Environmental Engineering Reference
In-Depth Information
Box 2.12
Direct estimation of catch and harvest mortality.
If the total harvest,
C
, and pre-harvest population size,
N
, are known or can be
estimated, harvest mortality is:
M
ˆ
C
N
SE(
M
ˆ
)
M
ˆ
CV(
C
)
2
CV(
N
)
2
The offtake-based methods for estimating population size (Section 2.3.5) are a
natural way to obtain the necessary data for this since they require offtake to be
known, and provide estimates of population size. The precision of estimated har-
vest mortality will obviously be higher if the total harvest can be recorded with-
out error (SE(
C
)
0). Unfortunately this will often not be possible, in which
case offtake must be sampled instead. If a proportion,
p
, of the total harvest is
sampled, giving a catch of
c
, estimated total catch is given by:
C
ˆ
c
p
SE(
c
)
p
SE(
C
ˆ
)
1
p
where SE(
c
) is derived from the empirical sampling variance of the catch.
Sampling catch can be done either by direct observation, or by asking harvesters
to report their offtake to you. In the case of
direct observation
, sampling can be
done in a number of ways, recording either the total offtake across all occasions
for a sample of harvesters, recording the total offtake across all harvesters on a
sample of occasions, or, less desirably, recording the offtake from a sample of har-
vesters on a sample of occasions. Whichever method is used, the sampling pro-
portion is calculated as the ratio of observed to total possible effort:
p
e/E
,
where the definitions of effort depend on the approach taken:
Sampling approach
Effort observed, e
Total possible effort,
E
Occasions
Number of occasions,
t
Total number of occasions,
T
Harvesters
Number of harvesters,
h
Total number of harvesters,
H
Both occasions and
Number of harvester
Total harvester occasions,
harvesters
occasions,
t
T
H
i
t
TH
-
h
i
i
1
t
i
1
For example, if 20 harvesters were monitored on 100 days over a year, during
which the average total number of harvesters active was 60, the sampling effort
would be (100
0.09.
In the case of
self-reported harvest
, the sampling proportion,
p
, is estimated as
the probability that any given harvest will be reported. This probability, the
reporting rate
, can be estimated by following up the initial voluntary survey with
20)/(365
60)
2000/21900
