Concurrent recruitment failure in gadoids and changes in the plankton community along the Norwegian Skagerrak coast after 2002

Abstract

Johannessen, T., Dahl, E., Falkenhaug, T., and Naustvoll, L. J. 2012. Concurrent recruitment failure in gadoids and changes in the plankton community of the Norwegian Skagerrak coast after 2002. – ICES Journal of Marine Science, 69: 795–801. Since 1919, annual beach-seine sampling has been carried out along the Norwegian Skagerrak coast with the main objective of measuring the abundance of 0-group gadoids. Repeated incidents of abrupt and persistent recruitment collapses in gadoid populations have been observed locally. These collapses have been linked to gradual eutrophication, which has resulted in abrupt changes in the plankton community and deprivation of adequate prey for 0-group fish. Since 2002, the recruitment of 0-group gadoids has been very poor along the Skagerrak coast on a regional scale. At the same time, major changes in the plankton community have been observed. A regular and pronounced autumn bloom dominated by red-tide forming dinoflagellates such as large Ceratium spp. and the toxic Karenia mikimotoi has practically vanished, and a linear decrease in oxygen concentration at intermediate depths (30 m) that has been evident since the 1960s, was reversed around 2002. These changes would normally indicate improved environmental conditions. However, the recruitment failure in gadoids does not support this perception. Rather, the evidence points towards a potential shift in the plankton community that has resulted in gadoid recruitment failure.

Mapping of marine benthic invertebrates in the Oslofjord and the Skagerrak: sampling data of museum collections from 1950-1955 and from recent investigations

“…In the Oslofjord and coastal waters of the Skagerrak, environmental changes over eight to ten decades and more have been investigated from time series on shallow-water fish fauna (Fromentin et al 1997;Johannessen et al 2012), repeated sampling of Petersen's (1915) early softbottom studies (Rosenberg et al 1987) and reconstruction of past foraminiferal species assemblages (Dolven et al 2013). The collections of benthic fauna from the 1950s may supplement these studies by providing data for numerous species and a number of habitats that are not included in the presently examined historic data sets.…”

Bifurcations in Marine Ecosystems

“…However, the abundance data suggest significantly decreasing trends in (Reid et al, 2003). During the Flødevigen time series (2000À2012), Paracalanus/ Pseudocalanus was most abundant in July and August (Johannessen et al, 2012), coinciding with what appears to be the critical period for survival of 0-group cod in these waters (Chapters 3 and 4). The other species for which the abundances have been significantly reduced, Oithona spp., Acartia spp., and T. longicornis, also have peak abundances in July.…”

“…Comprehensive testing in the field provided substantial evidence to suggest that the recruitment collapses were caused by abrupt shifts in the plankton community that deprived the young-of-the-year gadoids of adequate prey (Chapters 3 and 4). Here, various published time series have been updated that provide direct and indirect evidence of abrupt changes in the plankton community, for example, data on oxygen Dahl, 1996a, 1996b), gadoid abundance (Chapter 2), and phytoplankton and zooplankton (Johannessen et al, 2012). In addition, a more recent incident of concurrent recruitment failure in gadoids and changes in plankton community is presented (Johannessen et al, 2012).…”

“…Here, various published time series have been updated that provide direct and indirect evidence of abrupt changes in the plankton community, for example, data on oxygen Dahl, 1996a, 1996b), gadoid abundance (Chapter 2), and phytoplankton and zooplankton (Johannessen et al, 2012). In addition, a more recent incident of concurrent recruitment failure in gadoids and changes in plankton community is presented (Johannessen et al, 2012). In contrast to the previous events, this recent shift occurred during increasing temperature and decreasing eutrophication.…”

“…3.1) using the standard fluorescent method of Strickland and Parsons (1968). Trends in Chl a between 1989 and 2012 were studied on the basis of annual averages and separately for spring (Julian day 20À119), summer (day 120À214), and autumn (day 215À314; for details see Johannessen et al, 2012). The annual pattern in Chl a was estimated for two periods, 1989À2001 and 2002À2012.…”