Habitat use of arctic charrSalvelinus alpinus in Thingvallavatn,Iceland

Synopsis Habitat use by four morphs of arctic charr,Salvelinus alpinus, was investigated in Thingvallavatn, Iceland, by sampling with pelagic and benthic gill nets. Sampling was done in May/June and August/September. Greatest abundance of fish was recorded in the littoral and epipelagic zone in early autumn. Catches were low in early summer. The four morphs are partly segregated in habitat. Small (SB-) and large benthivorous (LB-) charr have a more restricted spatial distribution than piscivorous (PI-), and especially planktivorous (PL-) charr. Both benthivorous morphs are mainly found in the littoral zone, and occur in largest numbers in stony shallows at depths between 0 and 10 m. PL-charr usually dominates in numbers in all habitats. PI-charr is most abundant in epibenthic habitats, although numbers are always low. All morphs are caught in higher numbers at night than during the day, but the diurnal activity difference is highest among SB-charr. The habitat use by different morphs is as may be expected from their morphology and diets. Within the population of PL-charr, young and small fish are more abundant on the bottom than in the pelagic zone, and there is a surplus of females in the pelagic zone. Along the benthic profile, young, small and immature PL-charr are more abundant in deep than in shallow waters. The results are discussed in relation to food supply, competition and predation. Possible reasons for the occurrence of four arctic charr morphs are also discussed.Contribution from the Thingvallavatn project.     

Molecular genetic examination of the polymorphic Arctic charr Salvelinus alpinus of Thingvallavatn, Iceland

Thingvallavatn, Iceland contains two sympatric morphotypes (benthic and limnetic) of Arctic charr Salvelinus alpinus. Each morphotype is composed of two morphs and these differ markedly in ecology, behaviour and life history. We used molecular genetic approaches to test whether (i) genetic heterogeneity exists among morphs and (ii) if morphs arose in allopatry and came into secondary contact or arose sympatrically within the lake through genetic segregation and/or phenotypic plasticity. Direct sequencing of 275 bp of the mitochondrial DNA (mtDNA) control region, mtDNA restriction fragment length polymorphisms and single locus minisatellite analyses detected insufficient variation to test our hypotheses. Analysis of multilocus minisatellite band sharing detected no significant differences between morphs within the same morphotype. However, significant differences among morphs belonging to different morphotypes suggest some genetic heterogeneity in Thingvallavatn charr. Limnetic charr from Thingvallavatn were more similar to sympatric benthic charr than to allopatric limnetics from two other Icelandic lakes. This suggests that the Thingvallavatn morphs arose sympatrically within the lake rather than in allopatry followed by secondary contact.     

Alternative mating tactics of arctic charr,Salvelinus alpinus,in Thingvallavatn,Iceland

Synopsis During the spawning season the operational sex ratio of the large benthivorous (LB) arctic charr morph in Thingvallavatn is skewed in favour of males which compete intensely on the spawning site. The skewness is caused by males staying longer on the spawning ground than females. LB males employ two mating tactics. Either they guard the female or they attempt sneak-matings with guarded females. The tactics seem to obey a pure conditional strategy where relative size is the important criterion. Generally, the larger males use the guarding tactic and the smaller the sneaking tactic. Assessment of size is a part of the strategy. Relative size influences male behaviour irrespective of which tactic they employ. Guarding is more successful when courting is considered, both with respect to relative frequency and duration of courting acts. Females are aggressive towards the sneakers especially when they are small. This could be interpreted as female choice in favour of large males, but also as a defence against egg predation. A small dwarflike benthivorous morph (SB) is found in the same area. It is mainly active during night and on rare occasions SB-males are seen sneaking into the nest of LB-females. In such cases they are attacked vigorously by both LB sexes.     

Segregation in spawning and early life history among polymorphic Arctic charr, Salvelinus alpinus, in Thingvallavatn, Iceland

The relationships among time of spawning, incubation temperature, timing of first feeding and early growth were examined in four sympatric morphs of Arctic charr in Thingvallavatn, Iceland. Large benthivorous charr spawn in July-August at sites with cold ground-water flow. Planktivorous and piscivorous charr spawn in September-November and are not confined to ground-water sites. The spawning of small benthivorous charr overlaps with that of other morphs. Progeny of large benthivorous charr start feeding 2-3 months earlier than the progeny of autumn spawners. This results in differential size distribution and growth rates of young in the spring.     

Status of arctic charr (Salvelinus alpinus) farming in Norway, Sweden and Iceland

This article describes the status of the Arctic charr (Salvelinus alpinus L.) aquaculture industry in Iceland, Norway and Sweden. The purpose of the work was to give an update on the status of Arctic charr farming in these countries based on information obtained from the industry itself. The data were collected using questionnaires designed for the purpose with key personnel in each country interviewing the farmers or farm managers. The questions were divided on topics focusing on different sides of the production, including farm characteristics, biological challenges and markets. The aim was to identify strengths and weaknesses in the production related to country and production technologies. The information may prove useful for decisions and prioritising future research and development work on removing bottlenecks and improving sustainable production of Arctic charr.     

August diet of age-0 Pacific halibut in nearshore waters of Kodiak Island,Alaska

Synopsis The objective of this study was to describe the diet of age-0 Pacific halibut,Hippoglossus stenolepis, for the inshore waters of Kodiak Island, Alaska during August 1991. Stomach contents were identified from 170 age-0 halibut captured inshore of the eastern and southern coasts of Kodiak Island, and were analyzed in relation to halibut size, location, depth and substrate of capture. One hundred sixty-eight of 170 fish had eaten crustaceans, of which the predominant prey taxa were Mysidacea (34.3%), Cumacea (33.1%), Gammaridea (26.6%) and Caridea (3.9%). In five of six capture locations, mysids and amphipods were predominant prey. In the remaining area, Sitkinak Strait, cumaceans were the primary food source. At depths less than 10 m, mysids were the predominant prey taxa. Gammarid amphipods were of primary importance at depths of 10–30 m. Halibut captured from 30–70 m fed mainly on cumaceans. Cumaceans and gammarid amphipods were consumed by halibut caught on gravel substrate. Fish caught on substrates of sand and mud fed mainly on mysids and amphipods. Cumaceans were also consumed on sandy substrates. Fish 45 mm fed on cumaceans. An ontogenetically related shift in diet occurred at 46–55 mm TL, at which size the halibut's primary prey began to shift from cumaceans to mysids. Fish of 46–75 mm consumed increasingly greater proportions of mysids, amphipods and shrimps. The diet of age-0 Pacific halibut along the Kodiak coast during August was related to predator size, and location, depth, and substrate type of capture.     

Acoustical estimates of fish and zooplankton distribution in the Piaseczno reservoir, Southern Poland

Hydroacoustical surveys in the Piaseczno reservoir were performed in May and September 2002 using a Biosonics 101 dual beam echo sounder. They have revealed very scarce fish populations in pelagic waters with twice-higher abundance in autumn (530 fish ha⁻¹) as compared with spring (280 fish ha⁻¹). Small and very small fish (below 10 cm length) dominated. Apart from fish, Chaoborus larvae were producing acoustical echoes of the TS similar or slightly weaker than that of small fish. Invertebrates formed a thin layer, less than 2 m thick at the border of an anoxic zone, and were changing their depth position between 6 and 16 m, both diurnally and seasonally.     

Seasonal and ontogenetic shifts in the diet of Arctic charr Salvelinus alpinus in a subarctic lake

Seasonal and ontogenetic shifts in the diet of Arctic charr Salvelinus alpinus were studied in a deep, ultra‐oligotrophic lake in subarctic Finland from both stomach contents and the stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope compositions of muscle and liver tissues. Both diet and isotope results indicated that the S. alpinus population relied mainly on littoral benthic energy sources. The strong littoral reliance appeared largely independent of season or fish size, although the data lacked small (total length, L_T, <130 mm) and young (<3 years) S. alpinus. Liver isotope values of intermediate‐sized S. alpinus (200–350 mm), however, suggested exploitation of the increase in the abundance of pelagic zooplankton in the late open‐water season. The results suggest that, in general, a strong littoral reliance of fishes can be a feature in subarctic lakes throughout the year. Due to its faster isotopic turnover rate and thus higher resolution for temporal diet changes, liver could be more commonly used in stable‐isotope studies of fish trophic niche shifts instead of using only the less responsive muscle tissue.     

Constancy of synaptic ribbon numbers in the retina of the arctic chary,Salvelinus alpinus

Synopsis At high latitudes, such as in Iceland, the daily photoperiod varies from almost continuous darkness in winter to virtually constant light in summer. Previous studies of detailed retinal structure in vertebrates have shown significant daily and annual effects of photoperiod. We sampled arctic charr in Iceland during the summer, including fish that were both light- and dark-adapted, during both day and night. We observed retinomotor responses characteristic of light- and dark-adaptation, but found no difference in the number of synaptic ribbons in the retina. The morpho-physiological changes, appearing as retinomotor responses, are thus not expressed at the synaptic level.     

The effect of wind exposure and filamentous algae on the distribution of surf zone macroinvertebrates in Lake Esrom,Denmark

Distribution and composition of the epilithic macroinvertebrate fauna was studied on 21 different sites situated in the surf zone of Lake Esrom. A relative degree-of-exposure (windinduced wave activity) was calculated for each station from the specific number of wind days, wind velocity and a measured wind fetch. Macroinvertebrate abundance averaged 20 200 ind. m^–2 stone bottom, and was composed primarily of six taxa (70% of the mean densities). The distribution patterns of the 48 most common taxa were compared to exposure, the amount of filamentous algae and shore slope, by regression, cluster analysis and ordination analysis (PCA and RDA). The abundance of 25 taxa was significantly correlated to the amount of filamentous algae, and a protectional effect of algae growth was suggested for the exposed sites. Seventeen taxa showed significant difference in abundance between sheltered western stations and exposed eastern stations. Stations with a gentle slope were characterised by sand covered stones and a fauna that qualitatively was similar to the fauna at greater depth in the littoral. Redundancy analysis (RDA) showed filamentous algae and slope to be the most important factors controlling the species distribution, whereas the gradient of exposure in Lake Esrom was too short to explain the species tolerance range.     

The effect of suspended sediments on ninespine stickleback,Pungitius pungitius,and golden shiner,Notemigonus crysoleucas,in a current of varying velocity

Synopsis Ninespine stickleback, Pungitius pungitius, and golden shiner, Notemigonus crysoleucas, were exposed at 5 and 20°C to 0, 15, 75 and 150 JTU (Jackson Turbidity Units) of suspended sediments. Fish were tested in a trough inclined at 2.3° with an inflow rate of 27 ml sec^-1. Changes in swimming behaviour were only noted for golden shiner and at 20°C and 15, 75 and 150 JTU. Under these conditions golden shiners were more active, changing from location to location in the apparatus significantly more often at higher than at lower concentrations of suspended sediments. This behaviour is compatible with a fleeing response from a stress inducing agent.     

Seasonal dynamics and spatial distribution of chydorid cladocerans in relation to chironomid larvae in the sandy littoral zone of an oligo-mesotrophic lake

The seasonal variation of the principal macro- and meiobenthic taxa in the sandy littoral zone of the oligomesotrophic Dutch Lake Maarsseveen I was studied during two years. Population peaks of the different taxa were clearly separated in time. In early spring there were chironomid density peaks of Stictochironomus sticticus, in one year followed by a Polypedilum maximum. From June–July chydorid cladocerans dominated, with a peak of Monospilus dispar followed by a peak of Rhynchotalona falcata. In autumn the chironomid Cladotanytarsus mancus became dominant, remaining so throughout the winter.To study the spatial heterogeneity of the major macro- and meiofaunal taxa, samples were taken in a grid of 2.5 × 10 m. Distributions of all but one taxon were significantly different from random, with Morisita indices varying from 1.23 (chironomids) to 2.10 (the chydorid Monospilus dispar). Wind-induced disturbance presumably had strong and species-specific effects on littoral macro- and meiobenthic taxa. Chydorid heterogeneity increased immediately following the first autumn storm of the season; this may be explained by the tendency of these organisms to remain attached to detritus particles.Temporal and spatial correlation coefficients between macro- and meiobenthic taxa were generally low, suggesting that interactions between these groups are weak, with distribution patterns that are independent of each other. Among the meiofaunal taxa, positive spatial and temporal correlation coefficients were found; apparently, the similarity in both seasonal dynamics and spatial distribution is larger among the meiofaunal taxa than between macro- and meiofaunal taxa. Also among most of the chironomid species significant positive temporal correlations were found, except for Stictochironomus sticticus and Polypedilum sp.; these species have similar life-cycles, but were during one year temporally separated by several alternating settlement peaks. In the next year, Polypedilum disappeared completely following an extremely strong Stictochironomus settlement peak.     

Comparative ecology of prickly sculpin,Cottus asper,and coastrange sculpin,C. aleuticus,in the Eel River,California

Synopsis We documented species' distributions, size structure of populations, abundance in mainstem and tributary streams, habitat use, and diets of prickly sculpin, Cottus asper, and coastrange sculpin, C. aleuticus, in the Eel River drainage of California, to determine the processes allowing coexistence of these very similar fishes. We observed prickly sculpins at 43 sites and coastrange sculpins at 34. The species co-occurred at 26 sites. Young-of-year coastrange sculpins were only observed within 42 km of the ocean, but young-of-year prickly sculpins were present throughout the species range. Mean, maximum, and minimum lengths of coastrange sculpins were positively correlated with distance from the ocean but no significant relationships were found for prickly sculpins. Absolute abundance of both species was highest in mainstem habitat (prickly sculpins = 0.6 sculpins m^–2 and coastrange sculpins = 0.4 sculpins m^–2) . Tributary densities of both species tended to be less than 0.1 sculpins m^–2. The species inhabited very similar habitats and had very similar diets. Coastrange sculpin populations in upstream areas were maintained by immigration from downstream areas in contrast with prickly sculpin populations that produced young-of-year fish throughout their range. Densities were probably not high enough for interspecific interactions to be important. The factors limiting the upstream distribution of the species may include high water temperatures, stability of the stream bed, and behavior of the fish. In the past, the range of sculpins within the Eel River drainage probably fluctuated with changing physical conditions. Recent introductions of exotic species that compete with and prey upon sculpins, and ongoing human activities in the drainage could result in major reductions in the distribution and abundance of one or both species.     

Interannual and regional variations in body length, weight and energy content of age-0 Pacific herring from Prince William Sound, Alaska

Age-0 Pacific herring were surveyed in October of 4 years in a large northern Gulf of Alaska estuary, to determine the range of variations in length, weight and whole body energy content (WBEC). These parameters reflect their preparedness for surviving their first winter's fast. During the surveys there were distinct regional and interannual variations in all three parameters for individual groups of herring in Prince William Sound. Likewise, with each collection there was typically a large range of size and WBEC values. The average standard length was (±S.D.) 80±13 mm (range=40–118), the mean whole body wet weight was 5·7±3·0 g (range=0·7–29·2) and the average WBEC of all age-0 herring captured, regardless of year or site (n=1471), was 5·4± 1·0 kJ g⁻¹ wet weight (range=2·4–9·4). The large range of WBEC and size indicates that age-0 herring at different capture sites were not all equally prepared for surviving their first winter.     

Daily rations,diel feeding activity and distribution of age-0 brook charr,Salvelinus fontinalis,in two subarctic streams

Synopsis Diel variations in the stomach content weight of age-0 brook charr indicate that they fed primarily during the day in Matamek River and Rivière à la Truite, Québec. Age-0 charr in both rivers moved close to the shore at night. The mean weight of stomach contents in Rivière à la Truite was highest at the end of the daylight period, whereas in the Matamek River stomach content weights maintained maximum level during most of the day, except in August when high water temperatures reduced food intake. Daily rations were estimated at 5.09 and 4.09% body weight for charr from Rivière à la Truite in July and August, respectively. In the Matamek River, estimates were 7.13, 9.75, and 1.27%, in June, July, and August, respectively. Charr from Matamek River had a higher instantaneous growth rate than charr in Rivière à la Truite (3.96 versus 2.72% body weight per day) and were significantly heavier by mid-August. Differences observed between populations in daily rations and growth rates were attributed mainly to differences in water temperature regimes.     

Nutritional condition of larval milkfish,Chanos chanos,occurring in the surf zone

In order to clarify the nutritional conditions of larval milkfish in the surf zone, the following parameters were examined: 1) DNA and RNA content and RNA/DNA ratio of fed and unfed larvae collected from the surf zone and reared in the laboratory; 2) survival rate of the unfed larvae; and 3) total length, otolith increment counts and RNA/DNA ratio of wild larvae collected daily from the surf zone. The DNA and RNA content of the unfed larvae decreased, but increased in fed larvae. The RNA/DNA ratio decreased in unfed larvae, whereas in the fed larvae it decreased for the first three days after capture and increased thereafter. These results indicated that the values of DNA and RNA content and RNA/DNA ratio could be used as an indicator of nutritional condition of milkfish larvae after 6 days of starvation. Although total length of the wild-larvae did not show serial changes, their otolith increment counts showed continuous increases, indicating that the larvae sojourned in the surf zone for several days. In the same period, RNA/DNA ratios of the wild larvae decreased continuously, the ratios of larvae with fewer otolith increment counts being relatively higher than those of larvae with greater increment counts. Based on these results, the milkfish larvae remaining in the surf zone were concluded as being under insufficient nutritional conditions.     

神农架自然保护区川金丝猴食源植物的分布和丰富度

温带地区分布的灵长类动物在食物匮乏的时候容易受到食物不足的胁迫。川金丝猴就是分布于温带区域的灵长类动物之一,因此在不同的季节,食物对其有不同程度的影响。神农架自然保护区是川金丝猴分布的最东缘,该地区的研究对川金丝猴的保护有着非常重要的影响。本文通过对神农架川金丝猴食源植物和栖息环境 的植被调查,来探讨食物的分布和丰富程度对川金丝猴的影响。在神农架川金丝猴分布的核心地区,选择具有代表性的植被类型,按照海拔梯度设置88个样方,基于优势树种和数据分析,我们把川金丝猴的栖息地划分为12 个类型。阔叶林的食源植物种类要高于针阔混交林;而低海拔的类型中提供食源植物的种类要高于高海拔类型。尽管川金丝猴的食谱范围很广,但是它喜食的食物种类分布极不均匀,而且在其不同类型的栖息地内,食源植物的种类和数量随着季节的变化而变化。因此,季节的变化和食源植物的分布不均衡都会影响川金丝猴的取食行为和时间分配。我们建议借助人为改造森林结构,增加食源植物的数量,进而达到保护和扩大川金丝猴种群的目的。     

Numbers and distribution of the Great Cormorant in Iceland: Limitation at the regional and metapopulation level

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Comparison of low oxygen avoidance in a bimodal breather,Erpetoichthys calabaricus and an obligate water breather,Percina caprodes

Synopsis Behavioral responses of the African reedfish, Erpetoichthys calabaricus, (Polypteridae), a bimodal breather, and the logperch, Percina caprodes, (Percidae), an obligate water breather, to low concentrations of dissolved oxygen were examined in a two-choice experimental design. In contrast to logperch, which consistently avoided dissolved oxygen concentrations of about 2.0 mg 1⁻¹, reedfish did not exhibit strong avoidance to dissolve oxygen concentrations of 0.5 mg 1⁻¹ and less.