Running on empty? Freshwater feeding by spawning anadromous alewife Alosa pseudoharengus

Abstract

Anadromous alewife Alosa pseudoharengus (n = 202; mean ± S.D. fork length = 231 ± 14 mm) were captured from 10 May to 27 June 2018 in an upper watershed lake on the Isthmus of Chignecto, Canada (45 57 0 N, 64 14 0 W). Thirty individuals (mean ± S.D. fork length = 250 ± 12 mm) were captured in an adjacent estuary downstream of a tide gate on 25 April 2018. Comparing estuarine to freshwater specimens, mean gonad mass and gonadosomatic indices in males and females decreased approximately 40% and 60%, and 31% and 50%, respectively. Individuals were characterized as pre-spawners in the estuary and spawners in the lake. Males maintained similar body condition throughout the spawning run whereas female condition decreased 9.4% between the estuary and lake. Stomach fullness decreased comparing estuarine and freshwater specimens, yet 93% of stomachs examined from individuals captured in the lake contained prey. Most males fed throughout all spawning stages (3%-17% empty stomachs), while all females fed during pre-and post-spawning stages and some fasted during spawning (11% empty stomachs). Cumulative prey curve never reached an asymptote, either weekly or for the entire sampling period, so freshwater diet may not have been completely described. Calanoid copepods (79.3%I A ) were a diet staple, with the secondary prey of mayfly nymphs (O. Ephemeroptera) consumed more by females (13.6%I A ) than males (6.2%I A ).PERMANOVA and PERMDISP analyses revealed significant dietary differences in freshwater were weekly and not due to dispersion effects, thus most likely due to feeding on various development stages of insect species. Our results challenge the long-held paradigm that anadromous A. pseudoharengus fast during the spawning migration.

River- and Sex-Specific Annual Survival Rates of Mature Anadromous Alewife (Alosa pseudoharengus)

“…During lake spawning in our study area, both sexes fed; however, males maintained their body condition as in estuaries during the start of the spawning run, whereas female body condition decreased by 9.4% because of egg release from their larger ripe gonads [31]. Tis decrease in body condition of females during spawning runs may have accounted for some of the diference in apparent annual survival between the sexes.…”

“…Depending on annual environmental conditions and deployments, arrays monitored from the head of tide (tide gate, [27]) to spawning sites (Long Lake, [31]) from April to July. Each river had a minimum and maximum of two and seven antennas, respectively, that individual alewife had to swim past to reach spawning sites (Table 3).…”

Seasonal reproductive allocation in landlocked Alewife Alosa pseudoharengus, in the context of niche construction and eco‐evolutionary feedbacks

“…However, capital breeders could also prolong their lake residency by modulating their energy reserves through progressive reduction in reproductive investment. Stewart et al (2021) also challenged the paradigm of fasting in upstream migrating Alewife at the northernmost species distribution limit (Nova Scotia/New Brunswick, Canada) but also acknowledged that a proportion of spawning females showed decreased feeding or even empty stomachs. In that respect, tapering in energy investment to spawning batch formation in successive rounds of spawning is reasonable, considering that anadromous Alewife needs to cope with high energetic demands during its riverine phase, which includes the upstream migration, multiple spawning events, extended freshwater residency (up to ~80 days; Kissil, 1974; Gahagan et al, 2010; Rosset et al, 2017; Marjadi et al, 2019) and downstream migration.…”

“…Adult Alewife reside in the ocean and migrate into lakes to reproduce (Cooper, 1961; Kissil, 1974; Littrell et al, 2018). During freshwater residency, feeding is minimal (Post et al, 2008; Post & Palkovacs, 2009) (with an exception at the northernmost species distribution limit [Stewart et al, 2021]), and a substantial proportion of somatic mass is lost (Ganias et al, 2015). The young of the year spend their first summer in their natal lakes before they emigrate to the ocean (Gahagan et al, 2010; Post et al, 2008; Walsh & Post, 2011; Yako et al, 2002).…”

“…assumed that in situ energy acquisition through foraging is likely in anadromous Alewife based on the extended (48-76 days) residency in the spawning lakes. However, capital breeders could also prolong their lake residency by modulating their energy reserves through progressive reduction in reproductive investment Stewart et al (2021). also challenged the paradigm of fasting in upstream mi-…”