The European otter (Lutra lutra) was common in Denmark until the 1960s, but its present distribution encompasses only a minor part of the country. The aim of this study was to assess whether the recent population decline has resulted in loss of genetic variability and to gain further insight into the dynamics of the population decline. This was done by analysing microsatellite DNA variation in contemporary and historical samples, the latter encompassing DNA samples extracted from museum specimens covering a time-span from the 1880s to the 1960s. Tests for di¡erences in expected heterozygosity and the numbers of alleles in contemporary versus historical samples and a test for detecting population bottlenecks provided few indications of a recent bottleneck and loss of variability. However, a procedure for detecting population expansions and declines, based on the genealogical history of microsatellite alleles, suggested that a drastic long-term population decline has taken place, which could have started more than 2000 years ago, possibly due to ancient anthropogenic pressure. Finally, assignment tests and pairwise F ST values suggested weak but statistically signi¢cant genetic di¡erentiation between the extant population and historical samples of otters from other regions in Denmark, more likely re£ecting di¡erentiation among original populations rather than recent drift.

Acoustic positional telemetry systems (APTs) represent a novel approach to study the behaviour of free ranging aquatic animals in the wild at unprecedented detail. System manufactures promise remarkably high temporal and spatial resolution. However, the performance of APTs has rarely been rigorously tested at the level of entire ecosystems. Moreover, the effect of habitat structure on system performance has only been poorly documented. Two APTs were deployed to cover two small lakes and a series of standardized stationary tests were conducted to assess system performance. Furthermore, a number of tow tests were conducted to simulate moving fish. Based on these data, we quantified system performance in terms of data yield, accuracy and precision as a function of structural complexity in relation to vegetation. Mean data yield of the two systems was 40 % (Lake1) and 60 % (Lake2). Average system accuracy (acc) and precision (prec) were Lake1: acc = 3.1 m, prec = 1.1 m; Lake2: acc = 1.0 m, prec = 0.2 m. System performance was negatively affected by structural complexity, i.e., open water habitats yielded far better performance than structurally complex vegetated habitats. Post-processing greatly improved data quality, and sub-meter accuracy and precision were, on average, regularly achieved in Lake2 but remained the exception in the larger and structurally more complex Lake1. Moving transmitters were tracked well by both systems. Whereas overestimation of moved distance is inevitable for stationary transmitters due to accumulation of small tracking errors, moving transmitters can result in both over- and underestimation of distances depending on circumstances. Both deployed APTs were capable of providing high resolution positional data at the scale of entire lakes and are suitable systems to mine the reality of free ranging fish in their natural environment. This opens important opportunities to advance several fields of study such as movement ecology and animal social networks in the wild. It is recommended that thorough performance tests are conducted in any study utilizing APTs. The APTs tested here appear best suited for studies in structurally simple ecosystems or for studying pelagic species. In such situations, the data quality provided by the APTs is exceptionally high.

The diel activity levels and spatial distribution of roach Rutilus rutilus differed markedly between two shallow lakes of different environmental state. The movements of roach (12-25 cm L T ), with surgically implanted mini-radio transmitters, were monitored regularly during several 48 h tracking sessions in a clearwater and in a turbid lake. In both lakes, the roach in general were most active during dawn and dusk and least active during the night. Activity level in midsummer was lowest around noon in the clear lake and high around noon in the turbid lake. In summer, roach in the clear lake stayed passively in a restricted area of water lilies during the day and moved into the central part of the lake during the night. In the turbid lake, roach were dispersed all over the lake during the day and moved close to the shoreline at night. Predator : prey fish ratios did not differ in the two lakes, however the observed behaviour of roach in the clearwater lake may be explained by a larger predation pressure from fish and birds, both being favoured in the clear water. # 2004 The Fisheries Society of the British Isles

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Abstract— This study investigated interactions between zooplanktivores (roach and perch) and piscivorcs (pike and large perch) in experimental ponds (16 m2) with open water habitat and three densities of natural macrophytes. Fish habitat selection was determined both day and night and was supported by daytime observations to study anti‐predator behavioural patterns. Diel migration out from among macrophytes was seen in the absence of predators, particularly for roach, which changed from 13% of individuals being in open water during the day to 90% at night. The risk of predation from piscivores influenced the habitat selection of the zooplanktivores. Roach seemed to be the most vulnerable to predation from pike and selected the open water (90‐92%) during daylight hours, but kept a 1‐m distance from the macrophytes edge. The presence of pike thus reduced the use of macrophytes by roach, which in turn may improve macrophytes and the edge area as a refuge for zooplankton. Pike appeared to have less impact on the gross habitat selection of O+ perch, which were associated with the macrophytes (58–89%), though they were still vulnerable to predation. Adult perch, which was a generally less effective predator than pike, showed subdued behavior, concealing them selves in the macrophytes most of the time. No effect of the simultaneous presence of two predator species was found.