Ferox Trout (Salmo trutta) as `Russian dolls': complementary gut content and stable isotope analyses of the Loch Ness foodweb

1. Conventional collection methods for pelagic fish species (netting, trawling) are impractical or prohibited in Loch Ness, U.K. To investigate trophic relationships at the top of the Loch Ness food web, an alternative strategy, angling, provided samples of the top predator, the purely piscivorous ferox trout ( Salmo trutta ).
2. The gut contents of these fish provided further samples of prey-fish, and subsequent examination of prey-fish guts revealed their dietary intake, analogous to the famous nested `Russian dolls'. Each trophic level separated by gut content analysis provided further complementary samples for stable isotope analysis and thus information on the longer term, assimilated diet.
3. Ferox trout exhibited considerable cannibalism to supplement a diet of Arctic charr ( Salvelinus alpinus ). However, conspecifics stemmed from a lower isotopic baseline in relation to charr, so ferox trout exhibited a lower trophic level than predicted (4.3) by using the δ¹⁵N values. Charr displayed dietary specialisation with increasing length, and isotopic values supported by the gut data placed the charr at a trophic level of 3.5. The isotope data also indicated that charr carbon was primarily autochthonous in origin.     

Decline of the Migratory Form in Bull Charr, Salvelinus Confluentus, and Implications for Conservation

Large-bodied, migratory life history forms of bull charr, Salvelinus confluentus, were historically abundant in northwestern North America, but many remaining populations of this now-threatened species presently persist as small-bodied residents isolated in headwater streams. We examined whether the migratory form has been lost from headwater populations of bull charr and their potential for re-establishment. Upstream and downstream movement of bull charr and other salmonids from three tributary populations in the Bitterroot River drainage, Montana, was measured with weirs over a 17-month period. The migratory life history was rare or absent in two tributaries but still present at a low level in a third. In contrast, substantial numbers (n = 1745) of juvenile and adults of other salmonids (brown trout, Salmo trutta, cutthroat trout, Oncorhynchus clarki, and mountain whitefish, Prosopium williamsoni) were captured near tributary mouths, indicating a migratory life history was common in other species. Apparent decline of the migratory life history in bull charr was not directly related to damming suggesting other downstream mortality factors (predation, temperature) also are involved. Isolated, nonmigratory forms have increased risk of extinction, and restoration of the population connectivity via the re-establishment of migratory stocks is an important conservation goal for bull charr recovery. However, the factors governing migratory tendency remain unclear.     

Tracing of labile zinc in live fish hepatocytes using FluoZin-3

Intracellular zinc levels are homeostatically regulated and although most is bound, a pool of labile Zn(II) is present in cells. We show here that the zinc probe FluoZin-3 is useful to monitor zinc fluxes during fluorescent imaging of the trout hepatic cell line D11. Nuclei and bulk cytosol appeared to lack detectable labile zinc, while the punctuate staining pattern colocalized with a lysosome-specific probe. Applying extracellular zinc alone resulted in vesicular sequestration of the metal ion. Together with Na-pyrithione a delayed and toxic rise in cellular fluorescence was triggered. When using another ionophore, 4-Br A23187, a zinc buffering effect of the vesicular pools was evident. Secondly, N-ethylmaleimide induced a homogeneous fluorescence rise, which was strongly enhanced by addition of Zn-pyrithione and disappeared after TPEN washing. This suggests the involvement of thiol residues in controlling available cytosolic zinc. Our observations have implications for the interpretation of calculated intracellular Zn²⁺ concentrations.     

Gastro-intestinal transport of calcium and cadmium in fresh water and seawater acclimated trout (Oncorhynchus mykiss)

Transport of calcium (Ca) and cadmium (Cd) was examined along the gastro-intestinal tract (GIT) of freshwater and seawater Oncorhynchus mykiss irideus (FWT and SWTies respectively) using in vitro and in vivo experiments. Based on known physiological differences between FWT and SWT which aid in regulating ion levels and osmolarity, we hypothesized that SWT would have lower rates of Ca uptake. Also, we predicted that Cd rates would also be lower because Cd is known to share a common transport mechanism with Ca. Kinetics of Ca and Cd transport were determined using mucosal salines of varying concentrations [1, 10, 30, 60, and 100 (mmol L^− 1 for Ca, μmol L^− 1 for Cd)]. Linear and saturating relationships were found for Ca for FWT and SWT, but overall SWT had lower rates. Linear and/or saturating relationships were also found for Cd uptake, but rates varied little between fish types. Elevated Ca had no inhibitory effect on Cd transport, and Ca channel blockers nifedipine and verapamil had little effect on Ca or Cd uptake. However, lanthanum reduced Ca transport into some compartments. A 21 day in vivo feeding experiment was also performed where FWT and SWT were exposed to control diets or Cd-spiked diets (552 μg Cd g^− 1 food). Whole body Cd uptake between fish types was similar, but the majority of Cd in SWT remained in the posterior intestine tissue, while FWT transported more Cd through their gut wall. Overall it appears that large differences in Ca and Cd uptake between FWT and SWT exist, with SWT generally having lower rates.     

Effects of gravel size and peat material concentrations on embryo survival and alevin emergence of brown trout,Salmo trutta L.

Embryo survival and alevin emergence pattern of brown trout were studied in simulated redds with different homogeneous gravel sizes and different concentrations of peat material. Optimal survival (95%) occurred in 18 mm gravel and survival decreased with decreasing gravel size. High concentrations (40%) of peat material resulted in low survival (65%). The proportion of premature emerging alevins increased in finer gravels and at high peat concentrations. Premature alevins had a large yolk sac and are probably very vulnerable to predators.     

Unidirectional Na(+) and Ca (2+) fluxes in two euryhaline teleost fishes, Fundulus heteroclitus and Oncorhynchus mykiss, acutely submitted to a progressive salinity increase

Na⁺ and Ca²⁺ regulation were compared in two euryhaline species, killifish (normally estuarine-resident) and rainbow trout (normally freshwater-resident) during an incremental salinity increase. Whole-body unidirectional fluxes of Na⁺ and Ca²⁺, whole body Na⁺ and Ca²⁺, and plasma concentrations (trout only), were measured over 1-h periods throughout a total 6-h protocol of increasing salinity meant to simulate a natural tidal flow. Killifish exhibited significant increases in both Na⁺ influx and efflux rates, with efflux slightly lagging behind efflux up to 60% SW, but net Na⁺ balance was restored by the time killifish reached 100% SW. Whole body Na⁺ did not change, in agreement with the capacity of this species to tolerate daily salinity fluctuations in its natural habitat. In contrast, rainbow trout experienced a dramatic increase in Na⁺ influx (50-fold relative to FW values), but not Na⁺ efflux between 40 and 60% SW, resulting in a large net loading of Na⁺ at higher salinities (60–100% SW), and increases in plasma Na⁺ and whole body Na⁺ at 100% SW. Killifish were in negative Ca²⁺ balance at all salinities, whereas trout were in positive Ca²⁺ balance throughout. Ca²⁺ influx rate increased two- to threefold in killifish at 80 and 100% SW, but there were no concomitant changes in Ca²⁺ efflux. Ca²⁺ flux rates were affected to a larger degree in trout, with twofold increases in Ca²⁺ influx at 40% SW and sevenfold increases at 100% SW. Again, there was no change in Ca²⁺ efflux with salinity, so plasma Ca²⁺ concentration increased in 100% SW. As the killifish is regularly submitted to increased salinity in its natural environment, it is able to rapidly activate changes in unidirectional fluxes in order to ensure ionic homeostasis, in contrast to the trout.     

Effects of vitamin B1 (thiamine) deficiency in lake trout (Salvelinus namaycush) alevins at hatching stage

The objectives of this study were to examine the relationship between thiamine concentrations in unfertilized eggs and yolksac individuals of lake trout (Salvelinus namaycush), along with any associated histopathological changes in the tissues of alevins at the hatching stage. We address these questions in a lake trout population from different spawning grounds of Lake Michigan (North and South), known for compromised survival due to early mortality syndrome (EMS). However, a dichotomous forage base of lake trout spawning stocks, with a dietary thiaminase-rich alewife in the North, and dietary low-thiaminase round goby in the South, provides the basis for the assumption that different diets may lead to differences in severity of EMS between different stocks. Lake trout eggs of 18 females were collected and fertilized individually with the sperm of several males. The eggs, eyed embryos and newly-hatched alevins were sampled to examine thiamine utilization during embryogenesis. Progenies of females with low (< 0.73 nmol/g) and high (> 0.85 nmol/g) levels of thiamine were chosen for histological studies. The obtained results showed that total thiamine levels in the body and yolk of eyed embryos and alevins at hatching were influenced by thiamine levels of unfertilized eggs and it decreased during embryogenesis (to 51% in eyed embryos and 28% in newly-hatched alevins in comparison to unfertilized eggs). The survival of lake trout until hatching stage does not correlate with the thiamine level, however it was affected by collection site and was significantly higher in fish from the South site (Julian's Reef). At the hatching stage, no pathological changes were observed in the brain, olfactory lobe, retina or liver in embryos regardless of thiamine concentrations in unfertilized eggs. It has been concluded that an enhanced thiamine requirement for the fast muscle mass growth near the swim-up stage is responsible for overt and histopathological signs of EMS. Current study confirms earlier findings that lake trout suffering from EMS can be successfully treated by immersion in thiamine solution as late as at the swim-up stage.     

Increased accuracy of batch fecundity estimates using oocyte stage ratios in Plectropomus leopardus

Using the ratio of the number of migratory nuclei to hydrated oocytes to estimate batch fecundity of common coral trout Plectropomus leopardus increases the time over which samples can be collected and, therefore, increases the sample size available and reduces biases in batch fecundity estimates.     

Evidence for interspecific hybridization between native white-spotted charr Salvelinus leucomaenis and non-native brown trout Salmo trutta on Hokkaido Island, Japan

Hybrids between native white-spotted charr Salvelinus leucomaenis and non-native brown trout Salmo trutta were identified in streams of Hokkaido, Japan, using both appearance and genetic characters. The DNA analyses indicated that the specimens were hybrids between female S. leucomaenis and male S. trutta . Occurrence of such hybrids implies increased mating opportunities between these species in wild streams.