Abstract
The number of cormorants has rapidly increased in the northernmost Baltic Sea. In 2018, 50 km × 50 km ICES catch rectangle 55H1 had 3140 breeding pairs. To estimate the predation effect of ...cormorants on perch populations, we Passive Integrated Tags tagged 1977 perch and 9.9% of tags were found. The median instantaneous cormorant-induced mortality during the breeding time, with consumption by non-breeding individuals, was estimated at 0.23 and at 0.35 during the whole residing period. We estimated with a yeild-per-recruit model that the long-term maximum loss of perch yield of tagged sub-population would be at 80% probability interval 32–67%, and when extended to the entire 55H1, 10–33%, respectively. The cormorants’ share of the >2-year-old perch biomass and production would be 8%, while that of other natural mortality would be 63% and that of fishing 29% in 55H1. The yield-per-recruit-results should be interpreted as an estimate of maximum cormorant effect because the dependence of predation rate on prey density was not accounted for, and density-dependence of growth, mortality, and reproduction of perch could partly compensate the loss. The results indicate that high density of cormorants can reduce the perch stocks and catches locally.
Hatching time (HT) of autumn-spawning fishes depends strongly on the egg incubation temperature and especially on the warming of water in spring, which synchronizes the hatching with ice-out despite ...the large inter-annual variability in spring phenology. However, the relative roles of genetic and environmental effects on the HT have rarely been explored. We studied experimentally the parental effects on the HT and size of vendace (
Coregonus albula
(L.)) and whitefish (
C. lavaretus
L.) larvae under short and long winter conditions using a full-factorial breeding design. Both parents significantly affected the HT of vendace, mostly by additive genetic effects, and the difference between short and long winter treatment was also significant. In whitefish, the female × male interaction was significant, implying non-additive genetic effects. The maximum range of the HT of eggs between parent pairs within certain winter condition was 3 weeks and was clearly lower than the potential range for the temperature-adjusted HT. The size of eggs or hatched larvae did not correlate with the HT in either of the species. The variation in HT between eggs from different parents creates a basis for genetic adaptation to climate change and for local adaption of populations in their thermal environments.
► We compared two common survey methods of aquatic macrophytes in boreal lakes. ► More taxa in less time were found with a phytolittoral inventory (PI) than with a transect survey (TS). ► Metrics of ...taxonomic composition varied more by method than did a trophy-specific index. ► The methods did not differ in relation to among-observer variation. ► The PI method is recommended for biodiversity surveys and the TS for ecological status assessments.
To compare the performance of two common methods for surveying boreal lake aquatic macrophyte communities (the transect survey and the phytolittoral inventory) in lake bioassessment, we studied five small humic lakes with both methods. In addition to this pairwise comparison, we contrasted available independent sets of reference lake data of either methodological origin. Specifically, we compared the observed species richness and a number of ecological quality (community) metrics and their variability both between the methods and within them in relation to sampling effort and operator. With the phytolittoral inventory, more taxa (mean 38.8±s.d. 12.1) were observed than with transect survey (29.8±6.5) in a shorter time. Ecological quality metrics relying on taxonomic composition were sensitive to variation both between (reference lake data: t-test, p<0.001) and within methods (metric values varied with sample size), whereas a trophy-specific metric based on indicator species was more robust in this respect (ref. lakes: t-test, p=0.788; minor variation with sample size). The two methods did not differ in among-observer variation (CV of community metrics 5.0–8.7), which was, as indicated by an NMDS ordination, relatively low compared to among-lake variation. Overall, the results highlight the importance of establishing reference conditions using unified methods, which are also consistent with those used in the macrophyte-based ecological assessments. Even though the more cost efficient phytolittoral inventory method is more suitable for biodiversity surveys focusing on the overall species richness and rare species, we suggest that the transect survey better meets the requirements of ecological status assessment, as it produces information also on the zonation of the littoral vegetation.
The character and magnitude of predation by the invasive, ectothermic Pacifastacus leniusculus, a crayfish widely introduced to Europe and Japan from North America, on the eggs of coregonid fishes, ...vendace Coregonus albula and whitefish Coregonus lavaretus were examined by experimentation, modelling and field data. The present results showed that P. leniusculus has the potential to be very efficient predator of fish eggs under winter conditions, but the predation by P. leniusculus did not significantly decrease production of coregonid larvae during the years with a high P. leniusculus population in the study lake. Hence, the mortality caused by the novel invertebrate predator appeared to compensate for other yet unexplored mortality factors instead of having an additive effect on the present salmonids.
Stable isotope analyses are increasingly employed to characterise population niche widths. The convex hull area (TA) in a δ(13)C-δ(15)N biplot has been used as a measure of isotopic niche width, but ...concerns exist over its dependence on sample size and associated difficulties in among-population comparisons. Recently a more robust method was proposed for estimating and comparing isotopic niche widths using standard ellipse areas (SEA), but this approach has yet to be tested with empirical stable isotope data. The two methods measure different kind of isotopic niche areas, but both are now widely used to characterise isotopic niche widths of populations. We used simulated data and an extensive empirical dataset from two fish populations to test the influence of sample size on the observed isotopic niche widths (TA and SEA). We resampled the original datasets to generate 5000 new samples for different numbers of observations from 5 to 80 to examine the statistical distributions of niche area estimates for increasing sample size. Our results illustrate how increasing sample size increased the observed TA; even sample sizes much higher than n = 30 did not improve the precision for the TA method. SEA was less sensitive to sample size, but the natural variation in our empirical fish δ(13)C and δ(15)N data still resulted in considerable uncertainty around the mean estimates of niche width, reducing the precision particularly with sample sizes n<30. These results confirm that the TA method is less appropriate for estimating population isotopic niche areas using small samples, especially when considerable population level isotope variation is expected. The results also indicate a need for caution when using SEA as a measure of trophic niche widths for consumers, particularly with low sample sizes and when the distribution and range for population isotope values are not known.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The stomach content of pikeperch Sander lucioperca was studied in 24 lakes in central Finland. The most important prey species was smelt Osmerus eperlanus then perch Perca fluviatilis. The proportion ...of cyprinids as prey increased with pikeperch total length (LT) and lake phosphorus content. Prey LT and pikeperch LT correlated positively, but the prey‐to‐predator size ratio correlated negatively with pikeperch LT.
In the auto-regulative commercial vendace fishery, the fishing effort is increased if stock decreases in order to reduce interannual variation in supply and revenue. When the revenue per unit effort ...reaches a profitability threshold, fishing ceases. The performance of this strategy was compared with constant effort and constant revenue strategies combined with different threshold levels of fishing shut-down. A simulation model of an age-structured vendace population was applied. Qualitatively the results were very similar for the Ricker and Cushing stock-recruitment curves with or without negative dependence between consecutive recruitments. The interannual variability of revenue increased with an increase in mean revenue when approaching maximum mean annual revenue. Given a low threshold, the constant effort strategy performed best and constant yield worst with respect to the objectives of maximum mean revenue, low mean effort, and low risk for spawning biomass. For the constant effort strategy, the revenue variation was already high at low revenue levels. For the strategy aiming at constant revenue, the variation was low at a low revenue level but increased rapidly with increasing revenue. At a moderate revenue level, the intermediate strategy produced good trade-offs of rather high mean revenue with reasonable variation and risk to reproduction. A tendency for two-year-cyclicity appeared at a low mean spawning stock level. When the threshold of fishery shut-down was increased, the variability in revenue increased for every strategy and differences in their performance decreased. Simultaneously, the risk of low spawning stock decreased. The threshold regulation increased the instability of stock in cases of high fishing effort. Means for regulating the access to fishing should be established in order to ensure the possibility of flexibility of fishing effort and tolerable revenue variation. Some threshold regulation to protect the spawning stock is also necessary.
The effect of environmental factors on migratory activity of adult river lampreys entering the Kalajoki and Perhonjoki, rivers in Finland, for spawning was studied using correlation and regression ...analyses. Telemetric tracking of 60 individuals was utilized to study the migratory patterns and holding habitat requirements of adult river lampreys. The increases in the river discharge, wind forcing towards the river mouth and speed and magnitude of river water cooling had positive effects on the numbers of lampreys entering the rivers whereas the increase in the illumination intensity of the moon had a negative effect on their migration activity. Radio-tagged lampreys typically passed slow-flowing river sections as well as steep riffles during one night, and were holding in runs, glides and the lowermost section of riffles. Substratum dominated by large boulders (> 256 mm) was preferred during winter holding behaviour. The migratory activity of lampreys released in low (< 2 °C) and high (> 16 °C) river water temperatures was low. Lampreys tended to halt next to illuminated bridges, and we suggest that this behaviour markedly shortened migration distances. The passage efficiency through a natural-like fish ramp in the low-head barrier was 100%. The results can be utilized in mitigation actions like river restoration, transplanting of adults, and improving passage and water flow regulation.
We used warm summer temperatures as an analogue of climate change to estimate the potential impacts on a population of smelt (Osmerus eperlanus) in a boreal lake. A combination of hydroacoustics, ...seine and gillnet catches with temperature and dissolved oxygen profiles was used to study the changes in the abundance, vertical distribution and age structure of the smelt population. Smelt were squeezed between their temperature and dissolved oxygen tolerance limits during most summers. Conditions in 2002 were, however, extreme and mean surface-water temperatures exceeded 21°C while at the same time waters below 6 m had low (< 0.5 mg l.sup.-l) dissolved oxygen concentration. Pelagic fish density during summer declined from 5000 to 200 fish ha.sup.-1. However, some young-of-the-year smelt survived in 2002, leading to a recovery in abundance to 4500 fish ha.sup.-1 by 2004. A similar recovery is not likely if the high temperature and low oxygen conditions persist during critical periods over a generation time.
Numerical simulations and empirical measurements of swimming Atlantic salmon (Salmo salar) were used to describe the effects of fish behavior on side-aspect target strength (TS). Simulation results ...were based on the numerical solution of the Helmholtz equation with the finite element method (FEM). A three-dimensional geometric model approximated the shape of the swimbladder of an Atlantic salmon. Numerical simulations were used to study the dependence of TS on the fish length, orientation, and swimming behavior. The results showed strong variation in TS, both when the side-aspect angle was changed and when the swimbladder was bent to the direction of the sonar beam. A total of 11 swimming adult Atlantic salmon 62107 cm long were measured with a horizontally aimed echosounder (200 kHz) and video camera, and the experimental results were compared with the corresponding simulation results. The linear regression between mean TS and the logarithm of fish length (L, cm) was TS = 24.4log
10
(L) 72.9 dB. The strong variability of TS owing to the orientation and bending of the fish and large L/λ ratios reduces the usefulness of TS alone for fish size estimation or species discrimination.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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