Stig Pedersen

Effects of density on foraging success and aggression in age-structured groups of brown trout

The benefit of monopolizing a limited resource is influenced by competitor density and by the relative competitive ability of defenders and intruders. Nevertheless, few studies have investigated the effect of density on resource defence in groups with large asymmetries in competitive ability, as a consequence of, for example, age and/or body size. We used two age classes (i.e. size groups) of stream-living brown trout, Salmo trutta, to investigate this issue. While old (and large) trout are assumed to be superior during interference competition, younger individuals may be both numerically dominant and constitute more than half of the total population biomass. In this experiment, the ability of one yearling to monopolize a concentrated food source was observed at four densities of under-yearlings (zero, two, six and 12) in an indoor seminatural stream. We predicted that the success of defence would decrease with increasing under-yearling density and that the frequency of defence (i.e. aggression) would peak at an intermediate density. As predicted, yearlings made significantly more unsuccessful foraging attempts and adopted darker body coloration at high density of under-yearlings, suggesting increased stress levels. However, in contrast to our second prediction, the number of aggressive interactions increased progressively with density. These novel findings suggest that the cost of defence increases with under-yearling density, probably as a consequence of stress from interference with under-yearlings employing alternative competitive strategies. However, the difference in size seems to enable yearlings to defend the food resource at higher density of competitors than predicted from the resource defence theory.

Non-indigenous signal crayfish Pacifastacus leniusculus are now common in Danish streams: preliminary status for national distribution and protective actions

North American signal crayfish (Pacifastacus leniusculus) are invasive in Europe and pose a serious threat to indigenous European crayfish such as the noble crayfish (Astacus astacus). This is mainly because signal crayfish is the carrier of crayfish plague agent, Aphanomyces astaci, which freshwater crayfish from all other continents are highly susceptible to. Until recently, the distribution of signal crayfish in Danish streams has been considered local and restricted to a small geographical area. Here we present data demonstrating that signal crayfish are now widespread in Denmark, including the largest Danish river, River Gudenå. For one of the rivers where co-existing signal crayfish and indigenous noble crayfish were documented, sensitive molecular tests could not detect the crayfish plague agent Aphanomyces astaci in either species. Hence, it seems that not all signal crayfish are chronic carriers of the disease. For the remaining freshwater systems with the introduced signal crayfish, the infection status is presently unknown. Large areas of the freshwater systems in Denmark also remain unexplored with respect to presence/absence of signal crayfish and noble crayfish. Nevertheless, our preliminary data that covers about 14% of the Danish rivers, strongly suggests that signal crayfish should be considered as a common invader that poses an increased threat to the biota in Danish streams, in particular for the indigenous noble crayfish.

Habitat suitability indices development in Denmark: are international indices applicable under small lowland stream conditions?

The transferability of habitat suitability indices (HSIs) remains a controversial issue within physical habitat-based modelling. This issue is often exacerbated when the indices are obtained from international sources without adequate transferability tests being conducted. The aim of this paper was to assess the applicability of seven sets of HSIs (three local and four international) to small lowland Danish streams applied to large parr brown trout (Salmo trutta), an important ecological indicator in these systems. Using a simple hierarchical literature review, the applied indices were ranked firstly for their biological and physical similarities to the local Danish small lowland stream conditions, secondly on the methods used to devise the HSIs and collect the field data, and thirdly if they were locally or internationally sourced. The review ranked background information hierarchically (from most important to least) provided with the HSIs literature: (1) size class of species, (2) season, (3) river type, (4) discharge level, (5) HSIs type, (6) HSIs category, (7) data processing methods, (8) sampling methods, and (9) HSIs country of origin (local or international). To test the effectiveness of the hierarchical review-based system to rank the applicability of the HSIs, the impact of using the different HSIs for physical habitat-based modelling was tested using the hydraulic-habitat model RHYHABSIM. A set of locally derived indices were used as the baseline and compared to the other HSIs. There were large differences in the hydraulic-habitat model predictions of the maximum weighted usable habitat area (WUAmax, ranging from 0.459 to 1.023 m2m−1) and the corresponding optimum discharge (Q opt, ranging from 0.170 to 0.315 m3s−1). This emphasizes that model output is very dependent on the set of HSIs chosen; so careful consideration in the selection process is needed. The review ranking system compared favourably to the physical habitat model results, indicating that it is possible to transfer HSIs from international sources if the biological and physical conditions from their source study and stream(s) were similar to the target stream, irrespective of the country of origin. The study shows promising results for water managers wanting to use already developed HSIs by transferring international HSIs even when using simple transferability tests such as a hierarchical literature review.