Surfacing behavior and gas release of the physostome sprat (Sprattus sprattus) in ice-free and ice-covered waters

Abstract

Upward-facing echosounders that provided continuous, long-term measurements were applied to address the surfacing behavior and gas release of the physostome sprat (Sprattus sprattus) throughout an entire winter in a 150-m-deep Norwegian fjord. During ice-free conditions, the sprat surfaced and released gas bubbles at night with an estimated surfacing rate of 3.5 times per fish day−1. The vertical swimming speeds during surfacing were considerably higher (~10 times) than during diel vertical migrations, especially when returning from the surface, and particularly when the fjord was not ice covered. The sprat released gas a few hours after surfacing, suggesting that the sprat gulped atmospheric air during its excursions to the surface. While the surface activity increased after the fjord became ice covered, the records of gas release decreased sharply. The under-ice fish then displayed a behavior interpreted as “searching for the surface” by repeatedly ascending toward the ice, apparently with limited success of filling the swim bladder. This interpretation was supported by lower acoustic target strength in ice-covered waters. The frequent surfacing behavior demonstrated in this study indicates that gulping of atmospheric air is an important element in the life of sprat. While at least part of the population endured overwintering in the ice-covered habitat, ice covering may constrain those physostome fishes that lack a gas-generating gland in ways that remain to be established.

Winter and summer differences in probability of fish encounter (spatial overlap) with MHK devices

“…In contrast to summer, water column fish density in the winter was higher and much less variable. The majority of fish in the passage at that time was likely to be herring, whose presence was supported by frequent trails of bubbles seen rising from schools or individuals in the echogram (herring and other clupeids are known to release swim bladder gas through the anal duct [23,24]). Rainbow smelt and sticklebacks were also potentially present in the area based on what is generally known of their life histories [21], and acoustically tagged striped bass have been recorded repeatedly passing through Minas Passage in the winter [3].…”

Diel vertical interactions between Atlantic cod Gadus morhua and sprat Sprattus sprattus in a stratified water column

“…These fish lack gas secreting glands for adjustment of the swimbladder at different depths, so the swimbladder becomes successively compressed at increasing depth making the fish negatively buoyant in deeper water (Solberg & Kaartvedt 2014). Using bottom mounted, upward facing echosounders during wintertime in Bunnefjorden, Norway, Kaartvedt et al (2009) tracked individual sprat and showed that the fish continuously swam up and down at depths > 55 m, and this way maintained the same depth over time.…”

“…In the present study, the ascent of sprat from depths of 55 to 60 m at dusk when predation risk is reduced may therefore also be due to a strategy of minimizing swimming costs by reducing negative buoyancy. Secondly, Kaartvedt et al (2009), Solberg & Kaartvedt (2014 and Solberg et al (2015) tracked individual sprat performing short excursions to the surface at night and ascribed this swimming behaviour to refilling the swimbladder by gulping atmo spheric air. However, even in situations of inverse DVM, with sprat aggregating in deep water at night, surfacing did not seem to impact the vertical distribution pattern at large (Solberg & Kaartvedt 2014).…”

“…Secondly, Kaartvedt et al (2009), Solberg & Kaartvedt (2014 and Solberg et al (2015) tracked individual sprat performing short excursions to the surface at night and ascribed this swimming behaviour to refilling the swimbladder by gulping atmo spheric air. However, even in situations of inverse DVM, with sprat aggregating in deep water at night, surfacing did not seem to impact the vertical distribution pattern at large (Solberg & Kaartvedt 2014). It is therefore doubtful whether surfacing behaviour can explain the vertical distribution dynamics observed in the present study in the Baltic Sea.…”

Movement ecology of adfluvial bull trout (Salvelinus confluentus)

“…In addition to locomotion through body-muscle contraction, bull trout may also use their swim bladder to change swimming depth. In fishes, the swim bladder is airfilled sac located in the coelomic cavity below the vertebral column (Holbrook and Perera 2011 (Mehner 2012;Solberg and Kaartvedt 2014), it remains feasible that both swimming and swimbladder regulation are operational modes of bull trout motion capacity.…”