Structure,development and function of the branchial sieve of the common bream,Abramis brama,white bream,Blicca bjoerkna and roach,Rutilus rutilus

Synopsis The filter feeding organ of cyprinid fishes is the branchial sieve, which consists of a mesh formed by gill rakers and tiny channels on the gill arches. In order to establish its possible role during growth we measured the following morphological gill raker parameters over a range of sizes in three cyprinid fishes, bream, white bream and roach: inter raker distance, bony raker length, raker width, cushion length and channel width. At any given standard length common bream has the largest inter raker distance, roach the lowest and white bream is intermediate. In the comb model of filter feeding the inter raker distance is considered to be a direct measure of the mesh size and retention ability (= minimal size of prey that can be retained) of a filter. For the three species under study there is a conflict between the comb model and experimental data on particle retention. Lammens et al. (1987) found that common bream has a large retention ability whereas roach and white bream have a much smaller one. A new model, the channel model (Hoogenboezem et al. 1991) has been developed for common bream; in this model the lateral gill rakers can regulate the mesh size of the medial channels on the other side of the gill slit. The present data indicate that this model is not appropriate for white bream and roach. At any given standard length white bream and roach only reach 70% of the raker length of common bream, which means that in this model the gill slits should to be very narrow during filter feeding. The gill rakers consist of a bony raker and a fleshy cushion. The bony rakers have a rather long needle-like part outside the cushion in bream, but not in white bream and roach which have blunt gill rakers. Blunt gill rakers are not suited to reduce the diameter of the medial channels. The comb model seems more appropriate for white bream and roach, but doubts about the validity of this simple model remain. The sum of the areas of the medial channels is an approximation of the area through which water flows in the filter. This channel area therefore gives an impression of the capacity or flow rate of the filter. With this capacity estimation and an estimation of energy consumption we calculated an energy ratio of filter feeding. The energy ratio decreases with increasing standard length with an exponent close to the expected exponent of -0.40. The energy ratio is highest in bream, intermediate in white bream and lowest in roach.     

Causes and consequences of the success of bream in Dutch eutrophic lakes

In the last decennia eutrophication has caused a shift in the species composition of fish communities in Dutch fresh waters. The changes have led to the disappearance of vegetation in lakes and ponds; zooplankton and chironomids are now the most abundant food organisms for fish. In the turbid, open waters bream and pikeperch are the dominant fish species. Only small bream is vulnerable to predation, but because bream grows much faster than the other cyprinids the time span in which the fish is vulnerable is the shortest. The large bream (>20 cm) can coexist with pikeperch since it is not vulnerable to predation and still utilizes the food organisms efficiently. Eutrophication is accelerated if both bream populations are composed of small-sized specimens preventing large-sized zooplankton to develop, and if they are composed of large-sized individuals which can efficiently stir up the bottom sediments.     

Ecological consequences of a manual reduction of roach and bream in a eutrophic,temperate lake

Hydrobiologia - A biomanipulation experiment was carried out in the eutrophic lake, Frederiksborg Slotssø (Denmark). During 1987 and 1988, densities of roach (Rutilus rutilus) and bream...     

Reproductive biology and recruitment of the white sea bream in the Azores

The life history of the white sea bream Diplodus sargus in the Azores showed a pattern consistent with digynic hermaphroditism achieving sexual maturity during the second year of life, at 16·7 cm L _T. Spawning occurred from March to June at temperatures between 15 and 17° C and the onset and duration of spawning season in the sea bream appeared to be influenced by sea water temperatures. As latitude decreased, both in the northern and southern hemispheres, the spawning season of D. sargus populations started earlier and extended longer, highlighting the potential importance of temperature to the onset and duration of reproduction in this species. Settlement took place from late May to July, and settlers remained in the nursery area for c . 2·5 months. Emigration from the nursery area to join shoals of juveniles occurred from late July to September.     

Edge effects in patchy seagrass landscapes: The role of predation in determining fish distribution

Predation is often described as an underlying mechanism to explain edge effects. We assessed the importance of predation in determining edge effects in seagrass using two approaches: a video survey to sample predators at small scales across seagrass edges, and a tethering experiment to determine if predation was an underlying mechanism causing edge effects. Underwater videos were placed at four positions: middle of seagrass patches; edge of seagrass; sand immediately adjacent to seagrass and sand distant from seagrass. Fish abundances and the time fish spent in view were measured. The main predatory fish (Australian salmon, Arripis spp.) spent more time over adjacent sand than other positions, while potential prey species (King George whiting, Sillaginodes punctata (Cuvier), recruits) were more common in the middle of seagrass patches. Other species, including the smooth toadfish, Tetractenos glaber (Freminville), and King George whiting adults, spent more time over sand adjacent to seagrass than distant sand, which may be related to feeding opportunities. King George whiting recruits and pipefish (Stigmatopora spp.) were tethered at each of the four positions. More whiting recruits were preyed upon at outer than inner seagrass patches, and survival time was greater in the middle of shallow seagrass patches than other positions. Relatively few pipefish were preyed upon, but of those that were, survival time was lower over sand adjacent to seagrass than at the seagrass edge or middle. Video footage revealed that salmon were the dominant predators of both tethered King George whiting recruits and pipefish. The distribution of predators and associated rate of predation can explain edge effects for some species (King George whiting) but other mechanisms, or combinations of mechanisms, are determining edge effects for other species (pipefish).     

A note on the growth-rate and year-class strength of bream, Abramis brama (L.), in three eutrophic lakes, England

In this study some comparative observations on the growth-rate and year-class strength of bream, Abramis brama , in three Shropshire-Cheshire meres are given. All populations are exposed to variable recruitment. The occurrence of four strong year-classes (1959, 1966, 1969,1973) suggests that climate is an important factor influencing spawning success. Water temperatures ≥16°C in July and August may be particularly important. All other intervening year-classes are absent. The growth-rate of bream in Tatton Mere and Ellesmere Mere is good ( L ∞= 541–561 mm) but comparatively poor ( L ∞= 448 mm) in Cole Mere. The significance of population density, food supply and parasitic infestation by Ligula intestinalis is discussed.     

The restoration of shallow eutrophic lakes,and the role of northern pike,aquatic vegetation and nutrient concentration

The feasibility of biomanipulation is related particularly to reducing the production and recruitment of planktivorous fish stocks. For assessing the level of predation needed to suppress planktivorous fish stocks, the relation between fish P and B, on one hand, and nutrient concentration on the other were analyzed. The carrying capacity of shallow lakes in terms of biomass is related to the total phosphorus concentration and nature of the lake bottom substrate. The production of planktivorous fish was 60–80% of the maximum carrying capacity. It is argued that aquatic vegetation and northern pike are effective tools to maintain water quality, but these are limited by the maximum nutrient concentration aquatic vegetation can sustain.     

The role of plankton, zoobenthos, and sediment in organic matter degradation in oligotrophic and eutrophic mountain lakes

Intensity of organic matter degradation, assessed by the respiratory electron transport system (ETS) activity, was studied in microplankton, zooplankton, chironomid larvae as the dominant group of the macrobenthos, and sediment in mountain lakes of different trophic levels in summer months. The highest ETS activities per unit of surface were observed in sediments. Significantly lower activities were observed in microplankton, and lower still in zooplankton, and chironomids. The total ETS activity m^–2 was higher in eutrophic lakes (Jezero na Planini pri Jezeru and Krnsko jezero) than in oligotrophic ones (Zgornje Kriko jezero, Spodnje Kriko jezero, Jezero v Ledvicah). The contributions of communities investigated to total ETS activity m^–2 differed between lakes of different trophic level. Estimation of respiratory carbon loss through different components revealed that the most of the organic matter was oxidized in sediments of mountain lakes. The respiratory carbon losses were higher through zooplankton than through microplankton in all lakes. Carbon losses through plankton components and sediments were significantly lower in oligotrophic than in eutrophic lakes. The contribution of respiratory carbon loss through chironomids to total carbon loss m^–2 was higher in oligotrophic than in eutrophic lakes. Therefore, it seems that contributions of microplankton and zooplankton to mineralization processes increase, and contributions of chironomids and sediment surface decrease with increasing trophic level of the lakes.     

Mating patterns of first-generation hybrids of the roach, <Emphasis Type="Italic">Rutilus rutilus</Emphasis> (L.), and the silver bream, <Emphasis Type="Italic">Blicca bjoerkna</Emphasis> (L.)

Mating patterns among first-generation hybrids of the roach, Rutilus rutilus, and the silver bream, Blicca bjoerkna, between hybrids or with parent-species males, have been investigated under experimental reproduction conditions. The results reveal that the level of sexual activity of these hybrids is high. Hybrid females mated simultaneously and successively with all types of male, resulting in fertilised eggs. Individual participation of roach males in mating was never observed, however.     

The role of pre- and post-dispersal seed predation in determining total seed loss

Most seed predation studies focus on either pre- or post-dispersal predation and may therefore underestimate the role of predation in regulating plant populations. We therefore estimated total seed predation of an invasive tree, mesquite (Leguminoseae: Prosopis spp.), by examining the entire seed pool from tree to seed bank. The spatio-temporal dynamics of total seed predation was examined by sampling across its Australian distribution and through time. The main predator was a host-specialist multivoltine beetle, Algarobius prosopis L. (Bruchidae), previously introduced as a biocontrol agent. Seed predation exceeded 20% in all seed stages (in pods on and off the tree, and seeds within woody endocarps (capsules) and free seeds on and in the ground) but was consistently highest in capsules on the ground (up to 90%). Pre-dispersal predation contributed little. Total seed predation rates were primarily determined by predation rates on the most persistent seed stage, in this case fallen pods if only pods are considered and seeds in capsules for the total seed pool. This pattern was consistent across the surveyed taxa, regions, years and seasonally. Predation rate was relatively unaffected by seed density, potentially because densities were always low (<150 seeds m⁻²). Average total seed predation within a region reached 55%, but we conclude that any population regulation of mesquite by seed predation will principally be through reduced seed bank persistence. Our results highlight the need to consider the entire seed pool, especially the often cryptic and overlooked long-lived stages, when determining seed loss to predation and its likely population consequences.     

Seasonal and size-related changes in the food of the short-finned eel,Anguilla australis in Lake Ellesmere,Canterbury, New Zealand

Synopsis An approximately monthly sampling programme in Lake Ellesmere, Canterbury, New Zealand, from January 1974 to April 1976 yielded 487 eels. The stomachs were fixed in 10% neutralised formalin and the contents examined. Preliminary analysis indicated that the mollusc Potamopyrgus antipodarum, the isopod Austridotea annectens, the mysid Tenagomysis chiltoni, the amphipod Paracalliope fluviatilis, the midge larva Chironomus zealandicus and the teleosts Retropinna retropinna, Galaxias maculatus and Gobiomorphus cotidianus together made up the bulk of the diet. The pre-ingested dry weight (i.e. the reconstructed weight) of the most important of these prey species was obtained by relating the length of a digestion resistant part to actual dry weight in field collected specimens. Regression equations for this relationship in each season enabled the reconstructed dry weight of each stomach item to be calculated. In some instances reconstructed weight was less than the actual digested dry weight of the prey specimen. In every case the larger value was used. This method is referred to as Combination Dry Weight (CDW) and is believed to be new. These data, used in conjunction with the energy content of the species concerned, enabled the caloric dietary contribution of each prey species to be determined. Comparison of relative contribution to eel diet between CDW and energy values calculated from CDW and bomb calorimetry revealed large differences. Marked variations in diet between ⩽40 cm, 40.1–50 cm, and>50.1 cm size classes were also shown. Eels ≤40 cm feed primarily on invertebrates and become progressively more piscivorous as they grow. Eels >50.1 cm are almost entirely piscivorous. Seasonal differences in diet also exist within each size class examined.     

Specializations and limitations in the utilization of food resources by the carp,Cyprinus carpio: a study of oral food processing

Synopsis The wide variety of aquatic food is considered to be instrumental for the diversification in fish species. Yet their abilities and inabilities of handling food are poorly known. For these reasons the food processing and feeding repertoire of the adult carp, Cyprinus carpio, fed on a variety of food types, were analyzed by light and X-ray cinematography of the head parts and by electromyography of the head and body muscles during feeding. Nine stereotyped movement patterns (particulate intake, gulping, rinsing, spitting, selective retention of food, transport, crushing, grinding and deglutition) compose the feeding process, their sequence and frequency were adjusted to the type of food. Following quantitative morphological analysis at macroscopic, light- and electronmicroscopical level, the relations between the functioning and architecture of the feeding apparatus were established. The structure and dimensions of the mouth opening, the protrusible upper jaw, the slit-shaped pharyngeal cavity, the palatal and postlingual organ, the branchial sieve, the pharyngeal masticatory apparatus and the distribution of taste buds, mucous cells and muscle fibers along the oropharyngeal surface were the directive structural characters used for estimating the abilities in food processing. The specializations for utilizing food items and its limitations, derived from structural and functional data, are compared with diet data found in the literature in order to evaluate the relative position of the carp in competition for food in the aquatic environment. It is established that the ‘omnivorous’ carp is specialized in effective handling of several categories of aquatic food, even when these are mixed with non-food (bottom invertebrates <4% SL in diameter) since the palatal organ enables the carp to separate food from non-food. This includes very hard-skinned food items, processed with the powerful pharyngeal jaws of the fish, and to a lesser extent zooplankton (>250 μm). The carp is at the same time very limited in processing long and struggling prey (e.g. fish) as well as vegetable matter, due to the lack of oral teeth and the specialized morphology of its pharyngeal chewing apparatus. These feeding abilities agree with diet data from literature. The reported herbivorism of carp illustrates its opportunism in feeding behaviour. Specialization in feeding is discussed and the necessity to take into account the total series of post-capture feeding actions for a more complete view on trophic specialization. Food intake and the intra-oral food processing of carp are bound to the structures of its sensory, central processing and effector apparatus and to the plasticity in their functioning. These together determine its feeding efficiency in exploiting the available aquatic food resources. Next to ethological and ecological studies functional morphology is another important tool to explain the trophic interactions of fish.     

The role of phenolic compounds and nutrients in determining food preference in greater snow geese

Summary We tested Buchsbaum's hypothesis that food palatability in geese is determined by a hierarchy of feeding cues among which deterrent secondary metabolites (mostly phenols) have a primary role (Buchsbaum et al. 1984). In preference tests, greater snow goose feeding was slightly depressed when grass was sprayed with ferulic acid but not when grass was sprayed with p-coumaric and tannic acids. Extracts of Timothy grass, red clover or alfalfa sprayed on grass also failed to depress goose feeding. In a multifactor experiment, phenol and protein content and height of grass were manipulated simultaneously. When ferulic acid was sprayed, protein and phenol content interacted in determining goose feeding preferences; protein content had no effect in the absence of phenol but did have an effect when phenol was added. When tannic acid was used in a similar experiment, results were inconclusive because of a significant and complex interaction between protein content and height of grass. Our results generally failed to support Buchsbaum's hypothesis that phenol content of plants has a primary role in determining food preference in geese. Protein content of plants seemed to be a more important factor.     

The role of water abundance, thermoregulation, perceived predation risk and interference competition in water access by African herbivores

In African savannas, surface water can become limiting and an understanding of how animals address the trade-offs between different constraints to access this resource is needed. Here, we describe water access by ten African herbivore species in Hwange National Park, Zimbabwe, and we explore four possible determinants of the observed behaviours: water abundance, thermoregulation, perceived predation risk and interference competition. On average, herbivores were observed to drink in 80% of visits to a waterhole. The probability of drinking was higher in 2003 (474 mm) than in 2004 (770 mm), and at the end of the dry season than at its beginning. For larger species, this probability may also be related to risks of interference competition with elephants or other herbivores. For smaller species, this probability may also be related to the perceived risk of predation. We also investigate the time spent accessing water to drink. The influence of herd size and the presence of young on the time spent accessing water for most species suggests that perceived predation risk plays a role. Thermoregulation also affects this time: during the hottest periods, herbivores spend less time in open areas, unless when wind is strong, probably owing to evapotranspired heat loss.     

The role of growth,predation, and habitat selection in the population distribution of the crown conch,Melongena corona Gmelin

The intertidal gastropod Melongena corona Gmelin exhibits a size gradient along the shore as a function of habitat. Small, juvenile snails were found on the sand beach and larger adults were found on hard substratum habitats (shell rubble beach and oyster bar). A transplant experiment was performed to test three proximate explanations for this pattern:

• 1.(1) differential growth rate of conchs in different habitats,
• 2.(2) differences in predation intensity between habitats and
• 3.(3) active habitat selection by snails.

Shell scars on the last shell whorl were used as an index of predation pressure. Growth and predation were not significantly different for snails of similar size in different habitats, but snails were found to return to their original habitat when displaced. The ability to home did not differ between sexes or juveniles and adults. Active habitat selection appears to be a significant proximate factor maintaining the population distribution. A number of potential ultimate causes of the size class segregation are suggested.     

Aquatic arthropod communities in Nepenthes pitchers: the role of niche differentiation,aggregation, predation and competition in community organization

Summary The structure and organization of aquatic arthropod communities in Nepenthes ampullaria pitchers were studied at two sites (M in Malacca and K in Kuching) in Malaysia. The communities consisted mainly of aquatic dipteran larvae. Community M was dominated by a filter feeder, Tripteroides tenax, which reached a high density despite a strongly aggregated distribution. Community K had five trophic groups: carrion feeders, filter feeders, detritus feeders, nipping predators and hooking predators, each including multiple species. The summed density of filter feeders in Community K remained much below the level attained by filter feeders in Community M. Niche differentiation within each trophic group with regard to pitcher age and feeding behaviour was not sufficient to allow species coexistence through niche separation alone. Aggregated distributions directly reduced interspecific encounters. Nevertheless, species belonging to the same trophic group commonly shared the same pitcher, because of high occurrence probabilities of dominant species and positive associations between some taxa (due mainly to similar occupancies by pitcher age). Predator coexistence in Community K may have been facilitated by self-limitation of the large predators through intraspecific cannibalism strengthened by aggregation. Prey coexistence, on the other hand, may have relied more on population suppression by predation, especially the selective removal of old instar Tripteroides.     

Reproduction and feeding of the mountain mullet,Agonostomus monticola,in Trinidad,West Indies

Synopsis Reproduction and feeding were studied in mountain mullet,Agonostomus monticola, in northeast Trinidad during the period January 1987 to September 1988. Macroscopic and microscopic examination of the gonads indicated a single, distinct reproductive season from June to October during the wet season. Evidence implies only one spawning event per individual during the reproductive season. Females significantly outnumbered males resulting in a sex ratio of 20.4:1. The mean size at maturity for females was 135 mm fork length (FL). Fecundity (F) is related to size by the equations F = 167.3 FL^2.7 and F = 16430.7 G + 235079 (G = gutted weight).A. monticola was found to be an omnivorous feeder. The dominant food groups found in the stomachs were insects, prawns, fruits and algae, in order of decreasing importance.