Evidence of cormorant‐induced mortality, disparate migration strategies and repeatable circadian rhythm in the endangered North Sea houting (Coregonus oxyrinchus): A telemetry study mapping the postspawning migration

Abstract

Life history theory predicts a trade-off between migration and residency where migration is favoured when it infers elevated fitness. Although migration to more favourable environments may offer higher growth rates, migrants often experience increased mortality due to predation. Here, we investigated mortality and migration behaviour of the North Sea houting (Coregonus oxyrinchus), an anadromous salmonid endemic to the Wadden Sea. We used acoustic telemetry to map the migration of the only remaining indigenous population by applying stationary hydrophones combined with manual tracking. Data suggested a total mortality of 26%, with 30% of the total mortality attributed to predation by great cormorants (Phalacrocorax carbo sinensis), highlighting that North Sea houting conservation could be jeopardised by increased cormorant predation. Risk of cormorant predation was size-dependent, with smaller fish suffering higher risk of predation. The study found North Sea houting to exhibit disparate migration strategies and identified a lentic area in the estuary as an important habitat. Two newly established artificial lakes within the river system significantly reduced the migration speeds, possibly indicating constrained navigation through the lakes. The migration into the Wadden Sea correlated with temperature perhaps indicating osmoregulatory constraints of sea entry. Unlike many salmonid species, migration occurred both day and night. Moreover, fish exhibited repeatable individual differences in diel activity patterns, suggesting that individuals differ consistently in their migratory activity throughout the 24-hr period. Our study provides novel information on salmonid migration, which is crucial for the development of science-based conservation strategies. K E Y W O R D Sartificial lakes, circadian rhythm, cormorant predation, repeatability, telemetry, whitefish | 673 JENSEN Et al.

Investigating fish migration, mortality, and physiology to improve conservation planning of anadromous salmonids: a case study on the endangered North Sea houting (Coregonus oxyrinchus)

“…In 2009, two artificial lakes, Lake Nørresø and Lake Hestholm Kog, were established on the river (Fig. 1) to provide nursery habitats for juvenile North Sea houting (Jensen et al 2015b;2018b).…”

“…In total, 78 adult North Sea houting (total body length: 42.7 ± 0.7 cm (mean ± SE)) were captured in River Vidaa between 18 th November 2014 and 16 th March 2015 (Jensen et al 2018b). Fish were captured using electrofishing or gillnetting (Jensen et al 2018b).…”

“…In total, 78 adult North Sea houting (total body length: 42.7 ± 0.7 cm (mean ± SE)) were captured in River Vidaa between 18 th November 2014 and 16 th March 2015 (Jensen et al 2018b). Fish were captured using electrofishing or gillnetting (Jensen et al 2018b). Each fish was anesthetized using benzocaine (VWR Chemicals; Radnor, Pennsylvania, USA; 2.55 mg l -1 ) (Aarestrup et al 2015;Baktoft et al 2016), and surgically tagged with an individually coded acoustic transmitter (model V9-2L; VEMCO, Bedford, Canada).…”

“…Total body length (to nearest 0.1 cm) was measured while each fish was anesthetized. Fish were released near the capture location following operative recovery (Jensen et al 2018b).…”

“…On this basis, studies are increasingly examining the repeatability of various migratory behaviours (Vardanis et al 2016;Hasselquist et al 2017;Villegas-Ríos et al 2017). Although a previous study reported repeatable circadian rhythms in migrating North Sea houting (Jensen et al 2018b), the repeatability of other migratory behaviours in the species is unknown.…”