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Evaluation of techniques for estimating fishery assessment parameters in the Tasmanian Rock Lobster Fishery


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Frusher, SD 2001 , 'Evaluation of techniques for estimating fishery assessment parameters in the Tasmanian Rock Lobster Fishery', PhD thesis, University of Tasmania.

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Rock lobsters are one of the premiere seafood products around the world. High
demand has led to most lobster fisheries being over or fully exploited. The Tasmanian
rock lobster fishery is no exception and has become a major industry for Tasmania
since its rapid commercialisation in the early part of the last century. The Tasmanian
fishery, based on the southern rock lobster Jasus edwardsii, is the backbone of the
Tasmanian fishing fleet and provides valuable socio-economic input into many of
Tasmania's coastal rural towns. For this reason, the Government requires scientists to
try to provide accurate and precise assessments of this fishery for their managers.
The most recent change in the assessment of this fishery was the development of a
mathematical assessment model. In addition to assessing the current state of the
resource, the model has forward projection capabilities so that future harvest
strategies can be evaluated. Like all fishery models, the Tasmanian assessment model
is based on a number of assumptions for estimating biomass and egg production. In
addition, the model assumes that the dynamics of fishing remain constant from year to
year. However, the dynamics of the fishery are changing as management, technology
and markets change the behaviour of fishers. The change to an Individual
Transferable Quota management system in 1998 has seen fishers focus on the dollar
return per kilogram, rather than maximising their catch, as a way to improve
profitability. Global positioning and echo sounder technology enable fishers to locate
and chart lobster habitat better than ever before, and the rapid expansion of air
transportation has seen the opening of Asian markets for premium priced live lobsters.
To ensure that model estimates are reliable under changing patterns of exploitation,
model estimates need to be validated. This is best achieved by estimating the same
parameters using different techniques.Trials to obtain estimates of exploitation rate and biomass using change-in-ratio (CIR)
and index-removal (IR) techniques have provided encouraging results for southern
regions of the Tasmanian rock lobster fishery. The latter two methods require that
there be (at least) two surveys within a year, with harvest(s) occurring between
surveys. The two methods have generally provided similar results. However, on occasion, the CIR and IR results were widely divergent with the IR estimates of
exploitation rate much higher than the CIR estimate. I examined the assumptions
required to be meet for use of each of these techniques, especially the assumptions
regarding catchability. Diagnostic tests were developed for each of the techniques to
ensure that the assumptions of catchability are met. The CIR technique, which has a
weaker assumption of catchability, was more robust than theIR technique.
Application of the diagnostic tests resulted in several estimates being discarded.
Despite this, exploitation rate estimates were available for five of the six fishing years.
The diagnostic tests also demonstrated when an earlier than expected moult had
occurred in the fishery. This moult affected the end of season sample which could no
longer be used to obtain exploitation rate estimates.
In northern regions of Tasmania, moulting occurs within the fishing season and the
CIR and IR techniques can not be used. This thesis evaluates an alternative approach
to stock assessment using multi-year tagging studies to estimate fishing and natural
mortality in northern regions of the fishery. Data obtained from a tagging project
undertaken from 1992 to 1995 were analyzed. The most parsimonious model was
based on using three tagging events each fishing season, and estimating annual fishing
mortalities and a single natural mortality estimate over the duration of the study.
Fishing mortality was partitioned to the period of the year based on the amount of
fishing effort between tagging events. Natural mortality was partitioned to the period
based on the amount of time elapsed between tagging events. Although annual fishing
mortalities could be estimated for each sex, a more parsimonious model was obtained
when female fishing mortality was set as a proportion of male fishing mortality
dependent on the amount of fishing effort in the female fishing season relative to the
amount of effort in the entire fishing year. Tag reporting rate was also held constant in
the model over the period of the study. Results demonstrated that relatively precise
estimates of annual fishing mortality and tag reporting rate could be obtained but
natural mortality was imprecisely estimated. Annual estimates of instantaneous
fishing mortality were high, averaging around 1.0 to 1.2 per year, and were similar to
those obtained by the assessment model. The precision of annual fishing mortalities
estimated in the years after tagging ceased declined due to the low number of tags
returned. Low tag returns were associated with the high exploitation rates and low tag
reporting rate (estimated to be 22% ). Natural mortality was estimated for all years combined. The estimate of natural mortality was zero with a standard error of 0.2 per
year. Natural mortality is an extremely difficult parameter to estimate. Current
estimates, which are used in models for southern rock lobster throughout its range in
Australia and New Zealand, are based on a small number of long term recaptures from
southern Tasmania.
Often, in fisheries such as the Tasmanian rock lobster fishery, a major management
objective is to rebuild the stock and lower exploitation rates. Under declining
exploitation rate, the need for a precise estimate of natural mortality increases. This
thesis investigated four different ways to improve precision of estimated parameters
using multi-year tagging models. Simulations were patterned after that Tasmanian
rock lobster fishery and showed that the best gains in precision were obtained by
either increasing the tag reporting rate or increasing the duration of the study.
Although there was considerable potential to increase tag reporting rate as the
estimate from the above study was low, there can be no guarantee that either increased
rewards or improved publicity will result in an increase in tag reporting rate. The most
certain way of increasing the precision of natural mortality was by increasing the
duration of the study. This thesis suggests a design based on three years of twice a
year tagging followed by three years of once a year tagging.
This thesis also investigated selectivity estimates from the fishery because selectivity
is assumed to be constant from year to year in the assessment model. Selectivity was
found to change as the size composition of the lobster population changed. Large
lobsters were found to inhibit the catchability of small lobsters. As large lobsters are
removed from the population over time the catch rate of small lobsters can be
expected to increase. Thus selectivity can be expected to vary as a function of the
exploitation rate. Assessment models, which are based on size structure, need to
account for changes in selectivity as the size structure of the population changes.
Otherwise, if the increase in catchability of small lobsters is not accounted for, it is
likely that declining recruitment to the fishery will not be fully detected.
This study found selectivity to be an important parameter in some crustacean trap
fishelies and identified the need to validate the assumption of constant selectivity both
within and between fishing seasons.

Item Type: Thesis - PhD
Authors/Creators:Frusher, SD
Keywords: Lobster fisheries, Tasmania, mathematical models, management, Jasus edwardsii
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