Search

Hydroacoustic acquisition was performed by means of two different single beam systems, the Simradmodel EK15 from 2015 and the Simrad model EY-M from the 1980s to explore potential differences of fish density and target strength (TS) distribution between EK15 and EY-M-based estimates. The oligotrophic Lake Storsjøen (48 km2) with steep shores, was chosen for the survey. The pelagic fish stock is dominated by whitefish (Coregonus lavaretus L), and, recently, illegally stocked smelt (Osmerus eperlanus L), and a low proportion of Arctic charr (Salvelinus alpinus L.). The total density was estimated at two depth layers (18–32 and 32–48 m), and was quite similar for TS ≥ −56 dB: 755 and 498 fish ha−1, respectively for the EK15-based estimates and similarly 766 and 490 fish ha−1 for the EY-M estimate. Target strength distributions were similar for TS > −48 dB. The proportion of single fish detected with EK15 was negatively affected by the long pulse duration. Six acquisitions from 1986 to 2016 showed a dramatic increase of density of TS = −46 to −44 dB echoes (>10 cm) between 2013 and 2016. This was due to the growth of the introduced smelt population. [ABSTRACT FROM AUTHOR]

Introduction: It is important to understand the drivers leading to adaptive phenotypic diversity within and among species. The threespine stickleback (Gasterosteus aculeatus) has become a model system for investigating the genetic and phenotypic responses during repeated colonization of fresh waters from the original marine habitat. During the freshwater colonization process there has been a recurrent and parallel reduction in the number of lateral bone plates, making it a suitable system for studying adaptability and parallel evolution. Objective: The aim of this study was to investigate an alternative evolutionary path of lateral plate reduction, where lateral plates are reduced in size rather than number. Materials and Methods: A total of 72 threespine stickleback individuals from freshwater (n = 54), brackish water (n = 27) and marine water (n = 9) were analysed using microcomputed tomography (μCT) to determine variation in size, thickness and structure of the lateral plates. Furthermore, whole-body bone volume, and bone volume, bone surface and porosity of lateral plate number 4 were quantified in all specimens from each environment. Results: The results showed a significant difference in plate size (area and volume) among populations, where threespine stickleback from polymorphic freshwater and brackish water populations displayed lateral plates reduced in size (area and volume) compared to marine stickleback Conclusions: Reduction of lateral plates in threespine stickleback in fresh and brackish water occurs by both plate loss and reduction in plate size (area and volume). [ABSTRACT FROM AUTHOR]

Background: Studying how trophic traits and niche use are related in natural populations is important in order to understand adaptation and specialization. Here, we describe trophic trait diversity in twenty-five Norwegian freshwater threespine stickleback populations and their putative marine ancestor, and relate trait differences to postglacial lake age. By studying lakes of different ages, depths and distance to the sea we examine key environmental variables that may predict adaptation in trophic position and habitat use. We measured trophic traits including geometric landmarks that integrated variation in head shape as well as gillraker length and number. Trophic position (Tpos) and niche use (a) were estimated from stable isotopes (δ13C, δ15N). A comparison of head shape was also made with two North American benthic-limnetic species pairs. Results: We found that head shape differed between marine and freshwater sticklebacks, with marine sticklebacks having more upturned mouths, smaller eyes, larger opercula and deeper heads. Size-adjusted gillraker lengths were larger in marine than in freshwater stickleback. Norwegian sticklebacks were compared on the same head shape axis as the one differentiating the benthic-limnetic North American threespine stickleback species pairs. Here, Norwegian freshwater sticklebacks with a more "limnetic head shape" had more and longer gillrakers than sticklebacks with "benthic head shape". The "limnetic morph" was positively associated with deeper lakes. Populations differed in a (mean ± sd: 0.76 ± 0.29) and Tpos (3.47 ± 0.27), where a increased with gillraker length. Larger fish had a higher Tpos than smaller fish. Compared to the ecologically divergent stickleback species pairs and solitary lake populations in North America, Norwegian freshwater sticklebacks had similar range in Tpos and a values, but much less trait divergences. Conclusions: Our results showed trait divergences between threespine stickleback in marine and freshwater environments. Freshwater populations diverged in trophic ecology and trophic traits, but trophic ecology was not related to the elapsed time in freshwater. Norwegian sticklebacks used the same niches as the ecologically divergent North American stickleback species pairs. However, as trophic trait divergences were smaller, and not strongly associated with the ecological niche, ecological adaptations along the benthic-limnetic axis were less developed in Norwegian sticklebacks. [ABSTRACT FROM AUTHOR]