Changes In Fish Assemblages Research Articles

Components and drivers of change in European freshwater fish faunas

AbstractAimHuman‐induced loss of native species and introduction of non‐native species have altered richness and composition of species assemblages world‐wide. During the past 15 years many studies have focused on changes in taxonomic similarity and identified numerous yet often contrasting reasons for these changes. This study aims to quantify taxonomic changes in freshwater fish assemblages between the mid‐19th century and today, while explicitly separating its different components and drivers.LocationGeographic Europe, 251 river basins > 2500 km².MethodsPairwise catchment comparisons of historic and contemporary fish species inventories, with and without migratory fish, using Jaccard similarity; quantification of relative species turnover using a newly developed Reshuffling Index; determination of the threshold (here: number of catchments occupied) that displays how widespread a species must be to cause homogenization.ResultsThe European freshwater fish fauna changed profoundly since the mid‐19th century. All river catchments exhibited an average net gain of 5.7 species, leading to an overall increase in faunal similarity across Europe of 3.1% (4.6% if migratory species are excluded). However, species turnover was much higher than indicated by the net gain. On average, 20% of the historic assemblages became reshuffled. The native catchment range size of an introduced species primarily determined its impact on taxonomic similarity change, irrespective whether it is translocated within or introduced from outside Europe and whether the species is of fisheries importance or not.Main conclusionsThe concurrent use of multiple indices allowed disentangling the main components and drivers of taxonomic change. It became evident that prevention of intended or unintended species introduction will not lower the rate of taxonomic homogenization per se. However, most species actually caused taxonomic differentiation despite their range gain. All introduced species still considerably contributed to taxonomic change with potential negative effects on ecosystem functions.

Global synthesis of the documented and projected effects of climate change on inland fishes

Although climate change is an important factor affecting inland fishes globally, a comprehensive review of how climate change has impacted and will continue to impact inland fishes worldwide does not currently exist. We conducted an extensive, systematic primary literature review to identify English-language, peer-reviewed journal publications with projected and documented examples of climate change impacts on inland fishes globally. Since the mid-1980s, scientists have projected the effects of climate change on inland fishes, and more recently, documentation of climate change impacts on inland fishes has increased. Of the thousands of title and abstracts reviewed, we selected 624 publications for a full text review: 63 of these publications documented an effect of climate change on inland fishes, while 116 publications projected inland fishes’ response to future climate change. Documented and projected impacts of climate change varied, but several trends emerged including differences between documented and projected impacts of climate change on salmonid abundance (P = 0.0002). Salmonid abundance decreased in 89.5% of documented effects compared to 35.7% of projected effects, where variable effects were more commonly reported (64.3%). Studies focused on responses of salmonids (61% of total) to climate change in North America and Europe, highlighting major gaps in the literature for taxonomic groups and geographic focus. Elucidating global patterns and identifying knowledge gaps of climate change effects on inland fishes will help managers better anticipate local changes in fish populations and assemblages, resulting in better development of management plans, particularly in systems with little information on climate change effects on fish.

Are long-term fish assemblage changes in a large US river related to the Asian Carp invasion? Test of the hostile take-over and opportunistic dispersal hypotheses

Gizzard shad is a dominant planktivore/detritivore in the Wabash River, and populations crashed in the early 1990s. Previous work (1974–2008) identified a substantial shift in body-size structure and functional trait composition in the Wabash River fish assemblage between 1989 and 1996. Invasive Asian Carp appeared in the Ohio River basin including the Wabash River in the 1990s. Our goal was to test for temporal changes in assemblage composition and trophic structure relative to the invasion of Asian carp from the early 1990s. We hypothesized that establishment of Asian Carp was a contributor to the assemblage composition shift, and that their presence altered the trophic pathways and food sources of native fishes including Gizzard Shad (hostile takeover hypothesis). Alternatively, Asian Carp may have found success through capitalizing on an empty niche, likely left vacant by the decline in Gizzard Shad, or abundance changes in other trophic groups (opportunistic hypothesis). We utilized archival fish and mussel collections to test for trophic changes in the ecosystem using δ13C and δ15N stable isotope analyses. We examined stomach contents of the dominant planktivore/detritivore consumer, Gizzard Shad, from archival and recent collections to test for changes in the phytoplankton community. Stable isotopes indicated a community reliance on other, more deplete, carbon sources than indicated by the algae, and a slight increase between the δ13C time periods. Although all functional feeding groups of fishes indicated some reduction in δ15N, the differences were only significant for omnivores, mussels, and planktivore/detritivores. Although Asian Carp may have contributed to the collapse of Gizzard Shad populations, other stressors were likely more important drivers of their decline. This is the first, albeit indirect, evidence of opportunistic “invasion” as opposed to the historically presumed hostile takeover model.

Assessment of dam effects on streams and fish assemblages of the conterminous USA

Despite the prevalence of damming as a global disturbance to river habitats, detailed reach-based assessments of the ecological effects of dams are lacking, particularly across large spatial extents. Using data from nearly 50,000 large dams, we assessed stream network fragmentation and flow alteration by large dams for streams of the conterminous USA. We developed 21 dam metrics characterizing a diversity of dam influences operating at both localized (e.g., distances-to-dams) and landscape scales (e.g., cumulative reservoir storage throughout stream networks) for every stream reach in the study region. We further evaluated how dams have affected stream fish assemblages within large ecoregions using more than 37,000 stream fish samples. Streams have been severely fragmented by large dams, with the number of stream segments increasing by 801% compared to free-flowing streams in the absence of dams and a staggering 79% of stream length is disconnected from their outlet (i.e., oceans and Great Lakes). Flow alteration metrics demonstrate a landscape-scale disturbance of dams, resulting in total upstream reservoir storage volumes exceeding estimated annual discharge volumes of many of the nation’s largest rivers. Further, we show large-scale changes in fish assemblages with dams. Species adapted to lentic habitats increase with dams across the conterminous USA, while rheophils, lithophils, and intolerant fishes decrease with dams. Overall, fragmentation and flow alteration by dams have affected fish assemblages as much or more than other anthropogenic stressors, with dam effects generally increasing with stream size. Dam-induced stream fragmentation and flow alteration are critical natural resource issues. This study emphasizes the importance of considering dams as a landscape-scale disturbance to river habitats along with the need to assess differential effects that dams may have on river habitats and the fishes they support. Together, these insights are essential for more effective conservation of stream resources and biotic communities globally.

Monitoring demersal scalefish populations in the Browse Basin region: accounting for spatial variability and detecting change in key fish populations

One of the objectives of the Applied Research Program (ARP), funded by Shell and the INPEX-operated Ichthys LNG Project, was to establish the basis for evaluating the effects of any potential oil spill from the Prelude or Ichthys fields on populations of commercially important demersal fishes in the Northern Demersal Scalefish Managed Fishery. The ARP has delivered improved baseline understanding of the status and spatial variability in populations of commercially and ecologically important finfish of the Browse Basin region, in the vicinity of the Prelude and Ichthys fields, using both commercial-style fish traps and baited remote underwater videos (BRUVs) as sampling methods. We used available environmental-data layers and fish-distribution data, in combination with modelled plume projections, to design a study such that sites were balanced with respect to fish habitat and likely affected and unaffected zones. The sampling-design employed was statistically effective with power to show changes in fish assemblages, and relative abundance of as little as 30% could be detected with 80% certainty in the main commercial species, goldband snapper (Pristipomoides multidens), using BRUVs. For the second-most important species, red emperor (Lutjanus sebae), a larger decline of 50% would have to occur for this change to be determined with the same level of confidence. Traps were a more powerful tool for sampling L. sebae, allowing a 40% change to be detected; however, for all other species BRUVs were more powerful. Sampling of the demersal-fish assemblages surrounding the Prelude and Ichthys fields by using both fish traps and BRUVs demonstrated the feasibility of detecting change in the relative abundance of the fish assemblage as well as in the key commercial species.

Identifying and quantifying environmental thresholds for ecological shifts in a large semi-regulated river

ABSTRACTEcological shifts, between a clear macrophyte-dominated state and a turbid state dominated by phytoplankton and high inorganic suspended solids, have been well described in shallow lake ecosystems. While few documented examples exist in rivers, models predict regime shifts, especially in regulated rivers with high water retention time. Here I quantified ecological shifts in a large, semi-regulated floodplain river during a transition from a turbid- to a clear-water state using water quality, aquatic vegetation and fisheries data from a rigorous, standardized long-term data set. My findings indicate that significant changes occurred in total suspended solids concentration, aquatic macrophyte abundance, native and non-native fish biomass, fish functional feeding guild patterns, fish habitat guild assemblages and fish spawning guild assemblage patterns over a nearly 20-year period in Navigation Pool 8 of the Upper Mississippi River. Transitions in physical and biological indicators were examined to identify mechanisms underlying the ecological shifts. Environmental variables driving fish assemblage changes were identified (total suspended solids and aquatic vegetation) and management-relevant thresholds are presented. Awareness of management thresholds is critical for resource managers to implement measures to prevent the river from moving to a degraded state characterized by high non-native fish abundance and low predatory fish species abundance.

Local Stressors, Resilience, and Shifting Baselines on Coral Reefs.

Understanding how and why coral reefs have changed over the last twenty to thirty years is crucial for sustaining coral-reef resilience. We used a historical baseline from Kosrae, a typical small island in Micronesia, to examine changes in fish and coral assemblages since 1986. We found that natural gradients in the spatial distribution of fish and coral assemblages have become amplified, as island geography is now a stronger determinant of species abundance patterns, and habitat forming Acropora corals and large-bodied fishes that were once common on the leeward side of the island have become scarce. A proxy for fishing access best predicted the relative change in fish assemblage condition over time, and in turn, declining fish condition was the only factor correlated with declining coral condition, suggesting overfishing may have reduced ecosystem resilience. Additionally, a proxy for watershed pollution predicted modern coral assemblage condition, suggesting pollution is also reducing resilience in densely populated areas. Altogether, it appears that unsustainable fishing reduced ecosystem resilience, as fish composition has shifted to smaller species in lower trophic levels, driven by losses of large predators and herbivores. While prior literature and anecdotal reports indicate that major disturbance events have been rare in Kosrae, small localized disturbances coupled with reduced resilience may have slowly degraded reef condition through time. Improving coral-reef resilience in the face of climate change will therefore require improved understanding and management of growing artisanal fishing pressure and watershed pollution.

Mangrove habitat use by fishes in Southeastern Brazil: are there temporal changes in the structure of the community?

We compared fish community structure in Guaratiba Mangrove, SE Brazil, among three seasons of the hydrological cycle [(i) spring, increasing temperature and rainfall, decreasing salinity; (ii) summer/early fall, high temperature and rainfall, intermediate salinity; (iii) late fall/winter, low temperature and rainfall, high salinity] and between the years 2002–2003 and 2008–2009. The aim was to test the hypothesis that the fish community structure changes seasonally, associated with seasonal changes in environmental conditions, and that changes occurred between the two yearly periods because of increased anthropogenic activities. The sampling protocol for the two surveys was identical. A total of 63 species was recorded, with 38 species occurring in 2002–2003, and 53 species in 2008–2009. The main changes in the mangrove's fish assemblage between 2002–2003 and 2008–2009 were (i) an increase in species richness, fish density and total biomass; and (ii) an increase in the zoobenthivorous species of gerreids Eucinostomus argenteus, Diapterus rhombeus and Ulaema lefroyi, and a decrease in the planktivorous clupeid Harengula clupeola. Other abundant species, such as the opportunistic atherinopsid. Atherinella brasiliensis, the detritivorous mullet Mugil liza and the zoobenthivorous pufferfish Sphoeroides testudineus did not differ in abundance between the two yearly periods. Ten species occurred only in 2002–2003, and 23 species only in 2008–2009, indicating significant changes in community structure over the 6-year period. Seasonal changes in community structure were more conspicuous in 2008–2009, when species abundance and richness were greatest. The highest fish abundance was recorded in spring and in summer/early fall, and the lowest in late fall/winter. The increased abundance and richness over time may be at least partially attributable to protection policies because of the effective implementation of a biological reserve in the area.

Inter-annual changes in fish communities of a tropical bay in southeastern Brazil: What can be inferred from anthropogenic activities?

We assessed inter-annual changes in fish assemblages of a tropical bay which experienced a heavily industrialized process in the last decades. A highly significant difference in community structure among the bay zones, and a decrease in fish richness and abundance over time were found. Changes in fish richness and abundance between the two first (1987–1988 and 1993–1995) and the two latter time periods (1998–2001 and 2012–2013) were sharpest in the inner bay zone, the most impacted bay area, and in the middle zone, whereas the outer zone remained comparatively stable over time. These changes coincided with increased metal pollution (mainly, Zn and Cd) in the bay and with the enlargement of the Sepetiba Port. Spatial changes in the fish community structure among the bay zones were related to differences in salinity, transparency and depth with this latter variable acting as a buffer stabilizing temporal community changes.

Seasonal shift in boreal riverine fish assemblages and associated bias in bioassessment

Collated electrofishing data from wadeable riffles of boreal rivers in Finland revealed a substantial shift in the fish assemblage composition, accompanied by a decline in total fish density and a reduction in species richness from early summer to late autumn. As the major changes in fish assemblages, the density of cyprinids decreased sharply from June towards autumn, whereas salmonids peaked in September. These shifts were considered to originate mainly from spawning migrations, fish movements to winter refuge habitat and other temperature-related responses of rheophilic versus eurytopic fish species. Temporal change in fish assemblages induced a prominent variation in the monthly fish-based index values used for bioassessment. The proportion of sampled sites classified as high or good in ecological status within the Water Framework Directive (WFD) based on fish increased from 25.9% in July to 68.3% in October. These results, combined with the observed timing of young of the year (0+) fish recruitment to electrofishing catch, suggest that sampling of fish in boreal rivers for WFD monitoring should be restricted to a considerably shorter period than the prevailing practice in order to avoid temporal bias.

Spatial and seasonal dynamics of fish assemblage along river Soan, Pakistan and its relationship with environmental conditions

This study aims to discuss the seasonal and spatial dynamics of fish assemblage of River Soan in relation with environmental changes. Survey was conducted during pre monsoon and post monsoon seasons in 2008. Physico chemical and biological samplings were carried out at 26 sites which were selected along the whole stretch of the river. High concentration of total phosphorous and conductivity, low pH and dissolved oxygen were measured at urban part of midstream zone whereas high concentration of total nitrogen was measured at downstream zone. A total of 22 species were collected during the survey among which Barilius pakistanicus, Puntius ticto, Crossocheilus diplochilus and Labeo dero were distributed throughout the river stretch. Maximum fish diversity was recorded at suburban part of midstream while its urban part was appeared to be the most species poor part of the Soan River. Herbivore individuals constituted the dominant trophic group of fish assemblage. Significant seasonal and spatial changes in fish assemblage were determined at river downstream. Upstream- downstream differences in fish assemblage were more pronounced during pre monsoon and seem to be related to thermal gradient and hydrological changes. Seasonal variations in fish assemblage were significant only at downstream zone and were influenced by velocity, floodplains and migration patterns in fish assemblage. Understanding of spatial and seasonal trends in fish assemblage in relation with their environment is of great importance for the management and conservation of fish assemblage. Findings of this study will be helpful in future attempts to restore and conserve the aquatic assemblages of Himalayan streams.

Seasonal changes in water quality and fisheries of Tanbi Wetland National Park

This research was aimed at studying seasonal changes in water quality of Tanbi Wetland National Park (TWNP) and how these interact with the different land use types to influence fish assemblage. Physicochemical properties were measured in situ with a portable YSI multiprobe watermeter and fish sampling was done using a seine net of 100×4 m. To assess nutrient (nitrogen and phosphorus) input and chlorophyll a concentration (chl-a) in TWNP, spectrophotometry (fluorescence method) was applied. Subjecting the main water variables to a Principal Components Analysis (PCA) indicated a strong correlation between water temperature and pH, phosphate and pH, as well as water temperature and phosphate. A further Multidimensional Scaling (MDS) of the environmental variables indicated salinity as the main water variable influencing fish species richness in TWNP. Except at the settlement and agricultural zones, changes in fish assemblage appear not to be influenced by the specific land use types. As judged from a three-decade time series of rainfall and atmospheric temperatures, seasonal variations in environmental conditions have intensified and are negatively influencing fish assemblage in TWNP.

Spatial variation in benthopelagic fish assemblage structure along coastal East Africa from recent bottom trawl surveys

The structure of benthopelagic fish assemblages of the continental shelves and upper slopes along coastal East Africa (Kenya and Tanzania) was studied based on data from bottom trawls during 2012. These surveys are the most recent since the historical bottom trawls conducted in the 70s and 80s along costal East Africa. The bottom trawls sampled fishes in 27 stations along the Kenyan coast using FV Vega, while in Tanzania 24 stations were sampled by MV Mafunzo. A total of 66 fish species in 43 families were trawled in Kenya, while 40 species belonging to 22 families were sampled in Tanzania in depth ranges of 10 m to 230 m. The highest fish biomass was in shallow (<50 m) areas for both Kenya (123.08 kg/km2) and Tanzania (49.17 kg/km2). Numerically dominant species in Kenyan trawls included the largehead hairtail, Trichiurus lepturus (21.44%), the filesnout grenadier, Coelorhinchus denticulatus (9.50%) and the orangefin ponyfish, Photopectoralis bindus (7.57%), while in Tanzania, the hipfin ponyfish, Leiognathus leuciscus (27.09%), sulphur goatfish, Upeneus sulphureus (19.56%) and the finstripe goatfish U. taeniopterus (12.05%) dominated the trawls. The nMDS analysis indicated the fish assemblages to be influenced by both depth and area for Kenya, and mostly area sampled for Tanzania, while multivariate Correspondence Anlysis (CA) provided characteristic species associated with depth and area for both Kenya and Tanzania. Results of rarefaction curves showed the highest species diversity occurring in Tanzanian shallow depths (>50 m) of the south coast and shallow and mid-depths (50–150 m) of north coast. The lowest species diversity was associated with Kenyan samples of north coast in the mid-depth (50–150 m) and deep (>150 m) waters. The dominant species in the trawls differed with those documented in the historical trawls of the 1970–1980s. The results provide a taxonomic database on the fish species off coastal East Africa useful for monitoring spatio-temporal changes in fish assemblages in the face of climate change effects and increasing exploitation levels.

Trophic relationships, feeding habits and seasonal dietary changes in an intertidal rockpool fish assemblage in the Gulf of Cadiz (NE Atlantic)

This paper describes the use of resources and diet of nine resident fish species in the rocky intertidal zone of the Gulf of Cadiz and examines whether their populations are affected by trophic competition. A stomach content analysis of the nine species revealed that only one was herbivorous (Parablennius sanguinolentus), while the rest were mainly carnivorous (Gobius bucchichi, Gobius cobitis, Gobius paganellus, Zebrus zebrus, Salaria pavo, Lepadogaster lepadogaster, Scorpaena porcus and Tripterygion tripteronotum). The most frequently consumed prey were amphipods, isopods, polychaetes, decapods, chironomids, tanaidaceans, gastropods, copepods, cumaceans and ostracods. In most species, the occurrence of polychaetes and molluscs was higher in the cold season, whereas that of isopods, decapods, chironomids and fish increased in the warm season. In general, larger specimens consumed larger prey, with an increase in the occurrence of isopods, decapods and fish. An analysis of trophic niche breadth defined G. cobitis as generalist, G. bucchichi as opportunist and S. porcus as specialist, whereas the values obtained for the other species did not indicate a clearly defined strategy. Low diet overlap values and the segregation observed in several analyses indicated an adequate distribution of resources.

Changes of fish assemblages after construction of an estuary barrage in the lower Nakdong River, South Korea

In the Nakdong River, an estuary barrage was constructed in 1987, and it divided the freshwater and the seawater, resulting in a change in the ecosystem. To estimate the impact of barrage construction on fish assemblages, we evaluated 20 years of monitoring data before and after the construction of the estuary barrage, and evaluated the role of fishways. The barrage construction generated entire changes of fish assemblages. After construction, the number of fish species dropped sharply, and 36 species disappeared. Conversely, 18 species appeared at this site, including eight freshwater species, seven of which were exotic or translocated species. Barrage construction affected freshwater fish more severely than it did estuarine and marine species because of the existence of an estuarine environment below the barrage. We did not detect any evidence of recovery of fish assemblages. A total of 31 species were collected at fishways, and the number of individuals collected at each fishway was positively correlated with the amount of discharge from the estuary barrage, mean daily tide level, and water temperature. Migratory fish using a fishway had obvious occurrence periods. Therefore, the efficiency of fishway use can be increased if an appropriate management plan is prepared and implemented.

Use of functional traits to assess changes in stream fish assemblages across a habitat gradient

Abstract Functional traits are important for understanding the links between species occurrence and environmental conditions. Identifying these links makes it possible to predict changes in species composition within communities under specific environmental conditions. We used functional traits related to habitat use and trophic ecology in order to assess the changes in fish community composition between streams with varying habitat structure. The relationship between the species traits and habitat characteristics was analyzed using an RLQ ordination analysis. Although species were widely distributed in habitats with different structures, physical conditions did favor some species based on their functional characteristics. Eight functional traits were found to be associated with stream habitat structure, allowing us to identify traits that may predict the susceptibility of fish species to physical habitat degradation.

Long-term changes of fish assemblages associated with artificial reefs off the northern coast of Rio de Janeiro, Brazil

Summary Changes in fish assemblages associated with artificial reefs 5-km offshore (Southeast Brazil) were assessed. Bottom gillnets were used for 1 month per year for up to 15 years after the artificial reef had been established. Totals of 552 fishes were caught, with Chloroscombrus chrysurus and Conodon nobilis the dominant species and Scianidae the richest family. Species richness, abundance and biomass varied significantly with time, increasing gradually for several years but decreasing thereafter. The same unimodal response was recorded for abundance and biomass of planktivorous and invertivorous fishes and for Haemulidae, suggesting that this family accounted for most of the temporal variations recorded per species richness and CPUE values. The abundance and biomass of sharks decreased gradually after the first year but increased in the 15th year. While these variables increased steadily toward an apparent stable level in the later part of the study for Scianidae and Ariidae, they decreased linearly with time for Carangidae. Fish use of the artificial reefs may be modulated by a continuous species turnover, since there was no single species recurring throughout the surveys. This study stresses the importance of long-term assessments of artificial marine reefs in order to incorporate the effects of successional patterns of fish colonization into management actions.

Three decades of recurrent declines and recoveries in corals belie ongoing change in fish assemblages

Coral reefs are increasingly being altered by a myriad of anthropogenic activities and natural disturbances. Long-term studies offer unique opportunities to understand how multiple and recurrent disturbances can influence coral reef resilience and long-term dynamics. While the long-term dynamics of coral assemblages have been extensively documented, the long-term dynamics of coral reef fish assemblages have received less attention. Here, we describe the changes in fish assemblages on Tiahura reef, Moorea, from 1979 to 2011. During this 33-yr period, Tiahura was exposed to multiple disturbances (crown-of-thorns seastar outbreaks and cyclones) that caused recurrent declines and recoveries of coral cover and changes in the dominant coral genera. These shifts in coral composition were associated with long-term cascading effects on fish assemblages. The composition and trophic structure of fish assemblages continuously shifted without returning to their initial composition, whereas fish species richness remained stable, albeit with a small increase over time. We detected nonlinear responses of fish density when corals were most degraded. When coral cover dropped below 10 % following a severe crown-of-thorns sea star outbreak, the density of most fish trophic groups sharply decreased. Our study shows that historical contingency may potentially be an important but largely underestimated factor explaining the contemporary structure of reef fish assemblages and suggests that temporal stability in their structure and function should not necessarily be the target of management strategies that aim at increasing or maintaining coral reef resilience.

Rising from the ashes: Changes in salmonid fish assemblages after 30 months of the Puyehue–Cordon Caulle volcanic eruption

Events such as volcanic eruptions may act as disturbance agents modifying the landscape spatial diversity and increasing environmental instability. On June 4, 2011 the Puyehue–Cordon Caulle volcanic complex located on Chile (2236m.a.s.l., 40° 02′ 24″ S- 70° 14′ 26″ W) experience a rift zone eruption ejecting during the first day 950millionmetrictons into the atmosphere. Due to the westerly winds predominance, ash fell differentially upon 24millionha of Patagonia Argentinean, been thicker deposits accumulated towards the West. In order to analyze changes on stream fish assemblages we studied seven streams 8, 19 and 30months after the eruption along the ash deposition gradient, and compare those data to pre eruption ones. Habitat features and structure of the benthic macroinvertebrate food base of fish was studied. After the eruption, substantial environmental changes were observed in association with the large amount of ash fallout. In western sites, habitat loss due to ash accumulation, changes in the riparian zone and morphology of the main channels were observed. Turbidity was the water quality variable which reflected the most changes throughout time, with NTU values decreasing sharply from West to East sites. In west sites, increased Chironomid densities were recorded 8months after the initial eruption as well as low EPT index values. These relationships were reversed in the less affected streams farther away from the volcano. Fish assemblages were greatly influenced both by habitat and macroinvertebrate changes. The eruption brought about an initial sharp decline in fish densities and the almost total loss of young of the year in the most western streams affecting recruitment. This effect diminished rapidly with distance from the emission center. Thirty months after the eruption, environmental changes are still occurring as a consequence of basin wide ash remobilization and transport.

Immediate changes in stream channel geomorphology, aquatic habitat, and fish assemblages following dam removal in a small upland catchment

Dam removal is becoming an increasingly important component of river restoration, with >1100 dams having been removed nationwide over the past three decades. Despite this recent progression of removals, the lack of pre- to post-removal monitoring and assessment limits our understanding of the magnitude, rate, and sequence of geomorphic and/or ecological recovery to dam removal. Taking advantage of the November 2012 removal of an old (~190year-old) 6-m high, run-of-river industrial dam on Amethyst Brook (26km2) in central Massachusetts, we identify the immediate eco-geomorphic responses to removal. To capture the geomorphic responses to dam removal, we collected baseline data at multiple scales, both upstream (~300m) and downstream (>750m) of the dam, including monumented cross sections, detailed channel-bed longitudinal profiles, embeddedness surveys, and channel-bed grain size measurements, which were repeated during the summer of 2013. These geomorphic assessments were combined with detailed quantitative electrofishing surveys of stream fish richness and abundance above and below the dam site and throughout the watershed and visual surveys of native anadromous sea lamprey (Petromyzon marinus) nest sites. Post-removal assessments were complicated by two events: (1) upstream knickpoint migration exhumed an older (ca. late eighteenth century) intact wooden crib dam ~120m upstream of the former stone dam, and (2) the occurrence of a 10–20year RI flood 6months after removal that caused further upstream incision and downstream aggradation. Now that the downstream reach has been reconnected to upstream sediment supply, the predominant geomorphic response was bed aggradation and associated fining (30–60% reduction). At dam proximal locations, aggradation ranged from 0.3 to >1m where a large woody debris jam enhanced aggradation. Although less pronounced, distal locations still showed aggradation with a mean depth of deposition of ~0.20m over the 750-m downstream reach. Post-removal, but pre-flood, bed surveys indicate ~2m of incision had migrated 25m upstream of the former reservoir before encountering the exhumed dam, which now acts as the new grade control, limiting progressive headcutting. Approximately 1000m3 of sediment was evacuated in the first year, with ~67% of the volume occurring by pre-flood, process-driven (e.g., changes in base level) controls. The combination of changes in channel-bed sedimentology, the occurrence of a large magnitude flood, and the emergence of the new crib dam that is a likely barrier to fish movement was associated with major reductions in abundance and richness in sites downstream and immediately upstream adjacent to the former dam in post-removal sampling. At the same time, we documented the presence of four species of fish, including sea lamprey, which were not present above the dam prior to removal, indicating that upstream passage has been achieved; and we also documented lamprey spawning activity at sites immediately below the dam, which had previously been unsuitable owing to an excessively coarse and armored riverbed. Our results point to the importance of interactions between dam removal and flood disturbance effects, with important implications for short- and long-term monitoring and assessment of dam impacts to river systems.