Fish population dynamics

1. Concept of
Population
& unit
stock

2. Concept of Population:
Population:
 Population is all the organisms of the same
group or species who live in a particular
geographical area and are capable of
interbreeding.
 Sometimes, due to unavoidable bias it may be
necessary to differentiate between a ‘sample’
population and the real population, where the
sample population consists of the objects that
have an equal probability of being selected.

3. Cont..
o Population is the breeding unit of species
having common spawning grounds.
o A population is a group of individuals of the
same species that live together in the same
place, and that posses an average set of
properties, such as birth rates and death rates.

4. Cont…
 The simplest and least restrictive definition of
population is that a population is a group of
individuals of the same species that live
together in a particular area (e.g.,
Roughgarden, 1989).
 However, even though this definition is widely
used by ecologists, it gives rise to serious
difficulties and misinterpretations.

5. Cont…
 A more rigorous definition should define the spatial
dimension more precisely; for example, a group of
individuals of the same species that live together in an
area of sufficient size that all the requirements for
reproduction, survival and migration can be met (e.g.,
Huffaker et al. 1984).
 Local population is a group of organisms of the same
species that live together in an area where there is a
high probability of interbreeding.

6. Cont…
 Meta-population is a group of populations that
share occasional migrants.
 Absolute population is an estimate of the total
number of organisms in an area.
 Population density is an estimate of the number
of organisms per unit area (e.g., hectare) or unit
of habitat.

7. Cont…
 Size of population is determined by growth which is
influenced by natality (‘+’ influence) mortality (‘-’
influence) and dispersion (‘+ & -’ influence).
 There are various ecological density dependent and
density independent factors that are the factors that
controls the growth.
 For the purpose of fitting yield models, males and
females of the populations are treated as separate
units, if differences exist in all morphological and
biological characters.

8. Cont..
 For modelling purposes, population of different
species coexisting in a particular area having similar
characters can sometimes be treated as a single
stock.
 Assessment could be made on stock of a species,
only when biology of species is clearly understood
which includes its feeding, growth, spawning and
migration habits.

9. Malthusian theory of Population
 Malthusianism is the
idea that population
growth is potentially
exponential while the
growth of the food
supply or other
resources is linear.

10. Concept of a unit stock:
 A stock is a sub set of a “species”.
 The basic purpose of fish stock assessment is to
provide suggestions on the optimum exploitation
of aquatic living resource such as fish and
shrimp.
 A stock is sub set of a “species” which is
generally considered as basic taxonomic unit.

11. Cont..
 Stock is a subset of one species having the
same growth and mortality parameter, and
inhibiting in a particular geographical area.
 A stock as “an intraspecific group of randomly
mating individuals with temporal or spatial
integrity”. (Ihssen at al.,1981)

12. Shared stock:
 Several countries may exploit the same stock.
This is the case for many migratory stock, e.g.
tunas
National stock:
o Fish stock exploited by a particular country.

13. Why does the stock unit often fail..?
1. The full distribution area of the stock is not covered
by the data collected, so that only part of the stock
is considered. This is a typical example where
several independent fisheries are exploiting the
same stock.
2. Several independent stocks are lumped together,
for example, because their areas of distribution
overlap.

14. 3. Continuous immigration and emigration of the
components of one or more stocks from the fishing
ground. Taking into account that most of the
exploited marine resources undertake migration,
an essential element to perform stock assessment
is an understanding and knowledge of migration
routes.