THE DEEP SCATTERING LAYER

Balu. S# and N.G. Menon*

*Principal Scientist, CMFRI, Cochin-18

#ENVIS on Marine Ecosystem

The Ocean from top to bottom is divided into two- the pelagic, comparatively warmer waters and the benthic- the cooler aphotic waters of the bottom. The oceanic life is mainly concentrated in the upper pelagic waters where there is enough light and food where in contrast, the deeper water is scarce of food. The epipelagic waters where the major bulk of the marine life is concentrated contributes to the commercial fishing industry and these waters are being exploited, in some case over exploited, for food and profit. But the mesopelagic and deeper waters below remain mostly untouched. Over crowding of coastal waters and subsequent decrease in catch has turned the attention of the industry to the deeper waters. The little known habitats and resources of the deep sea need a lot of explorative studies and research surveys before harvesting it.

Hence a National Project was launched by the Department of Ocean Development, Govt. of India, on ‘Studies on Deep Scattering Layer’ with in the EEZ of India in 1997-98 with active participation from CMFRI (Nodal Agency), NIO, CIFT, FSI, Andhra University, CIFE and CMLRE of DOD. The aim of the study was to survey and to understand the DSL characteristics, importance, biocomposition, distribution, abundance and utilization of the major exploitable components like the Myctophids, Pelagic Shrimps etc from the EEZ in space and time.

The DSL

The Deep Scattering Layer (DSL) or the Sound Scattering Layer (SSL) as the name suggest, scatter the sound, mainly coming from the echo sounder or sonar used by the ships for scanning the bottom and this in turn appear as a ‘second bottom’ or ‘false bottom’ on the recorder. The Deep Scattering Layer is an assemblage of mostly negatively phototrophic, diurnally and vertically migrating organisms. These bioaoustic layers ascend to the surface during dusk and descend to the bottom during dawn. The existence of DSL or SSL or false bottom was discovered by Eyring, Christensen and Raitt of the Division of War Research, University of California in 1942. Ever since their discovery constant attempt has been made world over to study their biocomposition, migration, trophic interaction etc. In India also some preliminary survey studies were carried out by the Govt. of India research vessels viz. RV Varuna and FORV Sagar Sampada in the EEZ during different periods of time.

The DSL components

The micro nektonic components of DSL is represented by several species of epi and meso pelagic fishes of the families viz Mytophidae, Photichthyidae, Bregmacerotidae and Pelagic Shrimps, Swarming Crabs, Cephalopods; where as the macrozooplankton comprised of copepod, decapod, siphonophores, amphipods, chaetognatha, euphausiid, lucifer and several larval forms like Leptocephalii, zoea, megalopa, alima and phyllosoma. Although none of the micronekton and macrozooplankton are harvested commercially at present from the low latitude oligotrophic Indian DSL, the crustaceans pelagic shrimps and swarming crabs and the jelly fishes offer great potential for harvest and utilization in future either as direct source of food / feed or in a variety of value added forms as the former seems to have a nutritional value similar to prawns and crabs besides their greater role as a forage of major pelagics. Though these non utilized and under utilized species occur in the DSL in large concentration their high costs of harvesting relative to the returns have constrained their commercial development at present . The DSL is a network of numerous food chains, some of which end up in species of economic importance like pelagic sharks, tunas, cephalopods etc. The DSL organisms also act as an indicator for these economically important species. The ascend and descend of the nekton are influenced by the availability of favourite food items in the layer.

The DSL ecosystem and scope for future research

Though the DSL appears as single layer, at times they appear in multiple layers with two or three layers below the principal surface layer, in diffuse or dense conditions depending on the concentration of organisms. As the DSL is known to carry trophic network, a knowledge on the biomass and the biocomposition of principal groups which build up the layers is essential to study the food relationships in this ecosystem and its energetics from lower to higher levels of food web. It is also important to know the abiotic and biotic pattern and the process of DSL ecosystem and its biodiversity for the ecological as well as the economic management of oceanic fisheries. A thorough knowledge on the micronekton abundance in relation to abiotic factors; qualitative and quantitative patterns of biomass in the multiple layers; feeding energetics and prey-predator relationship; identification of indicator organisms for concentration of exploitable resources; GIS mapping of DSL biomass concentration and conduct of modeling exercises for their forecasting is also imperative. The present method (swept area) of biomass prediction of the DSL coupled with the bioacoustic technique could yield prediction models for a market oriented exploitation and utilization of the hitherto unfamiliar mesopelagic resources. Further research is also needed for developing appropriate harvest technique on the widely dispersed stocks and the post harvest sector require concerted endeavors for production of value added fishery products and by products from the low value non traditional mesopelagic shrimps and fishes.

Acknowledgement

The authors wish to acknowledge the Department of Ocean Development, Govt. of India, for the generous funding of the DSL project during 1997-2002 and also for making available the data of the study at their disposal without which the review was not possible.