Growth of cod and climate change

To evaluate the influence of a global warming in Europe with an associated rise in water temperatures on growth of cod from the North Sea, the Norwegian coast and the North East Arctic a scenario was developed based on the model equation gained from the growth experiments in this thesis. It has t o be mentioned that this scenario should function as a basis for a comprehensive discussion focussing On the effect of temperature On growth. Therefore, food limitation or migration due to temperature were neglected in this scenario.

However, these aspects will be discussed in chapter 5.

Fig. 14 (A) illustrates the monthly mean water temperatures at a depth between 0 -1 00 metres for the period 1900-1998 as calculated by Dippner (1999) from ICES data. The corresponding water temperatures in the case of a water temperature rise of 1.5OC have been shown in Fig. 14 (B).

1 2 3 4 5 6 7 8 9 1 0 1 1 12

rnonths

1 2 3 4 5 6 7 8 9 1 0 1 1 1 2

rnonths

Fig. 14: Monthly rnean water ternperatures at a depth between 0-100 rnetres for the period 1900-1998 as calculated by Dippner 1999 (A) frorn ICES Data and during a scenario of a 1.5OC rise in ternperature (B) frorn the Southern North Sea (black line), Norwegian coastal waters (grey line) and the Barents Sea (dotted line).

Results

Fig. 15 (A) illustrates the higher water temperatures and seasonality of the Southern North Sea between spring and winter with a peak of 15OC in summer, followed by lower temperatures of the Norwegian coastal waters and the

Fig.15: Daily growth rates in a yearly cycle of female cod frorn the Southern North Sea (black line), Norwegian coastal waters (grey line) and the Barents Sea (dotted line) calculated by the rnodel growth equation dL(T) = dLmax * e^ (T* Tmax" . T ernperatures based on the data from Dippner (1999) (A) and of a scenario of a 1.5"C rise (B), See fig.

14.

The actual water temperatures (Fig. 15 A) of the North Sea led to the highest length increment values and a strong seasonal variability for growth of cod living in this region. Here, two high peaks for growth existed: one in June and one in November, when the environmental temperatures for North Sea cod corresponds to the optimum temperature of 10.7 'C as found out in the growth experiments. Low peaks of growth for North Sea cod also resulted in winter when the water temperature of the Southern North Sea drops to 5OC and in summer when the temperatures can reach 15OC between July and September.

So growth is negatively affected by water temperatures below or beyond the optimal range.

The winter temperatures of Norwegian coastal waters are the Same as in the Southern North Sea, but throughout the yearly cycle the northern waters are colder and the temperature fluctuations are not so high. Here, a maximum of 8OC occurs in summer. So growth of Norwegian cod is more or less constant from February to June and increases to 0.07 cmlday in September. The environmental temperatures for cod from the Barents Sea never exceed 4OC and are more constant compared to the other regions. Growth of cod living in

this region does not fluctuate very much and length increments (between 0.02 and 0.03 cmlday) are much smaller than of specimens inhabiting the Southern North Sea and Norwegian coastal waters. In the case of a temperature rise by 1.5OC individuals growth would increase in three populations in winter and spring. Changes are highest for Arctic fish and length increments would be doubled. With increasing Summer temperatures growth of North Sea cod would decrease and in August the length increment of these individuals would drop down to a theoretical value of 0.048 cm per day. In this case, growth of North Sea specimens would be lower than of cod from the more northerly and colder areas.

4.3 Fecundity studies

On the basis of oocyte countings of North Sea and Baltic cod and literature data for North East Arctic cod (Kjesbu 1988) and Norwegian coastal cod (Botros 1962), absolute fecundity was calculated for each individual. High variability of fecundity of North Sea and Baltic cod was observed.

In Fig. 16 and Fig. 17 the estimated fecundities were plotted versus age (years) and body lengths (cm). The fecundity-age plots revealed an exponential relationship 'for Baltic Sea, Norwegian coastal and North East Arctic cod (Tab. 10). However, a linear formula appears to fit the North Sea cod data best.

Fecundity at age was highest for North Sea cod, followed by the lowest values in North East Arctic cod with intermediate results for animals from the Norwegian coast and the Baltic Sea (Fig. 16). For example, North Sea cod at four years produce more than two million eggs per season whereas cod from the Baltic produce less than about one million eggs per season at the Same age.

aae lvearsi

Fig. 16: Relationship between absolute fecundity and age (years) in cod of various regions (Southern North Sea, Norwegian coast, Baltic Sea, North East Arctic). Data for the North East Arctic by Kjesbu (1988) and or the Norwegian Coast by Botros (1962).

Regression analysis showed higher coefficients of determination (r2) for fecundity at length compared to fecundity at age. The fecundity-length plots revealed an exponential relationship for Norwegian coastal cod and North East Arctic cod (Tab. 10). Linear formulae fit the data of North Sea cod and Baltic cod best. In contrast to the age-fecundity plots, the length-fecundity plots revealed a higher absolute fecundity for Baltic Sea cod than for North Sea cod:

60 cm long cod from the North Sea produced about one million eggs, but about three million eggs were found for Baltic cod at the Same length.

Fig. 17: Relationship between absolute fecundity and length (crn) in cod frorn various regions (Southern North Sea, Norwegian coast, Baltic Sea, North East Arctic). Data for the Arctic by Kjesbu (1988), for the Norwegian coast by Botros (1962).

Results

Table. 10: Regression forrnulae for absolute fecundity of different cod populations Fabs = absolute fecundity, L = total body length (crn), A = age (years)

Fecundity versus age