RIVER LAMPREY Brief summary of Humber Basin Information

Transcription

1 RIVER LAMPREY Brief summary of Humber Basin Information Adults Population estimates Masters et al. (2004) estimated a population of adult returning river lamprey to the tidal River Ouse, upstream of the Wharfe confluence, in the order of 300,000. Other estimates made using alternative methodologies were 275,687, 62,403 and 160,020 giving a mean of 199,528 +/- 107,737 (APEM April 2007 draft). Jang et al. (2004) estimated the adult river lamprey spawning population for the 2003 season on the River Derwent SAC to be in excess of 6000 individuals at Stamford Bridge whilst APEM (April 2007 draft) estimated about 7,500 individuals for the River Derwent given that Stamford Bridge represents 80% of the current available spawning habitat. However, a further three years monitoring at this site suggested that the 2003 spawning population was far greater than what is likely to be a typical year s spawning total. Lucas (pers.comm.) has suggested that the typical spawning population of the river lamprey in the River Derwent might be 750. Rounding these estimates gives a population of adult returning river lamprey to the River Ouse, upstream of the River Wharfe confluence, of 200,000 and a typical spawning population of river lamprey in the River Derwent of 1,000. In 2006/07, Durham University studies (Greaves et al. 2007) revealed substantial adult river lamprey runs up the Trent, although relatively few penetrated upstream of Cromwell Weir. Since the trap Catch Per Unit Effort in the Trent was about one fifth of that in the Ouse, a total adult run of 40,000 up the Trent can be estimated. Although there is some suitable spawning habitat below Cromwell Weir and considerably more upstream, it is not known whether there has been recent successful spawning in the Trent. Limited monitoring of the screen trash at Keadby Power Station has resulted in only a few adults and no juveniles being recorded. Temporal variability and the relatively long life cycle of river lamprey also need to be borne in mind. Recorded adult river lamprey spawning totals In the Ouse tributaries for the years 2003 to 2005 are tabulated below: River Year Ure Swale not monitored Wharfe 0 (little effort) 212 not monitored Nidd not monitored

2 Although some individuals may have been counted more than once on different occasions, these figures are probably minima, considerably less than the total spawning number in each river. However, these data indicate that, in the River Ouse and its Pennine tributaries, river lamprey are probably self sustaining and variable in their distribution from year to year. Also, there is not a very marked a difference in the observed spawning populations in the River Derwent and the combined Pennine tributaries (Swale, Ure, Nidd and Wharfe). However, even allowing for a commercial catch of, perhaps, 60,000 per year, a large proportion of the estimated population in the River Ouse, upstream of the River Wharfe confluence, is unaccounted for. There may be spawning in areas which have not been monitored as well as considerable predation e.g. by pike, otters and piscivorous birds. The total number of adults leaving the Humber Estuary is, of necessity, highly speculative but using a basis of 200,000 passing into the Ouse upstream of the Wharfe confluence might be reasonably estimated to be 300,000. However, the only recent record of a river lamprey in the Humber Estuary, away from The South Humber Bank Power Station, is from near Hawkins Point in Migration behaviour Adult river lamprey migration into rivers mainly occurs between the beginning of October and the end of February, with some in September and March. Available information, particularly that resulting from recent studies by Durham University on the Humber tributaries, indicates that adult river lamprey exhibit, at most, only limited fidelity to their natal river. Thus, a maturing adult which entered the Humber Estuary from one river as a transformer might enter, and spawn in, any of the rivers flowing into Estuary. Determinants of migration orientation include increased flows and pheromones. Generally, upstream migration is stimulated at flows substantially above the median. Durham University studies indicate that ideal conditions for good river lamprey passage into the River Derwent occurred at flows of about 40 cumecs in both the River Derwent and River Ouse co-inciding with a neap tide. 40 cumecs constitutes a major flow in the River Derwent and a moderate flow in the River Ouse: Studies on river lamprey indicate that the adults are attracted to ammocoete larval odour during migratory phases. This would lure these lamprey towards rivers which already support ammocoetes and thus have suitable spawning conditions. If juveniles are not found in the Trent it may suggest that the pheromone pull is secondary to other environmental factors, especially flow.

3 Upstream migration from the Humber Estuary to spawning areas appears to be a two stage process especially since many adults enter rivers in autumn, several months prior to spawning which is usually in April. Unless temporarily impeded by obstructions, upstream movement is fairly rapid until the general vicinity of the spawning areas is reached. This initial migratory phase may show a tidal influence in tidal waters with most upstream progression on flood tides. However, in non-tidal rivers a most upstream movement is nocturnal. Once the vicinity of the spawning areas is reached, the lamprey tend to seek out protected areas of slack water, for instance in backwaters or amongst tree roots. Such behaviour may result in adults becoming entrained into intakes which would be avoided during the active migration phase. In the spring, adults make relatively limited movements between their shelter areas and the spawning beds. Uncertainties Accurate estimates are not possible and those estimates given above should be treated with a high degree of caution. Variability in adult river lamprey spawning totals for the some of the sub catchments over three years is shown in the above table. The longest adult lamprey data set in England and Wales spans a total of 11 years (1992/3 and ) on the River Dee at Chester weir fish trap, although this trap is does not capture adult river lamprey very efficiently. Adult river lamprey catches ranged between zero and 685. The renewed, and probably intensifying, exploitation of adults in the tidal Ouse may have the potential to adversely impact stocks in the medium to long term but, given the evident inter-year variation, many years of data will probably be required to identify such a decline. Although much knowledge about has been gathered about migration in recent years, many aspects still require further investigation. In particular, the flows required to attract adults upstream, as opposed to pass structures, requires clarification especially if successful lamprey passes are to be installed at barriers. Eggs Large numbers of small eggs are produced, perhaps twenty thousand per female. Natural mortality at this stage is considered to be high due to such factors as predation and lack of suitable habitat conditions. The reaches where spawning occurs, characterised by relatively shallow water, high velocities and stony substrates are, generally, not those where major abstractions are sited. Furthermore, such spawning sites are often well upstream of major intakes. Since drifting eggs are probably randomly distributed in reaches remote from spawning areas there is not the potential for selective entrainment due aggregation in the vicinity of intakes. Any drifting eggs are likely to non-viable even if they are not entrained. Due their small size ( 1 mm in diameter) and non-motility, it would be impracticable to

4 exclude eggs in most situations and, if impinged on very fine screens, would probably not survive even if the screens were periodically backwashed. As result of these characteristics, mortality of eggs due to entrainment is likely to be small relative to natural mortality. Furthermore, avoidance prevention of such mortality at active intakes is impracticable. Ammocoetes Population Estimates Generally, river lamprey ammocoetes are indistinguishable from those of brook lamprey, at least in field surveys, but Lampetra juveniles from the middle and lower reaches of rivers flowing into the Humber have been considered to be predominantly river lampreys. Most of the available quantitative information on juvenile river lampreys in the Humber Basin derives from a survey carried out in 2004 (Harvey et al. 2006). This was undertaken at sixteen sites where there was suitable juvenile habitat in close proximity to spawning sites. Thus, the results may portray a better situation than would have derived from a survey based on random site selection. Lampetra ammocoetes were recorded at all sites. The results are summarised below: River Mean density of Lampetra ammocoetes (no/sq.m.) Derwent 9.82 Nidd 2.14 Swale Ure Wharfe Yorks. Ouse catchment (excl. Derwent) In all rivers, mean ammocoete densities were greater than 2 per square and there were at least two size/age classes present. Overall, the ammocoete results indicated that the river lamprey populations in these rivers were in favourable condition. Apart from some ammocoetes, presumably those of brook lamprey, in the upper tributaries, no juvenile lamprey have been recorded from the River Trent. During October and November 2005, electric fishing surveys targeted at juvenile lampreys were carried out on some tributaries of the lower Trent but no ammocoetes or transformers were recorded (Jacklin, 2006). However, the substantial adult run might indicate the presence of juveniles, producing attractant pheromones, within the catchment. Informed opinion (Lucas, pers.comm.) suggests that it is likely there has been some spawning in recent years giving a run (of unknown size could be small or moderate) of transformers down the river. Migration behaviour

5 Generally, ammocoetes show restricted mobility and most downstream movement may be due to washout during spates. As a result, they are not very vulnerable to entrainment and impingement at abstraction intakes and mortality at such installations is likely to be low. Uncertainties Only a single quantitative, juvenile survey has been undertaken. Although providing valuable information, it needs to be borne in mind that this only gave a snapshot in one year of the populations at selected sites. Transformers Population estimates Transformers were recorded at fourteen of sixteen sites included in the juvenile survey (Harvey et al ). However, numbers were relatively low which is not surprising given that transformation probably occurs over several months and transformers appear to move to areas with a stonier substrate and faster flows than the typical ammocoete habitat targeted by juvenile surveys. No quantitative data on transformer runs in the United Kingdom appear to exist and most available information derives from impingement monitoring. Hopkins (2006) attempted to produce an estimate of transformers leaving the Derwent, the sub-catchment with the most comprehensive information, over the 2004 to 2005 winter. The best estimate, based on juveniles (transformers and large ammocoetes) in the survey by Harvey et al. (2006) likely to leave the river as transformers before the 2005 summer, was 28,000. Since the typical run of river lamprey adults into the Derwent is about 1,000, this suggests that 1 returning adult results from 28 transformers leaving the Derwent. Applying the 1 returning adult:28 leaving transformers ratio more widely, would indicate a transformer production of 5.6 million to produce the adults returning to the Ouse, upstream of the Wharfe confluence, and a total transformer production of 8.4 million to produce all the adults returning from the Estuary to the river. There are no recent records of transformers from the River Trent, however Lucas (above) suggests that there may be a run of river lamprey transformers down this River. Furthermore, Lucas (pers.comm.) considers that with the firm knowledge that there was a substantial run of adult river lamprey up the Trent last year (2006),and good anecdotal reports and non-target captures in recent years there may be a substantial transformer production from the Trent catchment in three to five years time. Migration behaviour

6 Direct observations on the migratory behaviour of river lamprey transformers are lacking and most information on this aspect derives from patterns of impingement at monitored intakes. This indicates that, within rivers, most downstream transformer migration occurs between the start of November and the end of March although there may also be some migration in October and April. At least in some situations, there may be two migration peaks separated by a mid-winter lull. Generally, there is increased impingement at increased flows and this may reasonably be attributed to increased downstream migration at such times. However, it might be that downstream migration is less variable but impingement increases at higher flows, perhaps due to reduced visibility resulting from increased turbidity or migration closer to the banks. Various evidence suggests that the amount of entrainment and impingement at different abstraction points is not, given present understanding, readily predictable from such factors as the volume of water abstracted. Thus, apparently similar intakes on the same watercourse can experience very different impingement rates presumably due to significant, but not obvious, differences in intake or channel morphology. Due to their small diameter, approximately 4 mm and their migration with the current, transformers are, generally, far likelier to become entrained into intakes, and penetrate many screens, than adults. Uncertainties Due to the paucity of data and the fragility of assumptions of made, even the best estimates of transformer numbers should be regarded as highly speculative. Mainly due to lack of understanding of transformer behaviour, any sizeable abstractions where transformers are known, or might reasonably be considered, to pass the intakes will need to be monitored to ascertain the numbers of transformers entrained. Dr David Hopkins References APEM (April 2007) Review of Recently Gathered Information on Lamprey Stocks and Conservation issues in Britain. Draft Report. Bubb, Damian H. & Martyn C. Lucas. (2006) Migration of adult river lamprey in the tidal Yorkshire Ouse and Derwent, October 2005 to Report of a joint project by the University of Durham with the Environment Agency, March Draft Report. Greaves, Rachael K., Damian H. Bubb, Martyn C. Lucas (2007) Adult river lamprey occurrence and migration in the River Trent in relation to barriers and

7 environmental conditions Draft Report. Harvey, Dr Jonathan P., Dr Andrew D. Nunn & Prof Ian G. Cowx (Hull International Fisheries Institute) (2006) Survey of larval lamprey (ammocoetes and transformers) in the Yorkshire Ouse and Derwent catchments, Final Report. Hopkins D.G. (2006) Preliminary estimates of river lamprey transformer production from River Derwent (Yorkshire). Rev Jacklin, T.E. (2006) The status of lamprey species in the River Trent with particular regard to the Humber Estuary SAC. Jang, Min-Ho, Kyong Ha & Martyn C Lucas (Durham University) (2004) Habitat availability, distribution and abundance of spawning lampreys in the Yorkshire Derwent system. Masters, Jerome, Min-Ho Jang, Kyong Ha & Martyn C Lucas (Durham University) (2004). Migration, exploitation and spawning of adult river lamprey in the tidal Ouse and Derwent (NE), September 2003 to May 2004.