Supplement to the knowledge of ecology of some Adriatic cartilaginous fishes (Chondrichthyes) with special reference to their nutrition
Abstract
This paper studies the nutrition of the fishes Scyliorhinus canicula, Squalus acanthias, Raja miraletus and Raja clavata. The material for this study was collected during 1968 and 1969 at 12 stations marked during the first Yugoslav Fishery and Biological Expedition »Hvar« (1948-1949) in the Central Adriatic and at three localities (banks), two south of the island Biševo (»Biševo I«, »Biševo II«) and south of the island Mljet (»Mljet«) (Fig. 1, Tab. 1). The stations are partly on the muddy bottom of the biocenosis Nephrops norvegicus - Thenea muricata (G a m u l i n - B r i d a, 1965), and partly on the sandyshelly bottom of the zoocenosis Turritella profunda (V a- t o v a, 1947 a, b) (Fig. 2).
Four hundred and seventy six stomachs were analyzed and the qualitative and quantitative composition of food was established for each separate species, thier preference for certain kind of food, succession of individual animal groups participating in the nourishment of Scyliorhinus canicula and Raja clavata depending on the size of the fish, parasitization (Nematoda), and the course of the annual and daily rhythm of nourishment by means of the statistical method by B l e v a d and Z e n k e v i č.
From the data obtained by the analysis of the material it is possible to conclude for each individual species as follows:
Scyliorhinus canicula L.
The food of this species consists of crabs (54.3%), fish (19.2%), cephalopoda (18.7%), and polychaeta (7.7%). (Tab. 4). Among the crabs only the higher crabs (Malacostraca) are fed upon, especially the decapod Alpheus glaber (58.6%), the stomatopod Squilla desmaresti (12.1%), and the decapod Upogebia sp. (7.7%). The smaller fish feed abundantly on Schizopoda (Mysidacea), and insignificantly so on Amphipoda. Fish are represented by smaller benthic species, as Gadiculus argenteus (12.1%) Maurolicus Pennanti (9.8%), Argentina sphyraena (9.8%), Merluccius merluccius (7.4%), and athers. The cephalopoda Sepiola spp. (27.5%), Illex illecebrosus coindetii and Todaropsis eblanae (together 12.5%) are tle most common. (Tab. 5).
The composition of the food changes with the size of the fish. (Fig. 4). With larger fishes the quantity of plankton crabs (Schizopoda) decreases gradually, and other animal groups, the quantity of which increases gradually with the growth of the fish, appear.
It is probable that selectivity exists in the nourishment. The species of crabs whose number is not very great in the environment like: Alpheus, Squilla, Upogebia are represented the most, and the species which come in much greater numbers like: Munida, Macropipus, Pontophilus, Macropodia and others are little or not at all represented. (Tab. 6). Nephrops norvegicus is not the object of nourishment. Selectivity within the other animal groups is not well expressed.
The annual rhythm of nourishment is the most intensive in the cold season of the year (from October to April) when the spawning of this species is the most intensive. The daily rhythm of nourishment shows its maximum in the early afternoon hours (about 3 p. m.). Tab. 7, Fig. 5.
The digestive tract (stomach, small intenstine) contained a larger number of Nematoda (over 70 pieces). A total of 84.7% fishes were invaded.
Squalus acanthias L.
The food of this fish consists most frequently of fish (52.3%), cephalopoda (33.8%), crabs (12.3%), and polychaeta (1.5%). (Tab. 8). Among the fish the pelagic forms are very frequent and the benthic ones less so. The most numerous are Maena vulgaris (8.8%), Cepola rubescens (8.8%), Sardina pilchardus (5.9%) and Gadus capelanus (5.9%). The crabs are represented by Decapoda, and especially the species Alpheus glaber (50%). The cephalopoda are mostly represented by larger forms as Ozaena spp. (36.4%), Illex illecebrosus coindetii and Todaropsis eblanae (13.5%) (Tab. 9).
Selectivity in food probably exists but it is not particular. The material from the stations contains chiefly the most numerous species of the environment (Gadus, Gobius, Cepola), while among the crabs the rarer species are represented ( Alpheus, Solenocera). (Tab. 11). Nephrops norvegicus was not found in the food.
The course of the annual rhythm of nourishment tallies with the bearing and forming of embryos. The nourishment is the least intensive at the beginning of the embryo’s development (October and April), and it increases gradually with the growth of the embryo until the very moment of its birth (September, March) when it reaches its maximum.
Parasitization is slightly expressed.
Raja miraletus L.
This species feeds on crabs (almost 100%) and on cephalopoda in a very small quantity (0.06%) (Tab. 14). Among the crabs Schizopoda (88.5%), Decapoda (9.02%) and Amphipoda (2.3%) are represented. Among the Schizopoda are Gastrosaccus lobatus (55.2%), Lophogaster typicus (25.4%), and Anchialina agilis (19.4%), and all the other species make the other 0.2%. Among the Decapoda are Pontophilus sp. (83.1%). Solenocera membranacea (9.8%), and others. (Tab. 13).
44.0% of fish were invaded by Nematoda whose number was usually about 1 to 7 pieces per stomach.
Raja clavata L.
It feeds on crabs (78.0%), fish (10.4%), polychaeta (9.4%), and cephalopoda (2.2%). (Tab. 16). Among the crabs are Malacostraca (Stomatopoda, Isopoda, Amphipoda, Schizopoda and Decapoda). The most numerous is the decapod Alpheus glaber (45.1%), the prawns Pandalina brevirostris (10.9%), and Solenocera membranacea (8.7%), and Munida bamffica (6.9%). In the nourishment of the juvenile forms Schizopoda (Mysidacea) are very abundant, and Amphipoda less so. Among the fish only the benthic forms are represented. The most frequent are Gobius-friesii-macrolepis (22.2%), Gadus capelanus (16.7%), Gadiculus argenteus (15.3%) and others. Among the cephalopoda are Sepiola spp. (46.7%) Illex ilecebrosus coindetii and Todaropsis eblanae (6.7%) (Tab. 17).
The contents of the food changes depending on the size of the fish (Fig. 8). The importance of Schizopoda in the nourishment decreases with the growth of the fish and then other animal groups are progressively introduced. Fish in the food come the latest.
Selectivity in nourishment probably exists. The crabs in the food are represented by those species whose number is not the greatest in the environment (Alpheus, Solenocera), or which were not established in that region (Pandalina). The more numerous species (Munida, Philocheras and others) are not well represented, and some (Macropodia, Macropipus, Parapeneus) could not be stated at all (Tab. 18). Nephrops norvegicus does not appear in the food. With other animal groups selectivity is not so evident (especially within the fish).
In the course of the year the greatest intensity in nourishment occurs in the winter and spring months (from October onwards) when intensive spawning is the most probable. The maximum of the daily rhythm of nourishment occurs in the late afternoon hours (about 5 p. m.) (Tab. 19, Fig. 9).
Nematoda were found in the stomachs and small intenstines of 35.1% of the fish. Their number was from 1 to 34 pieces per stomach.
Great similarity in the food composition is found between Scyliorhinus canicula and Raja clavata, which use crabs as their main food, less similarity appears between them and Squalus acanthias, whose food consists mostly of fish and cephalopoda, and the least exists with Raja miraletus, whose food consists of crabs almost exclusively. (Tab. 20, 21).
Due to a certain similarity in the food composition of these fishes definite competitive relations exist among them and these are best expressed between Scyliorhinus canicula and Raja clavata, and less so between them and Squalus acanthias and Raja miraletus. The competitive relations between these fishes and some commercially important species of Teleostei from this region, especially Merluccius merluccius, were also observed.
The obtained results of food composition tally essentially with such earlier data for Raja clavata (Č a n a d j i j a, 1956) and Scyliorhinus canicula (Č a n a d j i j a, 1961) in the Adriatic (Fig. 11). The differences are mostly in the representation of genera and species within the animal groups in the food, and less so in the representation of the very groups themselves. The differences in the food composition, in this case, result from the various composition of the biocenosis in which these fishes lived.
Considerable agreement exists with the data on the nourishment of the cartilaginous fishes in the Black Sea (B o r c e a, 1929, 1933; M a r t i, 1939; S t a n e s c u, 1958; S v e t o v i d o v, 1964, and others) and with the data by F o r d (1921), C l a r k (1922), E a l e s (1949) for the Atlantic and with the poor data by B i n i (1967) for the Italian coasts.
