Estuarine phase during seaward migration for allis shad Alosa alosa and twaite shad Alosa fallax future spawners

Abstract

– The timing and duration of the estuarine phase during seaward migration were addressed for allis shad and twaite shad, closely related sympatric species in the Gironde system. Otolith Sr:Ca and daily growth increments were used to retrospectively estimate the age at entrance in the estuary, the time spent in estuary and the age at seaward exit from returning adults. The two species exhibited different seaward migration strategies: allis shad entered the estuary at an older age than twaite shad (88 days vs. 59 days), stayed shorter in the estuary (11 days vs. 17 days) and exited to the sea at an older age (99 days vs. 77 days). In the Gironde system, the estuarine phase is very short and occurs within a particular time ‘window’ for the juveniles that will become future spawners. In this context, it might be suggested that the estuarine phase is a bottleneck for these sympatric shad populations.

Migration behaviour of twaite shad Alosa fallax assessed by otolith Sr:Ca and Ba:Ca profiles

“…). According to prior reported habitat preferences of A. fallax (Gerkens & Thiel, ; Maitland & Hatton‐Ellis, ; Stelzenmüller et al ., ; Lochet et al ., ) and lower salinities in the Wadden Sea area than in the open North Sea (Topcu et al ., ), distribution peaks were defined to represent fresh water (Sr:Ca ≤ 1·4 × 10 −3 ), Wadden Sea (2·3 × 10 −3 < Sr:Ca ≤ 3·2 × 10 −3 ) and marine water (Sr:Ca > 3·2 × 10 −3 ). Due to the indistinct threshold between the Wadden Sea and fully marine water, both environments were combined to one habitat (marine habitat).…”

“…The present finding of two different basic migration patterns during the first year of life was unexpected and indicated intra‐population differences in migration behaviour. The more frequent migration pattern, with a single downstream migration into the sea, resembled those of A. fallax and allis shad Alosa alosa (L. 1758) in the Gironde system (Lochet et al ., ) and A. sapidissima in the Hudson River (Limburg, , ). In contrast, the second basic migration pattern, with its clear back and forth movement from fresh to marine water and back to fresh water prior to the final seaward movement, has not been described regularly for Alosa spp.…”

“…The validity of this approach is supported by studies of Lochet et al . (, ) who found similar ranges (albeit, slightly lower thresholds) in A. fallax from the Gironde system in France. Similar to the procedure for Sr:Ca, a frequency distribution of all juvenile Ba:Ca was fitted.…”

Effect of temperature on growth, survival and respiratory rate of larval allis shadAlosa alosa

“…Coastal hypoxia has increased globally since the 1960s owing to eutrophication effects of river systems (Zhang et al, 2010). As estuaries play a physiologically and ecologically detrimental role in the life cycle of juvenile allis shad (Lochet et al, 2009), also indirect effects of high temperatures such as hypoxic periods should be evaluated as a multiple stressor scenario to uncover the mechanism of their widespread disappearance from European river systems. Furthermore, it has to be noted that the experienced increases in temperature are unlikely to threaten fish stocks from the perspective of lethal or sub-lethal effects, but rather have to be interpreted in a synecological context (Blaxter, 1992;Pörtner and Peck, 2010).…”

Prognose visstand in en rond het Haringvliet na invoering van het Kierbesluit in 2018

“…De larven en juveniele van elft groeien op in de rivier. Na het eerste en soms tweede jaar trekken de juveniele elften naar zee, waarbij een deel gebruik maakt van estuaria om op te groeien (Lochet et al 2009). …”