Benthic Food Resources Research Articles

Does invasion by armored catfish shift trophic ecology of native fishes? Evidence from stable isotope analysis.

Popular as aquarium fish, armored catfishes from South America (Pterygoplichthys spp.) have been introduced and become invasive in tropical and subtropical regions worldwide. These ecosystem engineers can deplete basal resources (e.g., periphyton and detritus), with potential negative effects for native fauna. We studied the trophic ecology of fishes in the Usumacinta River Basin, Guatemala, where Pterygoplichthys is now widespread and locally abundant. We analyzed stable isotopes (δ13 C, δ15 N) in fish tissues and basal resources to assess the potential impact of Pterygoplichthys on the trophic ecology of six co-occurring native fishes that feed at a similar trophic level (Astyanax aeneus, Dorosoma petenense, Thorichthys pasionis, Oscura heterospila, Poecilia mexicana, and Gambusia sexradiata). The study was conducted during the dry season in the La Pasion (LPR; high invasion) and San Pedro (SPR; low invasion) rivers. We compared isotopic spaces occupied by native fish and Pterygoplichthys, estimated isotopic overlap, and evaluated the trophic displacement of native species. We also evaluated the relationships of environmental factors, including the relative biomass of the invasive catfish, with δ13 C and δ15 N signatures. Except for P. mexicana, native species had lower isotopic overlap with the catfish in LPR. Native fish isotopic spaces were compressed and shifted toward higher trophic positions in LPR relative to SPR. Benthic food resources were important for Pterygoplichthys in both rivers, and water-column resources had greater relative importance (RI) for native species in LPR. Native fish δ13 C was significantly associated with Pterygoplichthys biomass, conductivity, and water flow velocity; and water depth and sedimentation had a significant association with native fish δ15 N. Findings provide evidence that invasive Pterygoplichthys, along with environmental factors, impact the trophic ecology of native fish in the Usumacinta Basin. Additional field research conducted over longer time periods and mesocosm experiments that account for fish assemblage and environmental variation could elucidate Pterygoplichthys impacts via food resource depletion or habitat alteration.

Habitat utilization, demography, and behavioral observations of the squat lobster, Eumunida picta (Crustacea: Anomura: Eumunididae), on western North Atlantic deep-water coral habitats

Deep-sea coral habitats, comprising mostly Lophelia pertusa (Linnaeus 1758), are well developed on the upper and middle continental slope off the southeastern United States (SEUS). These habitats support a diverse and abundant invertebrate fauna, yet ecology and biology of most of these species are poorly known. Ten cruises conducted off the SEUS (Summer–Fall; Cape Lookout, NC–Cape Canaveral, FL) from 2000 to 2005, and in 2009 provided an opportunity to investigate abundance and distribution of Eumunida picta Smith1883, a large-sized species of squat lobster commonly associated with these deep-water coral habitats. Video analysis from 70 manned-submersible dives documented occurrence, density, location on the coral colony, and behavioral observations for 5774 individuals of E. picta. Individuals collected (n = 178) from coral and adjacent habitats (e.g., rubble, soft sediments) were measured and their sex determined. Males and females were comparable in size (to 53.5 mm carapace length) and exhibited a sex ratio of approximately 1:1. Eumunida picta were most frequently observed as solitary individuals on high-profile coral matrix and were noted only infrequently on coral rubble, or rarely on soft substratum. Presence of coral habitat (i.e., live/dead L. pertusa), geographic region within the sampling area, and depth significantly influenced abundances of E. picta. Additionally, coral habitat (dead versus live coral), vertical position on the coral (upper, middle, or lower zone), as well as horizontal position in relation to the coral matrix (outer surface versus embedded in coral matrix) were significant factors influencing E. picta distributions within the coral habitat. More individuals were found on dead versus live coral, and most frequently occurred on the outer surfaces of coral branches located on the upper portion or near the tops of coral colonies. Eumunida picta were most often observed with claws extended into the water column. This unique hunting stance provides this squat lobster the opportunity to capture prey from the water column. An active predator, this species utilizes both pelagic (i.e., fishes, pyrosomes) and benthic (e.g., scavenging and grazing) food resources, and may function as an important trophic link between the water column and the benthos. Although considered a facultative reef associate in the strictest sense of the term, E. picta has a complex and intimate relationship with L. pertusa. Based on observations from dive videos, E. picta is a dominant and ecologically important member of the invertebrate assemblage associated with deep-sea coral habitats off the SEUS. As such, this species figures prominently in the structure and function of this ecosystem.

The relative importance of suspended versus benthic food resources to freshwater mussels in central Texas, U.S.A.

Abstract As unionid mussels continue to decline in North America much remains unknown regarding their primary food resources and feeding relationships. Our objective was to investigate spatial, temporal, intra‐ and interspecific variation in food resource use. In this study, we quantified stable C (δ13C) and stable N (δ15N) isotopic signatures for five mussel species and their potential food resources across four catchments and three seasons in central Texas, U.S.A. Across all species and sites, Bayesian mixing models suggest C derived from coarse particulate organic matter (CPOM; bulk detrital leaf packs) contributed on average 51% to the mussel diet and C derived from suspended particulate organic matter contributed 41% on average to the mussel diet. Mussel stable isotope signatures exhibited minimal variation across species within a site, but significant differences across sites and seasons. Although significant differences in food source contributions were observed between sites and species, differences were relatively small and either CPOM or suspended particulate organic matter were consistently the primary food resource. Mussels were potentially consuming microbial biofilms associated with CPOM pools, but preferential assimilation of detrital biofilms remains to be quantified. Carbon:nitrogen ratios of mussel soft tissue varied seasonally with summer ratios approximately 3 times higher on average than spring and autumn ratios, which is suggestive of poor food quality and thermal stress during the summer. Some species had significant positive relationships between isotopic ratios and shell length, which indicates changes in food resource incorporation with increasing mussel size. Together these data suggest the C in mussel tissues was of benthic and suspended origin. These findings provide further evidence that mussels effectively exploit benthic in addition to planktonic food resources which emphasises the need to maintain riparian habitats and allochthonous inputs.

The freshwater mysid Mysis diluviana (Audzijonyte & Väinölä, 2005) (Mysida: Mysidae) consumes detritus in the presence of Daphnia (Cladocera: Daphniidae)

AbstractFreshwater mysids of the Mysis relicta group are omnivorous macroinvertebrates that form an important link between fishes and lower trophic levels in many north temperate to Arctic lakes, where they exhibit diel vertical migration (DVM) to exploit subsurface food-rich layers at night. Benthic food resources have been assumed to be less important for mysid diets than pelagic zooplankton. Studies have nevertheless indicated that mysids consume benthic sedimented detritus, calling this assumption into question. We conducted a food-choice experiment to evaluate the feeding preferences of Mysis diluviana (Audzijonyte & Väinölä, 2005) by presenting field-caught specimens in individual foraging arenas with multiple choices of food. Experimental food treatments included a preferred pelagic prey (Daphnia), a presumed less desirable benthic resource (detritus), and a combination of both. We hypothesized that M. diluviana 1) prefers Daphnia over detritus and consumes only Daphnia in combination treatments, and 2) would not consume detritus except when detritus was the only food source available. Contrary to our hypothesis, M. diluviana readily consumed detritus in the presence of Daphnia. Mysis diluviana unexpectedly consumed more individuals of Daphnia in the presence rather than in the absence of detritus. Our results demonstrate that mysids take advantage of benthic food resources even in the presence of a presumably preferred zooplankton prey, calling to question the long-held assumption that benthic resources are unimportant when considering the trophic role of freshwater mysids of the M. relicta group.

Debris ingestion and nutritional niches in estuarine and reef green turtles

Little attention has been drawn toward the effects of marine debris ingestion in relation to nutrient acquisition and fitness consequences. We tested whether anthropogenic debris ingestion influence the nutritional niches of endangered green turtles (Chelonia mydas) in estuarine and reef habitats on the Brazilian coast. Our results showed that estuarine turtles consumed diets with lower proportional wet mass composition of protein (P) and water (W) than their reef conspecifics. The amounts of debris, mostly plastics, retrieved from the digestive tracts of estuarine turtles were higher compared with those individuals from reefs. The realized nutritional niche from estuarine turtles was subject to the debris density in the environment, lack of benthic food resources available and the surface foraging behavior, likely preventing them from reaching their nutritional goals and resulting in lower fitness. The study provides critical information for the management and conservation of ecologically threatened individuals, populations, and their natural habitats.

Bottom-up drivers of global patterns of demersal, forage, and pelagic fishes

Large-scale spatial heterogeneity in fisheries production is predominantly controlled by the availability of zooplankton and benthic organisms, which have a complex relationship with primary production. To investigate how cross-ecosystem differences in these drivers determine fish assemblages and productivity, we constructed a spatially explicit mechanistic model of three fish functional types: forage, large pelagic, and demersal fishes. The model is based on allometric scaling principles, includes basic life cycle transitions, and has trophic interactions between the fishes and with their pelagic and benthic food resources. The model was applied to the global ocean, with plankton food web estimates and ocean conditions from a high-resolution earth system model. Further, a simple representation of fishing was included, and led to moderate matches with total, large pelagic, and demersal catches, including re-creation of observed variations in fish catch spanning two orders of magnitude. Our results highlight several ecologically meaningful model sensitivities. First, coexistence between forage and large pelagic fish in productive regions occurred when forage fish survival is promoted via both favorable metabolic allometry and enhanced predator avoidance in adult forage fish. Second, the prominence of demersal fish is highly sensitive to the efficiency of energy transfer to benthic invertebrates. Third, the latitudinal distribution of the total catch is modulated by the temperature dependence of metabolic rates, with increased sensitivity pushing fish biomass toward the poles. Fourth, forage fish biomass is suppressed by strong top-down controls on temperate and subpolar shelves, where mixed assemblages of large pelagic and demersal fishes exerted high predation rates. Last, spatial differences in the dominance of large pelagics vs. demersals is strongly related to the ratio of pelagic zooplankton production to benthic production. We discuss the potential linkages between model misfits and unresolved processes including movement, spawning phenology, seabird and marine mammal predators, and socioeconomically driven fishing pressure, which are identified as priorities for future model development. Ultimately, the model and analyses herein are intended as a baseline for a robust, mechanistic tool to understand, quantify, and predict global fish biomass and yield, now and in a future dominated by climate change and improved fishing technology.

Dietary composition of black drum Pogonias cromis in a hypersaline estuary reflects water quality and prey availability.

The Baffin Bay estuary is a hypersaline system in the Gulf of Mexico that supports an important recreational and commercial fishery for black drum Pogonias cromis, a benthic predator. Seasonal measurements of water quality variables, benthic macrofauna densities and biomass, and determination of P. cromis food sources using stomach-content and stable-isotope analyses were carried out to determine how P. cromis food sources change with water quality and how this may affect P. cromis diet. Gut-content analysis indicated P. cromis selectively consumed bivalves Mulinia lateralis and Anomalocardia auberiana. Isotope compositions demonstrated that P. cromis relied on these benthic food resources produced in the Baffin Bay estuary year-round. Biomass and densities of these bivalves were influenced by changes in water quality variables, particularly salinity and dissolved oxygen. Thus, this paper demonstrates the relationship between water quality variables, benthic macrofauna, and their use as food resources by a carnivorous fish species, and emphasizes the need for integrated assessments when studying the effects of water quality on ecosystem function. Holistic approaches such as these can provide important information for management and conservation of fishery resources and can improve predictions of ecosystem response to climate variability.

Factors affecting trophic compositions of offshore benthic invertebrates at a sub‐Antarctic archipelago

AbstractFood availability is a key regulator of the distribution, metabolism, and success of benthic populations. In deep sea ecosystems, hydrodynamics and depth play fundamental roles in determining benthic food resources. Recent studies suggest that the Southern Ocean sub‐Antarctic front is shifting southward, with implications for primary production and food availability around the sub‐Antarctic Islands embedded in the Antarctic Circumpolar Current. We used fatty acid (FA) and stable isotope (SI) analyses to investigate the trophic pattern of benthic invertebrates and suspended particulate matter (SPM) at three Depths (shallow: 100 m, middle: 300 m, and deep: 600 m) in three hydrographic regions with different flow and productivity regimes around the Prince Edward Islands. Both region and depth affected the SI values of SPM, while feeding guild was the key factor influencing consumer trophic values. Depth affected the δ15N of all trophic groups and the FA compositions of suspension feeders. Deeper samples had a higher δ15N and showed a greater proportion of mono‐ and saturated fatty acids, reflecting greater remineralization of SPM and of food reaching the seafloor. Region affected the δ13C and FA values of SPM, suspension feeders and deposit feeder/scavengers, with differences between the interisland and open ocean regions. This was probably linked to the retention of nutrients and phytoplankton between the islands. Critically, the effects of depth and hydrographic region were taxon specific, indicating that long‐term responses to environmental change may have complex consequences for the feeding ecology and viability of benthic populations, with implications for the higher trophic levels that these populations support.

Benthic microalgae serve as the major food resource for porcelain crabs (Petrolisthes spp.) in oyster reefs: Digestive track content and pigment evidence

Suspension-feeding porcelain crabs (Petrolisthes spp.) are often the most abundant decapod crustaceans in oyster reef habitat. Analysis of water column and subtidal algal biomass from three Texas estuaries suggests that planktonic food resources are insufficient for porcelain crab growth. Pigment composition of porcelain crab muscle and digestive track contents included the diatom pigment fucoxanthin and cyanobacterial pigment canthaxanthin with digestive track samples containing attached (adnate) benthic diatoms as well as benthic cyanobacteria not found in the water column. Feeding appendages on porcelain crabs include numerous cirri with serrated edges as well as fewer more brush-like longer units. Benthic food resources are in sufficient supply to support porcelain crab biomass.

Behavioral responses of juvenile golden gray mullet Liza aurata to changes in coastal temperatures and consequences for benthic food resources

Temperature is an important factor for fish. Yet, little is known about temperature effects on the feeding behavior of fish and the subsequent consequences of these behavioral changes on the spatial distribution of resources. We analyzed the differences in the feeding behavior of two size classes of juvenile Liza aurata at two water temperatures (i.e. 10°C and 20°C), using laboratory mesocosms. We also examined whether potential temperature-induced changes in feeding behavior of the smaller size of L. aurata would affect the spatial distribution of the microphytobenthos (MPB) biomass, an important resource in coastal systems.Both the number of feeding events and the swimming velocity during feeding were higher at 20°C than at 10°C, independent of the fish size. The time spent feeding did not vary between 10°C and 20°C, while the distance covered during feeding was significantly smaller at 20°C than at 10°C. Grazing did not affect the mean MPB biomass, but did increase its spatial variance at the smaller scale (i.e. a few centimeters) at 20°C.A high number of feeding events, a high swimming velocity during feeding and a small distance covered during feeding in 20°C-acclimated L. aurata most likely represented an adaptation to an increase in metabolism, as well as to the need to reduce the energy costs of feeding at 20°C. Results also indicated that changes in feeding behavior of the 20°C-acclimated L. aurata were responsible for the increase in small-scale spatial variability in the MPB biomass but not an overall significant effect on the MPB mean. We suggested that the enhanced spatial patchiness due to grazing by fish at 20°C might yield a local increase in the mean MPB biomass, probably increasing photosynthetic efficiency of cells and algal growth that counterbalance the negative effect of algal removal by fish.

Fifty years later: trophic ecology and niche overlap of a native and non-indigenous fish species in the western basin of Lake Erie

Since the introduction of white perch (Morone americana) into Lake Erie over 50 years ago, the population size of native yellow perch (Perca flavescens) has decreased up to 79 % and significant changes to the ecosystem have occurred. We examined long-term population estimates and used stable isotopes of carbon (δ 13C) and nitrogen (δ 15N) paired with stomach content analysis to quantify the trophic ecology and niche overlap of adult yellow perch and white perch in the western basin of Lake Erie. We found that changes in yellow perch abundance since 1979 appeared to be better correlated with changes in fishery exploitation rates than with food competition effects from white perch. At the time of this study, yellow perch were found to have higher δ 13C values, indicating greater utilization of benthic food resources than white perch, and white perch occupied higher trophic positions based on δ 15N. The diets of both species varied spatially and seasonally based on stable isotopes and stomach contents, likely driven by changes in prey abundance. Comparison of niche widths using stable isotope population metrics and Schoener diet similarity index suggested a low to moderate degree of niche overlap between species. Isotopic niches of white perch were generally larger than those of yellow perch demonstrating broader resource utilization by this non-indigenous species. We submit that isotopic niche overlap comparisons are more appropriate for studies seeking to understand interactions among populations over course temporal scales, while diet overlap indices, such as the Schoener index provide a means to study fine-scale interactions such as ontogenetic and seasonal diet shifts.

Selective extinction of Triassic marine reptiles during long-term sea-level changes illuminated by seawater strontium isotopes

The relationship between cycles of sea-level change and marine diversity has long been the subject of debate. Large predators may be particularly sensitive to changes in habitat availability and marine productivity driven by changes in sea-level, especially those dependent on nearshore benthic food resources. To test this relationship, we compared the proportional diversity of differing marine reptile ecotypes through the Triassic with the isotopic composition of seawater strontium (87Sr/86Sr), a geochemical index linked to tectonically controlled sea-level change. The proportional abundance of marine reptiles adapted toward a diet of shelled prey rose during times of rapid sea-level rise and fell during times of rapid sea-level fall, while open water forms were more resistant to these changes. Our results indicate that the rate of sea-level change, rather than the absolute magnitude of sea-level or flooded shelf area, played a role in shaping patterns of ecological diversification and ecologically selective extinction during the Triassic. The link between the isotopic composition of seawater strontium and the evolution of marine reptiles demonstrates that sea-level change played an important role in the structuring of marine ecosystems over geologic time.

Responses to the Foraging/Predation Risk Trade-Off and Individual Variability in Population-Level Fitness Correlates

Foraging under the influence of interspecific interactions such as competition and predation risk can have effects on the energetic reserves of the forager. Measurements of condition in species such as fish are usually correlated with individual fecundity and, hence, fitness. From work in two study systems in which predation risk regulates habitat selection and foraging behavior of benthic fishes I examined whether risk dependence led to reduced variability in fish condition. In field populations of cottid fishes, observed in an estuarine system and in the near-shore habitat of an oligotrophic lake, I found that individuals that experienced higher predation risk showed reduced variability in CI. Estuarine cottids with high food availability and substantial predation risk varied less in CI among individuals than in the associated tidal creek. In the lake, where there is considerable heterogeneity in benthic food resources, a related cottid species showed reduced variation in CI with increasing predation risk from adults. Finally, I examine my previous experiments showing that the estuarine species is limited in its use of high resource availability in estuaries by competition and predation risk. Here I found that variability in individual condition index (CI) was higher when intraspecific and interspecific competition increased and did not increase in the face of predation risk.

Temporal changes in the diet of deep-water Penaeoidean shrimp (<i>Parapenaeus longirostris</i> and <i>Aristeus antennatus</i>) off Algeria (southwestern Mediterranean)

With the aim of analyzing the trophic dynamics of deep-sea shrimp over the central Algerian slope, stomach contents of 219 individuals of Parapenaeus longirostris and 263 Aristeus antennatus were collected at night at depths between 200 and 400 m on a seasonal scale in 2007. Fullness ( f ), which is a measure of feeding intensity, showed significant seasonal changes in Aristeus antennatus , and the highest f was found in pre-reproductive periods (3.3% in January; 3.0% in April) before the beginning of gonad maturation (April-September in the area). This trend was coupled with the period of high surface productivity off the central Algerian coast (November-April). In that pre-reproductive period A. antennatus consumed more crustaceans (e.g. large decapods such as Processa sp.) with a high energetic value. These kinds of seasonal oscillations in f were not found in P. longirostris , probably because it was distributed shallower than A. antennatus and does not have a single marked peak in its reproductive cycle. The dietary composition of P. longirostris and A. antennatus was clearly segregated in the MDS analysis, which suggests there is good resource partitioning between the two species. Foraminiferans, bivalves and polychaetes were the most frequent prey in the diet of both P. longirostris and A. antennatus . Small micro and mesoplankton prey (radiolarians, tintinnids, copepods and pteropods) were more important in the diet of P. longirostris , while A. antennatus fed more on benthos. Planktonic prey found in the diet of the two shrimp were more frequent in January-February and April-May, e.g. in periods of higher water homogeneity coupled with periods of higher surface production. The large contribution of zooplankton to the diet of the deep-water shrimp over Algerian slopes is probably due to the importance of eddies in the area, which may enhance water column production locally. The high surface temperature (lowest T in January of 15.2°C) may accelerate the degradation of the organic matter that reaches Algerian slope bottoms, thus reducing the biomass of macrobenthos. We hypothesized that this low availability of benthic food resources may favour night time migratory movements by benthopelagic shrimp off the Algerian coast in search of more productive, shallow bottoms.

Perch production in forest lakes: the contribution of abiotic and biotic factors

Horppila J, Olin M, Vinni M, Estlander S, Nurminen L, Rask M, Ruuhijärvi J, Lehtonen H. Perch production in forest lakes: the contribution of abiotic and biotic factors. Ecology of Freshwater Fish 2010: 19: 257–266. © 2010 John Wiley & Sons A/SAbstract – The biomass, growth and production of perch (Perca fluviatilis (L.)) in five humic forest lakes in southern Finland were estimated and the abiotic and biotic factors contributing to the between‐lake differences were explored. The average perch biomass in the study lakes varied from 13.1 to 45.6 kg·ha−1 and annual production from 6.0 to 16.5 kg·ha−1. The highest biomass and production were found in Lake Majajärvi with highest water colour and nutrient concentration. Variability in fish density rather than in individual growth rate seemed to be the main factor behind the between‐lake differences in perch production. No dependence of perch production on water transparency was found, because transparency had a stronger effect on individual growth than on fish density. The length increment of perch in their second year increased significantly with water transparency but at older ages no such dependence was found. At age of 4 years, the growth rate of perch decreased with perch density, indicating intraspecific competition for benthic food resources. At age of 7 years, the growth rate increased with increasing roach (Rutilus rutilus (L.)) abundance, suggesting that roach served as a resource for piscivorous perch. The effects of predatory top‐down control on perch populations were probably obscured due to the variable size and water quality of the study lakes.

Assessing feeding competition between lake whitefish Coregonus clupeaformis and round whitefish Prosopium cylindraceum

Abstract We collected lake whitefish Coregonus clupeaformis and round whitefish Prosopium cylindraceum from the main basin of Lake Huron and Georgian Bay in the Laurentian Great Lakes of North America to investigate details of diet and feeding tactics of these species in different seasons. Lake whitefish supports important commercial fisheries in Lake Huron and both species make use of habitats near the Bruce Nuclear Power Development, on the eastern shore of Lake Huron. Most fish of both species showed generalist feeding behavior, but some lake whitefish appeared to show specialist prey selection. The invasive spiny water flea Bythotrephes longimanus was an important component of the diet of both species. There was considerable dietary overlap between the whitefish species, but the ecological implications of these dietary overlaps are mitigated by the fact that dominant prey species differed in most seasons. We conclude that the potential for ecologically significant interactions between lake whitefish and round whitefish resulting from competition for similar benthic food resources in the main basin of Lake Huron is probably low.

How climate warming impacts the distribution and abundance of two small flatfish species in the North Sea

Climate change, specifically temperature, affects the distribution and densities of species in marine and terrestrial ecosystems. Here, we looked at the effect of temperature during winter and spawning period on latitudinal range shifts and changes in abundance of two non-commercial North Sea fish species, solenette ( Buglossidium luteum) and scaldfish ( Arnoglossus laterna). Both species have increased in abundance and moved to the north since the late 1980s, coinciding with a series of mild winters. In 1996, following a very cold winter, the abundance of both species temporarily decreased as they retracted to the south. The shift in temperature affected adult habitat conditions, allowing them to immigrate into new areas where they subsequently reproduced successfully. We can conclude this because recruitment improved following the increase in abundance. The recruitment relates significantly to the higher adult stock and higher temperatures. The predictions of higher average temperatures and milder winters in the North Sea make it likely that these species will increase further in abundance and move northward. The observed increase in abundance of these small flatfish species will affect the North Sea food web and therefore commercial species, e.g. plaice, by predation on juveniles and competition for benthic food resources.

Fish assemblage of restored Al-Hawizeh marsh, Southern Iraq

Al-Hawizeh marsh is one of the largest wetlands in south Iraq. During last two decades, 65% of a permanent marsh was drained and it led to a substantial loss of native aquatic flora and fauna. The marsh was reflooded in April 2003. The characteristics of fish assemblage in this marsh were described after three years of the restoration. A total of 4715 fishes of 15 species were caught since October 2005 to September 2006, using different fishing gears. The fish species in the marsh were divided into resident, seasonal and occasional groups. Liza abu was the most abundant species comprising 37.1% of the total numbers followed by Barbus luteus (29.4%), Carassius auratus (15.3%), Alburnus mossulensis (4.88%) and Aspius vorax (4.14%). Fish species diversity index ranged from 0.88 to 2.1, richness from 0.73 to 2.42 and evenness from 0.49 to 0.85. The diet varied among the fish species, most of them depended on two or three major food items. Several cyprinid species disappeared from the restored marshes or decreased in their abundance. This could be related to reduced water supply and effectively eliminated the spring flood pulses that sustained wetland ecosystems in the lower Tigris-Euphrates basin. Increase of salinity, scarcity of benthic food resources and competition with alien/introduced species have also detrimental effects on native cyprinid fishes.

Reproductive biology and diet of the southern fiddler ray, Trygonorrhina fasciata (Batoidea:Rhinobatidae), an important trawl bycatch species

The reproductive cycle of Trygonorrhina fasciata females in south-western Australian waters is shown to include a protracted period of embryonic diapause or delayed development. This characteristic is typically regarded as enabling young to be born when conditions are optimal. The birth of T. fasciata occurs in April/May, when water temperatures are still high and conducive to rapid growth. Furthermore, because gut fullness in this and subsequent months was similar to other times of the year, the prey of neonates, i.e. mysids, carids, pinnotherids, amphipods and isopods, were presumably abundant in that period. Ovulation occurs in April/May and embryos grow rapidly between December and birth in April/May. Females grew to greater lengths than males, i.e. 1460 v. 890 mm, and matured (L50) at larger lengths, i.e. 892 v. 678 mm respectively. As body size increased, teleosts, pilumnid and portunid crabs and molluscs became important dietary components. Overall, the diet consisted predominantly of crustaceans, fish, polychaetes and molluscs, with volumetric contributions of 73.4, 17.2, 5.3 and 2.8% respectively. Dietary composition underwent a cyclical seasonal change and differed among locations, indicating some degree of opportunistic feeding. Interspecific comparisons indicate that benthic food resources are partitioned among and within the main batoid species in south-western Australian waters.

Fishes and freshwater in southern African estuaries – A review

The functioning of estuaries relies on a natural dynamism imposed on these systems by riverine and marine influences. The increasing abstraction of fresh water from both large and small river catchments in southern Africa has had the effect of forcing some estuaries into artificial cycles, i.e. natural successions now have human imposed trajectories that are changing estuarine variability and forcing some systems into extreme states. The ichthyofauna has responded to these changes in a variety of ways. Where river flow has declined considerably, or ceased altogether for extended periods, fish recruitment has shown a considerable decrease. This can be related to the collapse in planktonic productivity which negatively affects zooplanktivorous fishes, as well as decreased amounts of olfactory cues entering the sea for the attraction of larval and juvenile marine fishes into these estuaries. Hypersaline conditions can result in both a reduced species diversity and abundance. However, where estuaries lose their normal estuarine salinity gradient and become “arms” of the sea, there is often an increase in fish species diversity due to stenohaline marine taxa entering the estuary. Unfortunately the gain in small numbers of marine stragglers is insufficient to compensate for the decline in estuarine dependent fishes that usually dominate these systems. Conversely, major river flooding often causes temporary decreases in both species diversity and abundance due to a rapid decline in salinity, increased suspended sediments, reduced dissolved oxygen levels, and a collapse in the availability of pelagic and benthic food resources. However, the “resetting” of estuaries by episodic events is part of the essential cycle that maintains and enhances estuarine productivity and habitat diversity. Recovery by estuary associated fishes from such events is usually rapid and linked to a variety of factors, especially estuary morphometry which has a direct influence on the flushing or retention of estuarine biota. Freshwater flows interact directly and indirectly with the fishes that inhabit estuaries, e.g. river floods directly influence estuarine morphometry, water temperature, salinity, pH, turbidity, nutrient status, organic inputs, dissolved oxygen concentrations and olfactory cues; and indirectly affect mouth status, tidal prism, habitat diversity, primary and secondary productivity, fish recruitment, food availability and competition. Depletion or removal of components of river flow to an estuary have major short and long-term negative impacts on the ichthyofauna, some of which can be ameliorated by the provision of an environmental freshwater allocation that is appropriate to that particular system.