggs. In late spring, population grows exponentially due to vigorous reproduction. In summer, high birth rates are balanced by high death rates (low food availability) and pop stays steady. In autumn reproduction decreases because of lower food availability + lower temps.
• Predation pressure by fish is largest during summer (high abundance of young fish + high activity due to higher temps). Summer species = smaller bodied (e.g. Daphnia), transparent or have high escape capabilities.
• Feeding rates of zooplankton on algae can be very strong - large effect on phytoplankton numbers. Larger zooplankton can feed on larger algae. So in presence of small cladocerans, phytoplankton community is dominated by large algae. Inedible species will eventually dominate = too large to be ingested (can be in colonies too), are toxic or can pass through intestine without being killed (are not digested).

Question 33

Invertebrate and vertebrate predators have a totally different impact on zooplankton communities and species. Explain. What happens if both types of predators are present?

Answer 33

• Vertebrates that feed on zooplankton: fish, amphibian larvae and birds - hunt visually so select larger prey. Adaptations = smaller, transparent, swarm
• Invertebrates: predatory zooplankton, aquatic insects, flatworms - prefer small zooplankton as they are easier to handle + predators are gape limited (can’t fit in their mouths)
• Only invertebrates: large bodied zooplankton dominate. Moderate density of planktivorous fish = medium-sized zooplankton. High density of planktivorous fish = small bodied zooplankton dominate as the fish eat all the invertebrate zooplankton predators (often larger than zooplankton) and small zooplankton no longer get eaten.
• Both are present, zooplankton have adapted to avoid both predators (see below)

Question 34

Why is fish predation pressure in temperate lakes much higher in summer than in spring?

Answer 34

As fish are ectothermic animals, their activity depends on temperature - during winter, fish rarely feed at all. With increasing temp, activity of fish increases and fish predation pressure on zooplankton increases. Also, the number of fish present in a lake is larger during the summer as a large number of young fish are present. Fish produce large number of offspring but many die young. Young fish are growing = extra hungry and can’t prey on larger predators so are more dependent on zooplankton

Question 35

How can one quantify the impact of fish on zooplankton communities?

Answer 35

Introduce or remove fish from a lake. Field experiments where zooplankton of a lake are incubated in situ where no fish were present or where density of fish was reduced or increased.

Question 36

Why does Leptodora, a quite large zooplankter, nearly always coexist with fish?

Answer 36

Leptodora = predatory cladoceran feeding on smaller zooplankton. It is really transparent but still suffers high mortality from fish predation BUT presence of fish results in a better size spectrum of zooplankton it preys upon = complementary feeding niche

Question 37

How does zooplankton cope with the ever-lasting changes in predation pressure in their habitat?

Answer 37

• Reduce their vulnerability to visual predators by becoming smaller in size, becoming more transparent, hiding in strata where light intensities are low, swarming e.g. Daphnia has a very small body (avoid fish predation) and also has long tail spine (avoid invertebrate predation). The spine is transparent so does not increase capture by fish
• Induced defenses in the presence of chemicals produced by predators. Higher impact of invertebrate predators during winter, higher impact of fish predation in summer - induced defenses allows them to cope with these seasonal changes
  > Morphological changes: induction of helmet in presence of invertebrate predators
  > Life history changes: reduction in size at maturity + size and number of eggs produced in the presence of fish. Increase in size at maturity + size and number of eggs in presence of invertebrate predators
  > Behavioral changes: swarming, escape behavior, diel vertical migration

Question 38

Why does one often observe a strong shift in size distribution of the zooplankton in the presence of fish?

Answer 38

Fish are visual predators. At among-species level, shift from larger taxa to smaller taxa. Within species, reduction in frequency of adults compared to juveniles

Question 39

Definition: size-efficiency hypothesis

Answer 39

Tendency of zooplankton populations in fishless water bodies to be dominated by large bodied species - due to presence of invertebrate predators AND higher competitive advantage:
- Large bodied zooplankton have on average a lower respiration rate = more efficient energy conversion - lower food threshold for maintenance of the population
- Large bodied zooplankton have higher starvation resistance
- Large bodied species have broader particle range that they can utilize as food (for filter feeders like cladocerans)
- Large bodied cladoceran species depress population development of small rotifer species through interference competition and predation - small rotifers can get ingested or damaged by being swept through filtering chamber of cladoceran

Question 40

Why are larger zooplankton better competitors?

Answer 40

• Invertebrate predators selectively prey on small bodied zooplankton
• Being transparent means increased susceptibility to the harmful effects of high light intensity. Also some pigments function as energy storage

Question 41

Why does large zooplankton dominate communities in fishless ponds?

Answer 41

Invertebrate predators prey on small zooplankton + large zooplankton have competitive superiority

Question 42

What is cyclomorphosis? How can it be explained?

Answer 42

Seasonal change in morphology of a species within a population e.g. change in shape of some part of an organism. Main reason is a reduction of vulnerability to predation (e.g. Daphnia helmet formation) but can also be due to temperature (e.g. Ceratium develops longer spines during summer to reduce sinking rate in the lower water viscosity)

Question 43

Diel vertical migration of zooplankton: what is it and what are the causes of this behaviour?

Answer 43

• Most commonly zooplankton rise above at night and sink during the day
• This migration is very costly because zooplankton move out of the epilimnetic water (high temp and rich in food = enabling fast population growth) during part of the day. Zooplankton move to deeper water layers during the day = lower temp and quality and quantity of food is lower (no actively growing algae populations), resulting in a reduced population growth. This is a predator avoidance strategy - animals reside deeper in water column during the day than at night. By moving to deeper water during the day, they reduce predation efficiency of fish (rely on light). So leads to reduced population growth but also reduced mortality due to predation
• Since it is to avoid fish predation, amplitude of migration is larger for lager bodied zooplankton
• Adaptive cause = avoid fish predation, proximal mechanism = change in light intensity. Can also be predator induced defense (e.g. some only migrate during summer when fish pops are high)

Question 44

What is the adaptive significance of diel vertical migration?

Answer 44

Reduce mortality by predation of fish

Question 45

Diel horizontal migration: adaptive significance.

Answer 45

• Many zooplankton species form swarms during the day to reduce predation by fish and then spread out to feed during the night
• Can also be due to habitat preference (e.g. preference for littoral or pelagial zone)

Question 46

What are the most important differences between tropical and temperate fish communities?

Answer 46

• In tropical waters, temp is always high and so is feeding activity - instead, changes in fish population density are correlated with water level or quality. Unlike temperate where activity is based on changes in temp.
• Tropical = more diverse. Oxygen solubility decreases with increasing temp so tropics more often have oxygen stress - e.g. have adaptations to use atmospheric air. Tropics have dry phases so some fish produce dormant eggs that can withstand desiccation (drying out)
• In tropics it is difficult to make distinction between river and lake fish species as many fish live in rivers during periods of low water and then move to lakes formed by floods during periods of highwater. Highwater = high food (nutrients) = high productivity

Question 47

Explain: trophic cascade hypothesis

Answer 47

• Dynamics of one level of the foodweb have a large influence on higher and lower trophic levels - effects ‘cascade’ to other levels
• Top-down hypothesis: reduction of planktivores results in an increase of zooplankton density and thus a decrease of phytoplankton density
• Bottom-up hypothesis: if you reduce density of planktivores there is no reduction of phytoplankton biomass as the phytoplankton biomass is determined by conc of nutrients

Question 48

How can one determine whether a trophic level is bottom-up or top-down controlled?

Answer 48

• In situ enclosure experiments or whole lake manipulations e.g. to determine if algae is bottom-up or top-down controlled, remove zooplankton and if they still grow they are bottom-up controlled. Then add nutrients to see if algae grow more
• They should not be seen as contradictory but complementary explanations of the variation in productivity of lakes

Question 49

Explain: alternative equilibria in shallow lakes

Answer 49

• Shallow lakes have internal eutrophication
• Clear water state: lot of macrophytes providing shelter for piscivorous fish (they wait for prey to swim by) = healthy population of piscivorous fish keeps planktivorous fish to a low density. Zooplankton community is composed of larger individuals in large numbers = efficient grazing of phytoplankton = no phytoplankton blooms
• Turbid water state: absence of macrophytes = ineffective hunting strategy for sit and wait predators = lower density of piscivorous fish = high density of planktivorous fish = smaller average size of zooplankton + reduced zooplankton population densities. Small zooplankton are less efficient in grazing phytoplankton + high productivity of system = phytoplankton blooms. Also increase in benthivorous fish which contribute to stabilization of turbid water state as their foraging stirs up sediments = increasing turbidity and releasing nutrients into water.

Question 50

Given that the two alternative equilibrium states of shallow lakes are stabilised by several feedback loops, which factors can force a shift from one equilibrium state to the other? And how is nutrient loading related to this capacity to shift?

Answer 50

• At very low nutrient concs, only clear water state can be observed as there are not enough nutrients for a phytoplankton bloom to occur
• At very high nutrient cons, only turbid water state will occur as macrophytes suffer drawback compared to algae in their competition for light (algae can aggregate near surface, plants stay at the bottom).
• In intermediate nutrient levels, the two alternative equilibrium conditions are stabilized by feedback loops and external influences are necessary to bring a lake form one state to the other:
  > Increase in turbidity through sediment loading (clear to turbid)
  > Presence of herbicides due to agriculture (clear to turbid)
  > Introduction of nutrients, especially P (clear to turbid)

Question 51

How can one make eutrophied, turbid lakes clear again?

Answer 51

• Nutrient reduction: in deep lakes, a reduction of P input results in gradual improvement of water clarity (not true for shallow lakes).
• Biomanipulation: involves planned changes in the community structure of higher trophic levels to reach a desirable clear water state - most importantly reducing densities of planktivorous fish either by directly reducing densities of planktivorous fish and/or introduction of piscivorous fish. Need to completely eliminate or reduce planktivorous fish density by more than 90%

Question 52

The success of restoration measures such as biomanipulation differs among lakes – which are the factors that may interfere with a sustainable shift to the clear-water equilibrium state?

Answer 52

• Nutrient loading - in hypertrophic lakes, also need measures to reduce nutrient loading
• Abiogenic turbidity - biomanipulation can only be effective if turbidity results from phytoplankton blooms but turbidity can also be from suspended material - need to dredge the lake. Especially for large lakes, biomanipulation is less successful due to wind suspension increasing turbidity in large lakes
• Limitations of size-efficiency hypothesis - large zooplankton are more efficient in grazing phytoplankton but they are also more vulnerable to the presence of cyanobacteria

Question 53

Biomanipulation as a restoration of eutrophied shallow lakes is less likely to be successful in tropical lakes than in temperate lakes. Explain. How can one restore lakes in the tropics?

Answer 53

• Max body size of zooplankton in tropics is smaller than in temperate zones so zooplankton based biomanipulation can be less efficient in controlling algal production even if fish predation pressure is strongly reduced. Instead, use phytophagous (e.g. carp) fish - however this implies introduction of non-native fish so must be very cautious AND they can contribute to even stronger blooms by increasing the amount of nutrients that cycle in the system
• Eutrophication in trophic lakes results in dramatic increase in biomass of floating vegetation = reduction in light that reaches lower layers. Nutrient reduction and mechanical removal can be a solution to this problem.

Question 54

How can one harvest natural populations in a sustainable way?

Answer 54

• Effective management must set a fishing rate that does not eliminate the fish population and that permits removal at a sustainable yield. Sustainable = fishing until biomass is reduced to half that of carrying capacity. Fishing efforts that reduce the population below half the carrying capacity will impede population recovery and therefore reduce catches in the future + risk of extinction
• Maximum sustainable yield: fishing level that maximizes production continually - always lower than the maximum fishing level
• Population growth is maximized at a population size that equals half the carrying capacity of the environment as population growth is limited when approaching the carrying capacity but a too small population will mean too few individuals to reproduce. Carrying capacity can be determined as the biomass of the fish population when it remains stable over years in the absence of fishing. (logistic model)

Question 55

How can one detect whether a fisheries is sustainable?

Answer 55

Overfishing can be identified by a decrease in catch per unit effort (CPU): as the stock is overfished, number of fish caught per day per boat per day decreases

Question 56

What are the factors that may interfere with the implementation of the theory on sustainable fisheries in practice?

Answer 56

• Carrying capacity assumes population is resource limited - so when fish stock is reduced they have access to more resources such that growth and reproduction is enhanced. However, not all fish populations are resource limited (bottom-up) - some are limited by predation (top-down) then the population observed in the natural habitat is already lower than carrying capacity - need to differentiate between fish densities that are kept constant by top-down vs bottom-up
• Fishing often selects larger individuals - on one hand this allows small fish to grow and reproduce but on the other hand reduction of fish stock to half its carrying capacity should not involve a reduction of almost all mature animals as adults contribute to the recruitment of the population. So, size limits should be imposed to regulate fish yield. Fish net size should be set so a substantial part of the population can reproduce before they are caught.
• Fish diversity - more than one species of fish exist in a given area. Size limits that are optimal for one species will be too small for another. Worse in tropic regions where diversity is high
• Problems with estimating max sustainable yield: logistic model is an approximation and it is difficult to estimate carrying capacity and carrying capacity is not constant as environmental conditions fluctuate from year to year

Question 57

Why are most fishermen in the world poor?

Answer 57

Cost of fishing increases with fishing effort. As soon as system becomes overexploited, their revenue decreases such that they have to increase fishing efforts to obtain enough income leading to further overexploitation and even higher fishing efforts = ever increasing cost

Question 58

If catches are declining, it is not helpful in the long run to increase harvesting effort or introduce more efficient fishing techniques. Why not?

Answer 58

One can easily drive population to densities where it cannot recover or to extinction

Question 59

Why might it be a good idea to create large marine reserves? Would such reserves also be useful for fishermen?

Answer 59

Reserves = areas where no fishing is allowed. These reserves will resupply fishing grounds + buffer the large year to year variations and uncertainties intrinsic to the estimation of max sustainable yield

Question 60

What are the risks of the introduction of exotic fish species for fisheries?

Answer 60

• Introduction of exotic species can increase fish production substantially, especially in new environments like man-made lakes. In man made lakes, major resources remain unexploited because specific niches are not filled with natural colonists
• However, it can create a devastation of the original ecosystem if the niche was not really empty so best to introduce in man-made systems that have not yet reached equilibrium. In natural lakes, introductions are not useful to obtain sustained increase in fish stock because the system has evolved to an equilibrium where different niches are exploited. E.g. nile perch in lake Victoria

Question 61

Phytoplankton

Answer 61

Photo-autotrophic organisms which float passively in water column