Changes In Fish Assemblages Research Articles

Fish assemblages and seasonal movements of fish in irrigation canals and river reaches of the middle Rio Grande, New Mexico (USA)

Abstract – Synoptic fish surveys of river and irrigation canal sites and a path‐analytic approach evaluate seasonal changes in fish assemblages in canal systems and river reaches of the middle Rio Grande, NM (USA). The objective was to assess how fish assemblages within the Isleta Reach respond to irrigation season and off‐season variations in water and fish contributions from upstream river channels (Albuquerque Reach) and from the Peralta irrigation system. Eighteen species of fish were found in the irrigation canals, 13 in the Isleta Reach and 15 in the Albuquerque Reach. The proportion of nonnative fishes was higher in canals (56%) than in the river (11%). Species rank abundances were used in path analyses of fish species compositions during the irrigation season and off‐season. During the irrigation season, path coefficients indicated fish movements downstream from the Albuquerque Reach (0.55) to the Isleta Reach exceeded contributions by the conveyance‐return (0.29) and drain‐return (0.05) canals. However, as the river de‐watered the return canals were important refuge habitats for native fishes. The 0.05 value represented a major decline relative to the path coefficient for the upstream Peralta drain canal (0.72). During the off‐season path coefficients indicated fish movements of 0.47 downstream in the river and 0.59 in the conveyance‐return canal but a negligible value for the drain‐return canal (‐0.06). Irrigation management could be modified to favour native fish ecology by controlling movement of nonnative predators in canal systems and in providing refuge habitats for native fish during periods when water demand exceeds the supply.

Changes in fish assemblages following 10 years of protection in Tasmanian marine protected areas

Most studies examining effects of marine protected areas (MPAs) on fish assemblages are potentially confounded, either because they are once off comparisons between fished and unfished locations, or because they are snapshot studies over a fixed period. Here we compare long-term changes within fully protected Tasmanian marine reserves with changes at external reference sites on an annual basis over the first ten years of protection. The results highlight the importance of long-term datasets for differentiating changes occurring over differing time scales. Notable results include a statistically significant increase in abundance of Latridopsis forsteri and large fish (> 300 mm) when examined across all reserves relative to controls, and a 10-fold increase in the abundance of large fish and a doubling of per site species richness of large fish within the Tinderbox Marine Reserve relative to controls. Short-term resident species that recruit sporadically show very different patterns in reserves compared to those that recruit regularly and have long-term age-class storage. While several recent reviews have suggested size of MPAs and duration of protection has little influence on the extent of recovery, our results suggest this is not the case and that responses can be slow, complex and species-specific. The extent of localised fishing pressure appeared to have a substantial influence on the degree of change detected, potentially confounding meta-analyses of recovery rates in MPAs if overlooked as a relevant parameter.

Evaluating day–night changes in shallow Mediterranean rocky reef fish assemblages by visual census

Ecological information on coastal fish distribution patterns and habitat use can be greatly improved by nocturnal samplings and observations. To this purpose, the structure of a Mediterranean fish assemblage inhabiting the shallow rocky littoral of Linosa Island (Sicily Strait, Italy) was examined by using visual census to detect possible diel variations in species composition and abundance. Day–night fish distribution patterns were investigated by multivariate and univariate analyses. Overall, 42 fish taxa belonging to 19 families were recorded: 35 during the day and 24 during the night. Seventeen species were common to both diurnal and nocturnal assemblages. Within the diurnal assemblage, Chromis chromis was the most represented species (37.2%), followed by Thalassoma pavo (23.2%) and Sparisoma cretense (10.8%). Within the nocturnal assemblage, the most abundant taxon was Atherina spp. (33.9%), followed by Apogon imberbis (26.4%) and Boops boops (11.5%). Our results indicated wide variation in the abundance and species composition during the day and during the night. Multi Dimensional Scaling plot showed a clear-cut separation between the two assemblages and analysis of similarities found significant differences as well. SIMPER analysis revealed that ten species individually contributed by more than 2.5% to the dissimilarity between diurnal and nocturnal assemblages, T. pavo, C. chromis and S. cretense being the first three species in order of decreasing percentage. ANOVA performed on species richness and fish abundance detected significant differences between diurnal and nocturnal assemblages, the latter showing far lower average values for both variables.

Long-term changes in fish assemblages of the Widawka and Grabia Rivers (Poland): pattern recognition with a Kohonen artificial neural network

The stability of occurrence, abundance and dominance of fish was compared between two sampling periods, 1963-66 and 2002-04 in two lowland rivers from the Warta/Odra system (Poland): the Widawka and its tributary Grabia (95.8 and 77.3 km long, respectively). The Widawka system has been under strong influence of a brown coal strip mine since the 1970s. Long stretches of the upper Widawka and of its certain tributaries were changed into concrete canals, which receive daily 660 000 m 3 of water disposed by the strip mine. The Grabia maintained its natural character. On the basis of the abundance of fish species, the Kohonen artificial neural network almost perfectly separated fish samples collected in 1963-66 from samples collected in 2002-04, which means that: 1) the differences in ichthyofauna between these two sampling periods were more drastic than spatial differences between the severely modified Widawka and the natural Grabia, and 2) in general, ichthyofauna at the most modified sites in 1963-66 was in better condition than at the least modified sites in 2002-04..In 1963-66, the species, roach, gudgeon, bleak and dace composed together 70% of the total fish number, while in 2002-04 the same joint dominance was gained by roach, perch and stone loach. The analysis of temporal changes in the Widawka and Grabia Rivers focused on the following 3 fish groups: (1) Non-psammophilous rheophils (NPR), which are most vulnerable to human-induced modifications, which is why their decline and/or extinction are typical for unbalanced lotic ecosystems. In this study, the total dominance of NPR decreased 5 times; (2) Psammophilous rheophils (stone loach and gudgeon), which often predominate in degraded smaller streams. In this study, the dominance of stone loach increased 3 times and was highest in the most affected stretches of the Widawka River, including those that had been canalised. Gudgeon was often dominant at moderately modified sites of the Widawka; (3) Roach and perch, whose high dominance is typical for disturbed larger rivers, which was supported also in this study by the fact that both species were most dominant in 2002-04. The dominance of roach increased almost twice and was highest in the lower courses of both rivers. A significant increase in abundance was recorded for perch, whose dominance was highest in the canalised section of the Widawka. The described changes were deep enough to influence the hardly sensitive CDI, Simpson and Shannon indices, which indicated a significant increase in dominance and a decrease in diversity of fish assemblages over the last 4 decades.

Short-term effects of a low dissolved oxygen event on estuarine fish assemblages following the passage of hurricane Charley

Hurricane Charley, a category 4 storm, passed directly over the Charlotte Harbor estuary and Peace River watershed on August 13, 2004. Following the storm's passage, dissolved oxygen in the Peace River fell below 1 mg l−1 and hypoxic conditions (<2mgl−1) extended into Charlotte Harbor. A long-term fisheries-independent monitoring program (1989 to present) was already in place in Charlotte Harbor, and sampling was intensified during the month after the storm. Changes in fish assemblages that resulted from the hypoxic event were determined using nonmetric multidimensional scaling and similarity percentages analysis. At the mouth of the Peace River and upper Charlotte Harbor, fish abundance decreased dramatically after the hurricane, and typical estuarine fish assemblages were replaced by those dominated by a few resilient estuarine and freshwater species, including the nonindigenous brown hoplo (Hoplosternum littorale) and sailfin catfish (Pterygoplichthys spp.). Fish assemblages in the estuarine portion of the Maykka River, located only a few kilometers west of the Peace River, were unaffected. The hypoxic event was short lived; dissolved oxygen and estuarine fish assemblages in the Peace River and upper Charlotte Harbor recovered within a month. The results of this study are consistent with other hurricane-related hypoxic events in the literature which have reported acute effects to estuarine systems in the short term, rapid recoveries, and long-term resilience.

Spatial Patterns of Fish Assemblage Structure in a Tributary System of the Upper Colorado River Basin

ABSTRACT This study was conducted to describe the distributions of both native and nonnative fishes and to identify spatial patterns in fish assemblage structure of Muddy Creek in the upper Colorado River basin of Wyoming using data collected from 77 reaches during 1999–2004. Fish assemblages in high-elevation reaches were characterized by brook trout (Salvelinus fontinalis) and represented a coldwater faunal zone. Reaches at lower elevations were characterized by warmwater fish species. The upper segment of the warmwater faunal zone contained four native (i.e., bluehead sucker [Catostornus discobolus], flannelmouth sucker [C. latipinnis], speckled dace [Rhinichthys osculus], roundtail chub [Gila robusta]) and two nonnative (i.e., white sucker [C. commersoni] and creek chub [Semotilus atromaculatus]) species. The lower segment of the warmwater faunal zone included species present in upstream segments and three additional nonnative species (i.e., common carp [Cyprinus carpio], redside shiner [Richardsonius baleatus], and fathead minnow [Pimephales promelas]). Differences in fish assemblage structure between coldwater and warmwater faunal zones were likely due to physiological constraints of species adapted for either coldwater or warmwater habitats. Changes in fish assemblages over the warmwater faunal zone are due to introduction and naturalization of nonnative fishes and anthropogenic barriers preventing upstream movements and colonization by some of these species.

Diel trophic structuring of seagrass bed fish assemblages in the Wakatobi Marine National Park, Indonesia

The faunal communities of seagrass beds throughout SE Asia are highly threatened by continued overexploitation, yet their ecology is poorly understood. Developing a greater understanding of the faunal linkages between seagrass beds and associated coastal habitats can facilitate more informed ecosystem level management. The present study used beach seine netting to sample seagrass bed fish assemblages in the Wakatobi Marine National Park, Indonesia, to investigate diel migrations of fish into and out of seagrass beds. These fish assemblages were found to be diverse relative to other studies within the region, with many species being economically important to local subsistence fisheries. The abundance, species richness and trophic structure of these fish assemblages changed with time of day indicating that fish populations are in a dynamic state. Mean fish abundance increased by ≈45% from day to night (Day: 8.61 ± 0.13 fish 100 m −2; Night: 15.6 ± 1.4 fish 100 m −2) while mean species richness increased from 6.6 ± 1.9 per seine haul to 11.4 ± 0.2. Increasing abundance and diversity of fish at night suggests migration onto these habitats from nearby habitats such as reefs, mangroves or deep water; and/or increased activity of those fish resident within seagrass habitats. Division of species into trophic categories enabled the trophic structure of changing fish assemblages to be examined. Assemblages were dominated during both the day and night by invertebrate and fish feeders; however, a major diel change in trophic structure occurred in the abundance of omnivores. During the day omnivores were abundant, but they were replaced at night by exclusive invertebrate feeders. We therefore propose that diel changes in seagrass fish assemblages are predominantly structured by food availability, although other factors such as increased night-time shelter provision were also found to be important albeit to a much lesser extent.

Effects of floods on fish assemblages in an intermittent prairie stream

Summary1. Floods are major disturbances to stream ecosystems that can kill or displace organisms and modify habitats. Many studies have reported changes in fish assemblages after a single flood, but few studies have evaluated the importance of timing and intensity of floods on long‐term fish assemblage dynamics.2. We used a 10‐year dataset to evaluate the effects of floods on fishes in Kings Creek, an intermittent prairie stream in north‐eastern, Kansas, U.S.A. Samples were collected seasonally at two perennial headwater sites (1995–2005) and one perennial downstream flowing site (1997–2005) allowing us to evaluate the effects of floods at different locations within a watershed. In addition, four surveys during 2003 and 2004 sampled 3–5 km of stream between the long‐term study sites to evaluate the use of intermittent reaches of this stream.3. Because of higher discharge and bed scouring at the downstream site, we predicted that the fish assemblage would have lowered species richness and abundance following floods. In contrast, we expected increased species richness and abundance at headwater sites because floods increase stream connectivity and create the potential for colonisation from downstream reaches.4. Akaike Information Criteria (AIC) was used to select among candidate regression models that predicted species richness and abundance based on Julian date, time since floods, season and physical habitat at each site. At the downstream site, AIC weightings suggested Julian date was the best predictor of fish assemblage structure, but no model explained &gt;16% of the variation in species richness or community structure. Variation explained by Julian date was primarily attributed to a long‐term pattern of declining abundance of common species. At the headwater sites, there was not a single candidate model selected to predict total species abundance and assemblage structure. AIC weightings suggested variation in assemblage structure was associated with either Julian date or local habitat characteristics.5. Fishes rapidly colonised isolated or dry habitats following floods. This was evidenced by the occurrence of fishes in intermittent reaches and the positive association between maximum daily discharge and colonisation events at both headwater sites.6. Our study suggests floods allow dispersal into intermittent habitats with little or no downstream displacement of fishes. Movement of fishes among habitats during flooding highlights the importance of maintaining connectivity of stream networks of low to medium order prairie streams.

Anthropogenic Stressors, Inter-Specific Competition and ENSO Effects on a Mauritian Coral Reef

Much of the western Indian Ocean suffered widespread loss of live coral in 1998 and interest is now focussed on the indirect effects of this coral loss on other components of the ecosystem, in particular fishes. However, it is just as important to identify changes in fish assemblages at locations that did not suffer coral mortality to understand local versus regional drivers. We surveyed benthic and fish communities on a reef flat in Mauritius five times between 1994 and 2005. The design allowed for comparison through time, along the coast and between inshore and offshore reef locations. The benthic community demonstrates a clear trend along the coast, likely in response to a dredged water ski lane, but little change through time. Branching Acropora colonies dominate much of the live coral and best explain patterns in the fish assemblage (P < 0.01). Few changes in overall fish species richness through time were identified, and observed changes were within fishery target families rather than species reliant on live coral. Departure from expected levels of taxonomic distinctness suggests degradation in the community associated with the dredged ski lane. Non-metric multi-dimensional scaling of the fish assemblage demonstrates a similar pattern to that seen in the benthos; greater differences along the coast (Global R = 0.34) than through time (Global R = 0.17) and no trend between reef positions. SIMPER analysis identified two species of Stegastes as the main drivers of trends in the MDS plot and the most dominant of these, S. lividus, appears to be reducing species richness of the remaining fish community. The study highlights Mauritius as a regional refugia of thermally-sensitive corals and specialised fish, suggesting a need for careful management.

Changes in Fish Assemblage Structure Upstream of Impoundments within the Upper Wabash River Basin, Indiana

AbstractThe effects of dams and impoundments on downstream fish assemblages have been well documented, but changes in fish assemblages in upstream tributaries have received little attention. We compared changes in abundance and composition in fish assemblages in streams fragmented by impoundments with those found in nearby unfragmented streams by sampling fish, in‐stream habitat, physicochemical factors, and drainage features in 22 agriculturally dominated streams during the summers of 2002 and 2003. Eleven sampling sites were tributaries upstream of impoundments, while 11 were tributaries of free‐flowing rivers. We tested the hypothesis that fish assemblages upstream of impoundments would differ from those found in streams without impoundments. Using multiple regression and canonical correspondence analysis, we partitioned the variation in species distributions into that explained by in‐stream habitat, reach‐level factors, drainage features, and temporal variation. Spatial patterns of species distributions indicated significant upstream effects of impoundment, fragmentation being the single largest predictor of species distributions. Mean fluvial specialist richness was significantly greater in unfragmented (6 species/reach) than in fragmented streams (3 species/reach), whereas mean fluvial generalist richness was significantly greater in fragmented (12 species/reach) than unfragmented streams (8 species/reach). A shift in piscivore abundance and composition was also observed, with smallmouth bass Micropterus dolomieu and redfin pickerel Esox americanus replaced by largemouth bass M. salmoides and white bass Morone chrysops in the streams fragmented by impoundments. Additionally, greater total richness and piscivore abundance was observed in tributaries upstream of impoundments. Unfragmented streams showed a significantly lower total abundance of piscivores (7 fish/reach) than upstream tributaries of impoundments (14 fish/reach). The downstream presence of an impoundment led to significant homogenization of fish assemblages through a significant increase in generalist richness and abundance in fragmented streams and a shift in the abundance and type of piscivores.

Fish assemblages in habitats dominated by Caulerpa taxifolia and native seagrasses in south-eastern Australia

Seagrass beds in estuaries are important habitats and nursery grounds for a great vari- ety of fishes, including many economically important species. The introduction of the invasive green alga Caulerpa taxifolia could potentially threaten the seagrasses of south-eastern Australia. This study examined the implications of the spread of C. taxifolia on ichthyofauna in 2 estuaries in central New South Wales. Fish assemblages were compared among adjacent habitats of C. taxifolia and 2 seagrass species (Posidonia australis and Zostera capricorni). Fish were sampled using a small beam trawl to test for differences among habitats in (1) the species composition of the fish assemblages, (2) total abundance and species richness of fishes, and (3) abundances of major fish families. Fish assemblages separated into 3 significantly distinct groupings based on habitat. Total abundances of fishes were similar among habitats; however, species richness was lower in C. taxifolia. The fish assemblages in C. taxifolia were largely characterised by high abundances of gobiid fishes, similar to those in Z. capricorni, and few or no syngnathid and monacanthid species when compared to sea- grass fish assemblages. This suggests that if C. taxifolia competitively replaces native seagrass beds in the estuaries of New South Wales, the resulting change in habitat may also cause a change in fish assemblages. This could reduce the abundances of some protected and economically important fish species but may also increase abundances of other opportunistic fishes.

Temporal change in fish assemblages in the lower Goulburn River, south-eastern Australia: comment on Pollino et al. (2004)

A recent analysis of fish assemblages in the Goulburn River, south-eastern Australia, used fish survey data collected between 1970 and 2002 to examine spatial patterns in the compositions and relative abundances of fish assemblages in the catchment. Based on this analysis, it was concluded that the native fish fauna of the Goulburn River is in a stressed condition and that introduced species dominate the fish assemblages. Fish survey data collected in 1982–1983 are compared with data collected in 2003–2004 to examine whether fish assemblages in the lower Goulburn River were stable at the temporal scale at which the data were aggregated in the previous analysis. The results show significant differences in fish assemblage composition between the two surveys, suggesting that fish assemblage structure in the lower Goulburn River did not remain stable between 1982–1983 and 2003–2004. The aggregation of data collected over several decades has the potential to confound analysis of spatial variations in fish assemblages and is unlikely to provide a reliable means of assessing their current condition.

Reply to the comment by Crook and Koster (2006) 'Temporal change in fish assemblages in the lower Goulburn River, south-eastern Australia'

In an earlier paper, multivariate statistics were used on historic fisheries data sets to show spatial differences between fish communities in the Goulburn Catchment (Victoria, Australia). In this reply paper, some further statistical analyses are presented to provide evidence for the assertion that fish communities at each site were stable over the temporal scale of the data was valid. Indeed, evidence for the stabilisation and persistence of fish communities after a major disturbance has also been recognised in other studies. Furthermore, in making judgements about the stability and persistence of fish communities, the scale of a study is an important factor, with patterns at the catchment scale often not being the same as those at smaller spatial scales.

Changes in benthic fish assemblages as a consequence of coastal works in a coastal lagoon: The Mar Menor (Spain, Western Mediterranean)

The benthic fish assemblage of the Mar Menor consisted of 37 species. Dominant species are: Gobius cobitis, Lipophrys pavo and Tripterygion tripteronotus on infralittoral rocks; Pomatoschistus marmoratus, Callionymus pussillus, Callionymus risso and Solea vulgaris on sandy bottoms and Gobius niger, Syngnathus abaster, Hippocampus ramulosus and Symphodus cinereus on Cymodocea nodosa– Caulerpa prolifera mixed beds. From 1985 to 1989 tourist development has led to the creation of new beaches and the installation of artificial rocky structures for retaining sediments. Dredging for the extraction of sand and subsequent pumping altered sediment characteristics causing a real stress leading to the substitution of typical sandy bottoms communities with Cymodocea nodosa by Caulerpa prolifera communities on mud. Soft bottom fish assemblages responded to changes in vegetation cover and substratum characteristics mainly changing the species composition, while artificial hard substrata contain a similar fish community than natural ones, harbouring even richer and more diverse assemblages. This positive effect of breakwaters should not obscure their likely negative effects on hydrodynamics and the subsequent changes of sediment quality and vegetation cover on the breakwaters’ area of influence.

Microhabitat-dependent changes in fish assemblages of Hungarian streams and creeks

Click to increase image sizeClick to decrease image sizeKey words: small watercoursesfish stockshuman impact

Interactive Effects of Environmental Variability and Military Training on Stream Biota of Three Headwater Drainages in Western Louisiana

AbstractWe collected fishes and macroinvertebrates seasonally from eight headwater streams in three different drainage basins (the Red, Calcasieu, and Sabine rivers) crossing Peason Ridge Training Area in west‐central Louisiana. Peason Ridge is part of the Fort Polk military training facility. We used multivariate analyses to test the effects of physical variables (i.e., current velocity, depth, and substrate), time (seasonal variability), drainage basin, and military training activities on assemblage structure. Tributaries of the Red River had the highest gradient and were predominated by shallow, swift‐flowing runs with sandy substrates. Southern tributaries of the Calcasieu and Sabine rivers were of lower stream gradient and contained deep, sluggish runs and pools with large amounts of woody debris and silt. Fish assemblages were structured primarily by military training and drainage basin. Faunal differences among drainage basins probably reflect differences in stream gradient. Fish assemblages in tributaries of the Sabine River were less diverse and showed evidence of historical degradation. Macroinvertebrate assemblages also were structured primarily by military training but had a strong seasonal component related to their short life cycles and high seasonal turnover. In contrast to fishes, macroinvertebrate assemblages were similar among drainages. Because military training was most intense in the Red River basin, its significance in these models probably represents differences in stream gradient among the basins because we found no negative effect on stream biota resulting from current training activities. Rather, historical logging and the resultant siltation coupled with isolation of populations by reservoirs are probable explanations for the changes in fish assemblages in tributaries of the Sabine River.

Long-Term Fish Assemblage Dynamics of the Alvarado Lagoon Estuary, Veracruz, Mexico

The fish assemblages of Alvarado Lagoon Estuary (a complex of coastal lagoons in the state of Veracruz, Mexico) have been surveyed intermittently by different researchers over the last 40 years. Assessing longterm trends in fish assemblage composition for this ecosystem is problematic due to differences in sampling efforts among the survey periods (1966–1968, 1987–1988, 1989, 1989–1990, 1990–1991 and 2000–2001) and by the inherent ecological variability of estuaries. To overcome these data limitations and better understand fish assemblage change over time, we used robust, simulation-based analyses to compare collections from the different surveys. The 107 fish species collected from the Alvarado Lagoon Estuary in these surveys represent 4 ecological guilds: marine stenohaline, marine euryhaline, estuarine, and freshwater fishes. The occurrence frequency of fish species representing each guild did not change significantly among the survey periods: the chi-square deviation statistic ( 2 = 8.53) was not significantly larger than the average value for 1000 simulated matrices ( 2 = 138.64; P = 1.00). A non-metric multidimensional scaling (MDS) based on Bray-Curtis similarities of fish species presenceabsence data showed that the 1966–1968 survey period was the least similar to the other survey periods. For the 1966–1968 survey, the range of Bray-Curtis inter-survey similarities was 40.4–58.6 (n = 5). By comparison, the remaining range of inter-survey similarities was 61.5–81.7 (n = 10). Average taxonomic distinctness (Δ+) and variation in taxonomic distinctness (Λ+), two sample size-independent measures of diversity, were calculated for all survey periods. Although Δ+ and Λ+ for all survey periods were within the simulated 95% confidence limits for expected values, these values for the 2000–2001 survey period were less than the average Δ+ and Λ+ values for the entire species pool. This suggests that the fish assemblage collected during the latest survey reflects a loss of both widespread higher taxa (reduced Δ+) and that the higher taxa lost are those with only a few representative species in the assemblage (reduced Λ+). These assemblage data show that fish assemblages of Alvarado Lagoon Estuary have not experienced significant changes over 40 years, but differences among the earliest (1966–1968), the latest (2000–2001), and the remaining survey periods indicate a recent decline in diversity.

Changes in algal, coral and fish assemblages along water quality gradients on the inshore Great Barrier Reef

Macroalgae, hard corals, octocorals, and fish were surveyed on 10 to 13 inshore coral reefs of the Great Barrier Reef, along a water quality gradient in two regions with contrasting agricultural land use. A water quality index was calculated for each reef based on available data of particulate and dissolved nutrients, chlorophyll and suspended solids. Strong gradients in ecological attributes occurred along the water quality gradient. Macroalgae of the divisions Rhodophyta and Chlorophyta increased with increasing nutrients, while Phaeophyta remained similar. Octocoral richness and abundances of many hard coral and octocoral taxa decreased, and none of the hundreds of species increased. At reefs in higher nutrient environments, hard coral and octocoral assemblages were composed of subsets of the many species found in lower nutrient environments, whereas fish and macroalgal assemblages consisted of contrasting suites of species. The study identifies species groups that are likely to increase or decrease in abundance with changing water quality.

Temporal Changes in Fish Assemblage from the Impingement Data at the Second Nuclear Power Plant,Northern Taiwan

The main purposes of this study are to find out the temporal fluctuation of marine fish community based on impingement data collected at the Second Nuclear Power Plant in recent years (September 2000 to May 2004), and to compare the recent fish community structure data to that of an earlier period, around 14 years ago (July 1987 to April 1990). Comparison of the data collected recently to that collected 14 years ago showed that there were no yearly or monthly differences. However, the community structures reflected by clustering dendrograms or MDS ordination plots were quite different between earlier and recent years. This difference is likely due to the drastic changes in the dominant species. Fourteen years ago, the most dominant species was mullet (Mugil cephalus), but now, it has changed to lower economic species like rabbitfishes (Siganus fuscescens) and spiny puffer (Diodon holocanthus). This phenomenon might reflect the declination in our fishery resources due to overfishing; a trend especially clear in those species with higher economic values.

Fish assemblage stability over fifty years in the Lake Pontchartrain estuary; Comparisons among habitats using canonical correspondence analysis

We assessed fish assemblage stability over the last half century in Lake Pontchartrain, an environmentally degraded oligohaline estuary in southeastern Louisiana. Because assemblage instability over time has been consistently associated with severe habitat degradation, we attempted to determine whether fish assemblages in demersal, nearshore, and pelagic habitats exhibited change that was unrelated to natural fluctuations in environmental variables (e.g., assemblage changes between wet and dry periods). Collection data from three gear types (trawl, beach seine, and gill nets) and monthly environmental data (salinity, temperature, and Secchi depth) were compared for four collecting periods: 1954 (dry period), 1978 (wet period), 1996–1998 (wet period), and 1998–2000 (dry period). Canonical correspondence analysis (CCA) revealed that although the three environmental variables were significantly associated with the distribution and abundance patterns of fish assemblages in all habitats (with the exception of Secchi depth for pelagic samples), most fish assemblage change occurred among sampling periods (i.e., along a temporal gradient unrelated to changing environmental variables). Assemblage instability was the most pronounced for fishes collected by trawls from demersal habitats. A marked lack of cyclicity in the trawl data CCA diagram indicated a shift away from a baseline demersal assemblage of 50 yr ago. Centroid positions for the five most collected species indicated that three benthic fishes, Atlantic croaker (Micropogonias undulatus), spot (Leiostomus xanthurus), and hardhead catfish (Arius felis), were more dominant in past demersal assemblages (1954 and 1978). A different situation was shown for planktivorous species collected by trawls with bay anchovy (Anchoa mitchilli) becoming more dominant in recent assemblage and Gulf menhaden (Brevoortia patromus) remaining equally represented in assemblages over time. Changes in fish assemblages from nearshore (beach seine) and pelagic (gill net) habitats were more closely related to environmental fluctuations, though the CCA for beach seine data also indicated a decrease in the dominance ofM. undulatus and an increase in the proportion ofA. mitchilli over time. The reduced assemblage role of benthic fishes and the marked assemblage change indicated by trawl data suggest that over the last half century demersal habitats in Lake Pontchartrain have been impacted more by multiple anthropogenic stressors than nearshore or pelagic habitats.