ICES mar. Sei. Symp., 199: 209-221. 1995 Decapod crustaceans in the diets of demersal fish in the Cantabrian Sea I. Olaso and E. Rodriguez-Marin Olaso, I., and Rodriguez-Marin, E. 1995. Decapod crustaceans in the diets of demer­ sal fish in the Cantabrian Sea. - ICES mar. Sei. Symp., 199: 209-221. The diets of 14 species of demersal fish, representing 81% of the fish biomass of the Cantabrian Sea (ICES Division VIIIc), were determined in the spring and autumn of 1988 from the analysis of 6536 stomachs. Decapod crustaceans represented 54.3% of the diet of these fish in frequency of occurrence and 22.1% in percentage by volume. The main prey taxa were: Solenocera membranacea, Alpeus glaber, the Crangonidae and Processidae families within the Natantia, the Paguridae and Galatheidae families within the Anomura, and Goneplax rhomboides and the Portunidae family within the Brachyura. Predator-prey linkages were described and each predator was assigned to the size group that would best demonstrate any size-related feeding pattern. I. Olaso and E. Rodriguez-Marin: Instituto Espanol de Oceanografia, Laboratorio Oceanogrâfico de Santander. Apdo240, 39080 Santander, Cantabria, Spain [tel: (+34) 42 2740431275033, fax: (+34) 42 275072], Introduction Studies of size, distribution, and abundance of prey and diet selection by predators were conducted because The Bay of Biscay forms a well-defined unit because of these parameters play a fundamental role in the selec­ its geographical location and semi-enclosed nature. The tion of diet and have the advantage of being easily southern part of this gulf, called the Cantabrian Sea, is a quantified (Murdoch and Oaken, 1975; Vince et al., transition zone and its fish and crustacean inhabitants 1976). The large majority of demersal fish are opportun­ are a mixture of tropical, subtropical species and boreal istic feeders and it is also known that prey size in relation species (Garaa-Raso et al., 1987; Olaso, 1990; Sanchez, to the size of the fish is a determining factor in the food 1990). In this area, seasonal reproductive and feeding composition of such feeders (Daan, 1973) and that migrations of commercial species (such as anchovy, differences in diet are influenced by preference for mackerel, red sea bream, hake, and tuna) take place, certain prey. Considering these principles and the ab­ and these migrations support intense fishing activity sence of such studies in this area, we were interested in throughout the year by a large and diversified fleet. determining some of the qualitative aspects of the inter­ In view of the economic importance of these fisheries, action between demersal and benthic fish. Hence, in the a series of research cruises were begun in 1980 to investi­ present article the role of decapod crustaceans as prey, gate the demersal fisheries and to provide information and the trophic linkages and differences in prey size on the distribution and abundance of the megabenthos selection are reported. (Olaso, 1990; Sanchez, 1990). In these cruises the analy­ sis of fish stomach contents was used to determine the alimentary habits of the fish and the relationship be­ tween the invertebrate megabenthos and demersal fish. Materials and methods In previous studies in this area it was determined that crustaceans are the most important prey group in terms The stomach contents of 14 species of demersal fish, of percentage of frequency (Olaso and Pereda, 1986; obtained from stratified random bottom trawl surveys in Sorbe, 1981). These authors noted the importance of the Cantabrian Sea in the spring and autumn of 1988, are crustaceans in the food of small-sized fish, while the rest given in Table 1. These species were selected because of the invertebrate groups were not very significant; they form a significant part (81%) of the fish biomass however, polychaetes and molluscs are important to (Sanchez, 1990) and/or are known predators of decapod specific species of predators (Olaso, 1990). crustaceans. Furthermore, these fish were sufficiently 210 /. Olaso and E. Rodriguez-Marin ices,™ , sd. symp , 199(1995) Table 1. List of predator species whose stomach contents were analysed. N = total number; %E = emptiness percentage. Spring Autumn Total Year Species N % E N % E N %E Aspitrigla cuculus 65 35.4 140 54.3 205 48.3 Aspitrigla obscura 36 16.7 35 54.3 71 35.2 Conger conger 134 41.0 216 39.8 350 40.3 Galeus melastomus 14 35.7 40 10.0 54 16.6 Lepidorhombus boscii 367 32.7 803 45.6 1170 41.5 Lepidorhombus whiffiagonis 484 44.4 696 51.0 1 180 48.5 Lophius piscatorius 89 80.8 141 69.5 230 73.9 Merluccius merluccius 434 57.8 760 53.4 1 194 55.0 Micromesistius poutassou 326 55.8 561 77.0 887 69.2 Mullus surmuletus 215 35.3 55 5.5 270 29.3 Phycis blennoides 121 27.3 121 27.3 Raja clavata 42 9.5 61 14.8 103 12.6 Scyliorhinus canicula 220 25.5 336 23.2 556 24.1 Trisopterus luscus 90 23.3 55 20.0 145 22.1 TOTAL 2516 43.0 4020 49.3 6536 46.9 abundant for analysis; a minimum of 50 stomachs of prey, and Nt is the total number of stomachs contain­ each species were analysed. ing the food analysed. The study area corresponds to ICES Division VIIIc, 2. The volumetric method. The volume of the stomach and is made up of sandy and muddy bottoms. Samples contents was measured applying the ecological feed­ were collected at depths of between 30 and 500 m. Fish­ ing method developed at Woods Hole (Bowman, ing was based on daytime trawling, with trawls of 30 min 1982), in which a “trofometro” is used, a piece of duration, using a trawl gear with a mesh size of 20mm. equipment comprising several different sized half­ Sampling was randomly stratified. The methodology cylinders built into a tray in such a way that they form used to select the stations was described by Sanchez and horizontal half-cylindrical moulds (Olaso, 1990). Olaso (1987). Percentage by volume, V: In each haul a maximum of 10 stomachs per length range of the more abundant fish species were sampled. V = v/V,*100 Fish with regurgitated food in their oral cavity were where v is the volume of a specific prey and V, is the rejected, whereas fish with empty stomachs but without total prey volume. apparent indications of regurgitation were not. The stomach contents were analysed on board ship. Examin­ ation involved separation of food items into taxonomic Each method has both advantages and limitations in categories. Fish, decapod crustaceans, and cephalopod evaluating the importance of a prey group. Frequency of molluscs were identified by species, but other groups occurrence produces a bias, overvaluing small and nu­ were combined into higher order taxa. merous prey. On the other hand, the volumetric method To evaluate the importance of the stomach contents overestimates the importance of large organisms (Hys­ two methods were used: lop, 1980). In this study the decapod crustacean prey are of relatively small size, and so both methods have been taken into account. 1. The frequency o f occurrence method. This was used The following information was collected for each prey to characterize fish feeding, and only stomachs con­ species: percentage in relation to the volume of stomach taining food were used for estimation (Dunn, 1954; contents, number of specimens per stomach, state of Kennedy and Fitzmaurice, 1972). This method does digestion and prey length. The prey which were undi­ not give quantitative information, but is rapid and gested and whose size coincided with those found in the requires a minimum of apparatus, giving a somewhat codend of the net were not considered, since predators qualitative picture of the food spectrum (Hyslop, could have fed while they were concentrated in the net 1980). Frequency of occurrence (percentage), F: during the haul (Brown and Cheng, 1946; Bowman, F = n/Nt*100 1986; Klemetsen, 1982). Measurements were taken from decapod crustaceans where n is the number of stomachs with a specific as follows: for the Natantia, the Macrura Reptantia ICES mar. Sei. Sym p., 199 ( 1995) Decapod crustaceans in the diets of demersal fish in the Cantabrian Sea 211 group and the Galatheidae family, total length was Table 2. List of prey groups found in stomachs of 14 demersal fish species. F = frequency of occurrence percentage; V = measured (distance from the post-orbital edge to the percentage by volume. telson apex, with the abdomen extended); for the Bra- chyura infraorder, carapace length was determined (dis­ Crustacea Decapoda F V tance from the foremost part to the end of the carapace) ; for the Paguridae family, céphalothorax length was Anomura Family Galatheidae measured (distance from the post-orbital edge to the Galalhea spp. 1.18 0.12 post-dorsal edge of the céphalothorax). Munida intermedia 0.26 0.12 If the stomach contents were in an advanced state of Munida perarmata 0.03 0.00 digestion or the prey was incomplete, the length of its Munida sarsi 0.43 0.19 Munida spp. 3.03 2.59 hard structure was measured - carapace or propodus Family Paguridae length in decapod crustaceans. In the laboratory, from Pagurus bernhardus 0.12 0.05 the material collected and preserved in 70% alcohol, Pagurus prideauxi 1.87 2.15 regression functions of size-weight, chela length- Pagurus variabilis 0.06 0.02 0.63 size, céphalothorax length-size, etc., were obtained, Other Paguridae 0.92 Other Anomura 0.17 0.01 from which total length and weight were estimated Total Anomura 7.72 5.88 (Rodriguez-Marin, 1993). Brachyura Atelecyclus rotundatus 0.63 0.78 Corystes cassivelaunus 0.23 0.81 Results Goneplax rhomboides 3.80 1.22 From the demersal fish selected, 6536 stomachs were Inachus leptochirus 0.06 0.01 Family Portunidae analysed (Table 1).