fish populations as indicators of long-term changes in uk estuaries

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Senckenbergiana maritima ] 31 ] (2) [ 169-175 ] Frankfurt am Main, 28.12.2001

Fish Populations as Indicators of Long-term Changes in UK Estuaries

MARGARET E. GILL & CHRISTOPHER L. J. FRID &2 KIRSTY HARWOOD & SARAH PEATY

With 3 Text-Figures and 1 Table

Keywords: Bioindicators, estuarine fish populations, beato trawling, water quality.

A b s t r a c t

[GILL, M. E. & FRID, C. L. J. & HARWOOD, K. & PEATY, S. (2001): Fish populations as indicators of long-term changes in UK estuaries. - Senckenbergiana marit., 31 (2): 169-175, 3 figs., 1 tab.; Frankfurt a. M.]

Trawling programmes in the Tyne estuary in North-east England have been conducted since 1982 by the Environment Agency, its predecessors and since 1990 by the Dove Marine Laboratory. Survey data have been combined and analysed to examine whether variations in the fish community can provide bio-indicators for changes in water quality following the installation of primary treatment works at Howdon.

Changes in fish abundance and diversity after 1988 suggest a general decline in water quality in the lower estuary, which may be attributable to increases in the discharge from the treatment works. In contrast in the upper estuary changes in fish populations ate commensurate with an improvement in water quality when outfalls were diverted to Howdon. Periods of change correspond with times of extension to the sewerage system. The Tyne fish community and the flounder population appear to provide reliable indicators of alterations in water quality.

Introduction

The Tyne estuary in the north east of England is some 32 km in length and has a catchment of approximately 2,900 square kilometres with an average daily freshwater flow of around 3.9 cubic million metres (NRA 1996a). Historically, inputs of crude sewage from an increasing population, combined with effluents from heavy industry, resulted in severe contamination of the estuary. This culminated in the middle of the 20 th century in marked reductions in fish populations and strong odours from the estuary during hot summers. Over the last two decades improvements in the water quality of the Tyne estuary

have been reflected in a substantial increase in the number of salmonids returning to the river each year (NRA 1994). This can largely be attributed to reductions in industrial and crude domestic discharges due to the decline of heavy industry in the region; along with the installation of a sewage treatment works (STW) at Howdon (Fig. 1). Routing of the region's sewage to the STW, termed the Tyneside Interceptor Scheme (TSIS) between 1982 and 1994, has meant that primary treated effluent discharged at Howdon has increased from circa 85,000 to over 1,200,000 population equivalents (NRA 1996b), causing a

Authors' addresses: MARGARET E. GILL, CHRISTOPHER L. J. FRm, Dove Marine Laboratory, Departmem of Marine Sciences and Coastal Management, University of Newcastle upon Tyne, NE1 7RU, U. K. - KIRSTY HARWOOD, WSP Environmental, Newcastle Business Park, Newcastle upon Tyne, NE4 7YG, U. K. - SAe, AH PEATY, Environment Agency, Tyneside House, Newcastle Business Park, Newcastle upon T,vne, NE4 7AR, U. K.

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localised decline in water qua]ity. Successive stages in the development of the TS[S led to a number of marked stepped changes in discharge Ioads. The greatest increases occurred in 1982, 1985, 1988 and 1990. In addition to these periodic increases there has been a general increase in discharges due to commercial developments and population expansion in the region.

Environmental monitoring is regarded as the most effective source of information for water management authorities to use both for the protection of natural habitats and for assessing environmental quality (Natio- nal Research Council 1990). Estuarine fish population studies can provide integrated indicators of water quality (ELLIOVT et al. 1988; POMrRET et al. 1988). The degree of estuarine contamination can be considered as either acute, whereby changes in the fish population occur through death or emigration, or chronic when deterioration in fish health and fecundity can lead to a general reduction in fish community numbers and species richness (ELL1OTT et al. 1988). Heavy pollution of estuarine environments can cause declines in fish abundance and diversity, ulti- mately to a point where a viable fish population can no longer be maintained (HEND~RSON & HAMILVON 1986; WILSON et al. 1988). Subsequent improvements in water quality have resulted in increases in the abundance and species richness of a fish community (W~AR~E et al. 1984). Analysis of the variation in total fish abundance and diversity can thus be used as indicators of the general health of estuaries. Variations in individual fish species may yield reliable predictors of total community abundance or provide probes, which are more sensitive to

changes in environmentai factors than the community as a whole. Analysis of the Tyne fish community should therefore reflect changes in estuarine water quality resulting from the installation of the TSIS.

The occurrence of flounder (Platichthys flesus) along the full length of the Tyne estuary (PoMrRET et al. 1988), their restricted spatial feeding range (PHrLLIVS 1980) and benthic habit allow the abundance of flounder to be used as a bio-indicator of water quality (B~vAN et al. 1985). Whiting (Merlangius merlangus), the other numerically dominant species in the estuary (PoMrREV et al. 1988) could also be used. Although the more mobile nature of whiting limits their use as specific bio-indicators long- term changes in population numbers may infer general changes in estuarine water quatity.

Over the last two decades a number of programmes monitoring variations of estuarine fish populations in the Tyne estuary have been undertaken (PoMFREV et al. 1988; GILL 1997). These surveys were initially carried out from 1982 by the Water Authority (NWA) and continued in 1989 by the National Rivers Authority/Environment Agency. Since 1990 the Dove Marine Laboratory has also conducted regular sampling o~ fish populations in the lower estuary. Although a considerable quantity of data was available from the various organisations, this has not previously been combined and reviewed within a single database. This study examines all data available in order to identify whether temporal or spat{al variations in abundance and species richness within the estuarine community reflected the changes in water quality expected from the diversion of sewage to the STW.

Material and Methods

Fish sampling surveys were conducted using a beam trawl deployed from the stern of respective vessels (PoMrRrV et al. 1988; FRm et al. 1995). Trawls were usually made around low water at sampling sites along the estuary and replicated as double tows in the Dove Marine Laboratory programme and triple tows by the NWA, NRA and Environment Agency.

Survey data were entered into a database designed by the Dove Marine Laboratory in which flounder abundance was grouped into size classes according to the ICES protocol (ANoN. 1989). Compatibility of the Dove and Environment Agency survey data was assessed to ensure temporal and spatial comparisons could be applied between respective surveys. Wbere sites trawled by the Dove and the Environment Agency coincided geographi- cally (Newcastle and Hebburn, Fig. 1) there was no significant difference between the number of flounder of whiting caught in either trawl in the same month from 1991-1996 (Mann-Whimey, p<0.05). This allowed all data to be pooled, producing a 6 station transect along the estuary.

For analysis all data were pooled into quarterly averages (January-March, April-June, July-September and October-December). This provided replication at each site yet maintained a seasonal resolution. The number of hauls for analysis each quarter varied between 2 and 5. Two groups of data were then analysed.

1. Total fish abundance and diversity for stations along the Tyne for the period 1982-1996 using time series, trend analysis and non-parametric multi-dimensional scaling.

2. The abundance of the dominant benthic (flounder) and demersal (whiting) fish (PoMFRET et al. 1988) along the Tyne for the period 1982-1996, using time series and trend analysis where significant temporal trends occurred.

Any temporal significance in the long-term variation was identified by regression using Minitab Release 10 (Minitab Inc., Pennsylvania, USA). In order to perform trend anaIysis any missing quarterly averages were esti- mated using a 3-point moving average procedure. Trends were then interpreted from fitting simple empirical models by applying time series analysis, whereby lower mean squared deviation (MSD) values indicate a more accurate representation of the model to the data observed.

AII non-parametric multi-dimensional scaling (MDS) analyses were performed using PRIMER 4.0/3 (Hymouth Marine Laboratory, Plymouth, UK), based on the Bray- Curtis similarity matrix (B~v & CI3RTIS 1957) on 4 ~h root transformed data to equally weight the occurrence of dominant and non-dominant species. For MDS plots the number of scaling iterations corresponded to the num-

h

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m,= Dove Manne LaDomtory slte S T W ouffall

Fig. 1. Map of Tyne estuary indicating location of Howdon sewage treatment works (STW), Dove Marine Laboratory and Environ- mental Agency Tyne trawl sites. - Shaded sections represent the different major stages of sewage interception to Howdon STW, illustrafing the dates and approximate areas diverted (adapted from Fig. 1, NRA 1996b).

ber of quarters in which observations occurred for each station. Differences between the fish community composition identified using MDS were analysed using a percentage similarity analysis (SIMPER) within the PRIMER package.

Identified changes in total fish abundance, species diversity, community structure and dominant fish abundance were compared to changes in water quality inferred from completion of the various stages of the Tyneside Sewerage Interceptor system.

Results

Temporal and spatial recordings of fish community data from the different surveys recorded 11 species of fish, which were used for abundance and community analysis. These were:

Flounder (Platichthys flesus) Plaice ( Pleuronectes platessa) Dab (Limanda limanda) Cod (Gadus morhua) Whiting (Merlangius merlangus) Herring ( Clupea harengus) Sprat (Sprattus sprattus) Viviparous Blenny (Zoarces viviparus) Pogge (Agonus cataphractus) Sea Scorpion (Taurulus bubalis) Saithe (Pollachius virens)

Pro]onged changes over several seasons suggesting short-term reductions in water quality were noted. A maintained general decline in both the total number and

species richness of fish was observed at Howdon frorn December 1986-December 1988 (Fig. 2a) and contrasted with the observed increase in fish diversity at Redheugh (Fig. 2c). A sustained suppression in the variation in both fish species richness and abundance was observed at the Newcastle site from December 1991 to 1996 (Fig. 2b).

In addition, changes were recorded over a period of several years or longer which inferred long-term changes in water quality. At the Howdon station the average number of fish species present showed a significant decline (Quadratic Regression P<0.05) over the 1982-1996 period investigated. The total abundance of fish at Howdon was best described by a quadratic model, increasing from 1982 to 1989 and then decreasing in the remaining period. Both abundance and species diversity at Newcastle showed a period of stability between 1982 and 1987 and subse- quently a decline between 1991 and 1996. In contrast at stations west of Newcastle i.e. Scotswood and Newburn a quadratic model implied an increase in both fish number (Quadratic Regression Scotswood P<0.05; Newburn P<0.05) and diversity (Quadratic Regression Scotswood P<0.001; Newburn P<0.002) between 1982 and 1991. This was followed by a period of stability until 1996.

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400 350 ~ I ~ (a) Howdon 3OO

400 (b) N e w c a s t l e

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300 6 o

2 5 0 5 r

200 I~ 4 m

._~ (c) R e d h e u g h F: ~1 " 1

1:~ 1200 7 c- " ~ .

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. 0 0 6 o 0 ~ ~ ~, 4OO 200

0 . . . . . . . . . . . . . . . . . . . . . . , , I , , , I . . . . . . . I I I , , ,

Fig. 2. Changes in average quarterly total fish abundance and diversity from 1982-1996 for (a) How- don; (b) Newcastle; (c) Redheugh; (d) Scotswood. - Arrows represent prolonged periods of change.

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Ta ble 1. Occurrence of significant differences in fish abundance of species richness for years preceding and following sewerage interception. - For the year of TSIS completion, data were split between the second and third quarter. Mann- Whimey significance of 95% or greater (P<0.05) represented as significant increase ('1 ~) or significant decrease (#).

Station Howdon Hebburn Newcastle Redheugh Scotswood Newburn Year of 1988 1990 1988-1990 1990 1990 1990 1992 TSIS impact Abundance ~ ~k ~ Species ~ ~ ~ ~

For each traw] site analysis of the results identified times when significant changes in both the abundance of fish and the diversity of species present, A comparison of these times with the dates when various stages of the TSIS had been completed suggested that a number of local impacts had occurred (Tab. 1). At Newcastle the quality of water would have improved in 1990 as effluent from the city centre and the brewery was diverted to Howdon. However removal of organic matter may also have been responsible for a subsequent reduction in abundance and diversity from 1992 at that site. A significant difference in both fish abundance and species diversity in the periods preceding and following 1990 at Howdon suggest a substantial change in water quality resulted from the diversion of the Newcastle city crudes to the STW (Tab. 1). In contrast increased abundance and diversity at the upriver stations followed TSIS extensions, which match the implied improvements in water quality.

A +

A

+A �9

+ 0

Non Parametric Multi-Dimensional Scaling and Similarity Aanalysis

A difference in fish community structure was observed between trawls conducted from 1980-1988 and those from 1989 onwards at Howdon. SIMPER analysis revealed the disappearance of herring, viviparous blenny and dab along with a reduction in the number of plaice and cod which accounted for 58% of the change in the fish community composition in the period following 1988 at Howdon (Fig. 3). There was some evidence for a similar shift in the fish community at Newcastle. No marked changes in the fish community were noted for any of the three upstream sites (Redheugh, Scotswood and Newburn).

Flounder and Whiting Numbers along the Tyne E s t u a r y 1982-1996

There was no significant temporal trend in the number of whiting at any station over the 1982-1996 period investigated nor in any periods associated with significant developments of TSIS. The differences in the fish community were therefore attributable to the populations of flounder or non-dominant species. At Hebburn there was a significant trend (Mann-Whimey P<0.001) in the number of flounder i.e. these fish were absent or rare

Fig. 3. iVŸ scaling analysis of average quarterly fish community composition at Howdon. - Values show separa- tion between groupings. Those between 1982 and 1996 are shown by octagons. Quarters identified by + refer to outliers where a concurrant decline in fish abundance and diversity was also observed.

between 1982 and 1988. The population fluctuated between 1988 and 1991 and then increased significantly. There was a significant decline in the number of flounder at the Newcastle site from 1990 onwards which coincided with the decline observed in total fish abundance (Mann-Whimey P<0.05). This period also corresponded with the completion of the Newcastle city section of the TSIS and contrasts with the increase in flounder number observed at the Hebburn site.

Significant temporal variations in the number of flounder at Scotswood (Mann-Whitney P<0.003) and Newburn (Mann-Whimey P<0.004) suggested an increase in the number of flounder from 1982-1990. This was followed by a period of stability at Scotswood. Numbers of flounder declined slightly at Newburn, but no significant differences were observed between populations at these sites preceding and following completion of the 1992 TStS stages.

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Discussion

A general improvement in water quality was expected following the installation of the Tyneside Interceptor Scheme (PoM~RET et al. 1988). This has been reflected in increases in the number of salmonids returning to the estuary (NRA 1994) and increases in the number of flounder in the upper estuary. However, analysis of fish abundance, diversity and structure in the lower Tyne implied a decline in fish number and diversity along with a shift in fish community structure. Such changes suggest localised environmental perturbation, which may be attributed to anthropogenic influences. This is supported by the fact that long-term changes in the fish community corresponded directly to changes in water quality expected from the TSIS. Events in the Tyne such as dredg- mg which may have a short-term influence cannot account for any longer term trends. Therefore longer term trends in species richness and community composition indicate that the improvements in water quality on completion of the TSIS has not been realised in the Iower estuary. This theory is compatible with the observed increase in sediment oxygen demand in the lower 10 km of the estuary (Environment Agency 1997).

Time series analysis suggests that whilst the abundance of fish at Howdon appeared to be stable in the 1980s and reduced during the 1990s, the species diversity at the site declined steadily over the whole period investigated. This change in fish community composition at Howdon in the periods preceding and following 1988 reinforces the significance of trends identified by time series analysis and comparative metrices of fish diversity. Moreover, since community variability has been linked to environmental perturbation a tight clustering of points from 1982 to 1988 infers that an undisturbed, stable fish community existed. In contrast, the dispersed distribution of data points from 1989 onwards is characteristic of a disturbed, unstable community resulting from fluctuations in the mortality or emigration of certain fish species more sus- ceptible to changes in biota, abiotic environment or anthropogenic factors (WARXVlCK & CLARKE 1991). The observed changes in community composition following the 1988 diversion of the Ouseburn crudes to Howdon were largely attributable to an absence of herring, viviparous blenny, cod and dab and suggest that a deterioration of water quality may have occurred. Such a decline in water quality may have pre-disposed the system so that the additional discharge, arising from the diversion of the Newcastle city centre outfalls in 1990, imposed a significant reduction in both fish abundance and species richness.

It is likely that increases in the volume of primary treated sewage discharged at Howdon over the last two decades has had a substantial impact on the water quality around the site. The prolonged change in both fish abun-

dance and diversity along with changes in the MDS plots suggest that the greatest change in water quality occurred around 1988, a period that corresponds with the diversion of crude discharges from the Ouseburn area to Howdon STW. Any increase in sewage loads at Howdon, may result in an enriched and more productive benthos cap- able of supporting a larger fish population (HALL et al. 1997). However, any maintained increase in organics in the sediments will produce a rise in sediment oxygen demand, creating a hypoxic environment, which in turn will impose detrimental effects on the fish community. Observed changes in total fish abundance, species richness and community stability all imply the latter and are rein- forced by observation of increases in sediment oxygen demand (NRA 1996a). While abundances of benthic fauna in the area remain high the fauna present are those indica- tive of high levels of organic matter (HALL et al. 1996). According to MATTHIESSEN et al. (1993) sediment concen- trations of organic chemicals are high enough to cause toxicity and be responsible for the impoverished fauna observed (HALL et al. 1996).

The marked reduction in fish abundance and diversity from 1990 onwards at Newcastle contrasts with the prolonged change at Howdon inferring a localised abrupt change in water quality around that time. The change in the fish community directly corresponds with the time at which Newcastle city crudes, including effluent from the brewery were diverted to Howdon. A significant reduction in the number of fish at the Newcastle site anda high sediment oxygen demand over the period 1982-1995 may suggest a Iocalised decline in benthic food availability and therefore the subsequent migration of the fish community. Whereas flounder are tolerant fish and can escape sub- optimal oxygen conditions by swimming away, anoxia can affect survival of benthic food organisms (RocERS & LOCKXVOOD 1990) making the site unattractive to fish.

There is a general trend at the two upper sites in the estuary, Scotswood and Newburn, where analysis suggests increases in fish number and species richness in the late 1980s followed by a period of stability. These changes following respective TSIS completion dates also support the suggestion of a general improvement of water quality and stabilisation of the fish community in the upper estuary. Examination of the benthic fauna across the same period indicates that abundances and number of species have increased particularly in the upper estuary (HALL et al. 1996).

This study has established a body of data, based on field surveys, which will be useful for predictive modell- ing studies and for future environmental monitoring. In particular the effects of changes within the system arising as a result of the installation of secondary treatment at Howdon, in late 2000, can be assessed.

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Submitted: 10 September 2000 Reviewed: 25 April 2001 Accepted: 22 May 2001

fish populations as indicators of long-term changes in uk estuaries

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