Studying Maugean skates

Tasmania, is home to what is possibly the rarest skate species in the world. With such a limited distribution, little has been known about the skate until now.  It was only discovered in 1988 and is confined to the upper reaches of Bathurst and Macquarie harbours. The skate appears to have disappeared from Bathurst Harbour, with no detections for decades. The only other population is thought to be about 3,000 strong in Macquarie Harbour. The Tasmanian Government admits the skate’s “numbers are low”, but does not have a recovery plan to protect the species. The EPA said skate population numbers appeared to be stable at the moment and two separate research projects were underway to understand fish farm impacts. Biologist Jeremy Lyle, said more research was needed to find out what the skates — and in particular their eggs — can cope with, but it was highly likely further drops in oxygen levels would reduce in the skate’s habitat, but to what extent remains unclear.

The Maugean skate lives in brackish waters, rich in tannins where there is very little other marine life. It is basically a deepwater species living in shallow waters. Of the 400 skate species identified worldwide, the Maugean Skate is the only one known to inhabit brackish water. Despite Macquarie Harbour’s remote location, the skate is competing with expanding aquaculture, tourism, and recreational net fishing. A recent FRDC/IMAS project has identified the distribution, habitat, reproductive dynamics, feeding habits and population of the Maugean Skate in Macquarie Harbour. It has also assessed the impacts of current and proposed marine farming operations on the population. Researchers spent 12 months monitoring the population of Maugean Skate using an extensive array of acoustic receivers positioned throughout Macquarie Harbour. Almost 60 Maugean Skate were acoustically tagged at multiple locations. Research fishing was also conducted over 15 months to assess 10 reproductive status and diet before releasing skate back into the water. All skate were microchipped before being released. During the monitoring, the tagged skate were detected at depths of six to 12 metres, 85 per cent of the time, and occasionally at depths greater than 55 metres. Depth utilisation appears influenced by water chemistry. Shallow waters have low salinity and high temperature variability. Deeper waters are stable in temperature and salinity but have low concentrations of dissolved oxygen. The intermediate depths that skate prefer are relatively stable in salinity, temperature and dissolved oxygen. While some skate left their core range for brief periods (days to weeks), almost all returned. There was no evidence to suggest long-term movement of skate out of the estuary.

This means that Macquarie Harbour’s Maugean Skate are likely to be a distinct population. The project estimated the population of the Maugean Skate to be around 3200 individuals, although this may be an underestimate. Maugean Skate were found widely distributed throughout Macquarie Harbour and displayed a high degree of site fidelity, with home ranges generally less than 10 kilometres square. Many skate showed an affinity for the Liberty Point/Table Head region, in the central, south-western side of the harbour. Maugean Skate were more active at night and moved into shallower water, which probably represents nocturnal foraging. Their diet was dominated by crabs and shrimp. While there was no evidence of feeding on aquaculture pellets, this cannot be ruled out. Preliminary estimates of age suggest the species is relatively short-lived. The maximum age observed was 11 years, but they may live to about 15 years. Maximum age (and size) is a useful proxy for productivity and suggests that Maugean Skate are probably relatively productive. Recreational fishers say gillnetting is one of the few options that allow recreational fishers to catch edible fish in this area – generally flounder. The local fishers suggested closure to gillnetting of most waters in the harbour deeper than five metres, and the closure of the Table Head/Liberty Point region. The project also found that direct interactions between Maugean Skate and aquaculture operations was likely to be limited. Many people have called on fish farms to financially support more research.