Spatial patterns of the temporal dynamics of three gadoid species along the Norwegian Skagerrak coast

Abstract

Time series from an extensive research survey of juveniles of cod Gadus rnorhua, p~l i a~k Pollachius pollachius and whiting Merlangius rnerlangus sampled from 1919 to 1994 at 38 stations along the Norwegian Skagerrak coast were investigated. Spatial and temporal analyses were performed to study the spatial pattern of the temporal dynamics of the 3 fish species. Spatially consistent variations were detected in abundance, year-to-year fluctuation as well as in periodicity. The spatial heterogeneity occurred at a mesoscale (differences hetween fjords) and at a local scale (differences hetween stations within a fjord) for the 3 gadoids. However, the magnitude of the spatial heterogeneity differed hetween species. Cod and whiting, which were more abundant in sheltered areas, showed higher spatial variahility than pollack, which was more abundant in exposed locations. In this way, the spatial pattern of abundance appeared to be linked to the scale of variability oi the species. AJl3 species exhibit periodic fluctuations of 2 to 2.5 yr in their optimal habitats, which probably resulted from intrinsic interactions in age-structured populations, such as density-dependent competition and cannibalism. In addition, all the species exhibited long-term trends possibly due to extrinsic forces, such as environmental changes or anthropogenic influences.

Long-term fluctuations in cod and pollack along the Norwegian Skagerrak coast

“…Results from modelling on the Norwegian Skagerrak cod (Bjmnstad et al unpubl., Stenseth et al unpubl. ) give strong support to the hypothesis that the 2 to 2.5 yr cycle was caused by biotic interactions, such as competition for space and food and/or cannibalism between the juveniles stages (Fromentin et al 1997). However, the origins of the long-term trends, which account for a n important part of the temporal variations, appeared more obscure and were not elucidated.…”

“…5 & 6). On the detrended series, no significant autocorrelation occurred for Calanus fjnmarchicus and only a significant negative autocorrelation at lag 1 for cod and pollack (which reflects the 2 to 2.5 yr cycle detected by Fromentin et al 1997). There was therefore no need for corrections for autocorrelation in these series.…”

“…1). In contrast to Fromentin et al (1997), we excluded the whiting Merlangius merlangus from this study, since this species displayed a high spatial het-erogeneity (much higher than cod and pollack). It would therefore have been difficult to obtain reliable conclusions about a possible impact of a large-scale oscillation on whiting.…”

“…From 1940 to 1944, during the Second World War, the sampling was interrupted at almost all the stations (see Fromentin et al 1997). Interpolating these 5 consecutive years was impossible because of the extensive variability inherent in these biological time series.…”

Invasive ctenophore Mnemiopsis leidyi in Norway

“…Ctenophore size was measured as the oral -aboral length. Only specimens from transitional stage and above are considered in the present data, as larvae may not have been adequately sampled by the nets used and (Fromentin et al, 1997;Durif et al, 2011). The sampling comprised 84-138 sites annually, grouped into 21 areas along the south-east coast of Norway (Figure 2).…”