Ecology and management of range edge populations: the case of toothfish species at the South Sandwich Islands

Abstract

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.