Environmental Engineering Reference
In-Depth Information
●
The catch is measured accurately;
●
CPUE is
directly proportional
to abundance.
Equal catchability
of individuals is often a problem, for the same reasons as in
mark-recapture analysis. The difference here is that the data contain no informa-
tion on heterogeneity in catchability, so we cannot estimate and control for it. This
is a major disincentive to the use of the method, or, at least, a reason to use it with
great care.
Again similar to mark-recapture methods, the
population closure
assumption
can be met by carrying out data collection over a short time scale relative to the nat-
ural rate of turnover in the population. However, this might be more difficult in
the case of offtake-based methods if the removal itself stimulates immigration by
creating unoccupied space attractive to colonising individuals, or if the harvesting
activity triggers emigration. This problem cannot be detected or controlled for
using catch data alone, and results should be regarded as suspect if there is any
suspicion that migration might be occurring. Even a relatively low rate of natural
mortality or emigration during the process can lead to dramatic under-estimation
of abundance, while net natural growth in the population can lead to equally
dramatic overestimation. Alternatively, it is possible to avoid the need for the
assumption by using an
open population model
instead of the closed model
shown in Box 2.5. In this case, catch and effort data collected annually can be used
to estimate key population parameters such as intrinsic growth rate, as well as
population size. This approach is covered further in Section 4.3.3.
If removal methods are applied experimentally, the assumption of
accurate
measurement of catch
is usually trivial (assuming that you keep good records), but
Hyperstability
Hyperstability
Proportionality
Proportionality
Hyperdepletion
Hyperdepletion
Abundance
Fig. 2.7
Possible relationships between catch per unit effort (CPUE) and
abundance. The ideal is direct proportionality, as this allows standard
catch-effort methods to be applied. However, CPUE may remain relatively high
across a wide range of population densities, only declining significantly as the
population approaches extinction (hyperstability). Conversely, CPUE may decline
strongly at high abundance, and remain very low over most of the abundance
range (hyperdepletion). Note that other relationships may exist.
