Diadromous fish in Mackay and Whitsundays face serious transit challenges

Mackay Whitsunday Fish Barrier Prioritisation Report
Diadromous fish move between the fresh and saltwater environments for some aspect of their lifecycle. The movement of Diadromous fish along our waterways is considered crucial to their life cycle and breeding patterns. Barriers such as dams, weirs, culverts, bed level crossings and causeways were stunting the growth of key species for commercial and recreational fishers. These structures have been built throughout the state for a variety of purposes such as irrigation supply, flow gauging and regulation, on-farm stock watering and irrigation supply, urban and industrial supply, flow management and flood control, prevention of tidal incursion, road crossings or simply for urban beautification and recreation facilities (Marsden et al. 2003).

Important recreational and commercial species such as barramundi, sea mullet, mangrove jack, tarpon, jungle perch, long finned eels all migrate between the two different habitats. By enabling connectivity to each species preferred habitat, you are increasing their numbers. In Mackay-Whitsunday there are 48 freshwater fish species and half of those are ‘diadromous’, where they migrate between freshwater and inshore marine habitats to complete their life cycle– they are a truly migratory species and need to transit between freshwaters and the sea, at various stages of their life cycle, including to breed. Queensland’s two most important and iconic in-shore commercial net species, barramundi and sea mullet (Williams, 2002) require unimpeded access between freshwater and estuarine habitats to maintain sustainable populations (Mallen-Cooper, 2000)

Fish movements within the river system

Native fish, such as barramundi, swim up the creeks as small juveniles and then use that safe habitat to find plentiful food away from predators. They may stay there more than a year before they grow big enough to swim back downstream and, as bigger fish, prosper in the major estuaries opening into the sea and spawn. If those juveniles are blocked from migrating upstream, they must instead take their chances among the bigger predators downstream and are much less likely to grow to adulthood. One study on a creek in the area showed about one barramundi heading upstream per hour. Multiply that by the number of hours in a wet season, and the number of creeks in the region, and the total fish affected becomes significant. Barriers impact fish communities in many ways, some barriers such as high dams form complete blockages, whereas other structures such as culverts present partial or temporary barriers, restricting passage during particular flow events (e.g. small, medium or high flows). Even small vertical drops downstream of road crossings and culvert aprons (>200 mm) are enough to form barriers for many fish, particularly juvenile and small bodied species. The swimming abilities of fish play a critical part in understanding the effects of barriers. Physiology, size, developmental stage and morphology all influence the ability of fish to ascend past barriers (Koehn and Crook, 2013). Generally, juvenile (Rodgers et al., 2014) and small bodied fish (Domenici, 2001) possess weaker swimming abilities than larger adult fish. Pertinently, many juvenile diadromous species undertake significant upstream migrations into critical nursery habitats, and less obvious barriers such as culverts and pipes can create velocities in excess of the swimming abilities of many species.

Fish migration in the Mackay Whitsunday region

Fish migration in the Mackay Whitsunday region is intrinsically linked to large seasonal variations in the annual hydrological regime. The annual ‘wet season’ increases stream flow conditions and creates an abundance of transitional wetland habitats and the largest king and spring tides also occur at this time of year. Weaker swimming ‘young of the year’ diadromous species such as barramundi and tarpon have evolved life history migration strategies to coincide with the increased summer flow conditions and higher tides. They utilise these favourable conditions to enter into and out of inter-tidal habitats before migrating upstream into low ordered streams and lowland wetlands. The cumulative impact of a series of barriers along streams has the ability to reduce upstream fish diversity, particularly diadromous species, and in some instances may cause localised extinctions upstream of the barrier (Bunn and Arthington, 2002). Therefore, the amount of connected in-stream habitat from the tidal interface upstream to the first barrier is extremely important. Simply, the greater the amount of connected in-stream habitat, the greater the diversity and abundance of diadromous species resulting in better condition fish communities.

The Mackay Whitsunday Fish Barrier Prioritisation Report 2015 (funded by the Australian Government, commissioned by Reef Catchments Ltd Natural Resource Management group and conducted by Catchment Solutions) has found fish in the Mackay Whitsunday area face up to 3974 potential barriers that prevent, delay or obstruct fish migration as they attempt to migrate across our region, leaving no aquatic ‘highway’ to move on. This is the first comprehensive fish barrier prioritisation study conducted locally. If fish movement continues to be blocked, it could lead to a serious decline in the native fish population long-term. Barriers that prevent fish connectivity also have an adverse impact on our local fisheries’ productivity and create environmental conditions favourable for invasive fish species – for example, tilapia. It is critical that works are undertaken to improve conditions for local fish.

From the 3974 fish barriers the report identified, the Mackay Whitsundays ‘Top 40’ most important fish barriers where identified. Barriers where the environmental benefits stack up best against the cost of a solution are rated most highly. The highest priority sites that show the most potential for effective outcomes, as well as value for money were included in this “Top 40” list. These sites are where attention and investment dollars need to be focused to build appropriately designed fishways, remove barriers and really start to improve life for our local fish. The report has named the “Top 40 barriers” for consideration by authorities, such as councils and government departments.

Highest priority waterways recommended for immediate fish passage works include: the O’Connell River, Flaggy Rock Creek, Cedar Creek, Marion Creek, Sandy Creek, Constant Creek, St Helens Creek, Jolimont Creek and Blackrock Creek.

By rebuilding fish passage at these sites, extensive areas of fish habitat will be opened up to migratory fish species. This is an important first step to ensure we keep genetic diversity and maintain healthy fish populations in Mackay Whitsunday waterways moving forward.

Off–Stream Wetland Barriers

Although off-stream barriers to fish migration were not part of the project objectives, they were considered to be very important fish habitats, therefore, potential barriers on these lentic habitats were identified during the initial desktop study. Following the identification process potential barriers were taken through one stage of selection criteria. The nature and extent of wetlands, consisting of low lying areas inundated by wet season rainfall events, combined with their predominantly freehold land tenure, means that on-site access to validate the presence and type of barrier can be challenging. Accurately assessing wetlands using GIS based stream network processing tools is also inherently problematic, as wetlands are generally not part of the stream network. Because of these difficulties, the authors recommend that a separate barrier prioritisation process specifically for wetland barriers is conducted in the future.

The majority of these potential wetland barriers within the MW region are located within and around the Goorganga wetland complex, near Proserpine. This wetland area is listed on the directory of important wetlands and they are significant on a national scale. Many anthropogenic barriers have been created on these important wetlands to prevent saltwater intrusion and pond freshwater to grow pasture including the Class 2 invasive weed species, Hymenachne. Further investigation is required to determine the extent of barriers in this area, particularly ponded pasture bund walls, but also to examine potential Hymenachne weed choke barriers. It is critically important that off-stream barriers are considered for future investigations as many of these habitats are located on coastal wetlands which are important nursery areas for many socio-economic diadromous species such as barramundi and tarpon.

Case Study: Forbes Road Rock-ramp Fishway

Fish Barriers & Mackay Whitsunday Fish Species
The Mackay Whitsundays region is blessed with a diverse array of freshwater fish species, many of which undertake migrations during their life history. Some of these migrations are short and confined wholly to freshwater habitats, while some migrations occur across vast distances and between varying habitats, including between freshwater and near-shore marine environments. Of the 48 freshwater fish species found to occur in the MW region (Moore and Marsden, 2007), almost half (48%) require unimpeded access between freshwater and estuarine habitats to complete their life cycle or maintain sustainable populations.
Migration strategies between key habitats have evolved for a variety of reasons, including;
• Feeding and reproduction purposes,
• Avoidance of predators,
• Utilisation of nursery areas (juvenile fish),
• Dispersal – to avoid being trapped in drying waterholes
• Maintain genetic diversity
However, barriers such as culverts, pipes, road crossings, weirs, dams, flow gauging structures, bunds (or ponded pastures) and sand dams prevent connectivity, impact fisheries’ productivity and create environmental conditions favourable for invasive fish species. Barriers to fish migration have the greatest impact on diadromous fish species – those that migrate between the sea and freshwater habitats to breed and/or sustain fish populations. These include important commercial, recreational and indigenous fishery species such as barramundi (figure 1), mangrove jack, sea mullet and jungle perch.

Man holding huge barramundi.
One metre long barramundi sampled (electro-fishing) while undertaking fish community monitoring in the lower freshwater reaches of the O’Connell River.

Forbes Road Concrete Causeway Fish Barrier
A concrete causeway on Forbes across the O’Connell River in Bloomsbury was preventing the migration of fish species and impacting local fish populations. The causeway barrier consisted of a series of pipe culverts which ‘dropped’ into the river below. The velocity of water flowing through these culverts was too fast for fish to swim through and the drop on the downstream side of the pipes into the river was too high – as native fish, unlike their northern hemisphere cousins; Atlantic salmon, can’t jump.
So in November 2013, Reef Catchments funded Catchment Solutions to construct a partial width rock ramp fishway (Figure 1) so that the fish can ascend through the fishway, negotiating the causeway barrier into important upstream habitats. Fish are then attracted to the fishway as flows pass through it. The fishway consists of a series of pools interspersed by rock ridges. The rock ridges consist of large 1.5 m boulders, with small gaps in-between. The drops between the pools are 75 mm. Fish use their ‘burst speed’ to negotiate each rock ridge in the fishway, before resting in the pool.

O'Connell rock-ramp fishway.
Nature-like rock ramp fishway, Forbes Road, O’Connell River.

Nature-like rock ramp fishway, Forbes Road, O’Connell River.

Mackay Whitsunday Fish Barrier Prioritisation Report