Spatial patterns in the distribution and early life characteristics of North Sea cod - Influence from environmental factors and climate change

Abstract

The North Sea cod stock, besides being fished down to its lowest level of spawning stock biomass in history, is also considered to be threatened by negative effects of climate change. As a stock close to the southern limit of the species’ latitudinal range it may particularly suffer from increasing temperature. However, climate change would not only entail increasing temperatures, but also changes in freshwater inflow, wind stress and large scale changes in current flow. While information is accumulating on the environmental effects on the early life stages, there is still limited understanding of how effects interact during the early life, from the spawned egg to the settled juvenile. The primary focus of the present PhD study was to investigate spatial patterns in the planktonic and early demersal stages of cod and relate these to biological and physical factors. The ultimate goal has been to enhance our understanding of interaction between factors in the light of potentially changing conditions due to climate change. Specifically addressed were i) the linkage between hydrography and the distributional patterns of cod eggs; ii) the influence of hydrography and the distribution of potential prey, on the horizontal and vertical distribution of fish larvae and iii) the description of suitable nursery grounds for settling juveniles in hydrographical and biological terms. Traditional methods of field sampling and identification based on morphology were combined with statistical models for spatial data, using Generalized Additive Models (GAMs), to describe the environmental conditions in spawning and nursery areas. While the main focus was on cod, other common gadoids and flatfish were also investigated, and the vertical distribution of fish larvae was examined in a comparative approach between species. During the study period for cod spawning in the North Sea the hydrographical patterns were quite stable, also in comparison to earlier studies. Likewise, the distribution of fish eggs and hence spawning exhibited only limited variation on the broad scale. This might stem from adults returning to the same spawning grounds. Within these grounds, there was some adaptability in spawning behaviour, as the centre of egg abundance varied in accordance with prevailing hydrographic conditions. Environmental conditions appeared to be more powerful descriptors for the presence/absence of fish eggs, while variations in the abundance were better explained by spatial dependency (i.e. greater similarity the closer two samples are together). During the study of fish larvae in the northern North Sea, we found aggregations near frontal systems. The larvae were concentrated in the upper and middle water column, forming two distinct assemblages during the day. Prey was abundant in the upper 40 m of the water column, and fish larvae aggregated between 20 and 40 m for foraging. When not foraging, the larvae remained at the same relative depth in relation to other species. During the 20-year period studied for the settlement distribution of 0-group cod, a strong decline in nursery area usage was observed. In later years, 0-group cod were mostly found in relatively shallow, warm and medium saline water. Modelling of the 0-group presence in these areas with a few temporally stable and temporally variable covariates allowed to predict the distribution in other periods to a fairly good level (rs>0.8), but also revealed that for long term predictions the dynamics within the population have to be taken into account. While the North Sea may have already become warmer, broad scale patterns of hydrography as depicted from bottom conditions have not changed substantially yet. However, more extreme climate forcing in the future, may change this. Effects of increased temperature and changes in Atlantic inflow, current patterns and freshwater influence might be beneficial for some species and at some stages during their life, but overall detrimental effects are likely, which may lead to reduced recruitment and stock size