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Catchability of the southern rock lobster, Jasus edwardsii

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Ziegler, Philippe Eric (2002) Catchability of the southern rock lobster, Jasus edwardsii. PhD thesis, University of Tasmania.

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Abstract

Catchability is estimated indirectly as a 'nuisance' variable in the spatially explicit
stock assessment model of the southern rock lobsters Jasus edwardsii in Tasmania,
Australia. This study attempted to identify the key mechanisms influencing
catchability to enable direct independent estimates of monthly catchability.
Seasonal variation in catchability of the southern rock lobster Jasus edwardsii was
estimated in a scientific reserve in southeast Tasmania by comparing estimates of
lobster density based on direct visual observations underwater with concomitant
estimates from trapping surveys. Underwater density estimates of undersized and
legal-sized male and female lobsters greater than 80 mm carapace length did not
change significantly over the 14 month study period with the exception of undersized
males (smaller than 110 mm carapace length). Sex ratios remained constant at
approximately 1:1. In marked contrast, catch rates of males and females and the sex
ratio of trapped lobsters varied strongly with season, implying that catchability varies
seasonally and with sex. Impact of capture on subsequent catchability appeared to be
weak, since the ratios of tagged animals in the population observed underwater
generally reflected recapture rates of tagged animals in trap catches.
Size-specific catchability generally increased with size, but also varied with sex and
season. During moulting and mating, size-specific catchability and relative
selectivity did not increase, and sometimes decreased for larger animals. The size-frequency
distributions of lobsters captured in traps therefore rarely reflected the
size-frequency distribution of the population on the ground. Negative associations
between small and large lobsters in traps were stronger in winter than in summer,
indicating strong behavioural interactions. These interactions can account for the
lower catchability of smaller lobsters. Relative selectivity estimates using tag-recapture
and size-specific catchability data provided similar results.
Seasonal variation in catchability of legal-sized males and females in the scientific
reserve was described by modelling the effects of water temperature, moulting and
mating. Seasonal changes in water temperature described 63% of the variation of
catchability for males, but were a poor predictor of catchability for females outside
winter. Both moulting and mating were highly synchronised, although males and
females moulted at different times of the year. Gaussian probability density functions
were used to represent the timing and intensity of moulting, mating and subsequent
compensation periods, and were added to the description of seasonal temperature
changes. Four Gaussian functions based on independent biological data considerably
improved the model fits for the catchability of males (R2 = 0.83). However, adding a
single Gaussian function to the temperature model, representing a combined
moulting and mating period based on independent biological data, provided a less
adequate description of the variation in catchability of females (R2 = 0.49). Only
models unconstrained by the observed timing of these events provided a good fit (R 2
= 0.74).
The seasonal catchability models developed for the reserve population were applied
to catchability over several years in two commercially fished regions of Tasmania.
Catchability was estimated using commercial catch and effort data and fishery-independent
estimates of exploitation rates. The seasonal catchability models suggest
that similar environmental and physiological processes were the main factors
determining seasonal catchability in the two fishery regions, but these factors varied
considerably in their relative importance between the two regions. Interannual
variation in relative catchability was correlated with density-dependent processes.
Full models described 72% of the overall variation in catchability over 6 years in the
south and 80% of the variation over 4 years in the north.
More work is required before direct estimates of catchability can be included in stock
assessment models. In particular, region-specific patterns of seasonal catchability,
and the relationship between density-dependent processes and the interarmual
variation in catchability need to be determined, before catchability can be reliably
predicted in future years and in other regions of Tasmania. Nevertheless, this work
has greatly improved our understanding of the processes that apparently underpin
seasonal catchability.

Item Type: Thesis (PhD)
Keywords: Lobster fisheries, Lobster fisheries, Jasus edwardsii
Copyright Holders: The Author
Copyright Information:

Copyright 2002 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2002. Includes bibliographical references

Date Deposited: 04 Feb 2015 23:39
Last Modified: 24 Aug 2016 04:36
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