The "marine heat wave" over this past summer
Anna Micha (Busselton Underwater Observatory) and Alan Pearce (Curtin University)
Regular visitors to the Underwater Observatory at the end of our Jetty may have noticed some unusual immigrants outside the Observatory windows during February and March this year. And those keen on water sports like swimming and diving would almost certainly have found the sea to be wonderfully warm, even more so than our usual balmy conditions in Geographe Bay.
The reason for this was that we had an exceptionally strong Leeuwin Current bringing down an extra dose of tropical creatures as well as a raft of warm tropical waters, much further south than normal for this time of year. In turn, this strong Leeuwin Current was associated with one of the strongest La Nina events for many decades (La Nina is the opposite phase of the well-known El Nino).
All in all, the stronger and warmer southward current brought with it some unusual visitors to the waters of southern Geographe Bay. By monitoring the marine life around the Observatory for many years now, the staff there have been able to show that some of the fish we observed this past summer are new arrivals, having not been seen previously (and most of them have not, or probably will not, survive through the autumn and winter). Some of these arrivals have been the Raccoon Butterfly fish, the Reef Banner fish and the Scissortail Sergeant Major (which has previously not been sighted south of Rottnest Island) *Photos attached. Each of these species appeared at the end of February and were sighted regularly for at least a month or longer.
Elsewhere in Geographe Bay, there were reports of Spanish Mackeral, Black Marlins and Tunas.
Because the water temperature at the Jetty has been measured for over 10 years in an ongoing monitoring programme, we have a very good idea of how warm the local waters actually were. Studies by the Department of Fisheries using satellite temperature data showed that the surface temperature was more than 3 degrees above the average for February over a large area along the west coast and far out to sea -- an unprecedented temperature rise over such a large area. Here at Busselton, summer water temperatures in February are usually between 21 and 22°C, but last February and March the monthly averages were over 23.5°C, almost 2°C above normal. The highest temperature ever recorded at the Observatory was 25.96°C in late afternoon on the 26th February (see the graph).
Higher up along the mid-west coast, some coastal areas experienced temperatures some 5°C above average, resulting in a number of fish kills as well as observations of whale sharks and manta rays way south of their normal range. The water temperatures have now returned to normal, and the Fisheries Department is preparing a report on a scientific meeting held specifically to examine the so-called "heat wave" and its consequences for marine life off Western Australia.
Graph of the hourly water temperatures at the Busselton Jetty Observatory between the 24th and 28th February this year- See attachment below.
Juvenile Raccoon Butterflyfish