The stock structure and migrations of plaice populations on the west coast of England and Wales

Plaice Pleuronectes platessa populations on the west coast of England and Wales are currently managed as two stocks: in ICES Division VIIa (Irish Sea, Cardigan Bay and St George's Channel), and ICES Divisions VIIf&g (Bristol Channel and Celtic Sea). A total of 13,784 plaice were fitted with Petersen tags and released in these areas during 1979–1980 and 1993–1996. Analysis of the 2788 recaptures received by June 2000 confirmed known spawning and feeding grounds in the region. It showed also that plaice >25 cm L _T tended to undertake extensive spatial movements. At this size, female plaice were likely to be mature or maturing for the first time, whilst males were probably mature. Tag recaptures indicated resident sub-stocks of plaice in the north-east Irish Sea, the south-east Irish Sea, Cardigan Bay and Bristol Channel, a contingent of plaice in all areas that undertook permanent dispersal to other areas, and a contingent which originated in the south-east Irish Sea and migrated to spawn in the Bristol Channel. Plaice originating in the Bristol Channel rarely moved north into ICES Vila. A general hypothesis of plaice population structure in the region is presented and discussed in relation to stock assessment.     

The influence of spawning and nursery grounds on the distribution of sole Solea solea (L.) in the Irish Sea, Bristol Channel and adjacent areas

In the coastal waters of west England and Wales, sole, Solea solea (L.), are found in greatest abundance in the north-east Irish Sea and the Bristol Channel. Data from research and charter vessel surveys in these areas showed differences in the spatial distribution of age-groups between the two regions. Juvenile sole (0-group, 1-group and 2-group) were found almost exclusively in the shallow (<20 m deep) parts of the north-east Irish Sea and adults (3-group and older) were also found in the same area and in deeper water. In contrast, while juvenile sole in the Bristol Channel were abundant in and around Carmarthen Bay, Swansea Bay and the Severn estuary, they also occurred frequently in deeper areas of the Bristol Channel down to depths of 40 m. Adult sole in the Bristol Channel were also relatively more abundant at depths >40 m than adults in the Irish Sea. The movements of sole which produce these patterns of distribution in the two areas are also shown by the recaptures of sole tagged in the respective nursery areas. The differences in the distribution of sole in the two areas may be related to the depths of their spawning grounds and the distances that maturing fish must travel to reach them.

In the north-east Irish Sea, sole spawning grounds are generally in waters of <40 m depth and within an extensive area of relatively shallow and gently shelving sediments close to the coastal nursery grounds. The bottom topography of the Bristol Channel is more steeply shelving and the spawning grounds of sole are at a depth of 40–75 m off Trevose Head, ≈100 km from the main nursery areas. The hydrographic conditions in the two areas are described, and it is concluded that one of the more important requirements of a successful spawning ground is the presence of suitable hydrographic conditions to transport eggs and larvae to nursery areas.     

Population stability over a ten-year period in the short-lived fish Liparis liparis (L.)

A ten-year study of the population dynamics of the sea-snail, Liparis lipuris , in the Bristol Channel, England is reported. This small fish which only lives for 1 year undertakes a regular seasonal migration from birth in marine waters into the estuary from which it retreats during the winter to return to the spawning grounds by early spring. At Hinkley Point, Somerset (salinity 22–3O%), abundance has been shown to be negatively correlated with water temperature presumably because of the avoidance of warmer inshore waters during mild winters. When the temperature effect is removed from the data the peak winter abundance of the population was found to have a coefficient of variation of only 27%. For a population which only comprises of age group individuals, this shows a remarkable stability. No statistically significant relationship was found between the abundance of sea-snail and either their predators or prey. However, there were indications of a negative relationship with the abundance of whiting, Merlangius merlangus , which was the most abundant predator. We conclude that the population of a short-lived marine fish living within a physically variable marine environment can be constrained within tight limits. Further. sea-snail reproductive success has been independent of variation in local physical conditions.     

Aspects of the biology and heavy metal accumulation of Ciliata mustela

The feeding habits, rates of growth, fecundity and other aspects of the general biology of five-bearded rockling from the Severn Estuary and the Bristol Channel are reviewed for the period 1974/76. Additionally the changes in heavy metal accumulation during this time have been measured and an attempt has been made to relate them to the diet of the fish.     

The Post-Fledging Mortality of the Blackbird in Great Britain

Some 3,000 pairs of Herring Gulls are now known to be nesting on buildings, mainly in coastal towns they are increasing most rapidly in eastern Britain, where recruitment is taking place from natural colonies. A smaller number of Lesser Black-backed Gulls has occupied rooftops in the Bristol Channel region.     

Pre- and post-barrage scenarios of the relative productivity of benthic and pelagic subsystems of the Bristol Channel and Severn Estuary

The relative importance of benthic and pelagic production within the ecosystem energy cycle, varies considerably for different regions of the Bristol Channel and Severn Estuary. In regions of high turbidity, pelagic primary production is negligible whereas, in the clearer waters of the Outer Channel, the much higher pelagic production contributes to both benthic and pelagic secondary production, The construction of a tidal power scheme would most probably reduce turbidity, so increasing the overall productivity of the enclosed estuary. A range of scenarios, and their implications, are discussed.     

Corrigendum

A re-evaluation of shell shape variation in Shetland dog-whelks.Mucelbi lapillus (I.) and their use as biological exposure indicators.Journal of Molluscan Studies.58: 315–328. Figure 6 and Figure 9 arc transposed. The caption of Figure6 is correct, but the illustration is that of Figure 9 and viceversa.     

Growth and abundance of O-group herrings, Clupea harengus L., in the Severn Estuary

Collections made from the cooling water intake screens of Power Stations in the Severn Estuary and the Bristol Channel have been used to investigate the biology of O-group herrings in this region. The abundance of herrings at Oldbury, the principal sampling site, was low in most years and in five successive seasons between July 1972 and December 1976, large numbers were caught only between the summer of 1975 and the spring of 1976. The herrings were first observed in July, when many had still not completed metamorphosis, and reached peak numbers in September and October during which period they were clearly growing. Numbers declined in December before rising dramatically in January at which time the length-frequency curves displayed a pronounced bimodality. On the basis of data from other parts of the Bristol Channel and Severn Estuary, it is suggested that this bimodality is due to the influx of some animals from other regions where the growth rate was greater. Although movement is apparently taking place at this time and the numbers decline markedly in subsequent months, the last remnants of the population do not leave the estuary until early May. Post-pelvic scute (K₂) and vertebral counts (VS) of 13.82 and 55.24 respectively, indicate that the Severn Estuary herrings are the product of a spring spawning stock, a view entirely consistent with length and field data.     

Polymorphic shell banding in the dog-whelk, Nucella lapillus (Mollusca)

The shell of the Common dog-whelk (Nucella lapillus (L.)) is white and unbanded at most places around the British Isles. However, high frequencies of banding occur on the Buchan coast, around Anglesey and the Menai Straits, on the Cower Peninsula, around the Devon–Somerset border in the Bristol Channel, and especially on the north Cornish coast (reaching a peak between Newquay and Padstow). The frequency of banding is significantly less in older than younger whelks in the same locality, and this change is uncorrelated with the selection against shell shape variation that takes place on exposed shores. It is concluded that banding is a pleiotropic manifestation of physiological variation, and that a study of such variation in different morphs could indicate the importance of different physiological stresses at different stages of the life history of N. lapillus.     

Notes on porbeagle sharks, Lamna nasus, from the Bristol Channel

In a sample of 71 Lamna nasus from the Bristol Channel, most were juveniles. Males, which outnumbered females, matured at about 165 cm. Their diet was composed of fish and cephalopods.     

A quantitative system of age analysis for the dog-whelk, Nucella lapillus

Past attempts to age classify dog-whelks are revised briefly. The most satisfactory method is a system which relies upon familarity of morphological characteristics with which to separate population samples into five different classes. An attempt has been made to quantitate this system. Definite standards have been suggested as the criteria with which to separate the five age classes, and a series of simple methods have been demonstrated which, when followed give finite values for each of these standards. The standards suggested are: (i) the lower limits for shell-length in mature individuals; (ii) the lip-thickness coinciding with maturity; (iii) the upper limits for shell-length in juvenile crevice dwellers. The methods of estimate for values for these standards are illustrated in practice with population samples from Blackrock, Sussex. The values thus obtained and the methods used in each estimation are discussed with view to their validity. It is reasonable to suspect that the quantitative age classification system can be used to classify dog-whelks from any shore provided large enough population samples are collected to estimate the values for each standard. Use of the system is further demonstrated by classification of animals from the shore at Martin's Haven, Pembrokeshire.     

Littoral communities and zonation on rocky shores in the Bay of Fundy, Canada: an area of high tidal range

The Bay of Fundy, on the east coast of Canada, has a maximum tidal range slightly higher than that of the Bristol Channel and similar environmental conditions, but a more extreme climate. However, the littoral community, despite the common presence of many species, differs greatly. Fundy shores show universal domination by Ascopkyllum nodosum under all exposure conditions and its biomass is very high. Funis vesiculosus is common but its distribution is unrelated to exposure. Fucoid algae normally cover all but a few percent of the midlittoral. As a consequence of this intertidal primary primary productivity is very high and a protected sub-canopy environment very extensive. The standard littoral zones do show a typical pattern but respond only slightly to exposure. Many typical and important species show constant zonation under alt exposure conditions. Typical of this group are A. nodosum and the other fucoid algae. Although the high tidal range does reduce the effect of wave action, this factor alone does not explain the results since they are not repeated in the Bristol Channel. Differences in the suite of herbivores, particularly the absence of Patella species in Fundy, is considered to be an equally important causative factor. Also significant is the modifying effect of the heavy algal canopy itself. Clearance experiments have shown that dominance by A. nodosum is not restored within 10 years and that herbivory, especially by littorinids and limpets, does play a critical role in succession.     

Occurrence of tripletail Lobotes surinamensis (Bloch, 1790) in the inner Bristol Channel,UK

A specimen of tripletail Lobotes surinamensis (total length: 402 mm, total mass: 1262 g) was caught in the shallow waters of the inner Bristol Channel on September 2, 2019, with this only the second known capture of this species from the British Isles. Given the cosmopolitan distribution of this species, the potential origin of this specimen is uncertain.     

Growth and movements of the twaite shad, Alosa fallax (Lacépède), in the Severn Estuary

Between July 1974 and April 1977, appreciable numbers of 0 group twaite shad, Alosa fallax , have been collected from the cooling water intake screens of the Nuclear Power Stations at Oldbury-upon-Severn and Berkeley on the Severn Estuary and at Hinkley Point on the Bristol Channel, England. Young of the year first appeared at Oldbury in July at a size of approximately 32 mm standard length, and grew to approximately 61 mm by October. The downstream migration in the late summer and autumn appeared to be closely correlated with water temperature since movement reached a peak soon after the temperature in the Estuary had declined below 19° C, and ceased altogether below 9° C. Thereafter, there was virtually no movement of shad until temperatures rose sharply above 7° C in the spring. Maturing adult twaite shad were captured in late April and May as they moved into freshwater to spawn, probably in mid-June. Males were recorded slightly earlier during the migration and were generally younger than females. Furthermore, mean lengths and weights for individuals belonging to the different age classes indicated that males were also smaller. Logarithmic relationships between length and weight of young and adult A. fallax and for the variation with size in the number of rakers on the first gill arch are presented.     

Population stability of the sea snail at the southern edge of its range

The sea snail Liparis liparis population in the Bristol Channel on the Atlantic coast of Britain is close to the south-western extremity of its range and yet it is one of the more abundant species present. Using a 17-year time series of monthly samples collected at Hinkley Point power station in Bridgwater Bay, Somerset, U.K., the coefficient of variation of annual abundance of sea snail was estimated as 70·48%. Analysis of variance of the six replicate time series showed that 94% of the total variance was attributable to between-year variation in local abundance and only 6% to sampling error. A comparison of the coefficients of variation in the annual number of captures of all the common fish found that the sea snail had one of the more stable populations in the estuary and bottom-living fish tended to show the lowest variability in population number. Arguments based on either the assumed maladaptation of populations at the periphery of their geographical range or the rapid response of short-lived animals to change would have predicted sea snail to have one of the more variable populations. As might be expected for a fish adapted to cooler, more northern and eastern waters, sea snail winter abundance was negatively correlated with seawater temperature. Using the unusual oscillation in water temperature observed during the winter of 1996/199 7 , it is argued that the correlation of sea snail abundance with temperature reflects changes in distribution rather than mortality, with the fish avoiding inshore waters in warm winters. It is hypothesized that demographic plasticity and biotic interactions such as parasite and pathogen attack can result in fish populations showing neither latitudinal nor longitudinal trends of increasing population variability from the centre towards the periphery of the range.     

Intertidal zonation of animals and plants on rocky shores in the Bristol Channel and Severn Estuary–the northern shores

The vertical distribution of common species of macroalgae and fauna on rocky shores extending through the length of the estuary is described. In the west, the shores are fully marine and exposed to Atlantic waves. In an eastward direction, greater shelter and an increasing tidal range occur along a gradient of salinity and turbidity. The flora and fauna change along this gradient. No single biological feature signals a transition from the Bristol Channel to the Severn Estuary but four arbitrary regions are recognized: a 'marine' section eastwards to Swansea, a 'transitional' section between Swansea and Cardiff, an 'estuarine' section up to Newnham, and the tidal river Severn.     

Regional climatic warming drives long-term community changes of British marine fish

Climatic change has been implicated as the cause of abundance fluctuations in marine fish populations worldwide, but the effects on whole communities are poorly understood. We examined the effects of regional climatic change on two fish assemblages using independent datasets from inshore marine (English Channel, 1913-2002) and estuarine environments (Bristol Channel, 1981-2001). Our results show that climatic change has had dramatic effects on community composition. Each assemblage contained a subset of dominant species whose abundances were strongly linked to annual mean sea-surface temperature. Species' latitudinal ranges were not good predictors of species-level responses, however, and the same species did not show congruent trends between sites. This suggests that within a region, populations of the same species may respond differently to climatic change, possibly owing to additional local environmental determinants, interspecific ecological interactions and dispersal capacity. This will make species-level responses difficult to predict within geographically differentiated communities.     

Appraisal of contaminants in sediments of the Inner Bristol Channel and Severn Estuary

In the inner Bristol Channel and Severn Estuary, most contaminated stations were in muddy, depositional areas of the estuary, particularly along the Welsh coast between the Rivers Taff and Usk, and also on the English coast between Avonmouth and Severn Beach. Some deeper areas, dominated by sand and mixed sediments, also showed contaminant concentrations in excess of those predicted from sediment texture, organic matter, and aluminium or iron concentrations.
Concentrations of lead, copper, chromium, nickel and zinc have decreased in the muddy sediments since the 1970s, but only lead has decreased in sand. With the exception of zinc, which is high in the Severn, trace element concentrations were comparable to those from other British estuaries.
Landward of the proposed barrage, contaminants are associated with the sub-estuaries and muddy areas at their confluence with the main estuary. Post-barrage effects on contaminants will be focused in these muddy areas. However, fine sediment deposition on what are now sandy areas would increase their contaminant burden unless calcium build-up from the resulting molluscan fauna has the antagonistic effect of contaminant dilution in the sediment.     

Benthic macrofauna and sediments in the Severn Estuary

In the Severn Estuary and Bristol Channel, a multivariate analysis indicated eight groupings of macrobenthic fauna, each associated with a different physical habitat. Samples were faunistically impoverished, especially in sublittoral areas, but greater species richness was associated with reefs of Sabellaria alveolata , which extended well into the estuary.     

Spatial distribution functions and abundances inferred from sparse noisy plankton data: an application of constrained thin-plate splines

As plankton biologists ask more detailed questions of necessarilysparse and noisy spatial data, the need for well founded methodsfor statistical analysis of such data grows. This note examinesthe utility of constrained thin-plate smoothing splines as atool for inferring underlying spatial distribution functionsfrom sparse noisy data. Constrained thin-plate splines are describedin a straightforward manner. An economical method of calculationis suggested, which sacrifices mathematical optimality for easeof computation. Using simulated data several methods for choosingthe complexity of the inferred distribution function are comparedand robustness to large amplitude noise is examined. Confidenceintervals are calculated and tested. The method is applied toegg data from Dover sole (Solea solea) in the Bristol Channel.