Pennington, United Kingdom
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Jolly M.T.,Marine Biological Association of The United Kingdom | Aprahamian M.W.,UK Environment Agency | Hawkins S.J.,University of Southampton | Henderson P.A.,PISCES Conservation Ltd | And 5 more authors.
Marine Biology | Year: 2012

Determining the magnitude of homing behaviour within migratory fish species is essential for their conservation and management. We tested for population genetic structuring in the anadromous alosines, Alosa alosa and A. fallax fallax, to establish fidelity of stocks to spawning grounds in the United Kingdom and Ireland. Considerable genetic differences were present among populations of both species, suggesting strong fidelity to breeding grounds and compatible with homing to natal origins. Moreover, the genetic structure of A. fallax fallax showed a clear pattern of isolation-by-distance, consistent with breeding populations exchanging migrants primarily with neighbouring populations. Spatial genetic differences were on average much greater than temporal differences, indicating relatively stable genetic structure. Comparing anadromous A. fallax fallax populations to the landlocked Killarney shad subspecies, A. fallax killarnensis (Ireland), demonstrated a long history of separation. These results demonstrating regional stock structure within the British Isles will inform practical management of stocks and their spawning habitats. © 2011 Springer-Verlag.


Henderson P.A.,Pisces Conservation Ltd | Magurran A.E.,Center for Biological Diversity
Proceedings of the Royal Society B: Biological Sciences | Year: 2014

To understand how ecosystems are structured and stabilized, and to identify when communities are at risk of damage or collapse, we need to know how the abundances of the taxa in the entire assemblage vary over ecologically meaningful timescales. Here, we present an analysis of species temporal variability within a single large vertebrate community. Using an exceptionally complete 33-year monthly time series following the dynamics of 81 species of fishes, we show that the most abundant species are least variable in terms of temporal biomass, because they are under density-dependent (negative feedback) regulation. At the other extreme, a relatively large number of low abundance transient species exhibit the greatest population variability. The high stability of the consistently common high abundance species-a result of density-dependence-is reflected in the observation that they consistently represent over 98% of total fish biomass. This leads to steady ecosystem nutrient and energy flux irrespective of the changes in species number and abundance among the large number of low abundance transient species. While the densitydependence of the core species ensures stability under the existing environmental regime, the pool of transient species may support long-term stability by replacing core species should environmental conditions change. © 2014 The Author(s) Published by the Royal Society. All rights reserved.


Magurran A.E.,University of St. Andrews | Henderson P.A.,Pisces Conservation Ltd
Philosophical Transactions of the Royal Society B: Biological Sciences | Year: 2010

Temporal variation in species abundances occurs in all ecological communities. Here, we explore the role that this temporal turnover plays in maintaining assemblage diversity. We investigate a three-decade time series of estuarine fishes and show that the abundances of the individual species fluctuate asynchronously around their mean levels.We then use a time-series modelling approach to examine the consequences of different patterns of turnover, by asking how the correlation between the abundance of a species in a given year and its abundance in the previous year influences the structure of the overall assemblage. Classical diversity measures that ignore species identities reveal that the observed assemblage structure will persist under all but the most extreme conditions. However, metrics that track species identities indicate a narrower set of turnover scenarios under which the predicted assemblage resembles the natural one. Our study suggests that species diversity metrics are insensitive to change and that measures that track species ranks may provide better early warning that an assemblage is being perturbed. It also highlights the need to incorporate temporal turnover in investigations of assemblage structure and function. This journal is © 2010 The Royal Society.


Magurran A.E.,University of St. Andrews | Henderson P.A.,PISCES Conservation Ltd
Proceedings of the Royal Society B: Biological Sciences | Year: 2012

How do species divide resources to produce the characteristic species abundance distributions seen in nature? One way to resolve this problem is to examine how the biomass (or capacity) of the spatial guilds that combine to produce an abundance distribution is allocated among species. Here we argue that selection on body size varies across guilds occupying spatially distinct habitats. Using an exceptionally well-characterized estuarine fish community, we show that biomass is concentrated in large bodied species in guilds where habitat structure provides protection from predators, but not in those guilds associated with open habitats and where safety in numbers is a mechanism for reducing predation risk. We further demonstrate that while there is temporal turnover in the abundances and identities of species that comprise these guilds, guild rank order is conserved across our 30-year time series. These results demonstrate that ecological communities are not randomly assembled but can be decomposed into guilds where capacity is predictably allocated among species. © 2012 The Royal Society.


Henderson P.A.,Pisces Conservation Ltd | Henderson P.A.,University of Oxford | Magurran A.E.,University of St. Andrews
Proceedings of the Royal Society B: Biological Sciences | Year: 2010

Species abundance distributions (SADs) are widely used as a tool for summarizing ecological communities but may have different shapes, depending on the currency used to measure species importance. We develop a simple plotting method that links SADs in the alternative currencies of numerical abundance and biomass and is underpinned by testable predictions about how organisms occupy physical space. When log numerical abundance is plotted against log biomass, the species lie within an approximately triangular region. Simple energetic and sampling constraints explain the triangular form. The dispersion of species within this triangle is the key to understanding why SADs of numerical abundance and biomass can differ. Given regular or random species dispersion, we can predict the shape of the SAD for both currencies under a variety of sampling regimes. We argue that this dispersion pattern will lie between regular and random for the following reasons. First, regular dispersion patterns will result if communities are comprised groups of organisms that use different components of the physical space (e.g. open water, the sea bed surface or rock crevices in a marine fish assemblage), and if the abundance of species in each of these spatial guilds is linked to the way individuals of varying size use the habitat. Second, temporal variation in abundance and sampling error will tend to randomize this regular pattern. Data from two intensively studied marine ecosystems offer empirical support for these predictions. Our approach also has application in environmental monitoring and the recognition of anthropogenic disturbance, which may change the shape of the triangular region by, for example, the loss of large body size top predators that occur at low abundance. © 2010 The Royal Society.


van der Veer H.W.,Netherlands Institute for Sea Research | Dapper R.,Netherlands Institute for Sea Research | Henderson P.A.,PISCES Conservation Ltd | Jung A.S.,Netherlands Institute for Sea Research | And 3 more authors.
Estuarine, Coastal and Shelf Science | Year: 2015

The ongoing daily sampling programme of the fish fauna in the Dutch Wadden Sea using fixed gear was analysed for the years 1960-2011. Spring sampling caught immigrating fish from the coastal zone and autumn samples reflected emigration of young-of-the-year. In total 82 fish species were caught with no clear trend in biodiversity. In both spring and autumn total daily catch fluctuated and peaked in the late 1970s. From 1980 to the present catches of both pelagic and demersal species showed a 10-fold decrease in total biomass. Mean individual biomass decreased in spring between 1980 and the present from about 150 to 20g wet weight. No trend was found in autumn mean individual biomass which fluctuated around 20g wet weight. The trophic structure remained constant for both the demersal and benthopelagic fish fauna from 1980 to 2011, whilst the trophic position of pelagic fish in spring fell from about 3.9 to 3.1. Min/max auto-correlation factor analysis showed similar trends in spring and autumn species biomass time series: the first axis represented a decrease from the 1960s followed by stabilization from the mid-1990s. The second trend showed an increase with a maximum around 1980 followed by a steady decrease in spring and a decrease and stabilization from 2000 in autumn. It is argued that the most likely explanatory variables are a combination of external factors: increased water temperature, habitat destruction in the coastal zone (sand dredging and beach nourishment, fishing) and increased predation by top predators for the first trend, and large-scale hydrodynamic circulation for the second trend. We conclude that both the trophic structure of the coastal zone fauna and the nursery function of the Wadden Sea have been reduced since the 1980s. Our findings corroborate that ecological change in coastal ecosystems has not only occurred in the past but still continues. © 2015 Elsevier Ltd.


Scott A.L.,Hampshire & Isle of Wight Wildlife Trust | Henderson P.A.,Pisces Conservation Ltd
Journal of the Marine Biological Association of the United Kingdom | Year: 2015

A study of an inshore southern North Sea population of lesser weever, Echiichthys vipera, on the Suffolk coast, England, found this small, abundant, benthic fish to reach an age of 15 years and suffer an adult mortality rate of only 0.23 y−1. The maximum length observed of 195 mm Standard length (SL) (225 mm total length, TL) was the greatest yet reported and many individuals >140 mm SL (163 mm TL) were caught between 2009 and 2012. Previous studies have reported a maximum of 160 mm TL and a von Bertalanffy asymptotic TL of 150.3 mm. Age structure analysis showed that recruitment into the local inshore Sizewell population continued until 5 or more years of age. A 6 year age of recruitment corresponds to the age when they have been reported to have disappeared from offshore locations and previously assumed to have died from old age. Regular seasonal changes in local abundance were observed with peak captures during May, presumably caused by seasonal immigration, followed by a summer minimum and a second, more variable, maximum in early autumn before the winter minimum. The winter minimum in captures may be due to either inactivity or offshore migration. Lesser weever has evolved a long-lived, slow growing, life history strategy unusual for small benthic fish in the southern North Sea. By spending long periods hidden in sand, using venom for defence and remaining inactive for an extended period each winter, lesser weever has adopted a strategy which favours high survival and increased longevity. Copyright © Marine Biological Association of the United Kingdom 2015


Henderson P.A.,Pisces Conservation Ltd. | Henderson P.A.,University of Oxford | Bird D.J.,University of the West of England
Marine Pollution Bulletin | Year: 2010

The species of fish and macro-crustacean living within the Severn Estuary are reviewed. The fish community is notably species rich and exceeds 100 species in total for the estuary. Standardised long-term sampling at Hinkley Point in Bridgwater Bay gives a total complement of 83 for a single locality and this number is increasing by about one new species every two years. Most of these new species are moving in from centres of population lying to the south of the estuary. Almost all species of fish and macro-crustacean living within the estuary undertake regular migrations so that they tend to move seasonally in waves up and down the estuary. For fish, both species richness and the total abundance reach a maximum in late summer and autumn. The timing of this peak varies between the upper and lower estuary. This seasonal maximum is primarily caused by the arrival of the new recruits which use the estuary as a nursery. In contrast, crustaceans tend to be at their most diverse and abundant in early to mid summer. Using a 30-year time series of fish and crustacean abundance collected at Hinkley Point it is shown that major changes in the structure of the community are now underway and there are considerable recent changes in the abundance. However, some abundant species, including sand goby, Pomatoschistus spp., whiting, Merlangius merlangus and sprat, Sprattus sprattus, the three most abundant species in the estuary, have shown no long-term trend. At present, approximately 20% of the fish and macro-crustaceans observed in Bridgwater Bay are undergoing rapid, typically exponential, change in abundance. For a numerically abundant, diverse, fauna composed of approximately 90 species such levels of change are unexpected and suggest that the system is presently far from equilibrium. In some cases, the observed changes can be related to recent warming and the North Atlantic Oscillation. The overall increase in fish abundance observed may reflect a general improvement in water quality and a reduction in other anthropogenic impacts such as mortality in cooling-water intakes. The potential impacts of tidal power generation in the Severn Estuary are reviewed. There is considerable potential for any major installation to impact the fish and crustacean populations as they migrate and also alter the nature of the habitat resulting in changes in community composition. A particular difficulty in predicting the future impact of harnessing tidal energy is that the present community is already changing rapidly. The ability of fish and crustaceans to pass through the turbines unharmed will be a key issue in an assessment of the impact of tidal power generation. © 2010 Elsevier Ltd. All rights reserved.


Henderson P.A.,Pisces Conservation Ltd | Plenty S.J.,University of the West of England | Newton L.C.,University of the West of England | Bird D.J.,University of the West of England
Journal of the Marine Biological Association of the United Kingdom | Year: 2012

A 30-year study of the estuarine population of yellow eel, Anguilla anguilla, abundance in Bridgwater Bay, Somerset, UK, shows that the population number has collapsed. Since 1980, the decline has averaged 15% per year. The abundance of eel in 2009 is estimated at only 1% of that in 1980. This is one of the greatest systematically quantified crashes of a fish population ever reported. Collections of eels impinged on cooling water filter screens were made monthly at Hinkley Point power station between 1980 and 2010 and from Oldbury power station between 1996 and 1998. Eels are always present in the Severn Estuary, although there are large seasonal variations in abundance. At Oldbury, in the upper estuary, eels are least abundant in January. In contrast, in the outer estuary in Bridgwater Bay, eels are most abundant between November and March. The size-distribution of yellow eels ranged from <200 to >700 mm indicating an age-range since the glass eel stage of 2 to >25 years. The mean size-range has not changed since the 1980s indicating that the population collapse is not caused by a sudden recruitment failure. It is suggested that there has been a continual long-term failure of recruitment to compensate for losses. The reason for this is unidentified, but is unlikely to be changes in the North Atlantic Oscillation or other natural environmental variability. A major effort to improve eel survival to adulthood is required if this species is not to gently fade to extinction. This would likely involve a cessation of elver fishing, a reduction in the volume of estuarine water extracted for power station cooling and other purposes during which eels are entrained and killed, and the removal of obstructions which increase mortality during migration. Copyright © Marine Biological Association of the United Kingdom 2011.


Henderson P.A.,Pisces Conservation Ltd. | Henderson P.A.,University of Oxford | Seaby R.M.H.,Pisces Conservation Ltd. | Somes J.R.,Pisces Conservation Ltd.
Journal of Experimental Marine Biology and Ecology | Year: 2011

Results from a 30-year study of fish and crustacean abundance at Hinkley Point, Somerset, England are reported. Standard community ecology metrics, including annual total species number recorded, alpha diversity and dominance indices, the rank-abundance curve and the assemblage of permanently present species, have all shown notable stability and no trend over the study period. In contrast, community structure has shown clear change which can be related to the fact that the abundances of many species have shown long-term trends. Of the 30 most abundant species, which together comprise more than 99% of the total species number and biomass collected, 17 have shown a long-term trend in log abundance indicative of exponential change. 9 species have shown approximately exponential increases, and 8 exponential decreases in abundance. This remarkable variation in individual species' abundance has been shown for some species to be related to changes in sea water temperature, the North Atlantic Oscillation Index, and salinity. While annual species richness has not increased, the number of species present each month has, on average, increased. This has been caused by changes in seasonal presence, with summer-autumn species extending their presence further into the winter. For fish, the dominant species show no trend, and it is argued they are likely to be under density-dependent control. It may be that while the most abundant species are constrained by resources, the majority of less abundant forms are dynamically unstable and more likely to be responsive to environmental change. © 2011 Elsevier B.V.

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