Team Sawfish: the extreme highs and lows of 2015

Team Sawfish’s 2015 was a year of extremes, not only for us researchers, but also for the sawfishes we study in Western Australia. Beginning the year with one of the driest wet seasons in the last 15 years, we knew we were likely to find low water levels and few young of the year sawfish, but we could not have imagined what surprises, good and bad, were in store for us.

Team Sawfish 2015. Dr David Morgan not pictured. Photo: Jeff Whitty

Our adventures began in early August, the early dry season, in the lower estuarine pools of the Fitzroy River. This time of the year is often ideal as the days are warm, nights are cool and rain is absent. By August, river discharge has fallen and the once flowing river is transformed into a chain of isolated pools. The small amount of river flow that remains and the relatively cold air temperatures keep the estuarine pools fresh and cool, with only the occasional spike in salt when the tidal waters intrude from nearby King Sound.  

The lower estuarine pool where we captured Dwarf Sawfish in August 2015. Photo: Jeff Whitty

 

During this early dry season excursion, we set out to continue our research on the Largetooth (Freshwater) Sawfish. However, the sawfish had other plans, as only Dwarf Sawfish, a marine species, were present. One after another, these fish filled our nets, an unusual occurrence for any sawfish species. We were not only surprised by the presence and numbers of the Dwarf Sawfish, but also by the salinity levels of the water, which were unusually high, being close to that of sea water, and the likely reason for the presence of the Dwarf Sawfish.

Knowing to never let a good opportunity pass us by, we decided to make lemon-aid from this lemon of a situation and tagged these Dwarf Sawfish with acoustic transmitters; a project (in collaboration with CSIRO) that was not due to start until October. This opportunity allowed for us to commence our study early, and collect months of data that we would have otherwise missed out on.  After deploying all of the tags that we had with us at the time, we departed with plans to continue to take advantage of this situation during our next field trip.

A Dwarf Sawfish tagged and released by Team Sawfish in the Fitzroy River in 2015. Photo: David Morgan

Returning in September, we once again were successful in finding large numbers of Dwarf Sawfish, at one point catching eleven within a single hour. Other fishers were also reporting unusual captures of marine fishes within the lower regions of the river, including hammerhead (likely Winghead) and Blacktip Sharks. We even had a surprise catch of our own. While fishing in the estuarine pools, we were ecstatic to catch the first ever Green Sawfish to be recorded within the Fitzroy River! Although we have observed Green Sawfish to use river mouths as nurseries, captures of this species in the King Sound are rare. 

Dr David Morgan with the Green Sawfish. Photo: James Keleher

Moving our efforts to the freshwater pools further upriver, we resumed our search for the Largetooth Sawfish. Despite high levels of effort, we encountered relatively few sawfish, a common trend over the last few years (likely due to the short wet seasons that have occurred during the same period). As the wet season size dictates the depth of the river, it also dictates how many sawfish can make their way to the safety and stability of the freshwater pools, and how many young of the year are recruited into the riverine nursery. As this dry spell has lasted several years, the only Largetooth Sawfish we observed in 2015 were those pupped in 2011. 

A sawfish cake. One of the few Largetooth Sawfish we saw on this trip.

In early December, our team found itself on the river once again, but this time amongst an unfortunate situation. Our team was informed by local residents that there had been a large die-off of sawfish and Bull Sharks in the upper reaches of the Fitzroy River.

Upon arriving at the site, the stench of death filled our noses and swarms of flies covered our faces. With the help of the locals, we recovered 12 Largetooth Sawfish, 8 Bull Sharks and a Whiptail Stingray, while also observing deceased catfish, cherabin (crayfish) and thousands of mussels floating on the surface of the water. Even arriving only a couple days after the deaths of the fishes, the now extreme heat and local scavengers had started to break down their bodies, making any autopsy and sampling of these animals near impossible. Despite the conditions, we collected what salvageable information we could, to ensure that this tragedy was not a total waste. 

Some of the Largetooth Sawfish and Bull Sharks killed by the low dissolved oxygen event in December. Photo: Jeff Whitty

Unable to determine the cause of death from the animals themselves, we turned to the environment to see if there was any evidence to suggest what happened. We deployed multiple sensors throughout the entirety of the affected pool to detect any abnormalities in the temperature, oxygen levels and pH of the water column; it was not long before we had identified the silent killer. There was very little oxygen below a depth of 1 meter and no oxygen below 2 m in this 10+ m deep pool. From interviews with local residents, we learned that a small rain event had washed oxygen-hungry organic matter into the pool. Without the additional input of freshwater, this organic sludge became concentrated within the single pool and likely absorbed the dissolved oxygen in the water, killing all bottom dwelling species. Similar occurrences have taken place in other seasonally flowing rivers in Australia but thankfully, residents said that this is a rare event in the Fitzroy River.

All in all, 2015 was an eventful year for the sawfishes of the Fitzroy River and Team Sawfish. Although we faced an number of unfortunate events, these unusual occurrences provided us with insight into how changes to the climate and environment can impact various species of sawfishes. As we move on to a new year of research, we are hoping to find a long and rainy wet season and a new batch of young of the year sawfish awaiting for us around the riverbend.

For more information about Team Sawfish and our work, please visit www.freshwaterfishgroup.com or like our Facebook page at https://www.facebook.com/Freshwater-Fish-Group-Fish-Health-Unit-342655079102312/?fref=ts. We would like to thank the Western Australia Marine Science Institute, Chevron Australia and CSIRO for funding these projects.