Acoustic measurements of schooling herring. Estimation of school biomass and target strength

The connections between reflected echo energy and dimensions of fatty herring schools were studied by a combination of multibeam sonar and a calibrated echo integration unit. A relation between the area and the biomass of the schools was established. A target strength relation was derived by preseining echo integration and sonar measurments of schools. Method deficiencies and improvements for school biomass estimation using sonar and preseining target strength measurements are discussed

Testing of acoustic stock estimation of spawning capelin

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Avoidance reactions of ultrasonic tagged cod during bottom trawling in shallow water

Introduction: Observations made by echo sounder (Ona 1988) have shown that cod (Gadus morhua L.) may avoid an approaching survey vessel horizontally and vertically. However, there has been a lack of information on the escapement behaviour of the individual fish. This short note discribes an experiment carried out in a shallow fjord in Northern Norway, where the reactions of single cod towards a small survey vessel during bottom trawling were studied, using ultrasonic tagging technique

Akustisk fangstovervåking i not – Utfordringer og løsningsforslag til bedre metodikk

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The response of mesopelagic organisms to artificial lights

Though mesopelagic fish respond to natural light (e.g., diurnal vertical migration), few studies have looked at how they respond to artificial light and if artificial lights could be used in commercial operations to improve catchability of mesopelagic fish. Here we present a preliminary study on how mesopelagic organisms respond to blue and green spotlights, as well as red and white diffuse lights in Masfjorden (Norway; max depth of 480 m). The response of organisms in each of the three sound scattering layers (SSLs) was observed when a) artificial lights were positioned in a layer or b) moved with a constant speed (generally 0.03 ms−1) towards a layer. The artificial lights were attached to a rig with a self-contained echosounder, which recorded the vertical and horizontal avoidance of organisms in each SSL to different artificial lights. Net hauls (MIC-net) and video footage confirmed that Maurolicus muelleri and siphonophores were present in the upper layer (100–150 m), while Benthosema glaciale were present in the deeper layers (∼200 m and ∼300 m to seabed). Our findings suggest that M. muelleri (SSL1) horizontally avoid blue spotlight and white diffuse light, while B. glaciale (SSL2 and SSL3) mainly avoid the same lights downwards and can be herded downwards over 250 m. Though this study should be regarded as preliminary, the observed avoidance/herding response suggests artificial lights could be applied to improve existing fish capture methods for mesopelagic fish.acceptedVersio

Akustisk sensor for fangstkontroll i notfiske - Spesifikasjoner for montering og bruk

Fangst- og redskapsovervåking gir kontroll over fangstprosessen og kan bidra til å redusere utilsiktet dødelighet og gi et mer målrettet og effektivt fiske. I prosjekt «Fangstkontroll i notfiske etter pelagiske arter» (FHF 901350) har vi undersøkt ulike akustiske metoder for fangstovervåking under notfiske. I denne rapporten presenterer vi forsøk der vi har tilpasset Simrads fangstkontrollsystem utviklet hovedsakelig for trål til fiske med snurpenot. De trådløse sensorene har et innebygget ekkolodd som informerer skipperen om fangstmengde i trålen. Vi har undersøkt om sensorene med noen tilpasninger til fiske med not kan gi like nyttig informasjon om fangstmengde og fiskens posisjon i nota. Resultatene viser at det gjenstår arbeid før systemet kan være en praktisk og nyttig løsning. Likevel har det helt klart potensial. God kontakt mellom sensor og hydrofon oppnås ved å ha hydrofon dypere i vannet. Stabil og robust montering av sensorer, fungerte best med sensor festet på nota noen meter under flåen, men også i snurperingene nederst i nota. Det var antydning til identifikasjon av fisk på ekkoloddene. Spørsmålet er om ekkoloddet i notsensoren har bra nok oppløsning for identifisering av fangst eller om det må vurderes flere smalere stråler istedenfor en bred stråle. I rapporten gir vi anbefalinger for montering og bruk av notsensor samt forslag til videre utviklingsarbeid.publishedVersio

Behavioural and welfare implications of a new slipping methodology for purse seine fisheries in Norwegian waters

The release of unwanted fish from purse seines whilst still in the water is termed slipping and may lead to significant mortality following release. The objective of this study was to determine the fish welfare implications of a new slipping methodology in which fish are released via a discharge opening formed in the bunt end of the purse seine net. Video analyses of collective and individual level fish behaviour were undertaken in the Norwegian mackerel and herring purse seine fisheries, to quantitively describe slipping behaviour and to determine its driving factors. The majority of fish escaped the purse seine with the schooling structure intact as part of large groups towards the end of slipping process, increasing their speed following escape. However, there was also a tendency (24% of all escapes) to escape in a manner likely to impact negatively upon their welfare, with a breakdown in schooling structure and physical contact with the fishing gear and conspecifics. The tendency to express such welfare compromising behaviour was higher for mackerel than for herring, but was also influenced by the vessel releasing the fish, the amount of fish being slipped, how long the discharge opening had been open and the particular slipping event. These results provide important information for future science-based development of welfare friendly slipping practises.publishedVersio

How and why acoustic detectability and catchability of herring change with individual motivation and physiological state in a variable environment: a multi-scale study on a local herring population in southwestern Norway

The understanding of distribution and aggregation in herring (Clupea harengus) can be enhanced by integrated multi-scale studies in small ecosystems. Hydro-acoustics, underwater cameras, herring and predator gillnet samples and oceanographical measurements were used to quantify herring schooling dynamics. During autumn (September) after an active feeding period, the herring was distributed in small and dense schools, mostly close to land and in relatively shallow water ( 40 m depth). All recorded herring schools were then vertically extended in the water column within the most variable temperature and oxygen profiles, presumably enabling individuals to adjust maturation rate to prevailing environmental conditions and synchronize spawning of individuals within the school. From late February prior to spawning, only one major pelagic school was observed, extraordinarily stationary at the only inlet to the inner basin. The pre-spawning herring aggregation was fairly easy to detect acoustically for more than one month. Just prior to spawning and during spawning, herring spread out and became extremely difficult to detect acoustically. Only underwater cameras and bottom set gillnets could then be used to identify herring and selected spawning areas. We argue that the dramatic seasonal changes in acoustic detectability and catchability we observe is best understood and predicted based on detailed knowledge of how herring react to a changing environment according to their physiological state and motivation. Such factors should also be taken more systematically into account when performing acoustic surveys in large marine ecosystems. We need to study in more detail the vital underlying processes behind the substantial variability observed in acoustic detectability and catchability of pelagic planktivorous fish species during their annual life cycle in order to better understand and quantify variability in acoustic surveys, and thereby improve our acoustic abundance estimation

Spatial and temporal overlap and trophic interactions between Atlantic Saithe (Pollachius virens) and Norwegian spring spawning herring (Clupea harengus) on herring spawning grounds

Atlantic saithe (Pollachius virens) and Norwegian spring-spawning herring (Clupea harengus) represent two of the most abundant and ecologically important fish species in the Northeast Atlantic. The two species co-occur in the Vesterålen/Lofoten area during the wintering of herring, but the temporal and spatial overlap between species in different scales as well as their trophic interactions is otherwise largely unknown. Saithe is a potentially important predator on herring. In the present study we found the commercial catches of saithe off western Norway to be closely related to the migration of herring into the coastal spawning grounds, indicating that saithe follow the high concentrations of migrating herring. Acoustic surveys within a defined study area demonstrated that the species overlapped spatially and temporarily. Degree of overlap varied both diurnally and with the maturity status of the herring (pre-spawning, spawning or spent). Concurrent catches of saithe and herring from trawling and gillnets supported an overlap also on the meso-scale. Observations on more fine-scaled behaviour obtained from an underwater rig with highsensity cameras positioned in high-density areas of herring demonstrated that the distribution and behaviour of herring and saithe was closely linked to the diel light cycle and spawning of herring. When looking at the actual outcome of the interactions between the species on a kind of temporally, integrated spatial microscale, it was observed that stomachs of large saithe (>50 cm) on average contained 2–3 herring, while the stomachs of smaller saithe were filled with herring eggs. This suggests a considerable predation pressure by saithe on different life stages of herring