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Distribution and abundance of pelagic shrimps from the deep scattering layer of the eastern Arabian Sea.

ABSTRACT The sonic scattering layer (SSL) or deep scattering layer (DSL) of the eastern Arabian Sea of India is found in depths between surfaces to 750 m with varying thicknesses and in multiple layers. There is a distinct resident community of pelagic shrimps that is widely distributed in the DSL biocomposition. Pelagic shrimps formed 19% of the total biomass and 47% of the micronekton biomass of the DSL. Many of them serve as important food resources to several pelagic/mesopelagic organisms. Pelagic shrimps concentrate in the deep scattering layer and diurnally migrate in the water column. The estimated average abundance pelagic shrimps ranged between 0.04-106/1000[m.sup.3]. Shrimp catches were higher during night at 50-200 m depth than day. The dominant families encountered in the catches were Thalassocarididae, Pasiphaeidae, Sergestidae and Oplophoridae. Dominant genera of pelagic shrimps present were Gennadas, Sergestes, Sergia, Lucifer, Oplophorus, Acanthephyra, Nematocarcinus, Leptochela, and Thalassocaris. Species Sergestes seminuds, Thallassocaris crinata and Leptochela robusta were present in swarms

KEY WORDS: pelagic shrimps, deep scattering layer, geographical distribution, vertical migration, swarming behavior


Since the challenger expedition (1873-1876), the occurrence of bright red pelagic shrimps in the mid and deep waters of the ocean has been known. Recently, much attention has been paid to discover their role in the productivity of the ocean. The importance of pelagic shrimps as sonic scatters, food for larger animals, and as agents in energy transfer to the deep sea is well documented (Omori 1974, Kiknchi & Omori 1985). Marine organisms aggregate at specific depths in the ocean and the scattered sound waves from these organisms can be recorded as a scattering layer on the echogram of an echo sounder. This layer is referred to as sound-scattering layer (SSL) or deep scattering layer (DSL) and has been observed in all the oceans (Sameoto et al. 1985, Iida et al. 1996). Hays (2003) stated that the DSL organisms, which ascend around dusk and descend around dawn presumably reflecting the predator-prey tracking. Often discrete layers are evident at different depths, each layer composed of different species or developmental stages (Tarling et al. 2001). The DSL is a layer of living organisms, ranging from almost microscopic zooplanktons like copepods to macroorganisms like shrimps, squids and fishes that prey from within and outside the DSL (Ingmanson & William 1973). According to Omori (1974), out of a total of about 2,000 species of shrimps recorded from the world oceans, as many as 210 species pass their complete life in the pelagic realm. In Indian waters, the earliest studying on the taxonomy of pelagic shrimps was that of Alcock (1901) who listed several species along with benthic forms from the collection of the marine survey ship "Investigator." Later Kemp (1917, 1925), Nataraj (1942, 1947) and more recently Suseelan (1984) have recorded many more species from the west and east coasts of India and studied their taxonomy. Pelagic shrimp form important food and forage to oceanic tuna, flying fishes (James et al. 1987, Philip 1998) and a number of fish species inhabiting the shelf waters that support commercial fishery. Pelagic shrimp are considered potentially important from the outer shelf and slope waters of the west coast of India (Venkatraman 1960, Suseelan & Nair 1990). Therefore a detailed investigation was carried out for understanding their distribution pattern, abundance and diel vertical migrations in the eastern Arabian Sea.


The pelagic shrimp samples were collected from the DSL during the 10 cruises of FORV Sagar Sampada as part of a DSL program in Indian EEZ between May 1998 to December 2000 in the area between 06-21[degrees]N and 66-77[degrees]E (Table 1). Pelagic shrimp catches were recorded from 123 stations (88%) out of the 140 stations sampled, covering 67 night and 56 day samplings (Fig. 1). The gear used for the collection was the Isaac-Kidd Midwater Trawl (IKMT). The IKMT was designed to collect meso/bathy pelagic specimens larger and more active than the specimens caught by plankton nets (Isaacs & Kidd 1951). The towing speed was maintained at 3 knots. Acoustic detection of DSL onboard was facilitated by two Echo sounders (Simrad, EK 400 and EK 200, 38 KHz/120 Khz). The IKMT was operated by observation of the DSL in the echogram, by making an oblique haul for 30 min. The sampling depth varied between 50 and 750 m and hauling depth depended on the concentration of DSL thickness as evidenced from echograms. The catches were preserved in 10% buffered formalin, after noting the volume, immediately after the haul. From the samples pelagic shrimps were sorted out and were used for the study. Identification of pelagic shrimps was done using the key and description of species by Alcock (1901), Hansen (1919), Kemp (1920), Holthius (1955), Yaldwyn (1957), Kensley (1971 a, b), Judukins (1978), Burkenroad (1981), Suseelan (1984) and Farfante and Kensley (1997). The pelagic shrimp abundance was estimated using "Swept area" method (Sparre & Vanema 1992). The abundance per 1-degree square was calculated by taking into account the DSL thickness (m), the average numerical abundance of shrimps (No/l,000 [m.sup.3]) in the collections taken from that area and these have been extrapolated to compute the biomass for catch 1-degree square. Because the sample size was small for a realistic estimation of biomass, the estimation presented here is only an indication of the resource abundance in the surveyed area.



DSL Characteristics

The deep scattering layers are found throughout the surveyed area in the Arabian Sea. Single layers are found more frequently than multiple layers. The scattering layer is generally found in depths ranging between 200-750 m during day and surface to 200 m during night, with varying thicknesses and in multiple layers both day and night. The DSL multiple layers were recorded at three locations (Table 2) with varying thickness and characters. All three multiple layers were found in the southern regions of the Arabian Sea. The multiple layers invariably recorded in depths between 10-90 m, 20-250 m and 40-400 m. Collections made at different times of day and night indicated that discrete bands or layers ascend to surface from the upper DSL by dusk and during the day it descend down.

Geographical Distribution and Abundance of Pelagic Shrimps

There is a distinct resident community of pelagic shrimps found to be widely distributed in the DSL biocomposition. Of the total biomass, pelagic shrimps formed 19% of the total biomass and 47% of the micronekton biomass of the DSL. The result showed that the pelagic shrimps numerically formed one of the major components of the micronekton in the DSL (Table 3). Figure 2 shows the general distribution of the pelagic shrimps (average values for each 1[degrees] squares were plotted) in the Arabian Sea. The north (15-22[degrees]N)-south (06-15[degrees]N) comparison showed an increasing abundance of pelagic shrimps towards south (64.8% of the total catch). The numerical abundance ranged from 0.04-106/ 1,000 [m.sup.3]. Along the southwest coast the maximum biomass recorded was at 08-74[degrees] square (106/1,000 [m.sup.3]) off Vizhinjam and off Cochin (100/1,000 [m.sup.3]), whereas along the northwest coast high catches was recorded off Ratnagiri region (51/1,000 [m.sup.3]). The family-wise estimated average biomass for the whole study area is presented in Figure 3. During the night the families Thalassocaridae and Pasiphaeidae were more abundant (>20/1,000 [m.sup.3]) followed by families Sergestidae (3.94/1,000 [m.sup.3]) and Luciferidae (3.94/1,000 [m.sup.3]), whereas the families such as Oplophoridae, Benthesicymidae, Nematocarcinididae, Pandalidae, Penaeidae, Solenoceridae and Stenopodidae were recorded in small numbers. In the day catches all families were recorded (<2.5/1,000 [m.sup.3]) in small numbers. This showed that pelagic shrimps aggregate mostly during the night.


Vertical Migration

Changes in the abundance of pelagic shrimps in the IKMT catches could throw light on the nature of their vertical migration.

To study the variation in the vertical distribution of pelagic shrimps, the data arbitrarily were group into 4-depth realm namely, 0-50, 50-200, 200-300 and >300 m for convenience of explanation. The pelagic shrimps abundance was rich at depth-ranges 50-200 m (777/1000 [m.sup.3]) and 0-50 (554/1000 [m.sup.3]) and low catches were yielded from below 300 m. During the night, they were concentrated more in the upper strata (0-200 m) and during the daytime they were more abundant >200 depth (Fig. 4). Among the pelagic shrimps, the dominant groups considered for vertical distributional studies were Sergestidae, Pasiphaeidae, Thalassocaridae and Oplophoridae (Fig. 5). Night catches of pelagic shrimps belonging to all families from surface waters (0-200 m) exceeded the day catches. Catches from these depths were higher during the day than at night. Some groups migrate to the shallowest depths possible at night, whereas the deeper-water forms prefer to migrate only to mid depths. For example shrimp belonging to Oplophoridae family were not captured either during day or night from the 0-50 m depth range. They were caught from 200-300 m-depth range during both day and night.


Species Composition

The pelagic shrimps of the DSL were made up of 29 species belonging to 19 genera and 11 families. Shrimps belonging to genera Oplophorus, Gennadas, Sergestes, Leptochela and Acanthephyra were dominant in the DSL catches (Table 4). Out of the 29 species obtained, 16 were restricted to a few locations and 13 were represented along the entire eastern Arabian Sea. The survey results showed that the caridean shrimps, particularly Acanthephyra sanguinea, Meningodora sp, Notostomus sp, Psathyocaris sp, Plesionika martia and P.alcocki, were recorded only from the southern parts. The species Funchalia dana, Solenocera hextii, Acetusjaponicus, and Nematocarcinus tenurostris were only from the northern parts and all other species were distributed along the entire Arabian Sea.

Swarming Behavior

The swarming nature of pelagic shrimps was evident from the strong concentration on various occasions during this study. In the oceanic waters Sergestes seminuds, Thallassocaris crinata and Leptochela robusta occurred in swarms (Table 5). Swarming nature was noticed mostly in the lower latitudes, and only a few swarms were noticed in the northern region at 16[degrees]14'N, 72[degrees]15'E. In some of the hauls, shrimps formed more than 75% of the total catch. Leptocheala robusta was found at 5 locations both during day and night, with maximum abundance at 10[degrees]29'N, 73[degrees]30'E in the depth of 50 and 90 m (256 & 99/1,000 [m.sup.3]) respectively. The Thallassocaris crinata was present only in the night collections at 3 locations, and the depth of occurrence ranged between 50-100 m and was caught during May and December with a maximum of 302/1,000 [m.sup.3] at 08[degrees]88'N, 74[degrees]34'E. The species Sergestes seminuds was found at two locations during the day collections at depths between 40-430 m during April and September. The abundance was <30/1,000 [m.sup.3] for these species.


The deep scattering layers are found in depth between surface and 750 m with the varying thicknesses and in multiple layers throughout the survey of the eastern Arabian Sea. Barham (1957) found that in Monterey Bay the number of layers changes throughout the year. Herring et al. (1998) recorded distinct multiple layers in the upper 350 m during the daytime off Oman coast. The striking feature noticed during this study was the presence of a high density of pelagic shrimps throughout the eastern Arabian Sea. The quantitative richness of the DSL in the Arabian Sea is mainly caused by the presence of micronektonic organisms like pelagic shrimps, swarming crabs, cephalopods and epi-mesopelagic fishes. Among the pelagic crustaceans occurring in the scattering layers of the sea, shrimps appear to occupy a prime position numerically (Suseelan & Nair 1990). Their distribution in fact shows a clear north-south variation as the abundance increased from north to south. The southern region was found to be more productive compared with the northern region of Arabian Sea, where their abundance was rather patchy and productive areas were less extensive. Overall biomass estimations showed that the maximum population densities were recorded off Vizhinjam (106/1,000 [m.sup.3]) and off Cochin (100/1,000 [m.sup.3]). Of the total 19% of the pelagic shrimps recorded from the DSL, species such as Leptochela robusta, Sergestes seminude and Thallassocaris crinanta, which are of high forage value, appeared in seasonal swarms during day/night. Their swarming is strongly seasonal and the fishing seasons correspond with the swarming periods. A study on the day-night variations in abundance revealed a pronounced vertical migration of these animals, being more concentrated in night samples than day. Herring et al. (1998) found that decapods (particularly species of Gennadas, Plesionika and Sergestes) contributed more to the biovolume among the micronekton of the DSL. Shrimps feed actively at night, however physiological evidence suggests that the lower meso and bathypelagic species are able to maintain their predatory activities throughout range of migration, by day as well as by night (Pearcy & Foress 1966). Waterman et al. (1939) stated that diurnal vertical migrations have long been known to play an important part in the lives of pelagic organisms. At night the animals were more densely packed in the depth ranges of 0-200 m, whereas, during the day these animals usually appeared at depths >200 m. Diel vertical migrations of pelagic shrimps are often extensive, hence accelerating the vertical flux of organic matter and recycling of chemical substances through the water column (Kiknchi & Omori 1985). Because fish such as tuna feed on pelagic shrimps, a positive relationship between the abundance of these two groups could be expected (Alverson 1961). Among caridean shrimps recorded during the present study, a species that deserves special attention is Leptocheala robusta, which was caught in large quantities throughout the west coast. This species is reported to form important forage to tuna and other pelagic fishes in the Lakshadeep and neighboring seas (George & Paulinose 1973, James et al. 1987). According to George et al. (1977) the maximum density of tuna and allied fishes in Indian EEZ is in the southwest coast and the oceanic islands. It is therefore reasonable to presume that the high productivity of pelagic shrimps might influence the abundance of tunas and allied fishes in these regions. The observation was made by James et al. (1987) that oceanic tunas that feed heavily on Leptochela robusta is indicative of selective feeding of tunas on such species of pelagic shrimps. Voracious feeding on species of these genera by tuna (albacore) and fin and sei whales (Balaenoptera borealis) has been reported from the pacific waters (Omori & Kawamura 1972, Omori 1974).


The first author thanks the Director, Central Marine Fisheries Research Institute; Scientist-in-Charge, Regional Centre, National Institute of Oceanography, Kochi for their encouragement and Department Ocean Development, Government of India, New Delhi, for granting a research fellowship. This is NIO contribution number 4196.


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P. K. KARUPPASAMY, (1) * N. G. MENON, (2) K. K. C. NAIR (1) AND C. T. ACHUTHANKUTTY (1) (1) National Institute of Oceanography, Regional Center, Kochi, India; (2) Central Marine Fisheries Research Institute, Kochi, India

* Corresponding author. E-mail:
FORV Sagar Sampada cruise details.

Cruise # Months Year

165 May 1998
167 June-July "
168 October "
169 November "
170 December "
173 May 1999
174 June "
183 April "
185 September 2000
190 December "


Multilayers in the study of the DSL.

 Depth of the Layer

 1 Layer 2nd Layer 3rd Layer
 Thickness) Thickness) Thickness)

10[degrees]29'N, 10 m 50m 90
 73[degrees]30'E (10 m) (30 m) (20 m)
08[degrees]29'N, 20m 250 m
 73[degrees]32'E (5 m) (10 m)
06[degrees]40'N, 40 m 90M 400
 77[degrees]30'E (20 m) (30 m) (100 m)


Total DSL biomass (per/[1,000.sup.3]) of numerical, volumetric
and weight from the DSL biocomposition.

 Day Night

 No Vol Wt No Vol Wt

Zooplankton 2884 205 297 4990 235 316
Nekton 1049 321 353 1020 252 319
Pelagic shrimp 372 62 63 1480 109 105

Species composition of Pelagic shrimp and location of occurrence

Species Name Size Range DSL Depth
 (mm) (m)
Infra order: Penaeidea
Super family: Penaeoidea
Family: Penaeidae
 1. Pelagopenaeus balboae 60 and 70 40
 2. Funchalia danae 60 60
Family: Benthesicymidae
 3. Gennadas praecos 60 60
 4. G. sordidus 20-40 50
 5. G. parvus 40 50
 6. G. scutatus * 26-42 50 to 700
Family: Solenoceridae
 7. Hymenopenaeus aquealis 25-40 370
 8. Solenocera hextii 40 205
Family: Sergestidae
 9. Sergestes seminudus * 26-46 50 to 350
 10. S. semissis * 15-35 50 to 400
 11. S. orientalis * 15-35 50 to 350
 12. Sergia inous * 30-60 50 to 500
 13. Acetesjaponicus 20-25 30
Family: Luciferidae
 14. Lucifer typus * 0.2-10 50 to 750
 15. L. penicillifer * 0.2-10 50 to 750
 16. L. hanseni * 0.2-10 50 to 750
 17. L. orientalis * 0.2-10 50 to 750
Infra order: Caridea
Super family: Oplophoridea
Family: Oplophoridae
 18. Oplophorus typus * 6-50 50-750
 19. Acanthephyra sanguinea 30-75 750
 20. Meningodora sp 50 750
 21. Notostomus sp 40-45 750
Super family: Nematocarcinoidea
Family: Nematocarcinidae
 22. Nematocarcinus tenuirostris 13-24 750
Super family: Pasiphaeoidea
Family: Pasiphaeidae
 23. Leptochela (Leptochela) 10-30 50-750
 aculeocaudata *
 24. L. (Leptochela) robusta * 10-35 50-750
 25. Psathyrocaris sp 35 750
Super family: Pandaloidea
Family: Pandalidae
 26. Plesionika martia 25-35 100
 27. P. alcocki 26-35 180
Family: Thalassocarididae
28. Thalassocaris crinata * 5-15 50-750
Infra order: Stenopodidea
Family: Stenopodidae
 29. Stenopus sp 15 30

Species Name Latitude Longitude
 ([degrees]N) (degrees]E)
Infra order: Penaeidea
Super family: Penaeoidea
Family: Penaeidae
 1. Pelagopenaeus balboae 13[degrees] 00' 69[degrees] 57'
 12[degrees] 30' 73[degrees] 30'
 12[degrees] 59' 69[degrees] 58'
 2. Funchalia danae 17[degrees] 30' 67[degrees] 24'
Family: Benthesicymidae
 3. Gennadas praecos 07[degrees] 07' 77[degrees] 12'
 4. G. sordidus 07[degrees] 07' 68[degrees] 32'
 10[degrees] 31' 77[degrees] 12'
 5. G. parvus 07[degrees] 07' 77[degrees] 12'
 6. G. scutatus *
Family: Solenoceridae
 7. Hymenopenaeus aquealis 13[degrees] 09' 73[degrees] 40'
 8. Solenocera hextii 16[degrees] 30' 72[degrees] 14'
Family: Sergestidae
 9. Sergestes seminudus *
 10. S. semissis *
 11. S. orientalis *
 12. Sergia inous *
 13. Acetesjaponicus 16[degrees] 78' 73[degrees] 85'
Family: Luciferidae
 14. Lucifer typus *
 15. L. penicillifer *
 16. L. hanseni *
 17. L. orientalis *
Infra order: Caridea
Super family: Oplophoridea
Family: Oplophoridae
 18. Oplophorus typus *
 19. Acanthephyra sanguinea 11[degrees] 26' 67[degrees] 34'
 10[degrees] 31' 68[degrees] 32'
 09[degrees] 59' 72[degrees] 01'
 20. Meningodora sp 14[degrees] 31' 67[degrees] 32'
 I1[degrees] 26' 67[degrees] 34'
 21. Notostomus sp 14[degrees] 31' 67[degrees] 32'
 11[degrees] 26' 67[degrees] 34'
 10[degrees] 21' 75[degrees] 34'
Super family: Nematocarcinoidea
Family: Nematocarcinidae
 22. Nematocarcinus tenuirostris 14[degrees] 31' 67[degrees] 32'
 16[degrees] 14' 72[degrees] 15'
 17[degrees] 30' 70[degrees] 25'
 07[degrees] 59' 74[degrees] 02'
Super family: Pasiphaeoidea
Family: Pasiphaeidae
 23. Leptochela (Leptochela)
 aculeocaudata *
 24. L. (Leptochela) robusta *
 25. Psathyrocaris sp 14[degrees] 31' 67[degrees] 32'
Super family: Pandaloidea
Family: Pandalidae
 26. Plesionika martia 07[degrees] 59' 76[degrees] 00'
 14[degrees] 31' 74[degrees] 09'
 27. P. alcocki 12[degrees] 28' 74[degrees] 09'
 11[degrees] 49' 74[degrees] 26'
 14[degrees] 31' 73[degrees] 08'
 10[degrees] 19' 75[degrees] 28'
 07[degrees] 59' 74[degrees] 02'
Family: Thalassocarididae
28. Thalassocaris crinata *
Infra order: Stenopodidea
Family: Stenopodidae
 29. Stenopus sp 16[degrees] 29' 73[degrees] 06'

* Species were collected from the area between 06-21[degrees]N
and 66-77[degrees]E.


Swarming species location and their numerical abundance of
eastern Arabian Sea

 (/1000/ Depths
Species [m.sup.3]) (M) Latitude

Leptochela 25 250 08[degrees]29'N
 robusta 49 10 10[degrees]29'N
 257 50 10[degrees]29'N
 99 90 10[degrees]29'N
 30 100 16[degrees]14'N
Thallassocaris 28 50 08[degrees]30'N
 crinata 302 75 08[degrees]33'N
 47 100 10[degrees]20'N
Sergestes 29 430 14[degrees]29'N
 seminuds 26 40 08[degrees]00'N

Species Longitude Day/Night Month

Leptochela 73[degrees]30'E Day May
 robusta 73[degrees]30'E Night May
 73[degrees]30'E Night May
 73[degrees]30'E Night May
 72[degrees]15'E Night November
Thallassocaris 73[degrees]25'E Night May
 crinata 74[degrees]34'E Night December
 73[degrees]26'E Night December
Sergestes 74[degrees]09'E Day April
 seminuds 75[degrees]00'E Day September
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Author:Achuthankutty, C.T.
Publication:Journal of Shellfish Research
Geographic Code:9INDI
Date:Dec 1, 2006
Previous Article:The growth of juvenile Chinese shrimp, Fenneropenaeus chinensis Osbeck, at constant and diel fluctuating temperatures.
Next Article:Indicators of performance in the fishery for shrimp Palaemon serratus (pennant) in Irish Coastal Waters.

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