Introduction
The Styx River (Figure 1) is a spring-fed, lowland, low-gradient river that arises in the northern suburbs of Christchurch City, then flows 21 km to discharge into Brooklands Lagoon (Hicks & Duncan, 1993). The much larger Waimakariri River, which contributes c. 80% of the water in the Styx River via aquifers (Woodward-Clyde, 2000), strongly influences river flows. The Styx itself moves along an old groove incised in the alluvial gravel outwash fan of the Waimakariri River and possibly represents a historic pathway taken by that river. Data for flows at the mouth are not available, but mean flows at Radcliffe Road (10.8 km above the mouth) are 1.48 m3s-1. Flows vary between 0.9 and 2.7 m3s-1 98.5% of the time, and the maximum flow over a 5 year period was 7.8 m3s-1 (Environment Canterbury, unpublished data).
The primarily rural catchment of the Styx River (55km2) is bounded to the north by the catchment of the south branch of the Waimakariri River, while the Avon and Heathcote Rivers drain the majority of urban Christchurch to the south. The Styx River has two main natural tributaries as well as a host of small drains. Smacks Creek (2km) is in the upper reaches of the Styx River and joins flows with the main stem above the Styx Mill Conservation Reserve, and Kaputone Stream (11km) converges with the middle reaches of the Styx River at Marshlands Road.
Background and history
Historically, the Styx River was a relatively clean-bottomed river because of low sediment inputs. Like most natural lowland rivers, the lower and middle reaches were probably soft-bottomed, while the spring-fed headwaters consisted of clean gravels that possibly extended downstream as far as Radcliffe Road. The river environment itself supported diverse communities of macroinvertebrates and fish (Robb, 1989; Eldon & Taylor, 1990; Taylor et al., 2000), as well as several species of aquatic macrophytes. The macroinvertebrate community included at least 15 species of caddisflies (Trichoptera) and also stoneflies (Plecoptera) (Eldon & Taylor, 1990). While historical data on the fishery is lacking, it undoubtedly supported several native fish species including Lamprey (Geotria australis), Inanga (Galaxias maculatus), Smelt (Retropinna retropinna), Giant Bullies (Gobiomorphus gobioides), Upland Bullies (Gobiomorphus breviceps), and possibly other species including Bluegilled bullies (Gobiomorphus hubbsi), mudfish (Neochanna burrowsius), and Giant Kokopu (Galaxias argenteus) (Taylor et al., 2000).
Around the turn of the century the North Canterbury Fish and Game Council introduced Brown Trout (Salmo trutta) to the Styx River from Tasmanian stocks. The trout adapted quickly and flourished on the plentiful invertebrate food supply, and the good access to clean spawning gravels. As brown trout compete aggressively with native fish species, the introduction undoubtedly displaced, or adversely affected, some of the native populations already present.
Despite the good trout population, the Styx River has not been heavily fished. This is probably due to other considerations such as angler perceptions of poor water quality and lack of isolation (Teirney et al., 1987), and also perhaps because of the abundance of comparatively better trout fisheries nearby (Eldon & Taylor, 1990).
With the draining of wetlands and the conversion of land to agricultural and horticultural uses, as well as increasing urbanisation, sediment inputs to the Styx River have almost certainly increased from historical levels. Compounding this, extra sediment inputs to the river have undoubtedly resulted from the construction of stormwater drains soon after the settlement of Christchurch (c. 1875) that increased the catchment area by 16km2 (Scott, 1963, cited in Hicks & Duncan, 1993). Additional problems may have also occurred as a consequence of installing tide gates near the mouth in 1964 to reduce flooding in the lower reaches. While this change has not led to dramatic collapses in the abundance and diversity of the biota, there is concern that cumulative effects have incrementally altered the character and ecology of the river. For example, stoneflies have not been found in the river since 1929 (Parrot, 1929) despite invertebrate sampling programs by the Christchurch Drainage Board in 1979 & 1988 (Robb, 1980, 1989). Other evidence presented by Eldon & Taylor (1990) suggests that the riverine environment has declined during the past century. In particular, anecdotal evidence suggests that the river bed is agrading in the middle reaches and into the upper reaches. Similarly, Hicks & Duncan (1993) found that sediment was agrading at a rate of 2mm/year above Radcliffe Road. However, Eldon & Taylor (1990) temper their observations by noting that macro-invertebrate diversity remains relatively high in the Styx; although this has declined in recent years (62 species in 1989 cf. 75 species in 1979). Freshwater crayfish (Paranephrops zealandicus), and shrimps (Paratya curvirostris) in particular seems to be reduced in distribution and abundance.
The Styx River catchment is currently undergoing a surge in urban development, and it is predicted that about 16% of the urban growth of Christchurch City by 2011 is likely to occur in the watershed (Couling & McCallum, 1994).
A recent decline in trout abundance
Over the past decade there has been increasing concern by community groups, river users, and scientists about an apparent decline in trout abundance in the Styx River. Given that the condition factor of adult trout is ‘fair to good’ (Taylor et al., 2000), and fishing pressure is not likely to have changed greatly, it seems likely that recruitment limitation is responsible for the decline. Some trout in the Styx River have been found to be reabsorbing eggs after the spawning season, presumably because good quality spawning habitat was not available (Eldon & Taylor, 1990). Trout redd surveys in 1990 (Eldon & Taylor) and 2000 (Mark Taylor, unpublished data) revealed a 50% decline in redd numbers, as well as a shift in redd distribution towards the upper part of the 1990 range. In 1990 most trout redds were reported to occur in the 500m reach above Styx Mill Road (Fig 1); but by 2000 most redds were found in the Styx Mill Conservation Reserve (in the upper part of the 1990 range), and very few were found in the 500m above Styx Mill Road.
Trout and other salmonids require clean, sediment-free gravels in which to spawn (Reiser & Bjornn, 1979). The interstices between the gravels allow oxygen-rich water to percolate through the substrate and oxygenate the developing eggs. If sediment begins to accumulate in the gravels, then the pathways for water movement become blocked and oxygen replenishment is reduced. Furthermore, if organic sediments are present they can promote the growth of oxygen-consuming bacteria which deplete the available oxygen. This is important because research in the USA (Maret et al., 1993) suggests that brown trout eggs and larvae suffer increased mortality at oxygen levels below approximately 6mgL-1, and that oxygen levels lower than 4mgL-1 are almost always fatal. Trout are reported to prefer spawning in sites, such as in the tailspills of pools, where hydraulic conditions reduce sediment inputs to the redd and ensure optimal conditions for the developing embryos (Lisel, 1989; Alonso et al., 1996).
Dissolved oxygen levels in redds in the upstream part of the spawning range were found to be reasonable (though not outstanding) at the time of sampling, but dissolved oxygen levels in the downstream redds were found to be lethally low (< 2 mgL-1). This suggests that redds located in the primary spawning reach of 1990 are no longer viable (Greg Burrell, University of Canterbury, unpublished data).
Finally, if sediment completely smothers the redd, or forms an armouring layer (Lisle, 1989), any larvae that hatch may find themselves unable to emerge from the gravels.
Deleterious effects on stream health associated with increased sedimentation are widely known to exist but very few studies have been done in New Zealand to demonstrate this (Ryan, 1991). While there are some exceptions (e.g., Jowett & Biggs, 1997), most of the knowledge about sedimentation processes in New Zealand has been inferred from the international literature. The Styx River exemplifies this. While there is an abundance of anecdotal evidence that sedimentation of the upper reaches of the Styx River is becoming a problem for trout and some macroinvertebrates, there are no studies that specifically investigate and quantify sedimentation processes in this part of the river.
Studies investigating sedimentation in the Styx River have focused on bed agradation in the middle and lower reaches (Hicks & Duncan, 1993), Brooklands Lagoon (Knox et al., 1978; Hicks & Duncan, 1993), and also storm inputs of sediment arriving in the middle reaches of the river (Radcliffe Road) or the lower Kaputone Stream (Hicks & Duncan, 1993). Currently, deep sediment extends well upstream of Radcliffe Road. Consequently, Hicks & Duncan may have underestimated sediment inputs to the upper Styx because of sediment deposition occurring above their study area. Presently, the most downstream patch of visible gravel exists immediately beneath the railway bridge east of Main North Road. Upstream of this patch, however, soft-bottomed substrates persist for a distance of several hundred meters. Embedded gravels become consistently visible amidst sediment bars and macrophyte beds about 400 meters downstream of Main North Road (pers.obs.)
Aims and Objectives
This study aims to investigate whether site differences in sediment accumulation rates (for clean gravels) might explain the reduction in trout spawning range and consequently the non-viability of redds at the most downstream spawning reach. While there is no historical data from the Styx River with which to compare the results, this study aims to provide baseline information for gauging future deposition rates, and to indicate if a pattern of faster sediment accumulation exists at the downstream sites used in this study.