Species-specific probe,based on 18S rDNA sequence,could be used for identification of the mucilage producer microalga <Emphasis Type="Italic">Gonyaulax fragilis</Emphasis> (Dinophyta)

Brook trout (Salvelinus fontinalis) in Appalachia experience prolonged periods of poor feeding conditions, particularly during summer and fall. To determine which prey organisms are important in sustaining brook trout populations, we monitored the feeding patterns of a population of brook trout over the course of 2 years with an emphasis on seasonal change. We employed a bioenergetics model to estimate whether or not each fish had obtained enough energy to meet daily metabolic demand. As a result, qualitative comparisons between fish feeding above maintenance ration (successfully feeding fish) and fish feeding below maintenance ration (unsuccessfully feeding fish) were possible. With the exception of winter, brook trout derived significantly more energy from terrestrial organisms than aquatic organisms. During each season, successfully feeding brook trout fed on greater proportions of specific prey types. Terrestrial Coleoptera and Lepidoptera consistently proved to be important prey during warmer seasons, while large organisms such as vertebrates and crayfish appeared to be important during winter. Our findings suggest that terrestrial organisms are more important than aquatic organisms in sustaining brook trout populations. Further, certain large and abundant terrestrial taxa are critical in providing energy to brook trout.     

Detection of conspecific alarm cues by juvenile salmonids under neutral and weakly acidic conditions: laboratory and field tests

A variety of fishes possess damage-released chemical alarm cues, which play a critical role in the detection and avoidance of potential predation threats. Recently, we have demonstrated that the ability of fathead minnows (Pimephales promelas) and finescale dace (Phoxinus neogaeus) to detect and respond to conspecific alarm cues is significantly reduced under weakly acidic conditions (pH 6.0). Rainbow trout (Oncorhynchus mykiss) and brook charr (Salvelinus fontinalis) possess an analogous alarm cue system. However, it is unknown if the trout alarm cue system is likewise affected by relatively small changes in pH. In addition, previous studies have not verified this phenomenon under natural conditions. We conducted laboratory and field trials to examine the potential effects of acute exposure to weakly acidic (pH 6.0) conditions on the detection and response of conspecific alarm cues by juvenile trout. Our laboratory results demonstrate that while juvenile rainbow trout exhibit significant increases in antipredator behaviour under normal pH conditions (pH 7.0–7.2), they do not respond to the presence of conspecific chemical alarm cues (i.e. response is not different from controls) under weakly acidic conditions. Similarly, a wild strain of brook charr in their natural streams near Sudbury, Ontario, failed to detect conspecific alarm cues in a weakly acidic stream (mean pH 6.11) while they responded to these cues in a neutral stream (mean pH of 6.88). This is the first demonstration that relatively small changes in ambient pH can influence alarm responses under natural conditions. These data suggest significant, sub-lethal effects of acid precipitation on natural waterways.     

Use of visual and olfactory cues of flowers of two brassicaceous species by insect pollinators

1. Pollinating insects exploit visual and olfactory cues associated with flower traits indicative of flower location and reward quality. Pollination can induce changes in these flower-associated cues, thereby influencing the behaviour of flower visitors. 2. This study investigated the main cues exploited by the syrphid fly Episyrphus balteatus and the butterfly Pieris brassicae when visiting flowers of Brassica nigra and Raphanus sativus plants. Whether pollen is used as a cue and whether pollination-induced changes affect flower volatile emission and the behavioural responses of the two pollinator species were also studied. 3. Pollinator preference was investigated by offering visual and olfactory cues individually as well as simultaneously in two-choice bioassays. Plant treatments included emasculation, hand-pollination and untreated control plants. The composition of flower volatiles from pollinated and unpollinated control plants was analysed by gas chromatography-mass spectrometry. 4. Both pollinators exhibited a strong bias for visual cues over olfactory cues. Neither pollinator used pollen as a cue. However, E. balteatus discriminated between newly opened and long-open flowers at short distance only when pollen was available. Flower visits by pollinators were influenced by pollination-induced changes in B. nigra but not R. sativus flowers. Pieris brassicae only responded to pollination-induced changes when visual and olfactory cues were offered simultaneously. The blend of volatiles emitted by B. nigra, but not R. sativus inflorescences was affected by pollination. 5. Collectively, the findings of this study show that different pollinators exploit different visual and olfactory traits when searching for flowers of two brassicaceous plant species.     

Nested species subsets of amphibians and reptiles on Neotropical forest islands

Nested species subsets are a common pattern of community assembly characteristic of many types of fragmented landscapes and insular systems. Here we describe nested subset patterns of amphibian and reptile occupancy on 23 forest islands in north-eastern Bolivia. We used observed occupancy patterns to differentiate five distributional guilds: widespread species, rare species, poor colonizers, area-sensitive species and supertramps. Amphibian occurrences were nested along a forest island isolation gradient, and when species from each of the distribution classes were removed from subsequent analyses of nestedness, we found that dispersal-limited poor colonizers were responsible for the association between nestedness and isolation. Amphibians associated with the grassland matrix at the study site showed a nested pattern linked with area, although this pattern did not scale up to all amphibians and could not be unequivocally attributed to any of the distributional guilds we recognized. There were no strong associations between two biological characteristics, body size and relative abundance in the matrix, and the likelihood of occupancy along either forest island area or isolation gradients. The relative importance of isolation in shaping nested patterns of amphibians on these forest islands may be a result of either (1) the greater range in isolation values included in this study compared with many others; (2) the long time since isolation in this landscape, manifesting a footprint of isolation not apparent in more recently fragmented patches; (3) the relatively homogeneous grassland matrix surrounding forest islands that likely provides little refuge for animals moving among forest islands.     

Prevalence-adjusted optimisation of fuzzy habitat suitability models for aquatic invertebrate and fish species in New Zealand

For many years, habitat suitability models for aquatic species have been derived from ecological datasets by model optimisation. Previous research showed that optimisation of the predictive model performance did not necessarily lead to ecologically relevant models due to the impact of the dataset prevalence. Therefore, the adjusted average deviation was presented as a performance criterion that allowed incorporation of ecological relevance in the model optimisation process. This paper aims to analyse the relation between the adjusted average deviation (aAD) and the training set prevalence for three species in different New Zealand river systems: caddis flies Aoteapsyche spp., large brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss. The aAD was implemented in a hill-climbing algorithm to optimise a fuzzy species distribution model for each species. Specifically, the hypotheses were tested that (1) similar relations between the aAD and the training set prevalence would be obtained, (2) training based on the aAD would lead to more accurate model predictions than training based on more frequently applied performance criteria such as CCI, and that (3) the final fuzzy model would produce a realistic model of habitat suitability. The approach in this paper may improve the transparency of the model training process and thus the insight into habitat suitability models. Consequently, this paper could lead to ecologically more relevant models and contribute to the implementation of these models in ecosystem management.     

Assessment of local predation risk: the role of subthreshold concentrations of chemical alarm cues

The ability to accurately assess local predation risk is criticalto prey individuals, as it allows them to maximize threat-sensitivetrade-offs between predator avoidance and other fitness relatedactivities. A wide range of taxonomically diverse prey (includingmany freshwater fishes) relies on chemical alarm cues (alarmpheromones) as their primary information source for local riskassessment. However, the value of chemical alarm cues has beenquestioned due to the availability of additional sensory inputs(i.e., visual cues) and the lack of an overt antipredator responseunder conditions of low perceived risk. In this paper, we testthe hypothesis that chemical alarm cues at concentrations belowthe point at which they elicit an overt behavioral responsefunction to increase vigilance towards other sensory modalities(i.e., visual alarm cues). Shoals of glowlight tetras (Hemigrammuserythrozonus) exposed to the subthreshold concentration of hypoxanthine-3-N-oxide(the putative Ostariophysan alarm pheromone) did not exhibitan overt antipredator response in the absence of secondary visualcues (not different than the distilled water control). However,when exposed to the sight of a visually alarmed conspecific,they significantly increased the intensity of their antipredatorresponse (not different from shoals exposed to the suprathresholdalarm cue). This study demonstrates that prey may benefit fromresponding to low concentration alarm cues by increasing vigilancetowards secondary cues during local risk assessment, even inthe absence of an overt behavioral response. By increasing vigilancetowards secondary risk assessment cues in the presence of alow risk chemical cue, individuals are likely able to maximizethe threat-sensitive trade-offs between predator avoidance andother fitness related activities.     

Pack-hunting in two species of catfish, Clavias gariepinus and C. ngamensis, in the Okavango Delta, Botswana

During the annual drawdown in the Okavango Delta, Botswana, large shoals of catfish migrate upstream in the main river channels. The examination of samples of the sharptooth catfish Clarias gariepinus and blunttooth catfish C. ngamensis revealed that they were pack-hunting and selectively preying on two species of mormyrids, the bulldog Marcusenius macrolepidotus and the churchill Petrocephalus catostoma . The predation on mormyrids may be related to prey size and abundance. In addition, catfish may sense the electrical discharge given off by the mormyrids when disturbed. The catfish feed intensively for a few months and build up body reserves and reproductive condition prior to spawning on shallow floodplains with the onset of the annual floods from Angola.     

Earwig preying on ambrosia beetle: Evaluating predatory process and prey preference

Earwigs (Dermaptera), such as Forficula auricularia L., are important euryphagous predators for a wide variety of prey and can markedly influence the populations of orchard pests. Most previous studies on earwig feeding behaviour have not used adult beetles of the prey species; few researchers have focused on prey preference in earwigs. Some fragments of beetle exoskeleton and an earwig adult, Anisolabella marginalis (Dohrn), were found in the same cage, where adults of ambrosia beetle, Euwallacea interjectus (Blandford), were emerging from the logs of a fig tree infected with Ceratocystis canker (fig wilt disease). Thus, A. marginalis was suspected of being a predator of E. interjectus. To shed light on this issue, in the laboratory, we set up a test arena and observed and recorded behavioural interactions between A. marginalis and E. interjectus. E. interjectus was collected from the logs of fig trees and reared on an artificial diet, along with six different ambrosia beetle species, which were collected from a trap (baited with ethanol) and a fallen maple tree. A series of laboratory experiments demonstrated that A. marginalis is actually a predator of E. interjectus and other species of ambrosia beetle, indicating its a potential for use in effective pest control in the field. The predators frequently consume and tend to select their prey depending on prey size, rather than sex and beetle species. Furthermore, earwigs have alternative predatory strategies for dealing with seven different species, although they use their forceps to cut the body of most tested beetles.     

Modulation of behavior in zebrafish,Danio rerio,according to female reproductive status and visual and chemical cues

Zebrafish, Danio rerio, is one of the most studied vertebrate models. However, there are still many aspects of its reproductive behavior to be elucidated. The aim of this work was to clarify whether males and females of this species display different behaviors according to sexual status, and the kind of cues they were exposed to (visual and/or chemical). Females in two different sexual status, pre- (PreS) or post-spawning (PostS), were exposed to single males cues (visual or chemical) or complementary cues (visual and chemical together). PreS females spent more time near the male’s compartment than PostS when they were exposed to complementary cues, but no differences were found when only one cue was present. Males showed a higher proximity to females when they were exposed to visual, and both cues together, but no difference was found according to females’ reproductive status. When no barrier was present, males and PreS females spent more time swimming together than males with PostS females. These results showed that females’ reproductive status, and the kind of cues to which females and males are exposed to, can modulate behaviors related to reproduction. Particularly, both cues together (visual and chemical) are necessary to trigger different behaviors in females according to their reproductive status.     

Effects of species invasion on population dynamics,vital rates and life histories of the native species

Invasions occurring in natural environments provide the opportunity to study how vital rates change and life histories evolve in the presence of a competing species. In this work, we estimate differences in reproductive traits, individual growth trajectories, survival, life histories and population dynamics between a native species living in allopatry and in sympatry with an invasive species of the same taxonomic Family. We used as a model system marble trout Salmo marmoratus (native species) and rainbow trout Oncorhynchus mykiss (non-native) living in the Idrijca River (Slovenia). An impassable waterfall separates the stream into two sectors only a few 100 meters apart: a downstream sector in which marble trout live in sympatry with rainbow trout and an upstream sector in which marble trout live in allopatry. We used an overarching modelling approach that uses tag-recapture and genetic data (>2,500 unique marble and rainbow trout were sampled and genotyped) to reconstruct pedigrees, test for synchrony of population dynamics and model survival and growth, while accounting for individual heterogeneity. The population dynamics of the two marble trout populations and of rainbow trout were synchronous. We found higher prevalence of younger parents, higher mortality and lower population density in marble trout living in sympatry with rainbow trout than in marble trout living in allopatry. There were no differences in the average individual growth trajectories between the two marble trout populations. Faster life histories of marble trout living in sympatry with rainbow trout are consistent with predictions of life history theory.     

Predator-induced changes in morphology of a prey fish: the effects of food level and temporal frequency of predation risk

In a series of experiments, we investigated the effects of food availability and risk frequency on the dynamics of predator-induced changes in growth and morphology of prey fish using goldfish (Carassius auratus) as our test species. In experiment 1, we fed goldfish high or low food rations and exposed them to either alarm cues from conspecifics, cues from swordtails or a water control. After 60 days, goldfish in the alarm cue treatment significantly increased their body depth and body weight but had smaller body length than goldfish exposed to swordtails cues or water, likely reducing their vulnerability to gape-limited predators. Importantly, food level had an impact on the amplitude of the morphological changes. In experiment 2, goldfish were exposed to two different frequencies of predation cues or a water control for 50 days. The cues were either continued or discontinued from day 51 to 100, and all cues were resumed from day 101 to 150. We found that goldfish exposed to predation cues increased their depth and weight at a faster rate than did the goldfish exposed to water, and of particular significance was the fact that frequency of risk had an effect on the amplitude of the change. When the cues were interrupted, the increase in growth rate parameters was reduced to the level of the goldfish exposed to water. However, when the cues were resumed, the rate increased to match the growth rate of the goldfish that were continuously exposed to the cues. Finally, we staged encounters between goldfish of differing morphologies and yellow perch (Perca flavescens) and found that deep-bodied goldfish had better survival than the shallow-bodied ones. These experiments illustrate the dynamic nature of inducible morphological defences.     

Ontogeny of feeding in two native and one alien fish species from the Murray-Darling Basin, Australia

Investigations into the feeding of the early stages of fishes can provide insights into processes influencing recruitment. In this study, we examined ontogenetic changes in morphology and feeding behaviour of two native Australian freshwater species, Murray cod, Maccullochella peelii peelii, and golden perch, Macquaria ambigua, and the alien species, common carp, Cyprinus carpio. Murray cod free embryos are large and well developed at the onset of feeding, whereas the other two species begin exogenous feeding much younger and are smaller and less-developed. Carp commence exogenous feeding 3 days earlier than golden perch, and show more advanced development of the eyes and ingestive apparatus. We conducted feeding experiments, presenting larvae of the three species with a standardised prey mix (comprising equal numbers of small calanoid copepods, large calanoid copepods, small Daphnia, and large Daphnia). Larvae of most tested ages and species showed a preference for mid-sized prey (300–500 μm wide). This was true even when their gapes substantially exceeded the largest prey offered. Daphnia were consumed more than similar-sized copepods. The results of this study suggest that survival through their larval period will be threatened in all three species if catchable prey <500 μm in width are not available throughout such time. They also suggest that interspecific competition for prey may occur, especially when larvae are very young. The precocious development of structures involved in feeding and the extended transition from endogenous to exogenous feeding of early carp larvae are likely to have contributed to the success of this species since its introduction to Australia.     

Variation in brook and brown trout survival within and among seasons, species, and age classes

The Program MARK was used to generate and test a plausible set of survival models for brook trout Salvelinus fontinalis and brown trout Salmo trutta to determine whether survival differed by season, species or age class. Apparent survival varied with time and age, but not by species. For the older (1999) age class, survival was lowest during the autumn of their age 1+ year whereas survival was lowest for the younger (2000) age class during the early summer of their age 1+ year. Additionally, estimates of survival for the younger age class exceeded those of the older age class in all but one interval (early summer 2001) but significant differences were observed in only one of these intervals (autumn 2000). To determine whether the observed seasonal differences in survival were related to seasonal differences in movement rates, multi‐strata spatial models were used within Program MARK to determine the probability of moving for each interval. In‐site movement rates were found to be low regardless of season (average for all cohorts combined was 5%). The ability of the multi‐strata modelling approach to detect temporal variability in movement, however, was potentially limited by spatial scale of the study reach ( c . 1 km). Differences in survival between different aged fishes could lead to selection acting on age at maturity or reproductive effort at a given age.     

Physiological responses of topmouth gudgeon, <Emphasis Type="Italic">Pseudorasbora parva</Emphasis>, to predator cues and variation of current velocity

Predators, either through direct or indirect encounter and current velocity, are frequently stressful to fish living in stream waters. In nature, fish may experience both current velocity stress and predation danger simultaneously. Experiments were carried out to clarify to what extent predation risk (with reference to different types of predatory cues) and current velocity can induce physiological stress in a running-water dwelling fish, topmouth gudgeon (Pseudorasbora parva). Fish were exposed to an alarm substance, predator odor, and visual cue, as well to combinations of predation risk and elevated current velocities. Metabolic rate, ventilation rate and fish activity were measured. Results showed that irrespective of the type of encounter, the presence of predator imposed physiological stress on fish. Metabolic rate were 0.983 ± 0.312, 0.641 ± 0.151, 0.572 ± 0.063, and 0.277 ± 0.016 mg O₂ W⁻¹ h⁻¹ following presence of alarm substance, visual cue, predator odor and control, respectively. Dramatic changes in ventilation rate and activity affirmed that alarm substance induced the strongest stress, followed by predator odor and visual cue. Reactions appeared to mirror the extent of fish perceiving danger of predation. Predation risk together with current velocity induces stronger stress, much stronger than if the current velocity works as a single stressor. However, the interaction between predation risk and current velocity did not have a significant effect on metabolic function; magnitude of metabolic response to high current velocity might mask the metabolic response to predator presence. Small fish living in stream habitats that face local predation risk would spend higher energy expenditure that may have negative impacts on growth, and hence their fitness.     

Inbreeding levels and prey abundance interact to determine fecundity in natural populations of two species of wolf spider

Long-term effective population size is expected, and has been shown, to correlate positively with various measures of population fitness. Here we examine the interacting effects of population size (as a surrogate for genetic factors) and prey consumption rates (as a surrogate for environmental quality) on fecundity in two sympatric species of wolf spider, Rabidosa punctulata and Rabidosa rabida. Population size was estimated in each of seven genetically isolated populations in each of 3 years using mark-recapture methods. Fecundity was estimated as the mean number of live offspring produced by ∼15 females sampled from each population of each species each year for 3 years. Prey consumption rates were estimated by sampling ∼300 spiders per population per year and assaying the proportion of spiders with prey. Larger populations have higher fecundity and more genetic diversity than smaller populations. Variation among populations in fecundity for a given year could be attributed most strongly to differences in population size, with variation in prey consumption rates and the interaction between population size and prey consumption playing smaller but still important roles. During the most stressful environmental conditions, the smallest populations of both species experienced disproportionately low-fecundity rates, more than doubling the estimated number of lethal equivalents during those years. The evidence presented in this paper for inbreeding-environment interactions at the population level and further evidence for a log-linear relationship between population size and fitness have important implications for conservation.     

Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem

Reductions in snow cover undera warmer climate may cause soil freezing eventsto become more common in northern temperateecosystems. In this experiment, snow cover wasmanipulated to simulate the late development ofsnowpack and to induce soil freezing. Thismanipulation was used to examine the effects ofsoil freezing disturbance on soil solutionnitrogen (N), phosphorus (P), and carbon (C)chemistry in four experimental stands (twosugar maple and two yellow birch) at theHubbard Brook Experimental Forest (HBEF) in theWhite Mountains of New Hampshire. Soilfreezing enhanced soil solution Nconcentrations and transport from the forestfloor. Nitrate (NO₃ ^–) was thedominant N species mobilized in the forestfloor of sugar maple stands after soilfreezing, while ammonium (NH₄ ⁺) anddissolved organic nitrogen (DON) were thedominant forms of N leaching from the forestfloor of treated yellow birch stands. Rates ofN leaching at stands subjected to soil freezingranged from 490 to 4,600 mol ha^–1yr^–1, significant in comparison to wet Ndeposition (530 mol ha^–1 yr^–1) andstream NO₃ ^– export (25 mol ha^–1yr^–1) in this northern forest ecosystem. Soil solution fluxes of P_i from the forestfloor of sugar maple stands after soil freezingranged from 15 to 32 mol ha^–1 yr^–1;this elevated mobilization of P_i coincidedwith heightened NO₃ ^– leaching. Elevated leaching of P_i from the forestfloor was coupled with enhanced retention ofP_i in the mineral soil Bs horizon. Thequantities of P_i mobilized from the forestfloor were significant relative to theavailable P pool (22 mol ha^–1) as well asnet P mineralization rates in the forest floor(180 mol ha^–1 yr^–1). Increased fineroot mortality was likely an important sourceof mobile N and P_i from the forest floor,but other factors (decreased N and P uptake byroots and increased physical disruption of soilaggregates) may also have contributed to theenhanced leaching of nutrients. Microbialmortality did not contribute to the acceleratedN and P leaching after soil freezing. Resultssuggest that soil freezing events may increaserates of N and P loss, with potential effectson soil N and P availability, ecosystemproductivity, as well as surface wateracidification and eutrophication.     

Segregation by species and size classes of rainbow trout,Salmo gairdneri,and Sacramento sucker,Catostomus occidentalis,in three California streams

Synopsis The hypothesis that Sacramento suckers, Catostomus occidentalis, compete with rainbow trout, Salmo gairdneri, for space in streams was examined by measuring microhabitat utilization of both species in three California streams. Two streams were similar in most respects except one contained only trout and one contained trout and a large population of suckers. The third stream, formed by the union of the first two, contained trout and a small population of suckers. The species overlapped in five of the six microhabitat variables measured: maximum depth, mean water column velocity, focal point velocity, surface water velocity, and substrate type. However, the species had strong vertical segregation; there was little overlap between species in focal point depth. Mean focal point velocities were also significantly different. Suckers roamed over and generally remained in contact with the bottom while trout held position in the water column. Microhabitat utilization by trout in the stream without suckers was similar to in the stream with a higher sucker density. Differences in microhabitat utilization by trout between the third stream and the other two was attributed to the larger size of the third stream. Both sucker and trout showed a similar within-species segregation of size classes - fish under 50 mm in length sought shallow water. Size-specific trends indicated ontogenic shifts in resource utilization which reduced overlap within species. These results suggest that competition for space between trout and suckers was not a major factor regulating microhabitat utilization of trout, although the possibility that larger suckers may displace small trout needs further study.     

Prey handling time and ingestion probability for Mesocyclops sp. predation on small cladoceran species Bosmina longirostris, Bosminopsis deitersi, and Scapholeberis mucronata

We compared the vulnerability of small cladoceran species (Bosminopsis deitersi, Bosmina longirostris, and Scapholeberis mucronata) to predation by Mesocyclops sp. in the laboratory based on prey handling time and ingestion probability. We also estimated the effects of ontogenetic changes on handling time by testing prey of various body sizes. All tested prey species showed an increasing pattern of handling time along with growth (increase of body size). For juveniles smaller than 0.3mm, the prey handling time was similar for B. deitersi and B. longirostris, but markedly longer for S. mucronata. Mesocyclops sp. rejected large (>0.35mm) B. longirostris and S. mucronata soon after capture and consequently showed a low probability of successful ingestion, whereas most large individuals of B. deitersi were consumed. The shorter handling time and higher ingestion probability of small juveniles indicate that the copepods ingest small juveniles more efficiently than larger individuals. Thus, it seems that the vulnerability of small cladocerans to copepod predation differs ontogenetically, with the greatest vulnerability being during the juvenile period. The ontogenetic reduction in the vulnerability of B. longirostris and S. mucronata was shown in the rapidly reduced ingestion probability as well as by the increase in prey handling time for large individuals. The results show that the vulnerability to predation by copepods of B. longirostris and S. mucronata reduces more quickly with increasing size than does that of B. deitersi, and it seems that the former two species are more resistant to copepod predation.     

Food consumption and diet composition of cod,Gadus morhua,inhabiting the southwestern Gulf of St. Lawrence

Synopsis Stomach content analysis of commercial size cod, Gadus morhua, inhabiting the southwestern Gulf of St. Lawrence are presented for the period May to November 1980. Season- and size-related changes in the diet composition are indicated and the results compared to those of earlier studies. The extent of cod predation on major prey groups is evaluated by estimating their consumption by cod age-groups 3–15. The evaluation is based on the estimate of food intake by individual cod of different ages derived with the aid of a procedure based on Ursin growth theory, estimates of numbers-at-age from cohort analysis and the results of stomach content analysis.With the exception of American plaice, Hippoglossoides platessoides, and snow crab, Chionoecetes opilio, the consumption of commercially important species was relatively low and restricted to older cod. Commercially important species accounted for less than 12% of the total food biomass consumed by cod during the study period. The remainder consisted of other fish (18%), decapods (22%), euphausiids (15%), molluscs (11%), annelids (8%), and other invertebrates (15%). Methods are suggested to improve the above estimates of total and individual prey biomasses consumed by the fish population.     

Species composition, seasonal dynamics and numerical responses of arthropod predators in organic strawberry fields

The species composition and relative abundance of predatory arthropod fauna were studied in organic strawberry fields in northwestern Turkey using sweep net sampling and pitfall trapping (activity density). Arachnida constituted 13.1 and 11.5% of the sweep net and pitfall trap catches, respectively. Among the predatory insects, the most abundant groups were Heteroptera (26.7%), Diptera (25.9%), Coleoptera (16.9%) and Orthoptera (10.8%) in sweep net samples. Coleoptera (84.2%) dominated the pitfall trap catches. Many aphid specific and polyphagous predators reached peak abundance during June and July. Pearson's two-tailed correlations showed a significant and positive relationship between syrphid, coccinellid or chrysopid predator numbers and strawberry aphid density. Aphid density was not significantly correlated with carabid or with nabid abundance. Examination of spatial distribution patterns of all predatory arthropod groups using Taylor's Power Law indicated that most arthropod predators, except carabids, exhibited aggregated dispersion patterns. Coccinellids changed their spatial patterns from a uniform to an aggregated distribution through the season. There was also a significant linear correlation between Anterastes sp. abundance (larvae+adults) and Isophya rectipennis+Poecilimon ricteri (larvae+adults) density at both locations. We observed, for the first time, Anterastes preying on Isophya and Poecilimon species. The seasonal abundance of the major predatory groups were described, and their potential importance in controlling strawberry aphid and other pests is discussed.