Erik Mortensen

Bio: Erik Mortensen is an academic researcher. The author has contributed to research in topics: Salmo & Trout. The author has an hindex of 1, co-authored 1 publications receiving 23 citations.

Production of trout,Salmo trutta, in a Danish stream

Abstract: The numbers of trout,Salmo trutta, in Granslev », Denmark, were estimated by the removal method on 18 dates from March 1974 to March 1976. Populations density varied from 0.39 to 0.74 trout m−2 in 1974–1975 and from 0.36 to 0.59 m−2 in 1975–1976 and at all times four or five year classes were present. The age structure of the population was unstable and the variable natural survival, immigration into and emigration from the study site could not be separated. An annual growth cycle with the most rapid growth for all year classes taking place from May to early August was found. Statistically significant differences between different years occurred in the growth of the 0,I and II age groups, but no evidence of density-dependent growth was found. The biomass ranged from 35.4 to 9.5 g m−2. The total mean annual biomass was 22.8 and 14.7 g m−2 in the two years and the II group made the greatest contribution, 44 and 48%, respectively. During 1975–1976 the mean annual biomass of each year class only was about two-thirds of that in 1974–1975. Annual production in the two years was 25.7 (range 24.7–28.5) and 12.6 g m−2 (range 11.7–15.0) and the II group accounted for about 46 and 38% of the production. In addition eel,Anguilla anguilla, produced about 0.5 g m−2 yr−2. The unstable age structure of the trout population was compared with trout populations from other streams. The importance of immigration as a recruitment process in middle and lower reaches of streams and of migrations as a mechanism to optimise utilization of the total stream habitat, as well as temperature as a factor controlling the growth rate are discussed.

Variations in the population dynamics of the European eel Anguilla anguilla (L.) along the course of a Cantabrian river

Abstract: – The size, density and production rate of eel were determined at 16 contrasting sites of 4 streams along the course of a Cantabrian river over the years 1990–1993. In addition, the diel cycles of feeding activity were determined monthly in another tributary over the year 1991–1992. Eels in the Esva were small, short-lived and mostly males (>99%). The seasonal patterns of eel density, feeding activity and condition were tightly fitted with each other, size-independent and regulated mainly by water temperature. Mean eel size increased with distance from the river mouth, but at each site, it remained similar between seasons and years. In contrast, density decreased upstream and showed marked seasonal and annual fluctuations. For all the sites, the number of eels increased in spring and summer and peaked by late autumn, coincident with higher water temperature and lower discharge. Mortality and migration rates were correlated with eel numbers for each stream, suggesting a densitydependent regulation. Production rates were correlated with distance from the river mouth, which explained only 10.2% of the variation, but they were also correlated with the initial and mean numbers, suggesting that site-specific factors acting upon density also influence production. Average production for the first year was 158.6 kg–ha–1. year–1 (range 56.5–378.0) but decreased to 104.2 and 89.7 in the next 2 years. The reduction in the numbers of eels rather than the later reduction in mean size was responsible for this decrease. Monthly water temperature, distance from the river mouth and the initial number and size of eels for each site explain most of the variation observed in the population parameters along the course of the Esva River.

Growth Pattern of Stream-Dwelling Brown Trout under Contrasting Thermal Conditions

Abstract: Growth of brown trout Salmo trutta was analyzed based on 3 years of simultaneous temperature and growth data from seven streams of contrasting chemical and biological character. A laboratory-based growth model was employed to examine geographical variation in growth performance among wild populations. A sensitivity analysis of model predictions was also performed based on simulated optimum temperatures for growth within the range of observed temperatures and on the optimum temperature used in the model. In spite of the diverse environmental conditions, the annual increment in body mass was not significantly different among most populations. However, the ratio of the actual growth rate to the maximum growth rate predicted by the model differed among rivers. A significant negative correlation was found between this ratio and mean annual water temperature. The growth model thus underestimated growth in the coldest rivers, suggesting that adaptations to local thermal conditions can occur at a small g...

Population dynamics of brown trout, Salmo trutta L., in a tributary in Brittany (France): spawning and juveniles

Abstract: The colonization by both resident and migrating spawner populations of brown trout and the characteristics of resident and migrating juveniles derived from the two populations have been studied in a brook and its tributary over 4 years. Resident trout spawns mainly in the upstream part of the brook and migrating trout in the downstream part. There are density and growth variations for the two age classes (0+ and 1 +) of juveniles in autumn according to the year and the environment. In the brook, the population of 0 + fish increases from downstream to upstream while the density of other age classes decreases. The migrating juvenile population of the brook changes annually and consists mainly of 1 s (one summer) individuals coming from the upper part. These individuals migrate generally in autumn and winter while young trout produced in the middle and downstream parts of the brook migrate mainly in the spring. The emigration process of the 0 + population decreases markedly from upstream to downstream and appears to be independent of the autumn length and sex ratio. In the tributary, most trout are 0+ years old, the population structure is different, and no migrating fish is observed. The results are discussed and a colonization strategy of the brown trout population in this brook is suggested.

Latitudinal and altitudinal growth patterns of brown trout Salmo trutta at different spatial scales.

Abstract: Spatial variation in growth of stream-dwelling brown trout Salmo trutta was explored in 13 populations using a long-term study (1993-2004) in the Bay of Biscay drainage, northern Spain. The high variability in fork length (L(F)) of S. trutta in the study area was similar to the body-size range found in the entire European distribution of the species. Mean L(F) at age varied: 0+ years, 57.4-100.7 mm; 1+ years, 111.6-176.0 mm; 2+ years, 155.6-248.4 mm and 3+ years, 194.3-290.9 mm. Average L(F) at age was higher in main courses and lower reaches compared with small tributaries and upper reaches. Annual specific growth rates (G(L)) were: 0+ to 1+ years, 0.634-0.825 mm mm(-1) year(-1); 1+ to 2+ years, 0.243-0.342 mm mm(-1) year(-1); 2+ to 3+ years, 0.166-0.222 mm mm(-1) year(-1), showing a great homogeneity. Regression models showed that water temperature and altitude were the major determinants of L(F) at age variability within the study area. A broader spatial analysis using available data from stream-dwelling S. trutta populations throughout Europe indicated a negative relationship between latitude and L(F) of individuals and a negative interaction between latitude and altitude. These findings support previous evidence of the pervasive role of water temperature on the L(F) of this species. Altitude appeared as the overall factor that includes the local variation of other variables, such as water temperature or food availability. At a larger scale, latitude was the factor that encompassed these environmental gradients and explained the differences in L(F) of S. trutta. In summary, L(F) at age in stream-dwelling S. trutta decreases with latitude in Europe, the converse of Bergmann's rule.

A conceptual model for examining density dependence in the growth of stream trout

Abstract: Determination of the occurrence and importance of densitydependent responses is central to understanding stream trout population dynamics. I propose a conceptual model of growth, based on a distribution of feeding site quality, that considers the effects of density on growth. The site quality model assumes that trout select the best feeding sites available and, as the number of trout increases, they will be forced to use less energetically profitable sites, resulting in decreased growth, but also an increase in variance of size or growth. Results from a 3-year study of a brown trout (Salmo trutta) population show that growth to age 1 was significantly reduced for the 1981 year class, which was about 4 times more dense than other year classes. The reduction in growth was not due to a decrease in the growth of all trout but rather an increase in the number of slower growing fish. These results are consistent with my site quality distribution model and suggest that individual fish growth, the distribution of growth, and the variance of growth rates should be considered in addition to mean size and growth.