Feeding Pattern And Diet Of First Feeding Brown Trout Fry Under Natural Conditions

Journal of Fish Biology (2006) 68, 507–521
doi:10.1111/j.1095-8649.2006.00938.x, available online at http://www.blackwell-synergy.com

Feeding pattern and diet of first feeding brown trout fry
under natural conditions
H. S K O G L U N D *

AND

B. T. B A R L A U P

Department of Biology, Laboratory of Freshwater Ecology and Inland Fisheries
(LFI-UNIFOB), University of Bergen,Thormøhlensgt. 49, N-5006 Bergen, Norway
(Received 28 December 2004, Accepted 8 August 2005)
The feeding habits of brown trout Salmo trutta fry were studied during the critical first
feeding period in a natural spawning and nursery stream. A low proportion of the fry
initiated exogenous feeding before emergence from the gravel, and while having nearly 30%
of the yolk sac remaining. This probablyreflected low feeding motivation or limited feeding
opportunities within the gravel environment. The majority of the fry started feeding after
emergence, and after most or all yolk was absorbed. Some fry emerged with large amounts
of yolk remaining, while others emerged after yolk exhaustion. The degree of stomach
fullness revealed that feeding was more efficient after a territory had beenacquired. The
diets of the young fry were dominated by chironomid larvae, followed by zooplankton and
Plecoptera larvae. Fry dispersing downstream and out of the nursery area were significantly
smaller than resident fry, indicating displacement due to competition for territories. The
majority of the downstream dispersing fry had initiated feeding, and there was at this point
no evidence of starvationin any of the fry. It therefore appeared that the later emerging fry
actively migrated out of the overpopulated nursery area to find available territories further
# 2006 The Fisheries Society of the British Isles
downstream.
Key words: brown trout; diet; first feeding; fry.

INTRODUCTION
Brown trout Salmo trutta L. spawn in the autumn, when the eggs are deposited
in nests in the streambedgravel. The eggs develop within the gravel until they
hatch in winter or spring. After hatching, the alevins remain in the gravel
interstices of the nest while utilizing the energy reserves stored in the yolk sac.
Sometime in spring or early summer, when the yolk supply becomes exhausted,
the fry must emerge from the gravel and start feeding exclusively on exogenous
food. Shortly afterentering the stream, the free-swimming fry establish feeding
territories that are aggressively defended against intruding competitors
(Kalleberg, 1958; Godin, 1982).
The initial period after emergence is critical for the fry, as high mortality often
occurs during the first weeks (Elliott, 1994). The most important cause of

*Author to whom correspondence should be addressed. Tel.: þ47 55 58 2221; fax: þ47 55 58 96 74;
email: helge.skoglund@bio.uib.no

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H. SKOGLUND AND B. T. BARLAUP

mortality is probably starvation of fry that do not manage to acquire a feeding
territory (Elliott, 1986). A successful transition to exogenous feeding is crucial
for fry survival. Since mortality appears to be highest for small fryand the latest
fry to emerge from the gravel (Einum & Fleming, 2000), it might also be
advantageous to initiate feeding as early during ontogenetic development as
possible, as this may enhance early growth and thereby increase the chance of
acquiring and holding a territory. Knowledge of the feeding pattern during first
feeding and the subsequent period is important for understanding the basicprocesses that govern survival during the critical early life phases of salmonid
fry. Although the period of emergence and first feeding in salmonids has
attracted much attention, studies differ on the timing of the onset of exogenous
feeding in salmonid fry. In addition, most studies on first feeding are laboratory
studies, where the main focus often has been when food first should be...