Hot Weather Stresses Fish and Service Staff

The IFS has received several reports over the past week from concerned anglers regarding heat-stressed fish at several shallow waters in the highlands.

Waters mentioned include Lake Paget, Second Lagoon and the back pond at Howes Bay Lagoon, all in the Nineteen Lagoons area of the Western Lakes, and Bruisers Lagoon near the turn-off to Tods Corner at Great Lake. Of these, only Lake Paget resulted in fish deaths with 16 brown trout up to 1 kg being removed last Thursday (11 February) morning. 

A number of these waters are stocked annually with adult fish transferred from Great Lake to cater for intense angling pressure during spring and summer. They are popular recreational fisheries, being wild trout waters in an untouched landscape yet accessible by foot for day trips. They provide anglers with some of the unique fishing experiences that Tasmania is famous for, such as sight fishing to good size wild brown trout, attracting many interstate and overseas visitors.

The Service is well aware of the threat to fish inhabiting small shallow waters due to high water and ambient air temperatures during summer months. As water temperatures rise, the levels of dissolved oxygen in the water decrease, and fish are unable to extract sufficient oxygen from the water siphoned through their gills. In severe situations, they can die from asphyxiation and turn ‘belly-up’.

Air and water temperatures generally drop over-night, particularly in Tasmania’s highlands, which should give the fish a chance to recover. However, the amount of oxygen available is complicated by the fact that plants photosynthesize during the day, which is when they produce most oxygen. Hence, there are a number of factors which govern the survivability of fish at these waters under extreme water temperature conditions, including bio-thermal mass (water volume and depth), type of substrate, wind exposure and the surrounding vegetation.

In past years, the number of fish stocked into these waters is depleted by anglers long before the summer temperatures peak, so the problem of heat stress and occasional fish death is normally avoided. Although there was no change to the number of fish stocked during the last stocking period, it appears that a good number of fish remain in several of these waters. This is great news for anglers – quality fish and excellent weather – the perfect time to visit these idyllic highland waters.

It is less enjoyable for Service staff, monitoring the condition of fish at certain waters and waiting for the hot spell to pass. While there was a slight reprieve for fish in waters that received some rainfall last weekend, the current hot weather is forecast to continue and there is a chance of further reports of fish suffering from heat stress in the following week.