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«ABSTRACT Like many other anchovies T. baelama feeds mostly on large zooplanktonic organisms. Euphausiids were the most important item of food ...»

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FOOD AND FEEDING HABITS OF THE SHORT-JAW ANCHOVY,

THRISSINA BAELAMA (FORSKAL), OF THE ANDAMAN SEA

R. MARICHAMY'

, Central Marine Fisheries Research Institute; Unit, Port Blair, Andamans

ABSTRACT

Like many other anchovies T. baelama feeds mostly on large zooplanktonic organisms. Euphausiids were the most important item of food supplemented by copepods, megalopae, prawns and amphipods. These major items in the diet of the fish did not differ significantly from year to year although the relative importance of particular item may vary slightly between diflferent months.

The smaller fish appeared to prefer smaller organisms like veligers, zoeae, Oithona, Pseudodiaptomus etc. while the adults preferred euphausiids, megalopae, prawns, amphipods and fish larvae.

The variations in the composition of the gut contents and feeding intensity are discussed in relation to environmental biota, size-groups and maturity of the fish.

INTRODUCTION

The anchovies constitute an important fishery in the inshore waters of Andaman Islands of which Thrissina baelama (Forskal) is the most important.

Sadasivan (1953) while giving a brief account on the biology of this species has stressed the need for an intensive observation to obtain a fuller and satisfactory picture. The present account deals with the food and feeding habits of this species.

The food and feeding habits of anchovies and allied species have been widely studied. Bapat and Bal (1950) have made a comprehensive study on the food of some young clupeids including Thrissocles kammalensis, T. hamiltonii, T. purava, T. dussumieri, Anchoviella commersonii and A. tri. Mookerjee and Mookerjee (1950) have described the carnivorous feeding habits of Engraulis hamiltonii. Jones and Menon (1951) have studied the nutrition of

1. Present address: Central Marine Fisheries Research Institute; Sub-station, Tuticorin.

98 R. MABICHAMY the Gangetic anchovy, Setipinna phasa. The food and feeding habits of Thrissocles mystax has been studied by Venkataraman (1956) and Ganapathi and Rao (1962). Basheeruddin and Nayar (1962) have briefly mentioned about the food of T. mystax and T. setirostris.

MATERIAL AND METHODS

Weekly samples of T. baelama for this study were collected from the fish market, where the catches from different fish-landing centres are sold. Shoreseines and cast nets were the two common gears used along this coast. All the samples were combined for the purpose of the study irrespective of the gears employed. 1,022 specimens of size range 57-134 mm (TL) were examined during 1968-70. Data on size, sex and stage of sexual maturity were recorded for each fish. Identification of gut inclusions upto genus and sometimes upto species level was possible, depending upon the state of digestion.

The percentage composition of the various items of the gut contents were analysed by the numerical method (Pillay, 1952). Since the food consisted of zooplanktonic organisms this method was found to be more convenient. The different food items were counted and its relative abundance expressed as percentage of the total number of items in the gut contents. The prevalence of each item of food among different size groups was calculated by the occurrence method (Hynes, 1950). In this method, the number of occurrences of all items was summed and scaled down to a percentage basis. The extent of feeding was determined by the degree of distension of the stomach and also the amount of food it contained, the stomach being graded as empty, trace, i full, i full, i full and full. The percentage occurrence of these different categories of stomach was calculated from the total number of fish examined in a month. The volume of the gut content was measured by displacement method using a graduated cylinder.

The study reveals that T. baelama feeds mostly on zooplankton. Euphausiids, mainly represented by Pseudoeuphausia sp., were the chief food, being consumed in considerable quantities throughout the period of investigation except April 1969. The average percentage for the period 1968-69 and 1969worked out to 44.0% and 26.4% respectively (Table 1.).

Copepods constituted a fairly good portion of the diet throughout the year.

The percentage composition of copepods varied between 3.1 and 65.1%. The following 10 genera of copepods were noticed in the stomachs of T. baelama,

1. Pontella, 2. Labidocera, 3. Pseudodiaptomus, 4. Temora, 5. Calanus,

6. Tortanus, 7. Corycaeus, 8. Oithona, 9. Anamalocera and 10. Sapphirina. Of these the first three items were very common in the diet of the fish.

–  –  –

were more common than Penaeus indicus. Amphipods occupied a secondary place, in November 1968.

A variety of larval forms represented by cypris, zoea, megalopa, veligers, alima and fish larvae were frequently observed in the stomach contents, of which megalopa were found in the guts almost throughout the year and occupied the first rank in the diet composition during November 1968 and March 1969.

VARIATIONS IN THE FOOD

The mean percentage composition of some important food items revealed that euphausiids (26.4-44.0%) were the most prevalent in the diet, the others in the order of importance being Po«;e//a spp., (10.4-17.3%) megalopa, (9.1Pseudodiaptomus spp., (6.1-11.4%) and Labidocera spp., (2.1-6.4%).





The fluctuations in the percentage composition of the essential food items such as euphausiids, copepods, megalopae, prawns and amphipods have shown some similarities in the two years (Table 1).

Whenever the euphausiids formed the main bulk of food the other common items such as copepods, megalopae and prawns showed a decrease. Similarly, when the copepods constituted the chief diet, a fall in the percentage composition of the other forms was seen. It is probable that these fluctuations may be due to changes in the availability of the various organisms in a particular environment.

Cypris was present in the stomach contents during most of the months.

It was maximum in April 1969 while in the corresponding month of the previous year it was absent. It appeared from the data that this item although represented in minor quantities was the one that was preferred by the fish almost throughout the year. Veligers were noticed in minor quantities iri the food but more common during 1968-69 than in succeeding year. Zoeae ranked third in the diet during February '69 and constituted only a minor percentage of the food during 1969-70.

It is evident that the high feeding intensity generally coincided with thi;

occurrence of eupausiids, megalopa or prawns or amphipods in appreciable percentages (March, September, November-December '68 and July-September 1969). A decrease in the feeding (0.1-0.3 ml) was observed to coincide when these items ranked either second or third in the diet and copepods were the dominant items in the gut as seen during June-July '68 and February, AprilJune '69 and December '69 to February '70. However the increase in the volume of food in May '68 and September '69 was due to the relative abundance of prawns and megalopae.

100 B. MABICHAMY

–  –  –

5 4 46.2 49.0. 5 2 14.5 25.6 26.4.

25.0 18.2 65.5 40.6 22.1 2.0. 1 3 14.6. 23. 48. 09. 30. 14. 33.

2.7. 30.

. 30. 23. 4.4 13. 52. 18.

4.5. 07. 05.

. 03.

. 16.

5 2 25.2 22.2 21.0 11.1 11.2 20.7 17.3.

15.9 11.2 17.0 43.7. 53. 64.

4.7 32.4 77.

11.4 10.5 4.5 09.

36.4 42.2 28.9 12.6 11.4 11.2. 0.05 3.0 0.4 03.

••. 2.0 09. 03.

••. 26.

. 2 8 10.4 17.7 15.7. 55.

.

. 13. 0.4 0.9 03. 0.2 06. 12.

6 3 13.2 18.4. 93. 46.

4 1 14.1. 8 7 13.4.

15.9 27.0 27.2 12.5. 2.7 55. 30. 53.

4.5 03. 65. 94. 29. 62.

18.2 12.5. 29. 2.8 06.

. 0.4 •• 0.7 03. 01.

•• •_• ••.

2.3 13. 19.

. 07.

18.2. 09. 03.

. 06. 0.4 07.

•• ••. 13. 06. 13. 26. 2.4 3.0 85. 14. 36.

•• ••

–  –  –

1968-69 1969-70 • •

–  –  –

In relation to plankton In pelagic feeders like clupeids a direct correlation between the distribution of plankton in the environment and its occurrence in the stomach contents can be expected. A comparison of food of the fish with the plankton collected at Port Blair during the corresponding period lead to the conclusion that the occurrence and the abundance of the food items in the stomachs of T. baelama have a relationship with their richness in the plankton in different months. Ths occurrence of Pontella spp., Labidocera spp., Pseudodiaptomus spp., Oithona spp., Temora spp., Calanus spp., Tortanus sp., Corycaeus spp. etc. in fairly high percentages in the stomach contents from May to August 1968, JanuaryJune, September 1969 and again during December '69 February 1970 coincided with its abundant occurrence in the coastal waters during the same period.

It is significant to note that swarms of eupausiids observed on 27-8-'69 and 8-10-'69 and their abundance in August and October 1969 coincided with their high percentage of occurrence in the stomachs of T. baelama. Among the larval forms cypris, veligers, zoeae and megalopae have been observed in the plankton almost throughout the year and noticed in the stomach contents also in most of the period.

Jn relation to different size groups As stated earlier the size range of T. baelama examined during the period

•of study was 57-134 mm. All the fish were grouped at 10 mm interval. The percentage occurrence of some important items of food in each size group was

•calculated and presented in Table III.

Copepods like Pseudodiaptomus spp., Labidocera sp., Oithona sp., were predominant in smaller individuals below 99 mm and occurred in very low percentages in the adults; while Pontella spp. megalopa, cypris larvae and pteropods were present in greater percentage in the fish of 100-129 mm size groups.

Euphausiids occurred in relatively high percentages in the bigger individuals.

There was a high percentage of fish larvae in size group 120-129 mm.

Thus, generally, the bigger organisms, such as euphausiids, megalopae, prawns, pteropods and fish larvae form the chief food of adult fish, while the young ones appear to feed mostly on small forms like veligers, zoea, Pseudodiaptomus, Oithona etc.

–  –  –

During the year 1968 a large percentage of fish were observed to feed actively during March and May and no empty guts were encountered in these months. In December 1968, besides the occurrence of such actively fed fish in appreciable numbers, a high percentage (34%) of specimens showed moderate feeding. Such condition was also noticed in the months August-September and November 1968.

The trend of feeding intensity was quite different in the subsequent year in which the active feeding was noticed during July-September '69. In the remaining period the condition of feed was poor and the occurrence of empty stomachs relatively high.

The variations in the feeding intensity were further assessed by measuring the actual volume of food. The volume of gut contents was high during March^ May-September and again November-December '68, coinciding with the occurrence of high percentage of actively feeding fish. Similarly, the data for the subsequent year revealed that during those months when the percentage of actively fed fish was high the volume of the food was also high. Thus, the 1969-1970

–  –  –

In relation to size offish In Fig. 1 feeding intensity is plotted against the monthly average length of the samples examined. A study of the curves of these variables does not seem to indicate any consistent parallel correlation in their fluctuations, it may be inferred that the seasonal variations in the intensity of feeding was due to the variations in availability of the Organisms in a particular environment.

In relation to sexual cycle Though mature fish are seen throughout the year, a high percentage of mature fish is noticed only during July-August and again from NovemberJanuary coinciding with the spawning seasons of the fish. Fish collected during July-September '69 had fed actively (table IV) coinciding with the first spawning activity of the fish. A rise in the feeding intensity was again noticed in November 1969 probably in relation with the second spawning. But feeding was poor during the succeeding months. The table for 1968-69 indicate a high degree of feeding only in March and May 1968 when the immature fish were predommant. However the active feeding as seen in November-December

–  –  –

1968 coincided with the second spawning period of this species although poor feeding was noticed in December 1969.

To understand more clearly the relation between feeding activity and spawning all the material was sorted out into two arbitrary groups. Immature and Mature, and the results depicted in Fig, 2. It can be seen that the variations in the intensity of feeding between these two groups showed a uniform pattern in May-August, October-November 1968 and in January-February 1969.

This trend was also noticed during 1969-70 and the values showed almost the same pattern of rise and fall even during the spawning season. Whereas an active feeding was noticed in mature fish during July-August 1969 (spawning period) the volume of food was less in the corresponding months of the year

1968. Similarly an active feeding (0.7 ml) was noticed during the second spawning i.e., in December 1968. But in the next year ia the same months mature fish fed poorly.

As the fluctuations in the feeding intensity of mature fish had not strictly coincided with the spawning period, it would be rather diflScult to conclude whether the fall in the feeding intensity was entirely due to the spawning activity of the fish.

–  –  –



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