Ekaterina Evseeva
Knipovich Polar Research Institute of Marine Fisheries and Oceanography, Murmansk, Russia Abstract
The success of young fish feeding in the first year of life is extremely important for the abundance dynamics of the commercial bottom fish species in the Barents Sea. The knowledge of the feeding pattern of fingerlings during this period makes it possible to predict the survival of the year-class and the abundance of the commercial stock recruitment.
Based on the quantitative analysis of feeding of 257 cod and 271 haddock collected in November-December 2015 and February-March 2016, the feeding conditions and peculiarities of cod and haddock fingerlings from 2015 year-class were studied. To estimate feeding conditions, their biological characteristics (average length and weight, fatness and condition) were also used.
The differences in the feeding intensity and food composition of fingerlings at the beginning and end of wintering were revealed. Euphausiids predominated in the diet of cod and haddock. The second most important feeding item of cod fingerlings was fish (capelin, sand lance, redfish, eel-blenny), of the haddock ones - hyperiids and polychaetes. From the beginning to the end of wintering, feeding intensity of both species increased, while fatness and condition decreased.
Moreover, feeding peculiarities of the 2015 year-class and the year-classes of the past were compared.
Keywords: cod, haddock, juveniles, diet, prey, biological parameters, Barents Sea Introduction
Fattening conditions of cod and haddock – the main commercial fish species in the Barents Sea – during the first years of their lifecycle are important factors for the further development of their populations. Variations in feeding habits during the first wintering related to the adaptation to new environmental conditions have a significant impact on the survival of a year class and its abundance in older age groups. Long-term observations over the interannual variations in the diet of juvenile cod and haddock in the Barents Sea provide precise predictions of the size of their stocks and correct estimates of their abundance.
Materials and methods
The samples for study were demersal juvenile cod and haddock collected on board PINRO’s research vessels “Fridtjof Nansen” and “Vilnus” in winter (in November-December 2015 and in February-March 2016). Using the quantitative-weight analysis method, the total number of examined specimens of cod and haddock was 257 and 271, respectively. Additionally, basic biological parameters including fat content and fish condition were studied as well.
Results and discussion
General diet composition
Twenty seven prey species from 15 large taxa were identified in the diet of juvenile cod and haddock. Species from nine taxonomic groups were found in the stomachs of cod in early and late
winter, however, these taxa slightly varied in the beginning and the end of the season (Figure 1). At the beginning of wintering, the diet of juvenile haddock included prey species from ten taxonomic groups and in February-March – from thirteen taxonomic groups (Figure 2).
Figure 1. Food composition of juvenile cod in November-December 2015 and February-March 2016.
Figure 2. Food composition of juvenile haddock in November-December 2015 and February-March 2016.
The spatial distribution of feeding intensity and food composition of juvenile cod and haddock is shown in Figures 3 and 4.
Cod
In November and December 2015, euphausiids (Euphausiidae) prevailed in the diet of cod by frequency of occurrence and weight. They comprised 33.6 % and 42.5 %, respectively. The frequency of occurrence of freshwater shrimp (Gammaridae) and hermit crabs (Pagurus) was low (10.9 %) and their weight percentage was 10 % and 6.61 %, respectively. Moreover, polychaetes (Polychaeta) (6.9 % by weight), hyperiids (Hyperiidea) (8.4 %), fish (11.1 %), as well as chaetognaths (Chaetognatha), mysids (Mysidacea) and cumaceans (Cumacea) (0.3-2.7 %) were observed in the stomachs of cod. The feeding intensity was relatively low. The average stomach fullness index (SFI) was only 63.3%οο whereas the portion of fish with empty stomachs did not exceed 13.0 %.
The diet of juvenile cod in February 2016 varied from the food composition in late 2015.
Euphausiids remained the main food items (78.9 % by weight), however, the frequency of their occurrence was low (5.4 %). In addition, hyperiids and copepodas (Copepoda) were relatively
abundant in the stomachs of cod (7.2 % and 5.4 %, respectively), however, their weight ratio was low (3.2 %, for both prey species). Mysids, polychaetes and fish (1.9 -5.6 %) were important food items as well. The feeding of cod was more intensive, the average stomach fullness index (SFI) increased to 100.1 %οο and the portion of fish with empty stomachs decreased to 8.1 %.
А B
Figure 3. Spatial-temporal dynamics in the diet of juvenile cod in November-December 2015 (A) and February-March 2016 (B).
А B
Figure 4. Spatial-temporal dynamics in the diet of juvenile haddock in November-December 2015 (A) and February-March 2016 (B).
Haddock
In November and December 2015, the diet of haddock included predominantly euphausiids as well.
Their weight percentage was 35.7 % with a frequency of occurrence of 28.5 %. The second important food items were fish including capelin, leptoclinus and redfish (31.3 % by weight and 6.3
% by frequency of occurrence) and hyperiids (7.3 % by weight and 25.2 % by frequency of
occurrence). The rest of the food was benthos (including freshwater shrimp, polychaetes, brittle stars (Ophiuroidea), ostracodes (Ostracoda), hermit crab, bivalved mollusks (Bivalvia) and mysids). The weight percentage of benthos comprised 0.05-6.1 %. The feeding intensity was relatively low. The average stomach fullness index (SFI) was 68.1%οο whereas the portion of fish with empty stomachs did not exceed 11.0 %.
In February 2016, euphausiids predominated in the diet of juvenile haddock (55.9 % by weight and 24 % by frequency of occurrence). The second important food items were polychaetes and freshwater shrimp (15 % and 7.7 % by weight, respectively) while the frequency of their occurrence was high (25 % and 22 % of the weight of food bolus, respectively). The other bottom-dwelling species (bivalved mollusks, cumaceans, mysids, hermit crab, brittle stars and isopoda (Isopoda)) and plankton (hyperiids), as well as fish species were far less abundant (0.03-2.7 %) regardless of their high occurrence (up to 6-8 %). The feeding of haddock was more active, the average stomach fullness index (SFI) increased to 75.7 %οο whereas the percentage of fish with empty stomachs was almost twice as high as in late 2015 (24.3 %).
The diet of juvenile cod and haddock. Comparative analysis
Euphausiids were prevalent in the diet of juvenile cod and haddock during their wintering in the entire Barents Sea (Figures 3, 4). An active consumption of euphausiids by cod and haddock gradually increased (from 32-41 % to 33-55 % by weight). Prey fish was the second important food item in the diet of juvenile cod and haddock. In early winter cod actively fed on prey fish in the northwestern Barents Sea and haddock – in the central areas. However, while the consumption of prey fish by cod eventually remained stable (13-14 %), the portion of prey fish in the diet of haddock decreased from 31 % to 8 %. At the beginning of wintering, in the diet of cod and haddock pelagic hyperiids (8.9 % and 7.3 %, respectively) dominated over polychaetes (6.5 % and 5.8 %, respectively). By February and March 2016, the weight percentage of polychaetes was higher than that of hyperiids (5.9 % versus 3.2 % for cod and 17.4 % versus 6.1 % for haddock).
The food spectrum of haddock was wider than that of cod. The diet of haddock included bivalved mollusks, brittle stars, ostracodes, isopoda and shrimps that did not occur in the diet of cod.
Nevertheless, copepoda were found in the stomachs of cod at the end of wintering. A relatively high occurrence of benthos in the diet of juvenile haddock may indicate the species’ earlier adaptation to the benthic mode of life and more accelerated development.
Biological parameters
At the beginning and the end of the first wintering, biological parameters of juvenile cod and haddock varied widely. These variations were caused by changes in their diets. Thus, fat content in fish of both species was higher by early winter (3.1 % for cod and 3.2 % for haddock) and it decreased when the wintering was over (up to 2.3 % and 2.2 %, respectively) (Tables 1, 2). Fish condition, as a more permanent indicator of fattening conditions than fat content, decreased in a similar way from November to March. At the beginning and the end of wintering, the condition of cod varied between 0.67 and 0.66, respectively, and the condition of haddock varied between 0.79 and 0.73, respectively. Low fat content and poor condition indicate that cod and haddock fed less actively and they consumed less food in early winter. However, the feeding of cod and haddock was more active by February, the average stomach fullness index (SFI) increased from 63.3 to 100.1 for cod and from 68.1 to 75.0 for haddock. Moreover, the percentage of cod with empty stomachs decreased (from 13.0 % to 8.1 %), whereas the percentage of haddock with empty stomachs was almost twice as high as in early winter (from 11.0 % to 24.3 %).
Table 1. Biological parameters of juvenile cod of the 2015 year-class.
Average data November-December 2015 February-March 2016
Empty stomachs, % 13.01 8.11
Length, cm 11.31 11.38
Weight, g 10.34 10.07
Fat content, % 3.14 2.28
SFI, %οο 63.32 100.09
Fulton's condition factor 0.67 0.66
Table 2. Biological parameters of juvenile haddock of the 2015 year-class.
Average data November-December 2015 February-March 2016
Empty stomachs, % 11.02 24.31
Length, cm 13.88 14.34
Weight, g 21.57 21.96
Fat content, % 3.17 2.33
SFI, %οο 68.12 75.73
Fulton's condition factor 0.79 0.73
Conclusions
The feeding intensity of cod and haddock increased by the end of wintering. The food spectrum of haddock was wider than in early winter. The number of prey species consumed by cod remained virtually unchanged, however, the food composition in the diet of cod varied.
From November to March, euphausiids prevailed in the diet of juvenile cod and haddock.
Additionally, prey fish were important food items for cod and polychaetes and gammaridae – for haddock. Species diversity in the diet of haddock was higher than in the diet of cod. A variety of prey species consumed by haddock was wider in early 2016. The occurrence of benthic organisms in the stomachs indicated that juvenile haddock had successfully adapted to the benthic mode of life.
Despite the fact that juvenile cod and haddock fed more actively, their biological parameters (i.e. fat content and fish condition) gradually decreased between November-December and March.
From November to March, stomach fullness of juvenile cod gradually increased whereas stomach fullness of juvenile haddock decreased. Nevertheless, the diet of juvenile haddock included a wider variety of prey species than that of juvenile cod.