Assessment of a vertical-slot fishway for non-salmonid fish at a tidal barrier on the sub-tropical Fitzroy River, Queensland

 Fishways designed for salmonids in temperate rivers have often been successful, but similar fishways provided for non-salmonid species in tropical and sub- tropical rivers have frequently failed. In 1970 a salmonid style fishway was built on a tidal barrage on the sub -tropical Fitzroy River, in Queensland, north-eastern Australia. Despite modifications to the design in 1987 assessment of the fishway in 1978 and 1992 demonstrated that it was ineffective; particularly as few barramundi (Lates calcarifer [Centropomidae]) were found at the top of the fishway. The inadequacy of the design appeared to be related to very high water velocities and turbulence within the fishway pools. Consequently, since construction of the tidal barrage many catadromous fish populations (species that live in freshwater but breed in saltwater) have declined in the freshwater reaches of the Fitzroy River. 

In 1994 the fishway was modified to a vertical -slot design. The study reported in this thesis is an assessment of the effectiveness of the new design from 1995 to 1997. In 38 paired samples taken at the top and bottom of the fishway, over 16 months, 29 fish species and over 23,000 fish were collected at a maximum rate of 3,400 per day. At the top of the fishway 15,471 individuals were collected and 7594 at the bottom. The migratory population consisted of 4 marine species, 9 diadtomous species and 16 potamodromous species. Common species using the fishway included blue -catfish (Arius graeffei [Ariidae]), bony herring (Nematalosa erebi [Clupeidae]), striped mullet (Mugil cephalus [Mugilidae]), barramundi, and long -finned eels (Anguilla reinhardtii [Anguillidae]). Shrimp  Macrobrachium australiense [Palaemonidae]), juvenile crabs (Varuna litterata [Grapsidae]) and long -finned elvers did not ascend the full length of the fishway and specific fishways for these species are recommended. Fish between 25 and 640 mm in length ascended the fishway, although the passage of smaller size classes of immature fish was restricted which may be important for the sustainability of these migratory populations. The bulk of the biomass (71% at the top and 66% at the bottom) was made up of the diadromous blue -catfish of which 81 % were immature. Furthermore, a number of other common fish species were represented mostly by juveniles, including striped mullet, bony herring and barramundi. 

This study is the first record of barramundi successfully migrating through an in situ fishway in that no significant difference was found between the size of fish at the top and bottom. A major finding was that 95% of the barramundi from the fishway were immature fish (200-640 mm long), but significantly larger barramundi (maximum length 980 mm) were captured by angling near the base of the fishway. Nevertheless, enlargement of vertical -slot width from 0.15 m to 0.45 m only encouraged a small number of larger fish (890 mm maximum length) to enter. Interestingly, very small barramundi (45-163 mm long) were also absent from the fishway, but these were found in a small tidal tributary below the barrage. Very young barramundi appear to inhabit specific off - channel habitat before migrating in the main river at a larger size. The implications of these results are that 150 mm slot widths are suitable for barramundi and that different life stages have different migratory requirements.