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

Abstract

We tested whether the long-term fluctuations in abundance of 0-group cod Gadus morhua and pollack Pollachius pollachius sampled from 1919 to 1994 at 38 stations along the Norwegian Skagerrak coast might be related to the North Atlantic Oscillation (NAO). The short-and long-term fluctuations of both species exhibited different spatial structures and were probably of different origins. The former, which were previously shown to be related to biotic interactions, displayed signif~cant highest similarities across the smallest scales (0 to 21 km). In contrast, the long-term fluctuat~ons were similar at a larger scale (equal to or larger than the Norwegian Skagerrak coast) and were probably due to extrinsic factors. We have shown that the climatic conditions along the Norwegian Skagerrak coast and the fluctuations of Calanus finmarchicus in the Skagerrak were closely assoc~ated with the NAO. Long-term fluctuations of cod and pollack were, however, not associated linearly or non-linearly with the NAO or C. finmarchicus. Trends of cod and pollack appeared to be related to the long-term fluctuations in the bottom sea grass coverage, mainly constituted by Zostera marina, and it cannot be dismissed that fishing has contributed to the recent decline of these fish populations. We also conclude that the postsettlement period is crucial for these populations because of competition for space.

Meteorological forcing of plankton dynamics in a large and deep continental European lake

“…The occurrence of warm winters over much of Europe during the late 1980s and early 1990s was associated with anomalously low pressures in the region of the Icelandic Low and with pressures higher than normal at lower latitudes, corresponding to a high positive NAO index (Hurrell 1995). There is growing evidence that the abundances of aquatic and terrestrial species were influenced by the NAO (Fromentin and Planque 1996;Fromentin et al 1998;Kröncke et al 1998;Post and Stenseth 1998).…”

Dynamics of coastal cod populations: intra- and intercohort density dependence and stochastic processes

“…A substantial sigmoid-like temporal trend is observed in most of the time-series (¢gure 2; see Fromentin et al 1997Fromentin et al , 1998. We have nevertheless chosen to perform our analysis without doing any form of detrending since moving average terms may contribute signi¢cantly to low-frequency £uctuations (BjÖrnstad et al 1999a;see Priestley (1981) for a general discussion).…”

“…SAS Institute 1990, Proc Model;Pindyck & Rubinfeld 1991). The mean c j , is allowed to vary between the series whereas the autoregressive coe¤cients (ar i , i 1, 2, 3) and the moving average term (ma 1 ) are assumed to be the same for all 54 stations; the global trend is incorporated though a thirdorder polynomial in time (poly(t,3)) (Fromentin et al 1997(Fromentin et al , 1998.…”

Building bridges across subdisciplines in marine ecology

“…However, Kornilovs et al (2001) postulate that the observed decline has had, at least in part, an environmental basis in reduced food supply (Calanus). In contrast, Fromentin et al (1998) examine the historic record in the Skagerrak and suggest no relation of cod populations to Calanus, whose abundance cycles with the North Atlantic oscillation. Rather, Fromentin et al (1998) relate changes in cod stocks to changes in amount of area of Zostera available for settlement of the larvae -in other words, a space limitation for juveniles.…”

“…In contrast, Fromentin et al (1998) examine the historic record in the Skagerrak and suggest no relation of cod populations to Calanus, whose abundance cycles with the North Atlantic oscillation. Rather, Fromentin et al (1998) relate changes in cod stocks to changes in amount of area of Zostera available for settlement of the larvae -in other words, a space limitation for juveniles. While no simple answer seems to be in sight, the potential influence of environmental variables cannot be discounted.…”