| dc.description.abstract | Continuous monitoring of the biomass in salmon cages is likely to increase the production
efficiency. It would optimize density-dependent farm procedures, e.g food and drug
administration, and provide a possibility for early detection of major escapes or thefts of fish.
The importance of a correct biomass estimate increases with growth in biomass and are of
great value for efficient delivery of fish quantities agreed in advance. Traditional methods for
biomass estimation are often labour intensive and stress the fish. The use of hydroacoustic
methods would be a gentle and efficient alternative, but require knowledgde of the acoustic
reflection properties (target strength, TS) of salmon to be accurate. The aim of the present
study was to investigate variations in ventralaspect TS of caged salmon, both by experimental
studies and by behavioural observations of caged salmon.
Change in swimming depth was simulated by a novel method where the swimbladder volume
was manipulated by inserting a thin polyethylen tube through the pneumatic duct of dead
adult salmon. A non-linear relationship between TS and swimming depth in the upper 20 m
was indicated. Experimental measurements of tilt angles indicated a maximum TS at -2 to -4'
in fish with a swimbladder volume giving neutral bouyancy at l m depth. The swirnming
behaviour of 1617 salmon (2.3 kg) in a 750 m3 cage changed with light intensity and had a
considerable effect on the acoustic backscattering from the cage. Swimming depth and surface
activity (leaping) was lower at nigth than day. The mean tilt angle, recorded from still-photos,
indicated a positive boyancy at nigthtime. A peak in rolling behaviour at dusk suggested that
the salmon actively regulated their boyancy prior to the "inactive" nigth period.
The observed variations in TS and behaviour have major implications for the use of the echointegration
method to estimate biomass of caged salmon and suggest that a special sampling
routine for farm conditions must be developed.NORSK SAMMENDRAG:Kontinuerlig overvåkning av biomassen i ulike merder på et oppdrettsanlegg ville kunne bidra
til en mer effektiv drift. Anvendelse av hydroakustikk til dette formålet ville være et skånsomt
og effektivt alternativ til tradisjonelle metoder. Dette krever imidlertid kunnskap om laksens
akustiske refleksjonsegenskaper (målstyrke). Målet med dette arbeidet var å belyse variasjon
i akustisk målstyrke i relasjon til svgrnmatferd hos laks i merd. Ekseprimentelle målstyrkemålinger
indikerte en ikke-lineaer sammenheng mellom laksens målstyrke (fra buksiden) og
svØmmedyp. Videre ble det etablert en stØrrelsesavhengig målstyrkefunksjon i tiltvinkelintervallet
-10 - +lo0. Atferdstudier viste at laksen sv~mrneatferd er avhengig av lysforholdene
og at dette har stor betydning for den akustiske refleksjonen. Laksen svømmer
dypere og hopper mer om dagen enn om natten. En klar Økning i rullfrekvens i skumringen
indikerte en aktiv regulering av oppdrift ved overgang fra "dagatferd" til nattatferd". Laksens
tiltvinkelfordeling indikerte positiv oppdrift om natten. Resultatene viser at hvis en skal
anvende akustikk til å mengdemåle oppdrettslaks, må det må utvikles en samplingsstrategi
tilpasset realistiske oppdrettsforhold. | en |
