Phase III – Modelling the effects of climate change on the distribution of shared fishery species in the subtropical Western Indian Ocean

Climate change that is linked to the build-up of greenhouse gases and aerosols in the atmosphere is now a widely accepted phenomenon that has led to increases in surface temperatures over the last 50 years. As a result, the water in the world’s rivers, estuaries, and seas is also heating up. The effects of climate change, such as habitat reduction, hybridization with introduced taxa, and habitat fragmentation have predictable effects on the genetic diversity of species. Although marine species generally face fewer constraints to their movement than terrestrial species climate change may pose a greater threat to species when their dispersal capabilities are limited or suitable habitat is unavailable.

The goal of this study was to explore the extent to which the range of seven selected shared fishery species endemic to the subtropical WIO might shift in response to changes in the surrounding environment with climate change. To study the genetic connectivity among different populations of three species and the existing ‘genetic variation’ of these species, as this provides the raw material for adaptation. This project aimed: (i) To determine which species are most susceptible to global climate change, (ii) To determine whether tropical and subtropical species will extend their distribution northwards (iii) To determine whether their overall range will contract or expand (iv) To estimate the genetic diversity of three commercially important species and their historical demographic patterns and genetic structure throughout their distribution range. The study was conducted in South Africa and Mozambique. The main beneficiaries are the scientific community, fisheries management and conservation organisations, policy makers, and local fishermen.

Three species, namely Chrysoblephus puniceus, Epinephelus andersoni, and Polysteganus praeorbitalis, were selected for genetic analysis where sample tissues were taken and their DNA extracted and sequenced. Spatial predictions of the effects of climate change on the distribution of species were derived from statistical distribution models (SDMs) that describe the relationship between the distributions of a species and environmental predictions. Both species distributions data and environmental data were used to model species niche. MAXENT (Maximum Entropy) models were used to model the ecological niche occupied by each species and how that niche might change relative to climate change. The potential thermal habitat for each species was projected for 20 years (2030) and 30 years (2040) using the projected temperatures.

Epinephelus andersoni analyses indicated strong genetic differentiation among localities throughout the distribution of the cat face rock cod, suggesting limited dispersal range as distance among localities, which may influence their connectivity. Microsatellite data provided strong evidence that C. puniceus exists as a single well-mixed stock through its distribution and can thus be considered a single trans-boundary stock. Polysteganus praeorbitalis mtDNA analyses suggest lower levels of genetic diversity for this species with considerably lower haplotype diversity in comparison to the slinger. A seasonal linear trend of OI-SST (1982-2010) reveals that coastal warming is occurring throughout the year along the east coast of South Africa (by as much as 0.32°C per decade). The models predict that by 2020 and 2030, E. andersoni will have lost 14% of its distribution, with the northern extent of the area of high probability of occurrence (which ranges from Richards Bay to Port St Johns) contracting from Richards Bay to Durban (Figure 8). Chrysoblephus puniceus is predicted to lose 5% of its distribution by 2020 and 12% by 2030. The areas of high probability of occurrence in KwaZulu-Natal (Stanger to Port St Johns) and Mozambique (Bilene to Zavora) are predicted to change. The KwaZulu-Natal area is predicted to contract, while the species will have a high probability of occurrence in southern Madagascar and not in Mozambique. Polysteganus undulosus is predicted to lose 14.5% of its distribution by 2020 and 34.6% by 2030. Spatial fragmentation for this species is also expected to increase, with gaps appearing in the geographic distribution in northern KwaZulu-Natal.

Information on genetic variation, distribution and their future predictions are crucial for management because connectivity among local populations is critical for species persistence, such that re-colonization from neighboring areas is enhanced by a continuous distribution but may be more difficult between fragmented areas. Predicted changes in species distribution are also fundamental for the management of marine fish species and the enhancement of marine parks and closures.

This project spun off the establishment of a collaborative linkage between SWIOFP through its South African institutional partner (Oceanographic Research Institute, Durban) and Dr Sean Fennessy on the genetic assessment of Chrysoblephus puniceus.