Environmental Engineering Reference
In-Depth Information
The population estimates are therefore:
250
(40
61
160
139)
250
19800
5750
N
ˆ
1
861
250
61
200
105
N
ˆ
2
861
200
661
Population confidence intervals estimated by maximum likelihood (using pro-
gram USER), before and after harvest are respectively 527-3461 and 327-3249.
Although the estimated density before harvest is fairly close to the true figure, the
precision of the estimates is very low, reflecting the border-line adequate change
in sex ratio. The surveys indicate a reduction from 0.42 (105/250) to 0.31
(61/200) males in the population, which is very close to Paulik and Robson's
(1969) rule-of-thumb minimum difference of 0.1. This result occurs despite a
fairly heavy harvest strongly targeted at males. Effective application of change
in ratios methods depends on a change in structure at least as strong as that
illustrated here.
cannot reliably be sexed, resulting in three categories (adult male, adult female and
juvenile). In principle, the method could also lend itself to the analysis of multi-
species offtake, although this has not been tested.
2.3.5.3 Catch-at-age
This heading covers a suite of methods in which data on the age structure of
the catch over time are used to
reconstruct the past population size
. To take a very
simple example, supposing that 10 individuals in their second year were caught
one year, and that this age class was the oldest ever observed, it might be assumed
that this catch represents the entire year two cohort. If we know independently that
the natural survival rate is 20%, these 10 must have been the survivors from 50
first years in the previous year, and if 100 individuals of this cohort were caught
that year, there must have been 150 in existence prior to the offtake. If 20 second
years were also caught in that year, the retrospective population estimate would
then be 170. This somewhat laborious process, known as
virtual population ana-
lysis
, or
cohort analysis
, can be useful in giving some idea of population trends and
harvest mortality rates in the past (e.g. Solberg
et al
. 1999; Fryxell
et al
. 2001).
However, it relies on the assumptions that the natural mortality rate is known and
constant, that there are no individuals alive beyond the maximum age observed,
and that there is no migration. Furthermore, in order to reconstruct cohorts that
have not yet fully passed through the population (in order to obtain the most
recent population size, allowing an assessment of current sustainability), it is also
necessary to assume that harvest mortality has remained constant over time. This
