This paper not to ba citetl withollt prior roference to the uuthor

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1 ,, l This paper not to ba citetl withollt prior roference to the uuthor Intornational Council fo~ the Exploration of the Sea C.M. 1979/tT:3 Marine l~als CO~T.ittce An Ecosystem Approach to Grey Seals and Fisher1es.,by Staffan Söderberg Swed1sh Museum of Natural History, 8ection for Vertebrate. Zoology, Stockholm 50, &weden. Introduction When effects of seals predation on fish stocks are discussed, the interactions between fish species thus utilized, the food web and the often complex connections existing,between the seal itself and its prey, are important to cons1der. When management of grey seals and its relation to management of fish stocks is stud1ed, collecting such ecological knowledge must be an urgent task. This paper tries to exemplify such interaction from an ecosystem point of view. For this purpose a very simple model of the situation in the Gotland area (southern Baltic Sea) is constructed, using the energy circuit language, 1ntroduced by H.T. Odum (1); the idea being to show how the effects of both fishery and seal predation may be expressed in terms of basic production of the marine ~cosystem. 'It should be emphasized, however, that the aim of this account is merely to serve as an example of how these relations can be converted into mathematical terms. Thus, the figures used should be qonsidered mainly as rough estimations adopted for the benefit of this account only.,, :~ Fishery in the'gotland area. The island Gotland, situated in the 50uthern pa~t of the Baltic Sea, i5 a considerable base for the 8wedish salmon fishery. Among other species. caught, being also important food items for the grey seal, herring, cod and flounders may be mentioned. The results of the fishery' efforts inthe area in 1973, are summarized in table 1. For use in the energy diagram, thc catch data have been transformed to heat equivalents, expressed in terms of kilojoule (kj) 109. Energy values for different rish ~pecies, taken from an,ordinary nutrition table, have been used for this calculation. (Cf. table 3.)

2 \ /' <. Predation by seals. '). The feeding habits of r,rey seals in the Gotland area have been investigated by Söderberg (2). The found average amount of different fish species enten by the seals is shown in tnble 4. For the purpose of this paper it is assumed that 50 adult grey seals are regularly present in the area. Provided that the averaee food supply for an adult grey seal can be estimated to 7 kg a day, the annual consumption by 50 seals may be calculated. (Cf. table 2.) In accordance with the catch dnta, these figures have been transformed into heat equivalents. Note, however, that the magnitude here is only kj 108! Energy flow diagram A simplified sketch model of tbe marine ecosystem in Gotland waters, as far as speeies mentioned are concerned, 1s shown in fig 1. Thc energy flow through thc system is given in 10 9 kj and 10 8 kj heat equivalents respeetively. 10% efficiency at each trophiclevel has been used, with exeeption for zooplancton - phytoplaneton, where the figure is 15%.. Since the average annual gross primary produetion (phytoplaneton) in the southern Baltie 1s known to be 150 g C/m2, wbere 1 g C equals 41,86 kj beat energy, the produetion basis for fisbery, as weil as for eonsumption by seals, may be ealeulated, eounting baekwards. Consequently, the pr1mary produetion basis for the fishery in the Gotland area (60fisbermen) in 1973 was: 950 km2 or 0,26% of the whole Baltie area ( km2). - The corresponding figure for the seals is 61,5 km2, or 0,02% of the whole Baltic area Thus, it may be eoncluded, that the seals around Gotland, in this example utilize a primary production basis,similar to 6,5% of that required for the fishery. (Cf. fig 2) o References 1. Odum, H.T. An energy eireuit language for ecologieal and social systems: its physical basis. Progr. rep.u.s. Atomic Energy Comm. Contr. At-C40-1) Söderberg~ S. Sälens födoval ach skadegörelse pä laxfisket i Ostersjön. Svenska Ostkustfiskarenas Centralförbund, pp.

3 \ 1'abJ c 1. Species - ) - Fisbery in thc Gotland arca (ace. to Central Bureau of Statistics) Catch in tons Tot: 3447 Heut equivalents in kj ,40 1,95 2,40 0,60 1,32 Table 2. Species Table 3. Consumption by seals in the Gotland area, Consumption Heat equivalents in kj 10 8 in tons 30,0 26,8 16,0 16,1 38,8 Tot: 127,7 2,0 1,4 0,5 Energy values for fish species (from ordinary nutrition table) 6660 kj/kg 3340 _"_ " 3320 n 2400 _"- Table 4. Fish species eaten by grey seals in the Gotland area~ in per cent of total consumption. Ace. to Söderberg (2) 23,5 % 21,0 % 12,5 % 12,6 % 30,4 %

4 l. - ~ Fishc>ry in thc Gotland aren 1973 l'redc!tion 01/ BeaZ" in tlw Gotland QY'ca fishery condumption by Dealn ,5 2,0 2,4 THE BALTIC 1 9 ) Encrgy flow in: 108 kj heat equivalents (fishery) 10 kj heat equivalents (seazs) 10 %efficiency at each trophic level, with exception for zooplancton - phytoplancton, where the figure is x) gross primary ~roduction: 150 g C/m /ycar, where 1 g C equals 41,86 kj heat equivalents.

5 '"...,... Scale 1 : y._~-- Primar,r production bnsia for the Gotlnnd fiahery o o 61,5 km 2 PrimaIj" p1"oduction baais for the Gotland grey seals