Ecological commonalities among pelagic fishes: comparison of freshwater ciscoes and marine herring and sprat

Systematic comparisons of the ecology between functionally similar fish species from freshwater and marine aquatic systems are surprisingly rare. Here, we discuss commonalities and differences in evolutionary history, population genetics, reproduction and life history, ecological interactions, behavioural ecology and physiological ecology of temperate and Arctic freshwater coregonids (vendace and ciscoes, Coregonus spp.) and marine clupeids (herring, Clupea harengus, and sprat, Sprattus sprattus). We further elucidate potential effects of climate warming on these groups of fish based on the ecological features of coregonids and clupeids documented in the previous parts of the review. These freshwater and marine fishes share a surprisingly high number of similarities. Both groups are relatively short-lived, pelagic planktivorous fishes. The genetic differentiation of local populations is weak and seems to be in part correlated to an astonishing variability of spawning times. The discrete thermal window of each species influences habitat use, diel vertical migrations and supposedly also life history variations. Complex life cycles and preference for cool or cold water make all species vulnerable to the effects of global warming. It is suggested that future research on the functional interdependence between spawning time, life history characteristics, thermal windows and genetic differentiation may profit from a systematic comparison of the patterns found in either coregonids or clupeids

Observing individual fish behavior in fish aggregations: Tracking in dense fish aggregations using a split-beam echosounder

Acoustic instruments are important tools for observing the behavior of aquatic organisms. This paper presents a simple but efficient method for improving the tracking of closely spaced targets using a split-beam echosounder. The traditional method has been a stepwise approach from the detection of echoes, rejection of apparently multiple targets and then tracking the remainder. This is inefficient because the split-beam angles are not included in the initial detection; rather they are only used in the rejection criteria before the subsequent tracking. A simple track-before-detection method is presented, where the phase angles, echo intensities, ranges, and times are used simultaneously, resulting in better detection and tracking of the individual fish. Two test data sets were analyzed to determine the effectiveness of this method at discriminating individual tracks from within dense fish aggregations. The first data set was collected by lowering a split-beam transducer into a herring layer. The second data set, also collected with a split-beam transducer, was from a caged aggregation of feeding herring larvae. Results indicate the potential of target tracking, using a split-beam echosounder, as a tool for understanding interindividual behavior

Planktivore vertical migration and shoaling under a subarctic light regime

Visually foraging planktivorous fish are prey of visual predators, and their foraging behaviour may be affected by light levels both in terms of gain and risk. The large seasonal change in day length throughout a subarctic summer at 69°N was used to show the influence of light on diel vertical migration (DVM) and shoaling patterns in a planktivorous fish assemblage consisting of two species (Coregonus lavaretus and Coregonus albula). Under the midnight sun in June, night and daytime behaviour was similar, with extensive shoaling and limited DVM. With increasingly darker nights towards autumn, the fish dispersed during the dark hours and showed more extensive DVM. Throughout the changing light regime of both the day and the season, the planktivores consistently chose depths with light levels compatible with visual foraging and reduced predation risk as revealed from reactive distance modelling of coregonids and their salmonid predators. The findings support the hypothesis that behavioural decisions are based on a trade-off between foraging rate and predation risk, and increased predator avoidance behaviour towards autumn suggests that this trade-off is state-dependent

Predation by brown trout (Salmo trutta) along a diversifying prey community gradient

Predation has a fundamental role in aquatic ecosystems, but the relative importance of factors governing prey selection by predators remains controversial. In this study, we contrast five lakes of a subarctic watershed to explore how prey community characteristics affect prey selection and growth rate of the common top predator, brown trout (Salmo trutta). The lakes constitute a distinct gradient of different coregonid prey fish, ranging from monomorphic common whitefish (Coregonus lavaretus) to polymorphic whitefish co-occurring with vendace (Coregonus albula). The brown trout was a morph–species- and size-specific pelagic predator, selecting the small-sized, pelagic whitefish morph or vendace over the benthic whitefish morphs. In all lakes, the average prey size increased with predator size, but small-sized prey were also included in the diet of large predators. The selection of small-sized, pelagic prey fish appeared to be a favourable foraging strategy for the brown trout, yielding higher growth rates and an earlier ontogenetic shift to piscivory. The findings emphasize that piscivory appear to be shaped by the diversity, size-structure, and abundance of available prey in a given community

The role of gill raker number variability in adaptive radiation of coregonid fish

Gill raker divergence is a general pattern in adaptive radiations of postglacial fish, but few studies have addressed the adaptive significance of this morphological trait in foraging and eco-evolutionary interactions among predator and prey. Here, a set of subarctic lakes along a diversifying gradient of coregonids was used as the natural setting to explore correlations between gill raker numbers and planktivory as well as the impact of coregonid radiation on zooplankton communities. Results from 19 populations covering most of the total gill raker number gradient of the genus Coregonus, confirm that the number of gill rakers has a central role in determining the foraging ability towards zooplankton prey. Both at the individual and population levels, gill raker number was correlated with pelagic niche use and the size of utilized zooplankton prey. Furthermore, the average body size and the abundance and diversity of the zooplankton community decreased with the increasing diversity of coregonids. We argue that zooplankton feeding leads to an eco-evolutionary feedback loop that may further shape the gill raker morphology since natural selection intensifies under resource competition for depleted prey communities. Eco-evolutionary interactions may thus have a central role creating and maintaining the divergence of coregonid morphs in postglacial lakes