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

Probability-based surveying using self-sampling to estimate catch and effort in Norway’s coastal tourist fishery

Recreational fishing as a tourist activity has become an increasingly important part of the Norwegian travel industry and may contribute significantly to the fishing mortality on Norwegian coastal cod (Gadus morhua). Quantifying catches in the tourist fishery is made difficult by Norway's intricate and long coastline, and the lack of a fishing licence system, a registry of businesses catering for fishing tourists, and a registry of charter boats. A probability-based survey was used to estimate annual catch and effort by boat for anglers associated with 445 tourist-fishing businesses during 2009. From a stratified random sample of businesses, fishing tourists were recruited systematically over time to record their daily catch and effort in diaries. Cod dominated the reported landed catch (harvest) north of 62°N, and saithe and mackerel dominated south of 62°N. The estimated total landed catch of all species taken by tourist fishers in the business sector during 2009 was 3335 t (relative standard error, RSE = 17%), of which 1613 t (RSE = 22%) were cod. It is concluded that surveys based on self-sampling can yield accurate estimates of catch and effort accounted for by the business sector of the Norwegian tourist fishery and that the tourist catch of coastal cod is insignificant compared with the commercial and recreational catch by local residents