Tatyana Nosova
Knipovich Polar Research Institute of Marine Fisheries and Oceanography, Murmansk, Russia Abstract
Snow crab Chionoecetes opilio from the Barents Sea is known as an object of fishery and an important diet component for some fish species at the same time. In the Kara Sea, the population of this species has not yet reached commercial significance, but, in recent years, its rapid growth indicates an increase in its importance in the sea ecosystem. By this reason, the investigations of the crab feeding are important, since feeding is one of the possible causes of the pattern of abundance dynamics and the character of the distribution area. Based on the analysis of 228 crab stomachs, collected in autumn 2014-2016, the differences in the diet of crab in the Barents and Kara Seas were revealed. Comparing of diet peculiarities showed that, in the Kara Sea, males with the carapace width (CW) less than 60 mm preferred to prey on ophiurans, while the individuals with a CW greater than 60 mm – on crustacean Hyas araneus. In the Barents Sea, S. typicus polychaetes dominated in the diet of males from all size groups. In the Kara Sea, females of two size groups (40
<CW <40 mm) fed mainly on ophiurans and bivalves, and, in the Barents Sea, – on polychaetes S.
typicus and sea urchins. The cluster analysis of the data showed that, in the Barents and Kara Seas, the greatest similarity was found among males with CW less than 60 mm and females with CW greater than 40 mm.
Keywords: Barents Sea, Kara Sea, snow crab, feeding Introduction
Snow crab Chionoecetes opilio from the Barents Sea is known as an object of fishery and an important diet component for some fish species at the same time. In the Kara Sea, the population of this species has not yet reached commercial significance, but, in recent years, its rapid growth indicates an increase in its importance in the sea ecosystem. The study of the pattern of crab feeding is important, since it is one of the possible factors determining the population dynamics and area.
Material and methods
The material for the study was crab specimens captured in the Barents (136 stomachs) and Kara (92 stomachs) Seas during the trawl surveys of PINRO in autumn 2014-2016 (Figures 1, 2).
A) B)
Figure 1. Biological characteristics of the investigated snow crab in the Barents (A) and Kara (B) Seas in September-October 2014-2016.
Figure 2. Location of stations for collecting snow crab feeding samples in the Barents Sea: - 2015, - 2016; in the Kara Sea: – 2014, – 2016.
To obtain the minimum value of the snow crab commercial stock biomass in the Russian EEZ, the density of crabs obtained in the “Loophole” in 2014 (before the fishery in the Russian EEZ) was extrapolated to the area of commercial sized crabs distribution in the Russian EEZ estimated in ecosystem surveys 2012-2017.
The results of feeding studies were summarized for the following groups:
Females:
Group I - immature juveniles with a carapace width (CW) of up to 40 mm Group II - mature adults with a CW of more than 40 mm.
Males:
Group I - immature juveniles with a CW of up to 60 mm;
Group II - mature adults with a CW of greater than 60 mm.
Results and discussion
Characteristics of snow crab feeding based on the occurrence of food items
The Barents Sea. In 2015-2016, in the Barents Sea, in the diet of the crab 63 taxa of marine organisms were recorded (Figure 3A). The most frequently occurred diet item were polychaetes and bivalve mollusks in the stomachs (more than 50% of the stomachs). On the second place of occurrence were crustaceans, ophiuroids, gastropods and sea urchins (from 10 to 20%), on the third - barnacles of genus Balanus and Foraminifera (<4%). Fish remains were registered in 4% of cases.
Other components of the diet (foraminifera, hydroids, priapulid worms and sipunculids) were met in less than 2% of the stomach. Also, in the stomachs of large males (≤ 100 mm), the individuals of snow crab (4%) was recorded.
The Kara Sea. The food composition of snow crab in the Kara Sea was less diverse (Figure 3B). In the stomachs of crabs, there were 45 taxa of marine organisms. The most frequently recorded items were bivalves (<50% of stomachs); polychaetes, crustaceans and ophiurans met in stomachs seldom (40%). Other components (hydroids, gastropods, barnacles and foraminifers) were found in less than 5% of the stomachs, and fish – in 3%.
A)
B)
Figure 3. The frequency of prey occurrence (in% of the total number of stomachs) in the stomachs of snow crab in the Barents (A) and Kara (B) Seas in 2014-2016.
Feeding of different size and sex groups of the crab
The analysis of the materials revealed significant differences in the intensity and change of the dominant groups of benthic organisms in feeding of the different size sex groups of the crab in the two seas (Figure 4).
The Barents Sea. Female juveniles consume mainly bivalves, S. typicus polychaetes and the sea urchin Strongylocentrotus pallidus. For mature females, the importance of mollusks and sea urchins was much lower than immature, polychaetes (in particular, S. typicus) have main importance. The significance of other groups (hydroids, foraminifera) in female’s diet of both groups was lower. In the feeding of immature males, the dominant role played also polychaete S. typicus, secondary role played ophiurans and bivalves, third-rate - sea urchins, barnacles, gastropods, shrimps and amphipods. For mature males, an important food item, except S. typicus, was Pectinaria hyperborea, the second place have shrimp Pandalus borealis and ophiurans. Less important in the diet of mature males have bivalves, fish, gastropods and worms Sipuncula. Own juveniles of Snow crab were noted only in the stomachs of males with width of carapace over > 100 mm and did not play a significant role in diet. The most intensively consume mature females and juvenile males.
The Kara Sea. In the Kara Sea, ophiurans were an important component of consumption of both groups of females, secondary importance have bivalves (Serripes groenlandicus and Yoldia
hyperborea). In juveniles of females, the value of crustaceans (in particular, the crab Hyas araneus) was much higher than that of the mature individuals. The importance of polychaetes was higher for females, they did not play a significant role in consumption of young crabs. The fish remains were noted only in the stomachs of immature females. In the feeding of young males, ophiuroids played a significant role, and for adults the most important were the Hyas sp. In juveniles and sexually mature males, bivalves (Yoldia hyperborea) have secondary importance, and polychaetes from the family Maldanidae and Spiochaetopterus typicus were less significant. Intensity of food consumption was higher for juvenile males and females.
Figure 4. Weight percentage (% of the food bolus weight) of snow crab food items according to data on the Barents and Kara Seas in 2014-2016.
Analysis of similarity of the crab food composition in the Barents and Kara Seas
Cluster analysis of data by the weight percentage of the prey organisms found in crab stomachs revealed a similarity in the food composition between the different size groups of females and males (Figure 5).
To assess the degree of similarity of the food spectrum, the index of the faunistic similarity Chekanovsky-Serensen was used.
In the Barents Sea, immature males (group I) and mature females (group II) have the most similar composition of food due to dominance in their diet of polychaetes S. typicus and ophiurans. In the Kara Sea, the greatest similarity of food was noted between the immature females and males, in the diet of which the ophiuroids, bivalves and various crustaceans predominated.
A) B)
Figure 5. Cluster analysis of similarity of different size groups of snow crab (in terms of the weight percentage of prey) in the Barents (A) and Kara (B) Seas.
Conclusions
In the Barents Sea, the widespread sedentary polychaete Spiochaetopterus typicus predominated by weight in the diet of males of all size groups. In the Kara Sea, adult males fed mainly on crustaceans, while juveniles - on ophiurans and bivalves. In the Kara Sea, young and adult females mainly fed on ophiurans and crustaceans (Hyas sp, Gammaridae g.sp), and, in the Barents Sea, - on polychaetes S. typicus and bivalves. A special feature of the food composition of crabs in the Kara Sea is the complete absence of sea urchins.
High values of the average total stomach fullness index of crabs in the Barents (12 o/ooo under 93%
of stomachs with food) and the Kara Seas (16 o/ooo under 96% of stomachs with food) indicated a good supply of crabs with food. Cannibalism was noted only in 4% of the stomachs in large males (CW > 100 mm) in the Barents Sea.
Adult mature males of the Barents and Kara Seas were more likely to eat mobile forms of benthos (crabs, shrimps) compared to young males and females. Detection of fish remains in the stomachs of immature females and males gives reason to guess that young snow crabs eat only dead fish, because the relatively small claws and the slow mobility do not allow young crabs to hunt actively to live fish.
The greatest similarity of the food composition of adult males and young females was found in the Barents Sea, while juvenile males and females - in the Kara Sea.
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