Surfacing behavior and gas release of the physostome sprat

(Sprattus sprattus) in ice-free and ice-covered waters Ingrid Solberg1 and Stein Kaartvedt1,2 (2014)

1Red Sea Research Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia.

2Department of Biosciences, University of Oslo, PO Box 1066, Blindern 0316 Oslo, Norway

Most physostome fishes (including sprat) are without gas secretion

organs and likely refill their swimbladder by “gulping”

atmospheric air at the surface. Gas is released through their anal

duct. However, little is known about the frequency and diel time

scale for such events. The potential effect of ice cover is also

unknown.

Assess the surfacing and gas release behavior of sprat

throughout an entire winter by the use of stationary upward-

facing echosounders

Reveal how the physostome sprat respond to ice covering

Study site: Norwegian fjord (150 m) that became ice-covered in

mid-winter

Three echosounders were deployed from Nov 09 - Apr 10

200 kHz floating at 30 m (enhanced resolution in surface waters

enabled studies of individual behavior), 120 kHz (80 m) and 38

kHz (150 m)

Field campaigns supplemented the acoustic studies

Acoustic post-processing was performed using Sonar_5 Pro

(Balk & Lindem 2005) and LSSS (Korneliussen et al. 2006)

Manual target tracking (TT) was used to record surfacing fish

and to assess vertical swimming speed during surfacing events

Gas release events were manually registered

Abundance of sprat was determined by echo integration in

order to calculate frequency of surfacing- and gas release per

fish per day

Floating upward-facing echosounder-rig, cabled to shore. No

limitation of power or storage capacity provided long-term acoustic

measurements at a temporal resolution of seconds

The rate of surfacing was particularly high in ice-covered waters,

while the rate of gas release was very low

Estimated surfacing rate in

ice-free waters:

~ 3.5 times per fish day-1

Estimated surfacing rate in

ice-covered waters:

12.5 times per fish day-1

The vertical swimming speeds during surfacing were considerably

higher in ice-free conditions

Discussion / Conclusions

The correlation between surfacing and gas release in ice-free waters suggests

that the sprat gulped atmospheric air during excursions to the surface. The

frequent surfacing behavior indicates that gulping of atmospheric air is an

important element in the life of sprat. Hampered access to the surface by ice

cover affected both surfacing frequency and swimming behavior, and ice

covering may constrain physostome fish in yet unknown ways.

The sprat released gas

many times per day

( >35) during ice-free

conditions

Gas release was strongly

reduced during ice cover

The swimming speeds were significantly lower in ice covered waters

(two-way ANOVA, p < 0.0000)

The average downward swimming speed after surfacing in ice-free

waters was particularly high (~ 28 cm s-1)

Results

Sprat surfaced frequently at night during ice-free conditions,

- and released gas in the following hours during nighttime

In ice-covered waters, the sprat displayed a different behavior by

repeatedly ascending towards the ice

Acknowledgments

Anders Røstad and Thor A. Klevjer were instrumental in conducting the acoustic studies. We thank the crew of

RV Trygve Braarud for support during cruises and Rita Amundsen for technical assistance.

References Balk H, Lindem T (2005). Sonar4 and Sonar5-Pro post processing systems (operational manual). University of Oslo, Oslo.

Korneliussen RJ, et al. (2006). The Large Scale Survey System: LSSS. Proceedings of the 29th Scandinavian Symposium on

Physical Acoustics, Ustaoset.

De

pth

(m

)

18:05 18:07

0

3

6

20:45:30 20:49:00

Time (h) UTC

Sv (dB)

-74

-68

-62

-56

-50

-80

0

5

10

Searching for the

surface?

Rate

of gas r

ele

ase p

er

fish p

er

day

0

20

40

60

80

before ice cover during ice cover after ice cover

72

2

35

Rate

of surf

acin

g p

er

fish p

er

day

0

4

8

12

16

during ice coverbefore ice cover

3.8

12.5

2.6

after ice cover

Introduction

Objectives

Materials and methods

0

10

20

30

21:35 21:40 21:45D

ep

th (

m)

Time (h) UTC

Sprat

Surfacing sprat

0

10

20

05:55 06:00

De

pth

(m

)

Time (h) UTC

Sprat releasing air

Gas bubbles