Current theory suggests that the life history of a species will vary in a predictable way from
the range centre where habitat is likely to be optimal, to range peripheries where it is not.
Moving towards cold water, high-latitude range edges, life history theory contends that
individuals should exhibit reduced growth rates to an increased average maximum length,
with delayed maturation and reduced somatic condition. In addition, increased recruitment
variability should be observed towards range edges.
Toothfish species Dissostichus eleginoides and D. mawsoni are large deep-water finfish
predators of the Southern Ocean region. Both are found at the South Sandwich Islands in the
South Atlantic Ocean, where they are situated on the edge of their distributional range. In
this study, data collected on board commercial fishing vessels were used to describe the
ecology, life history and dispersal patterns of range-edge toothfish populations, compared
with those of range centre populations (South Georgia D. eleginoides and Ross Sea D.
mawsoni). Two main research questions were addressed:
1. What limits the distribution of toothfish species populations at the South Sandwich
Islands and how do range edge life history and population dynamics differ from
those of range centre populations?
2. How can we use this information to more effectively manage the toothfish fishery at
the South Sandwich Islands and other fisheries on range edge populations?
A bioregional analysis made use of commercial longline data to resolve the position of the
ecotone between the Antarctic and Subantarctic bioregions at the South Sandwich Islands.
An abrupt transition was observed between the two bioregions around Saunders Island and
this was spatially coincident for finfish and invertebrate communities, indicating a dramatic
change in habitat type at this location. A correlative modelling analysis suggested that the
formation of toothfish species range edges is influenced by the extent of sea ice cover and
winter seawater temperature near the surface. However, differences between threshold
values of ice and temperature at the South Sandwich Islands and the Ross Sea suggest that
regional patterns in hydrography and topography may lead to the formation of range edges that do not necessarily correspond with niche requirements, with implications for studies
evaluating species redistributions in response to climate change.
Some, though not all of the hypothesised high latitude range edge life history traits were
observed in South Sandwich Islands D. eleginoides. Increased maximum size was observed,
though initial growth rates were similar to those of individuals at South Georgia and growth
is clearly not a factor limiting productivity towards the high latitude range edge. There was
no evidence at all for gonad maturation to spawning condition and this appears to relate to
improved somatic condition, which would not normally be expected in range edge
individuals. Highly episodic recruitment, also assumed to be a feature of range edge
populations, was observed in South Sandwich Islands D eleginoides population and this is
unlikely to be self-sustaining. This appears to be confirmed by an analysis of the otolith
chemistry of D. eleginoides, where a similar chemical signature was observed at the cores of
otoliths extracted from individuals captured at South Georgia and South Sandwich Islands.
In addition a mark-recapture model was developed to estimate the migration rates of postrecruitment
D. eleginoides between South Georgia and the South Sandwich Islands. Sexbiased
migration of males away from the cold water range limit were also evident from an
analysis of the sex ratio of the South Sandwich Islands population and have a considerable
impact on resident population age structure. Accounting for the emigration of males greatly
improved the accuracy of estimates from a CASAL assessment of the South Sandwich Islands
D. eleginoides population. However the episodic nature of recruitment negatively impacted
on the sustainable yield that could be taken according to CCAMLR harvest decision rules,
which do not appear to be appropriate for this and other range edge toothfish stocks.
Some aspects of range edge life history, including decreased growth rates, delayed
maturation and episodic recruitment will impact on the productivity of a population such
that it will be increasingly vulnerable to the impacts of exploitation. Where generalised
patterns in range position effects on life history can be discerned, this information can be
used to make predictions about the life history of exploited stocks that are otherwise data
poor and also will allow for improved predictions as to how species will respond to changes
in the environment such as those arising from climate change.