Cod distribution and temperature in the North Sea

ICES mar. Sei. Symp., 198: 244-. 1994 Cod distribution and temperature in the North Sea Henk Heessen and Niels Daan Heessen, H. J. L., and Daan, N. 1994. Cod distribution and temperature in the North Sea. - ICES mar. Sei. Symp., 198: 244-. Survey data on the distribution of individual age groups of cod in the North Sea exhibit marked annual differences in the ambient temperature. There is no indication that these differences are related to a certain preferred temperature, not at least for the juveniles. Juvenile cod are exposed to the highest water temperatures in late summer and autumn, and the lowest temperatures in winter. This analysis may provide a better basis for the study of annual growth differences and the effect of temperature on digestion rates. Henk Heessen and Niels Daan: Netherlands Institute for Fisheries Research, PO Box 68, 197 A B Umuiden, The Netherlands. Introduction One approach to the cod and climate problem is provided by the behaviour of the cod population in relation to annual variations in water temperature, with particular reference to distribution patterns. Mountain and Murawski (1992), for example, attributed changes in the distribution of several fish species on the northeast continental shelf of the USA to changes in temperature. In the North Sea, young herring surveys were initiated in February 1965, which have evolved over the years to highly standardized and internationally coordinated surveys (now called International Bottom Trawl Surveys, IBTS), aimed at estimating the year-class strength of a variety of species, including cod. Although preliminary analyses of the effect of water temperature on the distribution of cod have been presented earlier (Buijsse and Daan, 1986; Heessen, 199), an in-depth analysis is hampered by the fact that the hydrographical data collected during these surveys are entered in a different database at ICES than the fish data, and so far it has not been possible to link the stations properly. Analyses have therefore been limited to subsets of stations, for which temperature data have been made available on a national basis. Consequently these studies covered only some of the stations. However, upon request from the Multispecies Assessment Working Group, ICES has now made available a comprehensive data set giving the range, average, and standard deviations of the recorded temperatures in the database by statistical rectangle, quarter, and year. Although this set includes sources of information other than just the IBTS, it provides an excellent opportunity to study the relationship between the average cod catches during the surveys and the temperature regime in each year. The objective of this paper is to relate annual changes in distribution to variations in temperature. Methods The data available include the catches per hour fishing by age group by statistical rectangle for the IBTS in February 1977-1991. In addition to these data on the winter distribution of cod, quarterly surveys were carried out in 1981 and 1991 in connection with large-scale stomach sampling projects. During these additional surveys a variety of other gears was used, although in 1991 most of the countries involved employed the G OV trawl, which was also the standard gear for the February surveys (ICES, 1992). The catch data from the February surveys were made available through ICES; for the other surveys, data from the individual countries were sent directly to the Netherlands Institute for Fisheries Research. No corrections were made for possible differences in catchability between gears. Although information on hydrographic parameters could be included in the exchange format for the survey data, this is not done as a matter of routine, largely because such information is stored within ICES in a

ICES mar. Sei. Symp., 198(1994) Cod and climate in the North Sea 245 E5 E6 E5 E6 52 51 5 Quarter 1 1977 / 1991 Quarter 2 1991 49 48 47 46 V 7 45 44 58N 4 42 57N 41 4 9 8 7 6 5 4 2 1 29 28 27 11-56N 55N 5N 52N 51 N 52 51 5 Quarter 1991 Quarter 4 1991 49 48 6N 47 46 59N 45 44 1 58N 4 42 41 4 9 8 7 16. 6 14 18. 5 4 2 1 29 28 27, 12 T 11 1. 14 14. 55N 54N 5N 52N 51 N 5N 4W 2W 2E 4E 6E 8E 1E 12E 4W 2W 2E 4E 6E 8E 1E 12E Figure 1. Quarterly distribution charts of mean bottom temperature by rectangle, based on data from the ICES Oceanographic Data Bank. The position of the Dogger Bank is indicated. A. Winter 1977-1991; B. Spring 1991; C. Summer 1991; D. Autumn 1991.

246 H. Heessen and N. Daan ICES mar. Sei. Symp.. 198(1994) separate database. These two databases cannot be directly linked and consequently temperature information is not available for all individual hauls. However, the ICES Oceanographic Data Bank can export mean quarterly bottom temperatures by statistical rectangle with the associated ranges of observed temperatures and standard deviations. In order to meet the objective of this study, we replaced all temperature data in the IBTS database with the mean temperature data from the Oceanographic Data Bank. The winter temperature data cover almost the entire North Sea, but in other quarters there arc considerable gaps, particularly for 1981 and the last quarter of 1991. In cases of missing values, we interpolated temperature data if data were available for at least two neighbouring rectangles. Nevertheless, a large component of the cod population could not be associated with the ambient temperature and therefore the results for quarters other than the first one may be unreliable. In the subsequent analysis, the rectangles were classified in 1 C temperature bands. All observations of temperatures below C were pooled in the temperature band of -1 C. Subsequently, the number of rectangles in each temperature band and the average catch-at-age per hour per temperature band were calculated. The results were analysed in two ways: as densities in relation to temperature, and as percentages of the total population, relative to the surface area covered by each temperature band. Results Temperature In order to interpret the results, the quarterly temperature regime is indicated in Figure 1A-D. The data for the first quarter cover 15 years and therefore can be taken to describe the average situation fairly well. For the other quarters, only 1991 data have been used, because of the even larger gaps in the 1981 set. During the winter season (Fig. 1A), the average temperature in the northwestern North Sea is between 6 and 7 C, and the Dogger Bank marks the border of the colder waters found along the continental coast. The Shetland area, where Atlantic Water enters the North Sea, is characterized by the highest temperatures, and an inflow of slightly higher temperatures can also be observed through the Channel. The average bottom temperature in the North Sea during winter (Fig. 2) varies between 4.5 and 7.5 C, with notably high average values during the period 1988 199. The years 1979 and 1986 were relatively cold. For the entire North Sea, however, the temperature signal is weak. 6 5 4 R 2 1977 198 198 1986 1989 Figure 2. Indices (R) of abundance of 1-group cod (bars) and average North Sea bottom temperature in C (squares) during winter 1977-1991. The winter of 1991 was fairly mild and probably affected the temperatures recorded in the later seasons (shown in Figure 1B-D) compared to the average winter situation (shown in Figure 1A). The southeastern North Sea has started to warm up in the second quarter. The available data indicate major differences between neighbouring squares, which is undoubtedly caused by the fact that the ICES Oceanographic Data Bank is based not just on bottom trawl survey data but also on other sources, and the data are integrated over three months of observation. Depending on the actual date on which the data have been collected, considerable differences can be expected. In the third quarter there is a small strip of higher temperatures along the Scottish coast running southwards. Southeast of the Dogger Bank, temperatures above 1 C are recorded throughout. As far as temperature data are concerned, nothing has changed very much in the fourth quarter compared to the third. Altogether, these data imply only minor seasonal differences in temperature over a large central area north of the Dogger Bank. Distribution There is a marked overall age dependent relationship between cod density and temperature during the February surveys (Fig. ). On average, the highest densities of 1-group cod are found at the lower end of the range, even below C. The highest density of 2-group cod is at somewhat higher temperatures, whereas the older part of the population has its highest densities at the upper end of the temperature range. The general pattern appears to be consistent from year to year, although there are annual variations (Fig. 4). These observations are obviously strongly related to the general observations that 1- and 2-group cod are found predominantly

ICES mar. Sei. Symp.. 198(1994) Cod and climate in the North Sea 247 14 5 12 4 a 9 8 2 N/hr 8 6 4 2 age 2 1 5 Figure. Average numbers-at-age of cod per hour fishing by 1 C temperature bands in February 1977-1991. 4 2 in the southeastern North Sea, particularly along the continental coast, where temperatures are lowest. It has to be borne in mind when interpreting these data that fish are not free in selecting preferred temperatures, because the temperature range depends on the severity of the winter. In addition, the distance over which cod usually migrates is restricted (Daan, 1978). To circumvent this problem, we compared the fraction of the North Sea characterized by the different temperatures with the proportion of the population found at each temperature for the years 1977-1991 (Fig. 5). The percentage of surface area of the North Sea within each 1 C temperature band is based on the number of rectangles within that band. The percentage of the total population of each age group is given for ages 1-4+. This provides a better indication of the importance of a particular temperature in relation to the total distribution of the fish than the observed densities, because a large catch could be restricted to a single square. In that case, the particular temperature of that square would be overemphasized. Figure 5 shows that the largest fraction of 1-group cod is consistently found at the lower end of the range. To a lesser extent this is also true for 2-group fish, whereas the distribution of the older age groups is more variable. In general, however, the older age groups seem to be found towards the high side of the available temperature distribution. These pictures can be summarized by plotting the mean ambient temperature of the population by age group in each year against the mean North Sea temperature (Fig. 6). The ambient temperature of 1-group fish is indeed consistently below the average, and to a lesser extent this appears to be true also for 2-group fish. In contrast, -group and 4-1- cod have always been found in a slightly higher ambient temperature than the average. Figures 7 and 8 provide similar data for all quarters in 1981 and 1991. Note that the temperature data for the 1 5 4 2 1 5 4 2 1 1 1 2 4 5 6 Temperature ( C) 4 + 7 8 9 Figure 4. Numbers-at-age of cod per hour fishing (log scale) by 1 C temperature bands in individual years, February 1977-1991. 2nd, rd, and 4th quarters of 1981 and the 4th quarter of 1991 are far from complete and may be biased. Nevertheless, the picture emerging here is that the pattern of ambient vs. average temperature (Fig. 8) shifts markedly during the year. In autumn, - and 1-group cod live at higher ambient temperatures than the average North Sea values, whereas for the oldest age groups the opposite is the case.

248 H. H eessen a nd N. D aan ICES mar. Sei. Symp., 198 (1994) 1982 1987 L -1 1 2 4 5 6 7 1978 1988 % so; - 1979 1984 1989 198 1985 199-1981 1986 1991 EH bottom temperature age 4+ H age Figure 5. Fractions of the total cod population by age group observed in 1 C temperature bands by year in February 1977-1991. The shaded area represents the fraction of the total area characterized by each band. Ü age 2 H age 1

ICES mar. Sei. Symp., 198 (1994) Cod and climate in the North Sea 249 9 8 8» O o k. 4-««k. o Q. E c jq E < 7 6 5 4 7» 82 n m CD 88 8 age 1 age 2 age o age 4+ 87 2 2 4 5 6 7 8 9 Average temperature ( C) Figure 6. Average ambient temperature of different age groups of cod in relation to the average North Sea bottom temperature by year in February 1977-1991. Discussion The available data strongly suggest that there are marked differences in the temperatures at which the different age groups of cod live in the North Sea. However, it is unlikely that there is a specific preference involved for lower or higher temperatures, not at least in the case of the juveniles because of their exposure to the lowest temperatures in winter and to the highest temperatures in late summer and autumn. The older fish seem to avoid areas where temperatures fluctuate strongly and to buffer themselves against these by staying in those areas where the annual fluctuation in temperature is minimal. This suggestion is supported by the average distribution patterns of the different age groups (Fig. 9). This, of course, need not be a direct cause and effect relationship, because there are other parameters related to temperature, such as depth and the availability of food, which are more likely to be responsible. Also, for fish aged and older the distribution will be affected by spatial differences in exploitation rate. When comparing the situation in different regions, it is difficult to detect general patterns in the relation between distribution of cod and temperature. For example, Sinclair (1991) found that in summer on the Eastern Scotian Shelf ambient temperature decreased with increasing age, just as we found in the North Sea, whereas Rose et al. (1994) found older Labrador cod occurring in relatively warmer water. Observations in the fall in the Southern Gulf of St Lawrence showed distribution to be independent of age (Swain, 1991), as did Rose et al. (1994) for the Northeast Newfoundland Shelf. In studies of cod distribution in relation to temperature, conclusions are often drawn about the preferences involved without the annual and seasonal variations in possibilities for selecting the preferred' temperature being taken into account (Sinclair, 1991; Swain, 1991). Even if the density of 1-group cod in relation to temperature (Fig. ) suggests that, in winter, these fish prefer the 1-2 C temperature band, it must be taken into account that in warm winters their choice is restricted to a minimum of 6 C. Moreover, if 1-group fish prefer to remain close to the continental coast

H. Heessen and N. Daan ICES mar. Sei. Symp., 198 (1994) 1981 Q1 1981 Q 5 6 7 8 9 1 1112 1 14 15 16 17 18 19 1991 Q1 1991 Q % 5 5 6 7 8 9 1 1 1 1 1 1 1 1 1 1 1 2 4 5 6 7 8 9 1981 Q2 1981 Q4 % 5 5 6 7 8 9 111 1211415 R-n, jflaujja 5 6 7 8 9 111 1211415 1991 Q2 1991 Q4 % 5 5 6 7 8 9 1111211415 5 6 7 8 9 1111211415 il bottom temperature LJ age 4+ m age Figure 7. Quarterly fractions of the total population by age group of cod observed in 1 C temperature bands in 1981 and 1991.

ICES mar. Sei. Symp., 198 (1994) Cod and climate in the North Sea 1 I D " / ' * 1^4 2-12 fc. 4-* a km Q. E 8 «c «A E < 91/ 11/4 m 9 O 91/ 81/2 81/ A age O age 1 age 2 age O age 4+ 81/1 4 8 12 16 Average temperature ( C) Figure 8. Average ambient temperature of different age groups of cod in relation to the average North Sea bottom temperature by quarter in 1981 and 1991. irrespective of the type of winter, the apparent effect will be high densities in cold waters, because these are necessarily restricted to the coastal area at these latitudes. Because the warmer waters encompass a large area, where 1-group fish are not found, the average density in those is reduced. Therefore, differences in ambient temperature between age groups or years are not necessarily explained by differences in preference in the true sense of the word; much more intricate analyses are required if we are to arrive at any firm conclusions in this respect. The approach presented in this paper may be useful also because it provides a better basis for studying annual growth differences or effects of temperatures on digestion rates than a point estimate for the average temperature in the sea. The ambient temperatures of the individual age groups deviate quite unpredictably from the average. If the winter is cold, 1-group fish are exposed to much lower average values than when the winter is relatively warm. The physiological effect is thus much stronger than indicated by the average. Within the context of cod and climate change, it would seem unlikely that the distribution of cod in the North Sea would be very much affected by a gradual increase in temperature, because there is no direct evidence that the cod actively avoid high temperatures, at least not in the range observed so far. This contrasts with the findings for several fish species, including cod, on the northeast continental shelf of the USA; these were shown to compensate, at least partly, for the interannual temperature changes by migration (Mountain and Murawski, 1992). Water temperature may of course have other effects on the population dynamics of cod, particularly on recruitment, and in this context it is worth noting that the mature cod population (4+) appears to perceive a very limited temperature signal during spawning (January/February). Since the North Sea is well mixed at this time of the year, the surface and bottom temperatures are not that different and therefore it is unlikely that a severe winter will directly affect egg mortality to any large extent. If there is an effect, it is more likely to be found in the larval and early -group phase. However, the remarkable change in recruitment during the 196s to 198s (see Daan et a l., this volume) would seem very much stronger than could ever be explained by

2 H. Heessen and N. Daan ICES mar. Sei. Symp., 198(1994) FS F61 F7 1F8 [ F9 l G lg 1 1G2 F2 F5 I F6 l F7 i F8 F9 G G 1 G2 52 1-9 6 49 47 1-24 -99 2 48 5 8-45 44 5 8-44 5 7-42 5 7-5 5-8 5 5-4 29 Age 1 Age 2 E5 i E6 i E7 i E8 1E9 i FO i F1 if2 F F4i F5 i F6i F7 i F8 i F9 i G ig 1 1G2 52 E5 i E6 i E7 i E8 i E9 i FO i F1 i F2 i F i F 4 1F5 i F6 i F7 i F8 i F9 i GO i G11 G2 52 48 59 47 47 5 8-45 44 5 8-45 44 5 7-4 5 7-42 4 5 5-8 5 5 - To i r r Age 28 27 Age 4+ Figure 9. Average distribution of age groups 1, 2,, and 4+ of cod for the period 198-1987 (from Heessen, 199).

ICES mar. Sei. Symp.. 198 (1994) Cod and climate in the North Sea any temperature signal, and to that extent the effect of climate change on the North Sea cod can be expected to be subordinate to other much more important factors. Acknowledgments We are grateful to Dr Harry Dooley and Garry Hopwood from the ICES Secretariat for providing us with pre-processed temperature data from the ICES Oceanographic Data Bank and to various colleagues from national laboratories around the North Sea participating in the International Bottom Trawl Survey for providing us with the necessary data. References Buijsse, T., and Daan, N. 1986. Sources of variation in IYFS indices of abundance, a preliminary analysis. ICES CM 1986/ G: 55. Daan, N. 1978. Changes in cod stocks and cod fisheries in the North Sea. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 172: 9-57. Daan, N., Heessen, H. J. L., and Pope, J. G. 1994. Changes in the North Sea cod stock during the twentieth century. ICES mar. Sei. Symp., 198: 229-242. Heessen, H. J. L. 199. The distribution of cod (Gadus morhua) in the North Sea. NAFO Sei. Coun. Studies, 18: 59-65. ICES. 1992. Manual for the International Bottom Trawl Surveys. Addendum to ICES CM 1992/H:. Mountain, D. G., and Murawski, S. A. 1992. Variation in the distribution of fish stocks on the northeast continental shelf in relation to their environment, 198-1989. ICES mar. Sei. Symp., 195: 424-42. Rose, G. A., Atkinson, B. A., Baird, J., Bishop, C. A., and Kulka, D. W. 1994. Changes in distribution of Atlantic cod and thermal variations in Newfoundland waters, 198-1992. ICES mar. Sei. Symp., 198: 542-552. Sinclair, A. 1991. Distribution of Eastern Scotian Shelf cod with respect to age, depth and temperature. NAFO SCR Doc., 91/1. Swain, D. P. 1991. Annual variation in the distribution of cod (Gadus morhua) in the Southern Gulf of St Lawrence. NAFO SCR Doc., 91/14.