Techniques for surveying exploited species - Conservation and Sustainable Use - page 35

markers are not practical, marks specific to the capture occasion can be used, and
numbers of individuals having each possible capture history reconstructed later,
but this approach is not appropriate if individual covariates (such as size or age)
need to be included in the analysis.

2.3.5 Offtake-based methods
In principle, offtake (which is synonymous with catch or harvest) can be used as an
index of abundance. For example, harvest records alone have formed the basis for
some key analyses of exploited species' dynamics, most notably Canadian lynx
Lynx canadensis (Elton and Nicholson 1942) and red grouse Lagopus lagopus
(Hudson 1992; Haydon et al . 2002). These studies used long time series from
populations undergoing extreme fluctuations in order to look for retrospective
patterns in the data, and the sensitivity of catch as an index of abundance was
therefore not a major issue. In general, catch on its own is not a good indicator of
the current state of a population, for reasons discussed in Chapter 4. However,
given some additional information, catch data can be used to estimate abundance.
This is an attractive prospect for exploited species monitoring and research because
catch data are usually readily available, avoiding the need for further sampling
effort. Unfortunately, these methods are particularly prone to bias. Nevertheless,
they may be the only methods available in some cases, and they remain a key tool
in fisheries management.
The main categories of catch-based abundance estimation are catch-effort
methods (based on monitoring offtake, usually in combination with harvesting
effort), the change in ratios method (based on monitoring the offtake of different
categories of organism), and catch-at-age methods (based on monitoring the
age structure of harvest). All of these methods require substantial harvesting
pressure so that changes in catch reflect changes in abundance, rather than being
overwhelmed by sampling error. The methods are therefore only appropriate for
heavily exploited species.

2.3.5.1 Catch-effort methods
These methods minimally require data on the size of a catch, alongside some form
of population abundance index measured before and after harvest. To grasp the
basic principle, suppose that 100 individuals are harvested, and that the abun-
dance indices before and after harvest are respectively 1 and 0.6. This would sug-
gest that the harvest reduced the population by 40%, so the original population
size can be estimated as 100/0.4

250. Usually (but not always) Catch Per Unit
Effort (CPUE) is used as the abundance index, and a series of catches and associ-
ated efforts are recorded over time. Box 2.5 gives the details of how to apply this
method in practice. Catch-effort estimation of abundance makes a number of
important assumptions :
● All individuals are equally likely to be caught or detected;
● The population is closed (apart from offtake);