Exotic crayfish in a brown water stream: effects on juvenile trout, invertebrates and algae

1. The impact of the introduced omnivorous signal crayfish (Pacifastacus leniusculus ) on trout fry, macroinvertebrates and algae was evaluated in a brown water stream in southern Sweden using in situ enclosures. We also examined the gut content of all surviving crayfish in the enclosures. Two crayfish densities in addition to a control without crayfish were used in replicate enclosures (1.26 m(2)) in a 1-month experiment. Additionally, 20 trout fry (Salmo trutta ) were stocked in each enclosure to assess the effects of crayfish on trout survival and growth. 2. Detritus was the most common food item in crayfish guts. Animal fragments were also frequent while algae and macrophytes were scarcer. Crayfish exuviae were found in crayfish guts, but the frequency of cannibalism was low. 3. Trout survival in enclosures was positively related to water velocity but was unaffected by crayfish. 4. Total invertebrate biomass and taxon richness were lower in crayfish treatments. The biomass of all predatory invertebrate taxa was reduced but only three of six non-predatory taxa were reduced in the crayfish treatments. 5. Epiphytic algal biomass (measured as chlorophyll a , on plastic strips) was not related to crayfish density, whereas the biomass of epilithic algae (measured as chlorophyll a ) was enhanced by high water velocity and high crayfish density. The latter was possibly mediated via improved light and nutrient conditions, as active crayfish re-suspend and/or remove detritus and senescent algal cells during periods of low water velocity. 6. We conclude that the introduced signal crayfish may affect stream communities directly and indirectly. Invaded communities will have reduced macroinvertebrate taxon richness and the signal crayfish will replace vulnerable invertebrate predators such as leeches. In streams that transport large amounts of sediment or organic matter, a high density of crayfish is likely to enhance benthic algal production through physical activity rather than via trophic effects

Exotic crayfish in a brown water stream: effects on juvenile trout, invertebrates and algae

1. The impact of the introduced omnivorous signal crayfish (Pacifastacus leniusculus ) on trout fry, macroinvertebrates and algae was evaluated in a brown water stream in southern Sweden using in situ enclosures. We also examined the gut content of all surviving crayfish in the enclosures. Two crayfish densities in addition to a control without crayfish were used in replicate enclosures (1.26 m(2)) in a 1-month experiment. Additionally, 20 trout fry (Salmo trutta ) were stocked in each enclosure to assess the effects of crayfish on trout survival and growth. 2. Detritus was the most common food item in crayfish guts. Animal fragments were also frequent while algae and macrophytes were scarcer. Crayfish exuviae were found in crayfish guts, but the frequency of cannibalism was low. 3. Trout survival in enclosures was positively related to water velocity but was unaffected by crayfish. 4. Total invertebrate biomass and taxon richness were lower in crayfish treatments. The biomass of all predatory invertebrate taxa was reduced but only three of six non-predatory taxa were reduced in the crayfish treatments. 5. Epiphytic algal biomass (measured as chlorophyll a , on plastic strips) was not related to crayfish density, whereas the biomass of epilithic algae (measured as chlorophyll a ) was enhanced by high water velocity and high crayfish density. The latter was possibly mediated via improved light and nutrient conditions, as active crayfish re-suspend and/or remove detritus and senescent algal cells during periods of low water velocity. 6. We conclude that the introduced signal crayfish may affect stream communities directly and indirectly. Invaded communities will have reduced macroinvertebrate taxon richness and the signal crayfish will replace vulnerable invertebrate predators such as leeches. In streams that transport large amounts of sediment or organic matter, a high density of crayfish is likely to enhance benthic algal production through physical activity rather than via trophic effects

The different roles of crayfish in benthic food webs

I show that crayfish consume detritus, algae, macrophytes, and an array of invertebrates. Crayfish were found to reduce invertebrate densities. However, prey mobility seemed to be a key factor regulating the strength of interactions between crayfish and prey. Since crayfish seek to maximize growth, they benefit from feeding on invertebrates, shown to promote high growth rates. I found crayfish to increase their intake of invertebrate prey in eutrophic lakes. This suggests that crayfish have different roles in different lakes, they seem to be predators in eutrophic lakes and more like primary consumers in oligotrophic lakes. In lakes where crayfish are predacious they elongate the food chain between basal resources (i.e. detritus and algae) and top predators like perch. This since perch are likely to prey heavily on crayfish when these were available. Moreover, I found crayfish to be larger in eutrophic lakes, this indicates that they grow faster in eutrophic waters. The rationale behind this conclusion is that the mean sizes of crayfish, possible to catch in baited traps, at the beginning of each growing season (summer) would be very similar between populations due to intensive harvest the previous autumn. Consequently, I interpreted the differences in mean size of crayfish at the end of the growing season (when harvest starts) to be a measure of growth differences between populations. Furthermore, the results of this thesis indicate that omnivores like crayfish could derive carbon from some food sources and simultaneously derive nitrogen from other sources. I found periphyton to be the chief ultimate nitrogen source for crayfish in streams. About half of the nitrogen seemed to come from direct consumption of algae and the other half via consumption of invertebrates that in turn relied on periphyton as their chief nitrogen source. In contrast, I found no clear trend regarding ultimate carbon sources for stream crayfish. In addition, these findings suggest that omnivorous organisms potentially could be at different trophic level with respect to different elements

Fractionation of delta N-15 and delta C-13 for Atlantic salmon and its intestinal cestode Eubothtium crassum

Stable isotopes of nitrogen (delta(15) N) and carbon (delta C-13) were measured for Atlantic salmon Salmo salar and their intestinal cestode, Eubothrium crassum, sharing the same diet. Atlantic salmon muscle tissues were enriched in N-15 and depleted in C-13 compared to their prey (sprat Sprattles sprattus sprattus) and their intestinal cestode. There was no significant difference in delta N-15 or delta C-13 between E. crassum and the sprat. Differences in nutrient uptake and intestine physiology between Atlantic salmon and E. crassum are discussed, as well as how these may give rise to different fractionations of stable isotopes between a host and its parasites. Furthermore, Atlantic salmon contained a significantly higher lipid content than their prey, which may partly explain differences in delta C-13 values between the host and its cestode. In addition, cestodes inhabiting lipid-rich hosts were also lipid rich. Larger Atlantic salmon were enriched in N-15 compared to smaller fish. Cestodes inhabiting large hosts were also enriched in N-15 compared to parasites living in smaller hosts. The last two results were explained by larger fish possibly feeding from a higher trophic level, or from larger and older prey, that resulted in both a higher lipid content and an enrichment in N-15. (c) 2007 The Authors Journal compilation (c) 2007 The Fisheries Society of the British Isles

Lake trophic status and bioaccumulation of polychlorinated biphenyls in benthic organisms: a comparison between littoral and profundal invertebrates

We investigated the influence of lake trophic status on accumulation of polychlorinated biphenyls (PCBs) in littoral and profundal invertebrates from 19 lakes in southern Sweden. The PCB concentration in profundal chironomids (Chironomus sp.) was negatively correlated with lake trophy but the concentration in littoral crayfish (Pacifasticus leniusculus) was unrelated to shifting nutrient regimes. PCB concentrations in chironomids were negatively correlated, while crayfish did not correlate with lake total phosphorus. Two findings indicate that different processes determine bioaccumulation in littoral and profundal invertebrates: (i) a lack of correlation between PCB concentrations in crayfish and chironomids within lakes and (ii) some differences in congener distribution in the two species. We suggest a possible explanation in that chironomids are directly exposed to settling material from the pelagic zone, while crayfish are omnivorous and feed mainly in the littoral zone. A intensive investigation on one pond population could not reveal any individual factors, such as lipid content, size, or sex, influencing bioaccumulation of PCBs in crayfish

Biomagnification and polychlorinated biphenyl congener distribution in an aquatic predator-prey, host-parasite system

Biomagnification and polychlorinated biphenyl (PCB) congener distribution was examined in a predator-prey, host-parasite system, in which Atlantic salmon (Salmo salar) preyed upon sprat (Sprattus sprattus). Eubothrium crassum was an intestinal parasite in salmon that also "preyed upon" sprat, because the parasites gained access to foodstuffs via the host (salmon) gut. Salmon contained significantly higher concentrations of total PCBs compared to both parasites and prey (sprat), but no difference in PCB concentration was found between sprat and E. crassum. Salmon biomagnified several PCB congeners from their diet (sprat), whereas parasites did not, despite the fact that both salmon and their parasites ingested the same prey. Differences in nutrient uptake mechanisms between the host and their parasites, in addition to the lack of a gastrointestinal tract in the cestode, may explain the lack of biomagnification in E. crassum. No difference was found in PCB congener distribution between parasites, salmon, and sprat. and none of the animal types showed a preference for accumulating more or less lipophilic congeners (congeners with a high or low octanol/water partition coefficient [K-ow]). Biomagnification factors for individual congeners in salmon did not increase with K-ow-rather, they were constant, as shown by a linear relationship for congener concentration in prey and predator

Large variation in lipid content, ΣPCB and δ13C within individual Atlantic salmon (Salmo salar)

Abstract Many studies that investigate pollutant levels, or use stable isotope ratios to define trophic level or animal origin, use different standard ways of sampling (dorsal, whole filet or whole body samples). This study shows that lipid content, ΣPCB and δ13C display large differences within muscle samples taken from a single Atlantic salmon. Lipid- and PCB-content was lowest in tail muscles, intermediate in anterior-dorsal muscles and highest in the stomach (abdominal) muscle area. Stable isotopes of carbon (δ13C) showed a lipid accumulation in the stomach muscle area and a depletion in tail muscles. We conclude that it is important to choose an appropriate sample location within an animal based on what processes are to be studied. Care should be taken when attributing persistent pollutant levels or stable isotope data to specific environmental processes before controlling for within-animal variation in these variables. Lipid content, ΣPCB and δ13C vary to a large extent within Atlantic salmon, therefore, the sample technique for individual fish is of outmost importance for proper interpretation of data

Assessment of natal origin of pike (Esox lucius) in the Baltic Sea using Sr:Ca in otoliths

Spawning habitat of pike (Esox lucius) in the Baltic Sea include brackish water bays, brooks and rivers. Elevated salinity concentrations are one of several stressors that might increase the use and importance of freshwater habitats for spawning. In the Baltic Sea, one of the largest brackish seas in the world, freshwater species like pike, perch (Perca fluviatilis), whitefish (Coregonus sp), bream (Abramis brama), ide (Leuciscus idus), roach (Rutilus rutilus) and burbot (Lola iota) all undertake spawning migrations to freshwater. However, over the last decades populations densities of these species have declined, and recruitment failure has been argued to be at least part of the problem. The importance of brooks and rivers as spawning areas for these species have not been quantified and set in relation to spawning success in brackish bays. In this study, we collected 175 adult pike (Esox lucius) on their foraging grounds in the sea. Fish were collected in two regions on the Baltic coast, more than 600 km apart. Subsequently we determined their origin (freshwater or marine) using otolith chemistry. Sagittal otoliths were analysed for strontium using the PIXE-method. The results show that 80 of the 175 pike were recruited in freshwater, and several of the larger specimens showed reoccurring migration behaviour. Data show that freshwater is an important recruitment habitat for Baltic Sea pike, suggesting that habitat improvements in rivers entering the Baltic Sea might significantly contribute to population restoration