Lecture 10 Life History Strategies in Fish
Human impacts on lowland rivers
By Dr Rick Leah
Jones Building, School of Biological Sciences
It is recommended that you follow-up subjects from the lecture by further reading. These
notes are intended only as a memory prompt – The course website is being developed to
provide additional information but is NOT a substitute for reading scientific literature.
Life History Strategies
 Many factors affect the life history of a fish
Allocation of Energy
Variation in allocation of energy
 Input = food
 Output = Excreta and progeny
 Vary according to habitat and food availability (resources)
 Have to satisfy the Standard Metabolism
 This changes with habitat and therefore metabolic requirement
 Fast flowing river
 Then allocate to growth (somatic)
 Then to reproduction (gonadic)
Allocation of time
Allocation of time
 Split between feeding (essential), resting/predator avoidance and reproduction (mature
fish only)
 Feeding time is proportionally greater in immature fish
Feeding
 Feeding is dependent on the range of food items available and
Food Characteristics
 Size and visibility
Prey eaten (size and number)
 Too small to see or not worth the effort (pomegranites)
 Too large to capture (physically or swimming ability)
 Palatability – are they liked (contaminated toxic prey)
 Digestibility – are they digestable or worth the digestion energy (celery)
 Requirements of fish need meeting
Proteins, lipids, vitamins and essential ions and elements
 Food eaten depends on availability and abundance i.e.
Competition
Intraspecific (between same species)
 Particuarly in shoaling fish (bream) or as juveniles (when many about)
 Terretorial fish preclude others from area
 Therefore competition reduces food availability
Interspecific (between different species)
 Fish have a restricted habitat available to them
 Therefore come into competition with other species
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Can affect food size available (better, more efficient remove bigger ones so small are left
(bream and roach)
However if food resource is predictable in time and space can get specialisation
Need time to evolve it therefore tropics have more cases of this
 African Cichlids of lake Victoria and Malawi or South American Characins
South American Characins

Each species highly adapted morphology for specific feeding strategies they are not
omnivorous like temperate fish
Life History Variation
 Different species of fish have different ways to increase their population
 Tend to look at females when looking at life history variation
 They produce the eggs that limit the potential recruitment to the population

As mentioned earlier the allocation of energy and time changes throughout fishes life
cycle
Age and Maturity
 Trade-off between probability of successful reproduction vs. risk of death of the mother
to be
 A female that delays maturation until larger and hence older will produce more eggs but
risks dying before reproduction takes place
Fecundity and length
 Ovary size is often used to determine the reproductive potential of a fish and its
population and is called fecundity
 Bigger fish have greater fecundity as ovaries are bigger and more eggs (eg. Mature perch
ovaries form 20% of total weight)
F=aLb
 F = fecundity, L length and a and b are constants
 Studies have found b to be around 3
 This means that there is a direct cubic relationship
 i.e. volume increase with length
 fecundity increases with volume
Body Size
 Larger fish are less susceptible to predation
 Larger fish also store more energy and can swim faster and farther and overcome harsher
abiotic conditions
 Also territoriality dependent on size
 So beneficial for large size but reqiures more energy allocation to somatic growth
Longevity
 Longer life means more reproductive chances but depends on
 Age and Size at maturation (above)
 Availability of resources (is it reliable enough to let you live a long time) and
 Length of time between reproductive opportunities (ie 1 year 12 times vs. once a year for
8 years) although environmental conditions may mean that all 12 will die but spread it out
may mean that years 3 and 6 and 7 all live
Clutch Size
 Obviously no. of eggs depends on the condition of female and size/age
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No. of spawnings per season
Expect that females in populations where adult survival is poor will devote more energy
to reproduction than when survival is better
E.g. Commercially exploited species show increased fecundities compared to unexploited
populations European Flounder
Alternative is if large adults removed get earlier maturation at smaller and younger age
E.g. Nile Tilapia in lake George (now not commercially viable because all small)
Egg size and size at hatching
 Volume of ovary is obvious limitation on egg number and size
 Larger eggs mean lower fecundity
 But larger eggs mean larger yolk therefore more independent
 Therefore not looking for food and risking predation
 When they are hatched are larger
 So trade-off again between investment and survival
Fertilisation strategies
 Different strategies employed to produce enough fertilised eggs that survive to be
recruited into population
Broadcast Spawners
 Lots of small eggs sprayed out into pelagic water
 Often individuals synchronous by tides and moon
 Especially in tropics
 Huge mortality as predation and lack of fertilisation
Eggs in substratum
 Salmon
 Female makes depression in the gravel known as a ‘Redd’ and lays eggs within the
depression
 Male ejaculates sperm into the Redd
 Eggs are covered by more gravel left partly protected by gravel and when they hatch, the
young can live amongst gravel
Parental carers and guarded eggs
 Parents allocate a lot of energy
 Eggs bigger so more yolk and hatch bigger stronger
 Often when hatch get parental care
 Extreme case are the mouthbrooding Cichlids in African lakes
Migration
 Aims to maximise success at different stages of life history
Migration
 Different timings can be used at each migratory stage
 Depending on species e.g. sun intensity, temperature, olfactory cues (salmon)
 Migration and time often limited by life stages abilities i.e. swimming ability
 Juveniles have more difficulty moving through water have to attain size to overcome
hydrodynamic constraints e.g. grow bigger and stronger muscles
Two types
 Potadromy – within lake eg char and whitefish (known as the Gwyniad in Llyn Tegid)
 Diadromy –between two habitats eg between salt and freshwater
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Anadromous eg salmon
Catadromous eg eel
Human Impacts on Lowland Rivers
Floodplains
 Engineering and climate means less important in temperate areas
 Very important in tropical areas
 Especially when monsoon rains increase water flow massively
 Height can increase by several metres
 Therefore overflows and spreads out
 Zambezi River low water area 537km2, peak flood 10752 km2 (20X increase)
 Flood plains enriched with deposition
 Whole system very productive
 Many fish have lifecycles that use this increased food and increase in shallow
heterogeneous habitat for protection/nursery

In Britain and temperate regions in the past has allowed fish to disperse to new river
systems when flood recedes
Pollution Effects on Fish Distribution in Rivers and Lakes
Pollution sources
 People (sewage) – organic and phosphorus
 Industry – inorganic chemicals, thermal and organic-chemicals (PCBS)
 Agriculture and runoff – Organic-chemicals, silt, turbidity and nitrates
Organic Pollution
 Initial deoxygenation
 Affects fish directly so that they leave or ventilation increases (too low and they die)
 Affects fish indirectly as prey may disappear therefore no food
 Therefore species and population falls
 But like eutrophication primary productivity increases and some fish species take
advantage
 Effect is diluted as travel down stream
 And increased competition means population size returns to normal (next week)
Silt Effects
 General effect so affects the majority of fish in a similar way
 Gills get clogged
 Also turbidity can affect visually feeding fish and spawning sites get silted so no O2 to the
eggs
Toxic Pollutants
 Mercury from an ultraviolet lamp factory or Arsenic from a gold mine
 If high enough all organisms will die
 Fish downstream (either swim or current population)
 Some more tolerant however so population increases and diversity low
Thermal Pollution
 Same as toxic
 Fish have thresholds
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Different fish different thresholds
So some can die while others live
But sudden increase will shock fish and all die
Then down stream some more adapted than others
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Other toxins such as Organochlorines
However work differently
Polychlorinated Biphenyls (PCB’s) or DDD a pesticide
Hydrophobic, fat soluble and biologically stable
Therefore they are accumulated in body fats
They are biomagnified along food chain so effects are not seen until top carnivores are
reached
Fish can play a big part in this
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DDD in lakes
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Californian lake repeatedly sprayed 1949-1957 with DDD to control invertebrate midge
larvae (Chaoborus)
Non-biting but bothered bathers and anglers
Killed the larvae but also entered algae and zoop and herbivorous fish
Also along foodchain
Via predatory fish to Grebes
1954 found 100 dead Western Grebes
DDD in body fat was 1600 mg/kg
Bioconcentration of 80000 times that which was sprayed
Other places fish kills have occurred
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