Thesis submitted in fulfilment of the requirements for the degree
Master of Technology: Oceanography
in the Faculty of Applied Sciences
at the Cape Peninsula University of Technology
2014 / The Tristan lobster Jasus tristani is distributed among several isolated islands and
submerged seamounts in the South East Atlantic Ocean. This species occurs only at the
Tristan da Cunha group, a British Overseas Territory and the World’s most remote inhabited
island, and in international waters at Vema Seamount 1680 km ENE of Tristan. All these
populations are exploited commercially. The catch, processing and export of J. tristani is the
most important economic activity for the inhabitants of Tristan da Cunha, providing the
livelihood of many families and accounting for approximately 80% of the Island’s revenue.
Sustainable harvesting of this valuable resource requires accurate long-term data on catch
and effort, as well as information on the most important biological parameters such as
growth, recruitment, moulting and reproductive cycles. This information is critical for robust
assessments and management strategies. This thesis describes the history of the lobster
fishery at the Tristan da Cunha island group, reporting on catches and trends in Catch Per
Unit Effort between 1967 and 2010. A total of 247,014 lobster samples, both sexes
combined, was sampled for size composition and sex ratios, as well as 1,526 lobsters for
length/weight relationships, between 1997 and 2010. This confirms earlier findings that
females have broader and heavier tails than males for the same carapace length (CL).
Results show that males dominate catches at all islands, and their average size was larger
than that of females (83.5 ± 14.46 versus 73.4 ± 8.64 mm CL, respectively). Inter-island
differences in lobster population structure appear to be caused by differences of food
availability as well as in density-dependent growth and survival of young lobsters. The largest
lobsters were found at Gough Island (87.2 ± 15.13 mm CL), and the smallest at Inaccessible
island (73.2 ± 11.39 mm). Tristan was the next largest to Gough Island (84.0 ± 12.56 mm)
followed by Nightingale island (78.2 ± 11.33 mm). Lobsters caught inshore were larger than
those caught offshore, although this may be related in part to differences in catches between
fishing gear types.
This study showed that fecundity increases in a linear manner with CL, and although larger
lobsters clearly produce more eggs than smaller ones, the gain in fecundity is not as great as
in some lobster species where fecundity is more closely associated with weight. The study
showed no significant differences in egg size between islands, or between large and small
females at one island. The egg production per gram of body weight and mean egg diameter
both seem to be less than reported in an earlier study in the 1990s. While it seems likely that
this is due to differences in the way in which samples were collected (with only stage 2 ova
collected and measured in this study), the possibility of a decline in fecundity needs to be
investigated further.
A range of management measures have been developed over the history of the fishery, and
important current measures include an annual total allowable catch (TAC) for each island,
minimum size limits, and a closed season timed to protect egg-bearing females. The fishery
has recently been awarded certification by the Marine Stewardship Council. The study has
confirmed that current conversion factors are broadly correct and that different size limits
established for each island are justified. Concern is raised, however, by the fall in catch per
unit effort and the mean size of lobsters at the three northern islands over the past 7 years.
These trends will need to be closely monitored.
There are still many uncertainties over key parameters such as growth and recruitment and
the intention is to increase the knowledge base and our understanding of the dynamics of the
lobster stock. A research plan has been developed, so that progress can be monitored
through the gradual implementation of scientifically defendable fisheries management
procedures and increased research and monitoring capacity.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:cput/oai:localhost:20.500.11838/2033 |
| Date | January 2014 |
| Creators | Glass, James Patrick |
| Contributors | Venter, A., Mrs |
| Publisher | Cape Peninsula University of Technology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | http://creativecommons.org/licenses/by-nc-sa/3.0/za/ |
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