Fish and crustacean captures at Hinkley Point 'B' Power Station:
Report for the year April 2000 to March 2001.
P. A. Henderson & R. M. H. Seaby
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Details on operational power stations were correct at the time of writing; such details may now have changed.
ABSTRACT
This report is the annual summary of results from the long-term impingement monitoring at Hinkley Point 'B' Power Station. Data for the number of fish and crustaceans captured on the intake screens of Hinkley Point Power Station over the year April 2000 to March 2001 are presented together with an analysis of long-term trends in animal abundance. Data for 2000 have demonstrated that the previously noted trend of increasing fish and crustacean abundance within the Bristol Channel is continuing. Fish abundance in the estuary is probably 3 times higher than that recorded in the early 1980s and there is also a clear trend for increased species richness. The general conclusion is that conditions within the estuary are becoming more favourable for fish and crustaceans. Given the large amount of estuarine habitat available within the Bristol Channel and Severn estuary there can be little doubt that this region is an exceptionally important estuarine fish nursery and that it is increasing in importance. Using these data on the abundance and diversity of juvenile fish, there can be no doubt that the Severn Estuary and Bristol Channel is one of the most important fish nursery areas in Britain and quite possibly Northern Europe.
The trend of increasing water temperature that started at the end of the 1980s has continued. This has resulted in the decline in abundance of fish that favour cold conditions, such as dab and sea snail. There is evidence that recent warming is continuing to change the fish and crustacean communities. These changes can be viewed as generally beneficial, in that species richness has increased. If global warming is occurring then the Severn Estuary Data Set can be used to predict future community trends.
A particular feature of note in 2000 was the relatively high abundance of cod, herring, sole and prawn, Palaemon serratus, within Bridgwater Bay. This probably reflects large increases in abundance within the Bristol Channel and offers hope that the populations of these important commercial species are improving in this region.
It is shown that the recent closures of direct-cooled power stations in the region are coincident with the increased abundance of common fish and crustaceans at Hinkley Point. These observations do not prove that power stations have, in the past, reduced animal abundance. However, the SEDS data set will offer over the coming 2 years the best opportunity available in the world to test for the impact of direct-cooled power stations.
1. INTRODUCTION
This report presents the biological data collected during regular sampling at Hinkley Point Power Station for the year 2000/2001. These data contribute to the long-term time series of animal abundance which, together with physico-chemical and meteorological data, form the Severn Estuary Data Set (SEDS) - see reference list at the end of this report. The end of 2000 marked the completion of 20 full years of sampling at Hinkley Point. In terms of the number of species monitored, the proportion of the higher organisms living in the system observed, and the length of time of the observations SEDS is a unique ecological resource. It has four principal uses. First, it provides for the detection and analysis of ecological change caused by industrial water users such as power stations. Second, it provides a robust indicator of recent trends in animal abundance in the Bristol Channel. This benefits fisheries management interests, the examination of long-term trends in environmental quality, and the understanding of ecological systems. Third, it provides a superb database for the study of population dynamics and community ecology. Finally, it helps the Hinkley Point power stations to address the concerns of regulatory organisations. The fish time series for Hinkley Point, from January 1981 to March 2001, are supplied with this report as both Excel 97 and csv format files. Data on crustaceans and other variables can be obtained from Pisces Conservation Ltd.
2. MATERIALS AND METHODS
Quantitative monthly sampling of fish and crustaceans at Hinkley Point 'B' Nuclear Power Station started in October 1980: This report is the most recent in a series of annual reports on these catches. For the period April 2000 to March 2001 12 samples were successfully collected, using the established methodology (Henderson & Holmes, 1991). All sampling dates were chosen to work tides of intermediate range in the spring-neap cycle. On each visit, six consecutive one hour samples were collected in plastic baskets of 6 mm mesh size, positioned to collect all the debris washed from two of the four 'B' station drum screens. This debris was sorted, with the fish and crustaceans identified to species and the number captured per hour recorded. The standard length of the fish was measured. Plankton samples were also collected from the 'B' station intake forebay.
3. TEMPERATURE AND SALINITY
3.1 Temperature
2000 was a mild year with a low summer maximum and a winter minimum well above the lowest recorded. The 12-month moving average shows that 2000 was typical for years since 1989, which were notably warmer than the period between 1981 and 1986.
3.2 Salinity Salinities in Bridgwater Bay have been declining since 1995 and the December 2000 value of 19ppt was close to the record minimum of 18ppt recorded in February 1999. The salinity between November 2000 and March 2001 has remained consistently below the long-term average and reflects the unusually high rainfall and resulting river flow experienced by England and Wales over this period. There are indications of a small long-term trend of decreasing salinity within Bridgwater Bay. This reduction in salinity may affect the species captured at Hinkley Point, although it is unlikely to have much impact as it lies well within the range of tolerance of most of the species living in the region.
4. OBSERVATIONS ON FISH ABUNDANCE
In this section data collected over the period April 2000 to March 2001 are placed within the context of the longer-term trends over the period 1980 to 2001.
4.1 New records and unusual observations
The two new species recorded in 2000/2001 were Raitt's sandeel Ammodytes marinus and the Rock Goby, Gobius paganellus. Raitt's sandeel is an abundant offshore species that is known to occasionally come into shallow water and even enter the mouths of estuaries. It has been recorded from most marine power stations where long-term studies have been undertaken. The rock goby is a common species of rocky shores and the habitat in Bridgwater Bay is probably too muddy to allow the establishment of a resident population.
The three infrequent visitors captured were the three bearded rockling, Gaidropsaurus vulgaris; greater sand eel, Hyperoplus lanceolatus and blue whiting, Micromesistius poutassou. The three bearded rocking was recorded in March 2000 and April 2000, suggesting that a small migratory influx of individuals probably occurred during spring 2000. The greater sand eel may be increasing in numbers in some regions near to Bridgwater Bay, as this was the second summer in succession that the species was recorded. All of these are fish which can be commonly found in other habitats and should be viewed as occasional migrants. It is, however, notable that three such visitors should be recorded in a single year. This tendency for more visitors to be recorded during the most recent years partly explains the trend for increasing species diversity over the last 20 years (see Section 4.2).
4.2 Total number of species captured
Fig 2 shows the number of species captured per month over 6 hours of sampling from two of the four B station screens for the total 20 years of sampling. The normal seasonal pattern with a minimum in species richness in early summer and a maximum during the autumn and early winter was observed in 2000-2001. Both a 12-month moving average and simple linear regression (see Fig 2) shows that since 1980 there has been a gradual increase in the number of species recorded per month. The linear regression model gave the weak, but highly statistically significant positive trend:
Monthly catch = 0.0131 T + 12.886, r2 = 0.0599,
where T is time measured in months from January 1981.
Mean monthly species number has increased from about 13 in 1981 to greater than 16 in 2001. It can be concluded that the steady increase in the fish species richness of the estuary has continued during 2000.
4.3 Total number of individuals captured
Fig. 3 shows the number of individual fish captured per month over 6 hours of sampling from two of the four B station screens for the total 20 years of sampling. There is a clear seasonal pattern with maximum numbers recorded during the autumn or winter when sprat and whiting are resident within Bridgwater Bay. Since 1980 there has been a gradual increase in the number of individuals recorded per month which is shown on Fig 3 by both the 12 month moving average and by a linear equation fitted by least squares. The linear regression model gave the weak, but highly statistically significant, positive trend:
Monthly catch = 1.6529 T + 194.33, r2 = 0.032,
where T is time measured in months from January 1981.
Mean monthly number of individuals caught per month has increased from about 173 in 1981 to around 600 by 2001. It can be concluded that the steady increase in the fish abundance in the estuary has continued during 2000 and is now at a level which is almost 3 times that observed at the beginning of the 1980s.
4.4 Cod Gadus morhua
It has been previously reported that since 1986 cod have become more abundant within Bridgwater Bay, and from reports from fishermen it would appear cod have generally increased in abundance in the Bristol Channel and the waters surrounding Devon and Cornwall. Captures for the year 2000-2001 were the highest on record (Fig. 4). As almost all the cod captured are O group juveniles, this would suggest that recent recruitment has been good and is increasing. This is a most curious observation given the decline in North Sea cod stocks and the clear trend for sea water temperatures to increase.
4.5 Whiting Merlangius merlangus
During 2000/2001 whiting retained its position as the most abundant gadoid captured at Hinkley point. There is a steady long-term trend of increasing whiting numbers and whiting are now present in every month of the year.
4.6 Poor cod Trisopterus minutus
Abundance remained low during 2000. There is no evidence of a long-term trend in the numbers of this highly variable species.
4.7 Pout Trisopterus luscus
Pout abundance during 2000 was unexceptional. The species is highly variable in abundance and numbers are correlated with those of poor cod. There is no evidence of a long-term trend.
4.8 Hake Merluccius merluccus
No Hake have been caught over the last year and this once quite common fish can now be considered an infrequent visitor to Bridgwater Bay. There are indications that the abundance of the species is in decline.
4.9 Pollack Pollachius pollachius
Only a single individual was captured over the last year; while this species is never abundant, it seems likely that it is avoiding the area. It is possible that the reduced salinity makes the area less favourable.
4.10 Norway pout Trisopterus esmarkii
This species has continued to be caught in low numbers.
4.11 Five-bearded rockling Ciliata mustella
The abundance of this rockling has increased since 1997 and it is now appreciably more abundant than it was in the 1980s.
4.12 Northern rockling Ciliata septentrionalis
This species was once considered rare in Southern British waters. It is notable that while uncommon at Hinkley Point it is regular in its seasonal pattern and appears to be remarkably stable in abundance.
4.13 Twaite shad Alosa fallax The 'O' group of this species tend to be more abundant in warmer years (Holmes & Henderson, 1990). Thus abundance has declined from the level recorded between 1988 and 1991. During 1993/94 only a single specimen was captured. However numbers increased slightly in 1994/96 and fell again between 1997 and 1998. Above average recruitment occurred in 1999. No O-group shad were caught over the 2000/2001 winter suggesting poor recruitment in 2000. This species might show a 3-4 year cycle in abundance, in which case the present low abundance may be part of the normal long-term pattern. Hinkley Point abundance has been shown to be correlated with shad abundance in the River Severn. The present lack of recruitment may indicate a decline in the abundance of this protected species.
4.14 Bass Dicentrarchus labrax
Following strong recruitment in autumn 1997 bass numbers in the Severn Estuary and Bristol Channel have remained high. The number of young bass in 1997/98 was the highest ever recorded. This was anticipated, as the 1995 year class has now left the estuary allowing good 'O' group recruitment and the mean annual temperature was the highest ever recorded at Hinkley Point. Bass recruitment shows a very marked positive correlation with temperature. They are also cannibalistic, so that another strong recruitment will probably not be expected until the 1997 year-class leaves the estuary. This would be expected to occur in 2000. However, seawater summer temperatures in 2000 were not high so a strong year class was not observed.
4.15 Common eel Anguilla anguilla
The long-term decline in the rate of capture of this species has continued during 2000/2001 although there are indications that abundance is now stabilising at a new, lower, level. The reasons for this are obscure but are possibly related to a number of factors including excessive fishing for elvers, freshwater habitat destruction and introduction of the parasite Anguillicola crassus from Asia. There are no indications that power station intakes are responsible as a similar decline has been observed in rivers throughout Western Europe. It is notable that elver landings recorded in South Wales are highly correlated with power station patterns of capture. If the decline of this species continues, conservation measures will be needed. If this should occur mortalities of both elvers and larger individuals caused by power stations will come under greater scrutiny.
4.16 Conger Conger conger
This species has continued to be caught in low numbers. As most of the individuals captured are large juveniles it makes a far higher contribution to the total biomass than the numbers might suggest. It is probably one of the most abundant top predators within the vicinity of the intakes and appears to maintain a remarkably stable population. The numbers are less variable than would be expected if the rate of capture was a Poisson process.
4.17 Lumpsucker Cyclopterus lumpus
This species is only captured when large numbers of juveniles enter the region during the winter. This is an infrequent event and only one individual was captured in 2000. Increasing water temperatures may be having an adverse effect on this species, which has the centre of its distribution to the north of the Bristol Channel.
4.18 Sea snail Liparis liparis
The abundance of sea snail is negatively correlated with winter seawater temperature. Thus since 1987 the increase in mean water temperature has resulted in generally lower numbers being captured during the 1990s and 2000s as compared with the 1980s. The notable exception was the winter of 1996 when low water temperatures during January triggered a sudden migration into inshore waters. There is no evidence that the decline in abundance in Bridgwater Bay reflects an actual decline in the population of the species within the estuary. It is more likely that temperature influences the distribution of the fish within the estuary (Henderson & Holmes, 1990; Henderson & Seaby, 1999).
4.19 Thornback ray Raja clavata
This species became more abundant in Bridgwater Bay in the mid 1980s, when mean water temperatures were lower. During 1994/95 the second highest number of fish in a sample was caught and there are indications that abundance within the estuary increased further between 1993 and 1997. Only one individual was captured during 1998/99, none over 1999/2000 and three during 2000/2001. All of the rays captured were small juveniles and there were taxonomic difficulties in identification. This species does not favour estuarine waters with muddy sediments and the records probably reflect occasional inputs into the estuary of recently hatched juveniles.
4.20 Dab Limanda limanda
This is one of the most abundant flatfish within Bridgwater Bay. Most of the individuals captured during 2000/2001 were 'O' group juveniles which enter the Bay in late summer. In comparison with many marine fish, recruitment shows small between-year variation and there is no evidence of a long-term trend in population number. Henderson & Seaby (1994) and Henderson (1998) have reported a highly significant negative correlation between dab numbers and seawater temperature. Dab were not abundant in the estuary in 2000/2001; this probably reflects high water temperatures. Henderson (1998) also noted that dab in Bridgwater Bay grow faster during colder autumns and winters. If we are entering a period of increasing sea water temperature it is likely that dab will become less abundant.
4.21 Flounder Platichthys flesus
There are indications that flounder have become more abundant since 1986 although the variation is such that a longer time-series will be required to demonstrate the effect statistically. The total catch in 2000 was the highest recorded. As normal, the fish captured covered a wide range of age classes (Henderson and Seaby,1994). The effect of climate on the abundance of this fish is described in Henderson and Seaby (1994), who found that flounder abundance is negatively correlated with mean seawater temperature the previous year.
4.22 Dover sole Solea solea
This species has continued to be abundant within the estuary. Most of the sole captured were 'O' group juveniles which suggests that recent recruitment has, for the ninth year in succession, been above the long-term average. The effect of climate on the abundance of this fish is discussed in Henderson and Seaby (1994) who demonstrated a highly significant positive correlation between sole abundance and water temperature. The nine years showing the highest abundance have been recorded since 1990.
4.23 Plaice Pleuronectes platessa
Within Bridgwater Bay plaice is the least abundant of the common British flatfish. In the summer of 1996/97 a peak of 21 specimens were recorded. Since this period numbers have been much lower. However, there is some support for the view that numbers are gradually increasing.
4.24 Dragonet Callionymus lyra
This species does not live on a mud substrate within estuaries and is best viewed as an occasional visitor.
4.25 Grey gurnard Eutrigla gurnardus
After 1992/93, the first year during which significant numbers of this species were recorded few were caught until 1999 when another seasonal immigration of juveniles was observed. Few individuals entered the estuary in 2000.
4.26 Hooknose Agonus cataphractus
This species has remained in low abundance during 2000/2001. As in the case of the dragonet, Bridgwater Bay does not offer the preferred habitat of this species and it is best considered as an occasional visitor.
4.27 Nilsson's pipefish Sygnathus rostellatus
This species is clearly an occasional visitor that does not live within Bridgwater Bay.
4.28 Sand goby Pomatoschistus minutus
Abundance over the years 1999 to 2001 was the highest since records began in 1981. The previous record annual catch was in 1988. This species appears to be gradually increasing in abundance within Bridgwater Bay.
4.29 Transparent goby Aphia minuta
Numbers have remained low during 2000/2001 and there are indications of a long-term decline in abundance within Bridgwater Bay.
4.30 Herring Clupea harengus
Herring have dramatically increased in abundance during 2000/2001. It seems likely that there has been a considerable improvement in recruitment. It will be interesting to discover if this will continue over the coming seasons.
4.31 Sprat Sprattus sprattus
Sprat is the commonest pelagic fish captured at Hinkley Point and captures in December 1988 were the highest since recording began in 1980. Sprat were not abundant in Bridgwater Bay during the 2000/2001 winter; however, there are clear indications of a gradual increase in sprat abundance in the estuary over the last 20 years.
4.32 Grey mullets, Liza aurata, Liza ramada and Crenimugil labrosus
Owing to difficulties in assigning 'O' group individuals to species, all three grey mullet species are shown grouped in Fig. 5. From detailed examination of the young mullet caught at the station the majority are believed to be thin-lipped grey and golden-grey mullet. Mullets were exceptionally abundant in the autumn and winter of 1996. Their recruitment, like that of bass, is very dependent on temperature. Abundance during 2000/2001 is close to the long-term average.
5. CRUSTACEAN ABUNDANCE
5.1 Trends in abundance
The numbers of common crustaceans captured per month are presented in Figs 12 and 13. During 1999/2000 the normal patterns of seasonal abundance were observed for all species. There are clear indications that the numbers of crabs is increasing (Fig 12). This is particularly apparent in the time series of the edible crab, Cancer pagurus (Fig 12b).
Crangon crangon has remained the most abundant macro-crustacean or fish caught at Hinkley Point and the population remains remarkably stable, showing no clear trend in abundance over a 20 year period (Fig 13b). The second most abundant macro-crustacean the pelagic prawn Paciphaea sivado, which was exceptionally abundant in 1999, has maintained high numbers in 2000 and there are now indications of a trend of increasing abundance as shown by the upward trend in the 12 month moving average (Fig 13c). The pink shrimp, Pandalus montagui, has maintained an almost constant annual mean abundance since 1981 (Fig 13a). The large edible prawn Palaemon serratus is showing a clear trend of increasing abundance within the estuary (Fig 13d). In the 1998 report it was noted that the highest number of P. serratus in a single sample was found in October 1997 (403 individuals). This record was broken by the capture of 700 individuals in September 1998. In June 2000 this record was broken again with the capture of 1195 individuals.
6. THE EFFECT OF POWER STATION CLOSURES
Amongst a number of climatic and anthropogenic changes that may be contributing to the observed increase in species richness and abundance must be considered the recent closure of a number of direct-cooled power stations. Since sampling commenced in October 1980, Berkeley closed in 1989, Uskmouth in 1995, Pembroke in mid 1997 and Hinkley A in May 2000. All of these stations would have been killing fish and crustaceans that were members of the populations subject to capture at Hinkley B. It is highly unlikely that entrainment and impingement in power station cooling water systems would have changed species richness in the region because the estuary presents an open system that would receive a flow of recruits from other waters. However, if mortality rates are sufficiently high it is possible that direct-cooled power stations could reduce abundance by a detectable amount. Table 1 gives estimates of the number of fish > 3 cm in length that are captured per year by power stations in the Bristol Channel. The four power stations that have closed since 1989 were estimated to kill 3.44 x 106 individuals per annum. The number of small individuals that would have passed through the filter screens and been killed following entrainment has not been estimated, but would have been at least an order of magnitude greater.
This level of mortality would probably result in a detectable change if the total abundance in the region is between 108 to 109 individual fish, providing sufficient samples are available. For common crustaceans, which are caught and killed in far larger numbers by power stations, it may be possible to detect the impact of reduced mortality on populations of 1010 individuals. These numbers are probably in the correct order of abundance for the estuary suggesting that it may be possible to detect the increase in abundance caused by the reduction in mortality following power station closure. Such a test will require at least one more year of data to allow the impact of the closure of Hinkley A to be fully felt by the populations and give sufficient samples for analysis. However, there are indications that the increase in abundance of some species has occurred since power station closure as would be anticipated if power stations had been having an effect on population size. For example, Palaemon serratus showed a remarkably constant population size until 1998 after which it has been increasing almost exponentially (Fig 13 d). There are also indications that the Common shrimp, Crangon crangon, may have recently increased in average abundance (Fig 13 b). Amongst fish, sprat, whiting, flounder and sand goby abundance have all increased since the initiation of power station closures. Such coincidences cannot be considered proof of a causal relationship. However the SEDS data set should allow a more rigorous statistical analysis to be undertaken within the next 18 months.
Table 1: Estimated number of fish killed on the filter screens of marine and estuarine power stations situated in the Bristol Channel.
| Name | Type, cooling water volume m3s-1, screen mesh mm | Present status | Total number of fish killed on the filter screens per annum. Fish > 3 cm |
| Hinkley Point A | Nuclear, 40, 10 | Shut | 1.3 x 106 |
| Hinkley Point B | Nuclear, 30, 10 | Working | 9.9 x 105 |
| Oldbury | Nuclear, 26.5, 10 | Working | 2.5 x 105 |
| Berkeley | Nuclear, 26.5, 10 | Shut | 2.5 x 105 |
| Uskmouth | Conventional, 30.3, 5 | Shut | 2.9 x 105 |
| Aberthaw B | Conventional, 67, 10 | Working | 2.2 x 106 |
| Pembroke | Conventional, 50, 10 | Shut | 1.6 x 106 |
7. GENERAL OBSERVATIONS
The general trend of increasing water temperature observed within the Bristol Channel, which may be related to large scale climatic trends, is producing a gradual change in fish and crustacean abundance in Bridgwater Bay. There is clear trend of increasing species richness which is mostly due to the more frequent capture of warmer water species (bass, mullets, gurnards, trigger fish etc). However, while species that are close to the southern limit of their range in the Bristol Channel such as dab, northern rockling and sea snail have declined in abundance they are still common. Thus the net effect has been an improvement in species richness. It may also be possible that the environment of the estuary is improving for fish and crustaceans in general. 2000 has continued the twenty-year trend of increasing fish and crustacean abundance within the Bristol Channel. The general conclusion is that conditions within the estuary are becoming more favourable for fish and crustaceans. Given the large amount of estuarine habitat available within the Bristol Channel and Severn estuary there can be little doubt that this region is an exceptionally important juvenile estuary and that it is increasing in importance. There are, however, species, most notably the common eel, which are not increasing in abundance and may possibly decline further without vigorous conservation effort.
A particular feature of note in 2000 was the relatively high abundance of cod, herring, sole and Atlantic prawn, Palaemon serratus, within Bridgwater Bay. This probably reflects large increases in abundance within the Bristol Channel and offers hope that the populations of these important commercial species are improving in this region.
It has been shown that the recent closures of direct-cooled power stations in the region are coincident with the increased abundance of common fish and crustaceans at Hinkley Point. These observations do not prove that power stations have, in the past, reduced animal abundance. However, the SEDS data set will offer over the coming 2 years the best opportunity available in the world to test for the impact of direct-cooled power stations.
8. REFERENCES
CLARIDGE, P.N. & I.C. POTTER, 1987. Size composition and seasonal changes in the abundance of juvenile sole, Solea solea, in the Severn Estuary and inner Bristol Channel. J. Mar. Biol. Ass. U.K. 67: 561-569.
CLARIDGE, P.N., I.C. POTTER & M.W. HARDISTY, 1986. Seasonal changes in movements, abundance, size composition and diversity of the fish fauna of the Severn Estuary. J. Mar. Biol. Ass. U.K. 66: 229-258.
HENDERSON, P. A. 1998. On variation in dab, Limanda limanda recruitment: a zoogeographic study. J. Sea Research 40, 131-142.
HENDERSON, P.A. & R.H.A. HOLMES, 1990. Population stability over a ten year period in the short-lived fish Liparis liparis (L). J. Fish Biol. 37, 605-615.
HENDERSON, P.A. & R.H.A. HOLMES, 1991. On the population dynamics of dab, sole and flounder within Bridgwater Bay in the lower Severn Estuary, England. Neth. J. Sea Res. 27: 337-344.
HENDERSON, P.A., D. JAMES & R.H.A. HOLMES, 1992. Trophic structure within the Bristol Channel: seasonality and stability in Bridgwater Bay. J. Mar. Biol. Ass. U.K. 72: 675-690.
HENDERSON, P.A. & SEABY, R.M.H., 1994. On the factors influencing juvenile abundance in the lower Severn Estuary, England. Neth. J. Sea Res. 33; 321-330.
HENDERSON, P.A. & SEABY, R.M.H., 1999. Population stability of the sea snail at the southern edge of its range. J. Fish Biol. 54, 1161-1176.
HOLMES, R.H.A. & HENDERSON, P.A., 1990. High fish recruitment in the Severn Estuary: effect of a warm year? J. Fish Biol. 36, 961-963.