Contrasting structural complexity differentiate hunting strategy in an ambush apex predator.

Structural complexity is known to influence prey behaviour, mortality and population structure, but the effects on predators have received less attention. We tested whether contrasting structural complexity in two newly colonised lakes (low structural complexity lake-LSC; high structural complexity-HSC) was associated with contrasting behaviour in an aquatic apex predator, Northern pike (Esox lucius; hereafter pike) present in the lakes. Behaviour of pike was studied with whole-lake acoustic telemetry tracking, supplemented by stable isotope analysis of pike prey utilization and survey fishing data on the prey fish community. Pike displayed increased activity, space use, individual growth as well as behavioural differentiation and spent more time in open waters in the LSC lake. Despite observed differences between lakes, stable isotopes analyses indicated a high dependency on littoral food sources in both lakes. We concluded that pike in the HSC lake displayed a behaviour consistent with a prevalent ambush predation behaviour, whereas the higher activity and larger space use in the LSC lake indicated a transition to more active search behaviour. It could lead to increased prey encounter and cause better growth in the LSC lake. Our study demonstrated how differences in structural complexity mediated prominent changes in the foraging behaviour of an apex predator, which in turn may have effects on the prey community.

Freezer on, lights off! Environmental effects on activity rhythms of fish in the Arctic.

Polar regions are characterized by acute seasonal changes in the environment, with organisms inhabiting these regions lacking diel photoperiodic information for parts of the year. We present, to our knowledge, the first high-resolution analysis of diel and seasonal activity of free-living fishes in polar waters (74°N), subject to extreme variation in photoperiod, temperature and food availability. Using biotelemetry, we tracked two sympatric ecomorphs of lake-dwelling Arctic charr (Salvelinus alpinus n = 23) over an annual cycle. Charr activity rhythms reflected the above-surface photoperiod (including under ice), with diel rhythms of activity observed. During the dark winter solstice period, charr activity became arrhythmic and much reduced, even though estimated light levels were within those at which charr can feed. When twilight resumed, charr activity ensued as diel vertical migration, which continued throughout spring and with increasing day length, despite stable water temperatures. Diel activity rhythms ceased during the polar day, with a sharp increase in arrhythmic fish activity occurring at ice-break. Despite contrasting resource use, circannual rhythms were mirrored in the two ecomorphs, although individual variability in activity rhythms was evident. Our data support conclusions of functionally adaptive periods of arrhythmicity in polar animals, suggesting maintenance of a circannual oscillator for scheduling seasonal behavioural and developmental processes.

Fine-scale behavioural differences distinguish resource use by ecomorphs in a closed ecosystem.

Temporal differences in habitat use and foraging specialisms between ecomorphs represent aspects of behavioural phenotype that are poorly understood with regard to the origin and maintenance of ecological diversity. We tested the role of behaviour in resource use divergence of two Arctic charr (Salvelinus alpinus) phenotypes, a slim, putatively pelagic-dwelling morph and a robust, putatively littoral-dwelling generalist morph, over an annual cycle, using biotelemetry and stable isotopes. Pelagic morph charr exhibited significantly greater δC(13) depletion, concordant with increased zooplanktivory, than for the Littoral morph. Although three-dimensional space-use of the morphs strongly overlapped, on average, the Littoral morph used that habitat 19.3% more than the Pelagic morph. Pelagic morph fish were significantly more active, further from the lake bed and at greater depth than Littoral fish (annual means respectively, Pelagic, 0.069 BL s(-1), 8.21 m and 14.11 m; Littoral, 0.047 BL s(-1), 5.87 m and 10.47 m). Patterns of habitat use differed between ecomorphs at key times, such as during autumn and at ice break, likely related to spawning and resumption of intensive foraging respectively. Extensive space-use overlap, but fine-scale differences in habitat use between charr ecomorphs, suggests the importance of competition for generating and maintaining polymorphism, and its potential for promoting reproductive isolation and evolution in sympatry.

Allometric scaling of intraspecific space use.

Allometric scaling relationships enable exploration of animal space-use patterns, yet interspecific studies cannot address many of the underlying mechanisms. We present the first intraspecific study of home range (HR) allometry relative to energetic requirements over several orders of magnitude of body mass, using as a model the predatory fish, pike Esox lucius. Analogous with interspecific studies, we show that space use increases more rapidly with mass (exponent = 1.08) than metabolic scaling theories predict. Our results support a theory that suggests increasing HR overlap with body mass explains many of these differences in allometric scaling of HR size. We conclude that, on a population scale, HR size and energetic requirement scale allometrically, but with different exponents.