In accordance with Huet’s zonation model, limnophilic cyprinids are not well suited to the higher flow characteristics of the upper catchment and prefer deeper, slow flowing, lowland habitats. Focus on these species is therefore centred on the Lower River Irwell, the MSC and Salford Quays. Should EC FFD water quality targets be achieved, in tandem with the provision of appropriate physical habitat structure, the MSC and Salford Quays could potentially support the following species: roach, rudd, common bream, silver bream, common carp, tench, perch and pike. In addition, gudgeon would be supported throughout the MSC, as indeed they are present in the Turning Basin area.
Because of contrasting water quality between those areas that currently do and do not benefit from amelioration measures, factors constraining limnopils will be considered for each of these areas in turn.
In considering the lower River Irwell and the MSC, the water quality data presented within this report (and in greater detail in (APEM, 2007)) clearly indicate that DO and ammonia levels are not currently meeting EC FFD target levels for cyprinids. These data also provide information regarding the seasonality of water quality issues and also fine scale detail of the diel cyclic pattern of oxygen availability. Because of the limited availability of oxygen within these habitats, fish populations are seasonally constrained by the areas of the Canal they can exploit, with both longitudinal and vertical ranges becoming minimal during the summer months. According to an extensive search of the relevant literature, hypoxic condition can affect fish in many ways, - including increased susceptibility to pollution, growth, reproductive capacity and ability to avoid predation. Although growth rates and condition factors of the adult stock remain favourable, this may be due to low levels of fish density currently present. Should stock density increase then the health of these populations is likely to be put under greater pressure with food availability becoming limited within the areas of the Canal that the fish are able to tolerate.
As mentioned above, being subject to inadequate oxygen levels, the sustained swimming capability of fish is also compromised, making them more vulnerable to predation. In addition, predation risk is likely to be further elevated by the effective
‘herding’ of fish shoals within areas of acceptable water quality. This may be manifested as either vertical redistribution of the population to the surface layers or towards confined areas where oxygen levels are more favourable such as downstream of weirs. Where this happens fish are likely to be at greater risk from avian predators, such as cormorants, which frequent the Canal. Vertical habitat shifts by adult fish may also be impacting directly on the survival of 0+ fishes. This is reported to shift trophic interactions, with benthic invertebrates becoming inaccessible, larval fishes occupying the upper littoral zones become a more viable food resource for adult fishes.
The oxygen levels currently observed in these habitats are brought about by a combination of chemical, biological and physical factors. Although reducing sewage inputs will benefit the aquatic ecosystem in many ways, it alone will not combat the combination of the physical structure of the Canal, the historic deposition of organic sediments and the lack of physical mixing which causes the system to stratify.
Sewage effluents are also clearly affecting fish populations within the MSC as a consequence of endocrine disruption. Although the direct ecological relevance of these impacts is not clearly understood at present, improvement in inputs and associated oestrogenic compounds are likely to maintain a more balanced sex ratio, increase male fertility and may ultimately increase reproductive success.
This may also potentially show immediate benefits to the survival of successfully fertilised fish eggs. Limnophilic cyprinid eggs are no more tolerant to the affects of
‘sewage fungus’, as they are deposited in areas with little or no flow to refresh them.
Exposure to such bacteria may play a very important role in influencing the survival of embryonic and larval stages of development. However, as stated earlier, colonisation of the upper MSC with coarse fish has already successfully occurred, with excellent growth and condition. Whether recruitment is constrained by sex reversal requires more detailed investigation.
Other major factors threatening the fish of the upper MSC and lower Irwell include the immediate effects of episodic input from storm overflows, resulting in mass mortalities from the effects of organic pollution. The lack of appropriate ‘off river’
sanctuaries from floods is also a major issue potentially retarding recruitment in this area of the catchment. This is a potentially critical area for the future development of coarse fish populations within the MSC, and indeed is the central area of investigation in this study. However, it should be recognised that the physical structure of the MSC – with effectively impassable pounds between locks (i.e. Mode Wheel to Barton, Barton to Irlam and possibly from Bollin Point to Latchford Locks) prevents fish movement away from areas of deteriorating water quality. This issue is of great significance, as was seen with the substantial fish kill that occurred in May 2006, when low oxygen concentrations in the Mode Wheel to Barton pound resulted in the death of several thousand fish. Despite similar water quality conditions in the Turning Basin, relatively few dead fish were observed, indicating that most of the fish previously present (from APEM/EA sonar surveys) had apparently moved upstream towards the Irwell where channel morphology affords a degree of vertical mixing and hence maintains higher oxygen levels. With the lower pounds the option for fish to move to more favourable conditions is either absent or extremely limited, hence the observed mortality. Ironically, with the success of the water quality improvements, fish have been encouraged to colonise these areas, but unwittingly, are then trapped with no avenue to escape during the episodic, poor water quality events.
9.1.1Oxygenated areas
The oxygenation programme was primarily initiated to improve the aesthetic value of the upper MSC. In addition to achieving these goals, these areas extend the spatial habitat available to fish populations throughout the summer and during periods of hypoxia, offer an alternative refuge from the oxygenated water below Adelphi Weir.
Despite these benefits the artificial injection of oxygen to the Canal currently offers little more than a temporary life support system to individuals, with little benefit at the longer-term population level. This is primarily due to physical habitat constraints within these areas, which if rectified could prove highly beneficial to the self-sustainability of limnophils within the Canal. With the spawning of limnophils typically occurring between May and July, the availability of appropriate habitats within areas where oxygen levels can be maintained are extremely limited, with nocturnal oxygen sags at this time of year resulting in complete depletion of oxygen in the surface layers throughout the majority of the Canal’s entire length. Ideally the provision of a littoral shelf 2-3 metres wide with a mean depth of 1m would encourage the colonisation of macrophytes and the associated communities of macro invertebrates and zooplankton. In turn these habitats would provide ideal nursery habitats with appropriate food supplies for larval fish and because oxygen levels are maintained at desirable levels, the provision of appropriate spawning media would increase egg survival. The creation of such habitats throughout the rest of the upper Canal would also be of great benefit to fish recruitment. The softer bank engineering elsewhere, may facilitate the creation of such habitats, more easily than in the sheer banked areas of the Canal, which currently benefit from oxygen supplementation.
9.2 Salford Quays
Within Salford Quays fish populations depend on the maintenance of oxygen levels via ‘Helixor’ mixers. This alleviates the immediate problems arising from low DO levels, thus highlighting the physical factors that are currently affecting the fish populations. In effect this provides an insight into the future regarding the future for fish populations in the MSC, once water quality problems have been resolved. The key physical issue within the Quays is a lack of littoral habitats, which are needed to provide macrophytes for spawning, a sanctuary from predation and the provision of zooplankton production on which larval fish can feed.
On hatching, limnophils are poorly developed and rely on yolk reserves in order to gain enough energy to swim to the surface and fill their swim bladder. Until this point,
‘free embryos’ have negative buoyancy and in the absence of macrophytes to support them, will fall to the sediment where cutaneous respiration has to be maintained until enough energy has been absorbed from the yolk to make the journey to the surface. In order to cope with such conditions, some species such as pike have developed adhesive cement glands, upon hatching, with which they attach themselves to macrophytes to avoid sinking to the lower anoxic layers (Braum et al., 1996). With the shear nature of the banks within the Quays and mean depths of 7 metres,
the lack of such marginal habitats is likely to be a key issue retarding the natural recruitment of roach and perch while apparently, severely limiting the recruitment of bream, carp, rudd and tench.
Although there is still evidence of endocrine disruption in the roach population of the Quays, this watercourse no longer receives any sewage input and therefore any oestrogenic compounds present must have been locked into the Quays at the time of their isolation from the MSC. As the MSC sediments are similar in origin and nature, it stands to reason that endocrine disruption is likely to continue to influence the
condition of fish within the MSC in the future, even following water quality improvements.