Techniques for surveying exploited species - Conservation and Sustainable Use - page 37

if many harvesters must be monitored in order to estimate the total harvest, it can
be very difficult to satisfy this assumption. Approaches and pitfalls in measuring
catch from harvesters are discussed in Section 2.4.2.4.
The crucial assumption behind catch-effort analysis is that the abundance
index used is directly proportional to true population abundance . Unfortunately,
this assumption is often likely to be violated, particularly when CPUE is used as
the index. Of the two types of violation (hyperdepletion and hyperstability,
Figure 2.7), hyperstability is both the most likely to occur (Harley et al . 2001), and
the outcome with more serious consequences for conservation because it leads to
overestimation of abundance. While catch-effort data alone cannot tell us whether
the proportionality assumption is violated, some idea can be gained by assessing
whether any likely causes of violation are present in a given case (Table 2.2).
Of the issues raised in Table 2.2, inappropriate analysis is clearly the one that
can most easily be avoided. This requires that data are gathered at a scale at which
sampling is essentially homogeneous, ensuring that no significant areas are left

Table 2.2 A summary of the key reasons why the assumption of proportionality
between catch per unit effort and abundance might be violated.

Type of violation Type of cause Specific cause

Hyperstability Animal behaviour Groups remain easy to find, despite

reduced group sizes

Animal behaviour Individuals aggregate more as

depletion proceeds due to habitat
selection or conspecific attraction

Harvester behaviour Offtake limited principally by high

handling time rather than searching

Harvester behaviour Increasing efficiency of harvest

method over time

Inappropriate analysis Aggregating data over a wide area

when harvesters continually move to
patches with the highest abundance

Hyperdepletion Animal behaviour Individuals learn to avoid capture

Animal behaviour Heterogeneous capture probabilities—

vulnerable individuals are caught first

Harvester behaviour Interference between harvesters

Harvester behaviour Decreasing efficiency of harvest

method over time (see Box 2.5)

Inappropriate analysis Aggregating data over a wide area

when harvesters remain in heavily
depleted localities while the wider area
remains less depleted

Note: Hyperstability and hyperdepletion are defined in Figure 2.7.