At the prospect of global warming, an understanding of how increasing temperatures may affect both species and communities becomes increasingly important. In 2002, a regime shift occurred along the Norwegian Skagerrak coast in relation to increasing sea temperatures. The regime shift included a substantial reduction in recruitment of 0-group gadoids, reduction in copepod biomass and changes in phytoplankton.
In this study, the fish community before and after this regime shift is described using a fine-meshed beach seine. Six locations near Arendal were sampled biweekly from May to October, and data obtained in 1997-1999 were compared to new data from 2016-2017. This revealed a substantial decrease in the abundance of small, short-lived species (one-year life cycle) from the first to second period, hereunder two-spotted goby (Gobiusculus flavescens F.) and painted goby (Pomatoschistus pictus M.). This is discussed in context of the 2002 regime shift and the change in abundance and timing of plankton in general. Evidence suggests that a change in the zooplankton community with a substantial decrease in Para-/Pseudocalanus and other small-sized copepods, may have deprived the juvenile gobies of adequate prey and thus preventing growth into adult individuals. During the first period, these small-sized species formed high concentrations in the autumn and probably served as important prey for a variety of predators during autumn and winter, among these the 0-group cod (Gadus morhua L.). Before the regime shift in 2002, the year-class strength of cod was mainly determined at the 0-group stage. However, after 2002 beach seine catches of I-group cod have been very low, also after strong year-classes at the 0-group stage. Based on evaluations of potential causes, including predation from seals and sea birds and changed behaviour in cod, it is probable that the survival during autumn and winter is low due to reduced food availability from the collapse of the short-lived species. Low prey abundance may cause starvation mortality, but also increased predation as the small cod must spend more time searching for food and thus become visible to predators. On the other hand, there is evidence to suggest that predation on both the small, short-lived species and larger species has been lower after the regime shift in 2002. In conclusion, it appears that the regime shift in 2002 resulting in substantially reduced abundances of copepods has led to low survival in both the small, short-lived species and in 0-group gadoids. Deriving from this, the shift in the plankton community appears to have affected cod both directly and indirectly, by lower survival during the first summer because of low copepod abundances, and indirectly by low survival during autumn and winter due to low abundances of the short-lived species.