Sculpins (Cottidae) – is one of the most characteristic families of fish in the northern part of the Pacific Ocean, representatives of which are found from the tidal zone to a depth of more than 2 km (Shmidt, 1950; Sheyko, Fedorov, 2000; Parin et al., 2014). Many species of sculpins in the Far Eastern seas have a fairly high abundance and biomass, and therefore play a significant role in bottom ichthyocenoses as predators and potential food competitors of commercial fish, and can also be objects of coastal fishing (Borets, 1995; Fadeev, 2005; Tokranov, 2009; et al.,). Currently, about 60 representatives of this family are reliably registered in the Kamchatka waters of the Okhotsk, Bering Seas, and the Pacific Ocean, 16 of which are classified as numerous (Sheyko, Fedorov, 2000). Since the sizes and habitats of different species of sculpins vary significantly, the composition of their food and the methods of foraging differ significantly. Generalization of materials on nutrition of 27 species of family Cottidae, collected in 1978-2008, provides an opportunity to get an idea of the trophic groupings of sculpins in the Kamchatka waters.
The work is based on the results of the analysis of stomach contents of 27 species of sculpins (Myoxocephalus polyacanthocephalus, M. stelleri, M. jaok, Megalocottus platycephalus, Hemilepidotus gilberti, H. jordaniand H. zapus, Gymnacanthusdetrisus, G. galeatus, and G. pistilliger, Icelus spiniger, I. spatula, I. canaliculatus, I. ochotensis, I. perminovi, Thyriscus anoplus, Artediellus camchaticus, A. ochotensis and Artediellichthys nigripinnis, Enophrys diceraus, Microcottu ssellaris and Rastrinus scutiger, Melletes papilio, Stelgistrum stejnegeri, Triglops pingeli, T. forficatus and T. scepticus), collected in different seasons of 1978−2008 in the Kamchatka waters of the Okhotsk Sea, the Bering Sea and the Pacific Ocean (Fig. 1).
Fig. 1. Scheme of sampling points on nutrition of sculpins (red rectangles) in near-Kamchatka waters in 1978–2008
During the research period, fishing was carried out by bottom trawls, snurrevods (bottom nets) and shore seines at the depth from 0.5 to 850 m. The collection and processing of stomach contents was conducted in accordance with the “Methodical manual ...” (1974). In total, more than 8.5 thousand stomachs of the listed species of sculpins were processed by quantitative and weight method. In addition, about 14 thousand stomachs of the most mass representatives of this fish family (respectively 4 thousand of Myoxocephalus polyacanthocephalus, 2 thousand of M. jaok, 5 thousand of Hemilepidotus jordani and 3 thousand of H. gilberti) were dissected and analyzed in the field conditions. This allowed us to expand significantly the idea of the qualitative composition of their food and the size of the organisms consumed by them. Statistical processing was performed according to generally accepted methods (Lakin, 1980).
Analysis of the food spectra of 27 studied species of sculpins allows us to allocate among them 6 trophic groups: ambush predators, benthoichthyophages, nectobenthoichthyophages, benthophages, nectobenthophages and benthomacroplanktophages. Representatives of each of these groups differ in the type of nutrition, composition and size of forage organisms. The following is a brief description of the food composition of representatives of these trophic groups. More complete information is presented in earlier published works devoted to the study of the characteristics of their diet (Tokranov, 1995, 2009, 2018; Tokranov, Orlov, 2013; etc.).
Predators-ambushers. According to the way of foraging, Myoxocephalus polyacanthocephalus, M. stelleri, M. jaok and Megalocottus platycephalus are typical ambush predators which lead a relatively sedentary lifestyle, when foraging masked on the ground and grab prey at a distance that does not require significant movement (Tokranov, 2009, 2018; Tokranov, Orlov, 2013, etc.). The feeding spectra of sculpins, which differ significantly in size are very diverse and include about 100 representatives of 18-22 taxa. However, the basis of biomass (over 80-90%) of each of them is formed by only two groups of organisms – fish Pisces and decapods Decapoda (Table. 1), represented mainly by crabs of the families Majidae and Atelecyclidae. But in Megalocottus polyacanthocephalus, fish waste (13.1%) and developing fish caviar (3.4% of food weight) also play a significant role in the diet. Sculpins have pronounced age-related changes in food composition. In this regard, as they grow, invertebrates in their diet are gradually replaced by fish (Borets, 1997; Napazakov, Chuchukalo, 2003; Cuchukalo, 2006; Tokranov, 2009; Napazkov, 2015), i.e. there is a transition from optional to obligate ichthyophagy. The food spectra of Megalocottus platycephalus living in the estuaries and estuary zone of rivers is also quite diverse and subjected to age-related changes. However, the basis of its food is formed by small schooling fish (primarily three-headed stickleback Gasterosteus aculeatus) (76.6%) and fish waste (11.5% of the food mass). In the third year of life of this sculpin species, bottom and near-bottom crustaceans in its diet are replaced by fish, i.e., as in Myoxocephalus, it proceeds to optional ichthyophagy. The considered ambush predators are characterized by high plasticity of feeding, which makes it possible to use a significant set of potential forage organisms, as well as fish, fish and food waste and developing fish caviar (Tokranov, 2009, 2018; Tokranov, Orlov, 2013).
Table 1. Food composition (in weight %) of mass species of sculpins of the ambush predator trophic group in the Kamchatka waters
|Component||Megalocottus polyacanthocephalus||M. jaok||Megalocottus platycephalus|
|The number of fish examined||1567||975||650|
Note. «+» hear and in tables 2 and 5 means<0,1% in mass
Benthoichthyophages. All three representatives of the genus Hemilepidotus (H.gilberti, H. jordani and H. zapus), according to our data, can be attributed to benthoichthyophages with a broad food spectrum, which includes representatives of 16 different groups of invertebrates and fish (Tokranov, 1995, 2009, etc.). Although, according to V. I. Chuchukalo (Chuchukalo, 2006), they are more nectobenthophages, as fish component of their diet plays a secondary role. Despite the possibility to use a significant set of food organisms, the main objects of nutrition (over 60% of the food mass) in these sculpins during the year are only 2-3 groups of bottom and near-bottom crustaceans (in H.jordani – mainly Decapoda, in H. gilberti – Amphipoda and Decapoda, and in H. zapus – Cirripedia, Amphipoda and Decapoda) (Table. 2). In addition to them, small fish, fish waste and developing fish caviar play a significant role in the diet of those fish. The total share of these types of food is 28.3, 12.0 and 21.6% of the food weight, respectively. That is, in our opinion, can attribute Hemilepidotus to the group of benthoichthyophages. Icelusspiniger, according to its type of diet, is also benthoichthyophag with quite diverse food spectrum (Tokranov, 2009). However, the main part of its biomass in the Kamchatka waters (about 81%) is formed by decapods (mainly Crangonidae, Pandalidae shrimps and Paguridae hermit crabs) and juvenile fish (mainly flounder and pollock Theragra chalcogramma) 32-110 mm long (Table. 2).
Table 2. Food composition (in weight %) of sculpins of the benthoichthyophages trophic group in the Kamchatka waters
|H. jordani||H.gilberti||H. zapus|
|The number of fish examined||1362||841||244||438|
Nectobenthoichthyophags. Of the studied species of sculpins in Kamchatka waters only Thyriscus anoplus and Icelus spatula can be attributed to this group, because, along with benthic organisms, they consume juvenile fish and shrimps, and the value of the latter is the greatest (Table 3). The first of them is characterized by a relatively narrow food spectrum, and the basis of its biomass (about 89%) is formed by three groups of food organisms –Decapoda, Amphipoda and Pisces (Tokranov, 2009). With the increase in size of the Thyriscus anoplus, consumption of various groups of forage organisms changes significantly: while the main food for small individuals are amphipods and shrimps of the Thoridae family (96% in weight), the basis of the diet in the largest specimens are the last of them (29%) and juvenile fish (43%).
Table 3. Food composition (in weight %) of sculpins of nectobenthoichthyophages trophic group in the Kamchatka waters
|The number of fish examined||63||80|
In the diet of Icelus spatula, along with shrimps (representatives of the genera Nectocrangon, Lebbeus, Eualus, Spirontocaris) and amphipods, small fish fry of various species (families Cottidae, Agonidae, Liparidae, Stichaeidae) 12-55 mm long play a significant role (about 41% by weight) (Tokranov, 2009). However, unlike most other species of family Соttidae, in which fish food dominants only in the diet of large individuals, the relative importance of fish is the greatest (63.2 % by weight) in the diet of juvenile of Icelus spatula up to 100 mm in length. With the increase in the size of the Icelus spatula, the share of this forage object in its diet is significantly reduced, to 39.9% in the largest individuals. Such a character of age-related changes in the relative value of fish in the food of the Icelus spatula is most likely due to the fact that the consumed fish fry is small and comparable in size with other food organisms, while its weight indices significantly exceed those of crustaceans. As the Icelus spatula grows, the size of the consumed shrimps increases sharply, while the length of the fish fry consumed does not change so much.
Benthophags. All three investigated species of hook-eared sculpins (Artediellus ochotensis, A. camchaticus and Artediellichthys nigripinnis) are typical benthophages by type of food (Table. 4). Although their food spectra are quite different, the main forage objects of the first species are Polychaeta, Amphipoda and small Decapoda (96.4% by weight), of the second one – Polychaeta and Amphipoda (about 90%), and of the third one – only Amphipoda (84.7%) (Tokranov, 2009). But as Artediellus ochotensis and A. camchaticus grow, the consumption of Polychaeta increases. The Gymnacanthus galeatus, G. pistilliger and Melletes papilio can be characterized as benthophages with broad food spectra (Chuchukalo, 2006; Tokranov, 2009). However, according to our data, in the Kamchatka waters the main food items of the first of them are sea anemones of the genus Metridium (51%), of the second one – various Polychaeta and Echiurus echiurus. (about 70% by weight). The Melletes papilio, according to our data, mostly consumes Decapod crabs and Amphipods, whereas the Microcottus sellaris and Enophrys diceraus consume primarily benthic organisms, among which Amphipoda and Polychaeta dominates. Similar dominant groups of forage organisms for the Enophrys diceraus in the Kamchatka waters are indicated by V. N. Chuchukalo (Chuchukalo, 2006).
Table 4. Food composition (in weight %) of sculpins of benthophages trophic group in the Kamchatka waters
|The number of fish examined||62||621||100||125||50|
Nectobenthophags. The group of nectobenthophages includes seven of the investigated sculpins species, the food organisms of which are typically benthic organisms or those living in the bottom layer (Table. 5). The main food of the Gymnacanthus detrisus (more than 50% by mass) are various ctenophores and jellyfish (Napazakov, Chuchukalo, 2003; Tokranov, 2009), of the Triglops pingeli (93.7 %) – opossum shrimps Mysidacea and shrimps, of the Stelgistrum stejnegeri (about 100%) – opossum shrimps Mysidacea, shrimps and Amphipoda, and of other four species (Icelus canaliculatus, I. ochotensis, I. perminovi and Rastrinus scutiger) – shrimps and Amphipoda (respectively 93.4, 95.8, 91.8 and 71.5% by mass) (Tokranov, 2009).
Table 5. Food composition (in weight %) of sculpins of nectobenthophages trophic group in the Kamchatka waters
|The number of fish examined||210||336||22||110||105|
Benthomacroplanktophages. This group of the studied sculpins species includes only Triglopsforficatu sand T. scepticus, which, along with typically benthic organisms, use in food planktonic crustaceans that temporarily appear in the bottom layer (Tokranov, 2009). During the year the main food objects are two groups of crustaceans – Euphausiacea and Amphipoda (about 97-98% by weight) (Table. 6). Among the former, Thysanoessa raschii has the greatest importamne of both Triglops, whereas the latter in T.forficatus are represented mainly by amphipods (Anonyx nugax, Ampelisca eschrichti, A. macrocephala), and in T.scepticus – by amphipods (Anonyx nugax, Ampelisca eschrichti) and Hyperiidea (Parathemisto libellula, P. japonica).
Table 6. Food composition (in weight %) of sculpins of benthomacroplanktophages trophic group in the Kamchatka waters
|The number of fish examined||248||256|
The results of the data analysis on nutrition of 27 examined species of sculpins allow us to distinguish 6 trophic groups: predators-ambushers, benthoichthyophages, nectobenthoichthyophages, benthophages, nectobenthophages and benthomacroplanktophages. Representatives of each group differ in the type of nutrition, composition and size of consumed food organisms. Predators-ambushers are characterized by large size and high plasticity of nutrition, which gives them the opportunity to use a significant set of potential forage organisms, as well as fish and food waste and the developing fish caviar. All members of this trophic group show the pronounced age-related changes in the food composition: as they grow, invertebrates in their diets are gradually replaced by fish, i.e. there is a transition from optional to obligate ichthyophagy.
Although the food spectra of benthoichthyophages are even more diverse than those of predators-ambushers, despite the possibility to use a significant set of forage organisms, the main food objects for Hemilepidotus during the year are only 2-3 groups of bottom and near-bottom crustaceans: in H. jordani it is mainly Decapoda, in H. gilberti – Amphipoda and Decapoda, and in H. zapus – Cirripedia, Amphipoda and Decapoda. In contrast, Icelus spiniger mostly consumes shrimps of the families Crangonidae and Pandalidae, hermit crabs and fish fry. Representatives of benthoichthyophages differ in some way from nectobenthoichthyophages, which are characterized by rather small size and relatively narrow nutrition spectrum, as well as the fact that along with benthic organisms, they feed on fish fry of the families Cottidae, Agonidae, Liparidae, Stichaeidae and shrimps of the family Thoridae, and the value of the latter is the greatest.
The main feeding objects of the majority of representatives of bentophages are various polychaete worms, amphipods and small decapods. However, some species of sculpins that are included in this trophic group have a certain specialization. For example, the main role in the diet of Gymnacanthus galeatus plays the actinia of the genus Metridium and echiurus Echiurus echiurus, the value of which is even greater in the food of Gymnacanthus pistilliger.
The food composition of nectobenthophages is formed both by typically benthic and living in near-bottom level organisms such as Mysidacea (Triglopspingeli and Stelgistrum stejnegeri), small karenia shrimps, hydroid jellyfish and ctenophores (Gymnacanthus detrisus). Finally, representatives of the trophic group of benthomacroplanktophags (Triglops forficatus and T. scepticus) along with such typical benthic organisms as amphipods, use in food a large quantity of planktonic crustaceans (mainly Thysanoessa raschii), concentrating temporarily in the bottom layer.
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The author is grateful to all the staff of the Kamchatka Branch of Pacific Institute of Geography, Pacific Research Institute of Fisheries and Oceanography, Pacific Research Centre of Fisheries and Russian Research Institute of Fisheries and Oceanography, which participate in the collection and processing of materials on feeding of sculpins in the Kamchatka waters in 1978−2008.