In this work, the aim was to produce a realistic assessment of yearly mortality
of Archipelago Sea pike perch during the period 1997-2012. The utilized data
origins from the mark-recapture experiment carried out by the Finnish Game
and Fisheries Research Institute (FGFRI). In this mark-recapture experiment,
returnings of the marks were based on voluntary tag reporting by the fishermen
gaining small monetary rewards. In this study design, the count of returned tags
is affected by the size of the release cohort, efficiency of the fishing method used
by a fisherman and the fisherman’s willingness to return the tag. In addition,
each year a proportion of the tags become detached from fish, which means that
those tags cannot be returned. All these factors were taken into account in a
hierarchical model, which was developed in the same fashion as the well-known
Cormack-Jolly-Seber model. Data from the yearly total catch were not used in
this work because those data will be used
in the subsequent research utilizing
results of this work.
The objective of this work was to estimate fishing gear specific catchability
coefficients and mortality rates, including natural mortality rate. The amount
of data and number of parameters to be estimated set their own limitations,
so it was decided to estimate parameters of interest by splitting the data into
only three fishing fleets: professional fishermen, recreational net fishermen and
recreational line fishermen.
The estimability of the hierarchical model developed for mark-recapture data
was studied using simulation experiments. One was able to find such a model
configuration, where the parameters concerning mortality estimates may be es-
timated without significant systematic errors in the estimated posterior distri-
butions. Simultaneously, the tag reporting probabilities were estimated for each
of the three fishing fleets although systematic errors remained for these param-
eters.
The final mortality estimate indicates that about half of the Archipelago Sea
pike perch population is removed annually. For the recent years about half of
this mortality was caused by professional fishing, and almost the same amount
was due to natural death. The mortality caused by recreational fishing is the
smallest mortality component. The estimate concerns population similar to
released cohorts. The produced estimate is sensitive to many factors, whereas
effects of environmental change, or changes in seal or cormorant abundances,
were beyond the scope of this work.
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