Diet of Risso ’ s dolphin ( Grampus griseus ) in the western Mediterranean Sea

is a moderately large and cosmopolitan dolphin species that lives in deep tropical and warm temperate oceanic waters. Data on its distribution in the Mediterranean are very scarce but sightings seem to indicate that the species prefers the slope, mostly where the slope is closest to the coastline (Notarbartolo di Sciara et al., 1993). There are few studies on the diet of Risso’s dolphin and they are frequently based on one or two individuals, except for data coming from the east coast of South Africa (Cockroft et al., 1993). In general its diet has been considered mainly teuthophagous with fish as an occasional component (Goodall and Galeazzi, 1985). The diet of Risso’s dolphin includes neritic, oceanic and bottom dwelling cephalopods. Data on its diet in the Mediterranean comes from studies on one harpooned (Pilleri and Gihr, 1969) and one or two stranded individuals, which were summarised by Clarke (1996) and which corroborate the teuthophagous character of the diet. The aim of this note is to provide more complete information about the poorly known diet of this dolphin. The gastrointestinal tracts analysed in the present work came from fifteen Risso’s dolphins stranded on the Mediterranean west coast between 40o25’N SCIENTIA MARINA 70 (3) September 2006, 407-411, Barcelona (Spain) ISSN: 0214-8358

Risso's dolphin, Grampus griseus (Cuvier, 1812) is a moderately large and cosmopolitan dolphin species that lives in deep tropical and warm temperate oceanic waters.Data on its distribution in the Mediterranean are very scarce but sightings seem to indicate that the species prefers the slope, mostly where the slope is closest to the coastline (Notarbartolo di Sciara et al., 1993).There are few studies on the diet of Risso's dolphin and they are frequently based on one or two individuals, except for data coming from the east coast of South Africa (Cockroft et al., 1993).In general its diet has been considered mainly teuthophagous with fish as an occasional component (Goodall and Galeazzi, 1985).The diet of Risso's dolphin includes neritic, oceanic and bottom dwelling cephalopods.Data on its diet in the Mediterranean comes from studies on one harpooned (Pilleri and Gihr, 1969) and one or two stranded individuals, which were summarised by Clarke (1996) and which corroborate the teuthophagous character of the diet.The aim of this note is to provide more complete information about the poorly known diet of this dolphin.
The gastrointestinal tracts analysed in the present work came from fifteen Risso's dolphins stranded on the Mediterranean west coast between 40º25'N 00º32'W and 37º35'N 00º45'E from April 1987 to January 2003.Gastrointestinal tracts were stored deep frozen (-20ºC) and the contents were subsequently flushed through 0.4 mm and 0.2 mm mesh and preserved in 70% ethanol.
Single strandings of Risso's dolphins have occurred over the last 15 years in this part of the western Mediterranean coast, with an average of 1.4 individuals stranded per year.The cause of death was not established but seems to be natural.None of them were injured; only one 260 cm length male was caught by trawling fisheries.Table 1 shows some biological data and the stranding dates of the dolphins analysed in this study.
A summary of the contents of the digestive tract of G. griseus is shown in Table 2.One female (165 cm length) was excluded from the table as its gut only contained a single cephalopod eye lens.Cephalopod remains, lenses and especially beaks were the common remains in the gastrointestinal tract, particularly in the stomach.Minute remains, including a few cephalopod beaks, lenses and teuthoid statoliths, were the most frequent hard remains found in the intestine.Cephalopod remains consisted mainly of beaks, which were identified using guides (Clarke, 1986a) and reference material.Taxonomic keys were used to identify thaliaceans (Godeaux, 1998) and isopods (Richardson, 1972); cephalopod mantle length was estimated from the morphometric relationships given by Clarke (1986a), Würtz et al. (1992) and Smale et al. (1993).
The smallest individual (165 cm length) with hard remains in its stomach was a calf less than one year old according to age determination from reading GLGs (A. Raduán, 2004, personal communication); this early solid food intake in the life cycle of Risso's dolphin shows that lactation may be quickly complemented with solid food as occurs in other marine mammals (Stewart and Stewart, 2002).
The teuthophagous component of the diet of G. griseus in the western Mediterranean was made up of 25 species of cephalopods belonging to 13 families.Prey were mainly oceanic and pelagic (97.2%) with a muscular body (79%) and only 27.9% were luminous species.Squids and octopods were the main component of this dolphin's diet, pelagic octopods (mainly A. argo) were the most abundant prey (average numerical abundance = 41.28%;SD±34.32).Biological data on these species items have already been published in Blanco et al., 2005.

Species of families Ommastrephidae and
Histioteuthidae have been cited previously as the main component of the diet of Risso's dolphin in the Mediterranean (Clarke, 1996), but this study confirms the importance of octopods in its diet in the western Mediterranean, as found in other seas (Clarke, 1996;Cockroft et al., 1993).Nevertheless, the pelagic character of most octopods, mainly A. argo, contrasts with the benthic character of the most frequent octopod prey, Eledone cirrhosa in the Atlantic (Clarke and Pascoe, 1985;González et al., 1994;Santos et al. 1996) and is similar to the results concerning diet for the southern Indian Ocean (Cockroft et al. 1993).Pelagic octopods seem to be characteristic of its diet, which does not apply to other cetaceans in the Mediterranean (Meotti and Podestá, 1997;Blanco and Raga, 2000;Roberts, 2003) and other seas (Clarke, 1996).Benthic octopods appear in the diet of bottlenose dolphins (Blanco et al., 2001) in the Mediterranean.The fact that they are benthic species shows that these two dolphins have different feeding habitats.
Despite the numerical importance of pelagic octopods, ommastrephids, mainly O. bartramii and Todarodes sagittatus, may be assumed to be more important in terms of biomass due to their larger size.Furthermore, the mantle muscle has a higher calorific value than in other medium sized squids with an ammoniacal mantle, such as histioteuthids (Clarke, 1986b).This dolphin's most common squid prey, T. sagittatus, Ancistroteuthis lichtensteinii, Histioteuthis reversa and H. bonnellii, which are most frequent in 600 to 800 m depth (Quetglas et al., 2000), have been considered indicator species of the middle  slope in the western Mediterranean (González and Sánchez, 2002); they are also present in the 200 to 600 m depth stratum where other species of the family Ommastrephidae are abundant.Onychoteuthis banksii and some teuthoid species which are less frequent in its diet such as Ancistrocheirus lesueurii, Brachioteuthis riisei, Chiroteuthis veranii and Heteroteuthis dispar have only been found in the aforementioned deeper stratum (Belcari and Sartor, 1993).The occasional catches of A. argo in the Mediterranean provide little information about its specific distribution, but it is considered to be an epipelagic species of oceanic waters.A similar offshore distribution in the western Mediterranean is also inferred from occasional catches of other species.Species belonging to families Mastigoteuthidae and Cranchiidae are considered to be oceanic; Galiteuthis armata catches have been associated with bottoms deeper than 1000 m, as found for O. bartramii (Villanueva, 1992); A. lesueurii has been caught on slopes deeper than 500 m (D'Onghia et al., 1997); only Octopus macropus, which represents 0.3% of the prey, is considered a littoral species.
A colonial ascidian, two similar small teleostean cycloid scales (<2.5 mm) and a bryozoan were found in the intestine of a juvenile (172 cm length); Salpa fusiformis, Iasis zonaria and Pyrosoma atlanticum were found partially digested in the stomach of an adult female (292 cm length) stranded in April; three specimens of isopods were found undigested in the most anterior part of the oesophagus of a female (210 cm length).Cephalopods are assumed to be the main component of the diet of G. griseus; however, the importance of thaliaceans may be underestimated because they are easily degraded  in the gut and their remains are difficult to identify.They have previously been reported as dolphin prey in Commerson's dolphin (Goodall and Galeazzi, 1985) and in beaked and bottlenose whales (Dixon et al., 1994, Walker and Hanson, 1999, Walker et al., 2002).In other marine mammals pelagic tunicates have been considered appropriate prey because juveniles have a more limited ability to capture swift prey (Bowen and Siniff, 1999).In this study, pelagic tunicates also appeared in the adult diet.We suggest that thaliaceans should be considered as an occasional component of the diet of Risso's dolphin, which would perhaps take advantage of the high biomass occasionally reached by some thaliacean species (Andersen, 1998).The ascidian colony, fish, bryozoans and isopods may be indirect or incidental prey, considering their infrequent occurrence and smaller size.We conclude that Risso's dolphin feeds in the Mediterranean along the slope but preferentially on the middle slope (600 to 800 m depth).Further studies on feeding habitats of other top predators are needed to explain why this dolphin is preferentially associated with an area of the slope where cephalopod biomass and species richness is low compared to shallower strata (Quetglás et al., 2000;González and Sánchez, 2002).

TABLE 2 .
-Gastrointestinal content of Risso's dolphin (N=14 ) in the western Mediterranean.N, number and F, frequency of cephalopod prey; LRL, lower rostral and LH, hood cephalopod beak length (mm); ML (mm), estimated mantle length of cephalopods and total length in other taxa.