Background
The authors write that "at a broad geographical scale, species richness and diversity decrease as latitude increases both north and south of the equator," and in marine systems they say that this distribution pattern "has been linked most consistently to variation in sea temperature," primarily via studies conducted in north-temperate seas, such as those of Fisher et al. (2008) and Hiddink and Hofstede (2008).

What was done
Noting there is a paucity of such studies from the Southern Hemisphere, Lloyd et al. analyzed measurements of sea surface temperature and spear-fishing records pertaining to 84 species of marine fish personally harvested by one of their team: Gyula Plaganyi, a highly skilled and experienced spear-fisher, diving from a small boat anchored at off-shore reef sites located between latitudes 28.0 and 31.6°S on the east coast of South Africa. This they did for each day of diving over the years 1989-2007, over which time interval there was a 0.47°C increase in mean sea surface temperature, which rose from an average of 23.36°C for the period 1989-1996 to an average of 23.83°C for the period 2002-2007.

What was learned
The five Australian researchers report that over the specified time interval, "the proportion of the catch made up by temperate species, in terms of both the number and mass of fish, consistently decreased, whereas the proportion of the catch made up by tropical species consistently increased between the two time periods," while "the contribution of broadly distributed species to the overall catch remained approximately the same." In addition, they found that "average species richness and diversity increased 33 and 15%, respectively, between the two time periods."

What it means
Lloyd et al. write that their results "are broadly consistent with a predicted poleward shift in species ranges and a predicted increase in species richness and diversity with increasing sea temperature," confirming that "large-scale climate change causing a widening of the tropical belt and subsequent ocean warming is having a profound impact on marine species abundance patterns and community composition at a local scale in the sub-tropics." And these changes would appear to be of a positive nature.

References
Fisher, J.A.D., Frank, K.T., Petrie, B., Leggett, W.C. and Shackell, N.L. 2008. Temporal dynamics within a contemporary latitudinal diversity gradient. Ecology Letters 11: 883-897.

Hiddink, J.G. and Hofstede, R.T. 2008. Climate induced increases in species richness of marine fishes. Global Change Biology 14: 453-460.