Implementation Plan for a Full-Scale Study of Pelagic Populations

Fish Stocks
Mesozooplankton
Approach
Conclusion

Arabian Sea processes are attractive for U.S. GLOBEC studies in two respects:

The Arabian Sea provides a comparison between two different types of upwelling systems in close geographic proximity. These are a coastal upwelling regime along the Somali and Arabian Peninsula shores and an oceanic upwelling regime under the Findlater jet. It is obvious that the ecological requirements of these two upwelling regimes for mesozooplankton will be different, but the responses of planktonic species are not known. In addition, the monsoon climate offers alternating seasons in a tropical-subtropical setting, which it can be anticipated will offer some of the advantages for study of population processes usually found only in high latitudes.
The Arabian Sea is peculiarly rich in myctophid fishes, home to one of the largest, single species stocks in the world. Production of this fish, Benthosema pterotum, amounts to 100 million tons per year. Its population processes must be coupled to the extremely high production of the region which is based on nutrients supplied by monsoonal upwelling. This coupling seems to be unique but is not understood, and a U.S. GLOBEC program could elucidate the mechanisms.

The implementation plan offered below seeks to pursue these attractive features of the ecology of the Arabian Sea. Proposed studies of both mesozooplankton and fish stocks will be greatly enhanced by the prolonged operation of a dedicated, fishery research vessel in the northern Arabian Sea. An appropriate vessel is described in the section below on fish research. Zooplankton work is flexible enough to fit on this vessel or any other general oceanographic ship.

Fish Stocks

Hypotheses and Tests of Hypotheses: Study of the unusual fish stocks of the Arabian Sea can be based on some general and specific hypotheses. The latter are really questions. In many respects the population processes are not well enough understood for formulation of testable hypotheses.

General Hypothesis: The large stocks of myctophid fish and their high growth rates in the Arabian Sea are a function of regional physical conditions, including the dual coastal/oceanic upwelling system and the large region of suboxia below 150 m.

Specific Hypotheses:

These myctophids are protected from daytime predation at depth by descent into dysoxic waters.
Rapid growth is promoted by the unusually high productivity associated with both coastal and open sea upwelling.
Variations in myctophid abundance are related to topography.
Much myctophid production goes to non-predatory mortality, with the decomposing biomass ending up contributing to benthic nutrition.

A Modified General Hypothesis: It was suggested at the Denver workshop that possibly the dominance of myctophids in the Arabian Sea upwelling regimes contrasts with the dominance of clupeids in the Peru and California ecosystems for a different reason. In those latter systems, forcing winds for upwelling are episodic, they have an event scale (shorter than the seasonal scale) on which the winds lapse, then resume. During the lapses the mixed layer stabilizes and particulate foods can aggregate above a threshold for larval feeding. In the Arabian Sea both coastal and oceanic upwelling are prolonged and without lapses. The wind blows night and day for up to 3.5 months (but see the essay above on monsoonal variations). In the coastal regime there may be no mechanism for returning developing zooplankton stocks to the coast, probably a function served by lapses in upwelling off Oregon (Peterson et al. 1979) and Peru. In the oceanic upwelling region of the Arabian Sea, where much of the primary production may be by coccolithophorids, the food chain may be longer, less efficient, and less capable of supporting large stocks of clupeids, than in upwelling regions elsewhere. Myctophids in the Arabian Sea, then, essentially are filling a niche left open by the failure of clupeids.

Tests of these hypotheses will require varied sampling efforts, both in the water column and of the sediments. First, the levels of stock abundance and the growth rates that have been reported must be confirmed. There is reason for doubt about the accuracy of the earlier Norwegian sonar surveys, although they cannot be wrong in showing the stocks to be enormous. For this purpose we require a recurring, geographically dispersed stock estimation program using both carefully calibrated, echo-integrating sonar and large, preferably sonar-guided trawls. The trawls will provide secondary, immediate calibration checks for sonar results as well as large quantities of fish for direct study (species composition, size-frequency, condition factors, gut contents, reproductive status, enzymology and proximate analysis). Second, plankton sampling studies are required for study of food selection by comparison to gut contents and for study of the timing of larval development and geographic differences in larval distribution and success. Third, sites should be sought for evaluation of sediment accumulation of scales and otoliths so that temporal changes in myctophid stocks can be examined. Since it is a region of extensive suboxic, and on the seabed occasionally anoxic, conditions, it is possible that, as in the California, Peru and Benguela Current areas (Soutar and Isaacs 1974; Shackleton, 1987), sites can be found with very slight degradation and very detailed temporal records of fish remains.

An ambitious program of study can focus its seasonal comparisons along a single, onshore-offshore transect line, and the line out of Oman proposed for U.S. JGOFS work is an excellent choice. To provide a range of physical conditions and to investigate myctophids in both the coastal and oceanic upwellings, one station should be located on the shelf, 2-3 over the slope, and 2-3 along the line offshore. It should not be necessary to proceed beyond the limit of eutrophic conditions established by the SW monsoon beneath the Findlater jet. In addition, it will be necessary for studies to be dispersed alongshore, certainly at least from Cape Guardafi east to the southeastern limit of Pakistan. A tour of this perimeter on a seasonal basis would produce useful information on the response of the stocks to the variations of the monsoon regime.

Evaluations of our specific "hypotheses" may be developed as follows:

Sonar and trawl studies will evaluate the diel migratory behavior of B. pterotum and other myctophids, examining the relationship of these movements to suboxic conditions at the bottom of the epipelagic zone (ca. 150 to 1500 m) and to the distributions of potential predators. Gut contents of larger fishes and squid captured by the trawls will be examined for myctophid remains. The hypothesis predicts (1) that myctophids will spend the daylight period in the suboxic layer, and (2) that larger fish and squid predators will be greatly reduced or absent in that layer. Some evaluation of the impact of very large predators, like tunas, can be obtained by cooperation with the fisheries for them, obtaining distributional and gut content data. The hypothesis also predicts (3) that B. pterotum, D. arabicus, and possibly other species are metabolically well suited for prolonged occupation of suboxic waters. They will be anaerobically poised with high activities of lactate dehydrogenase and other glycolytic enzymes. This can be examined with the copious materials we anticipate from our trawling program.
Growth rates can be determined by classic ichthyological techniques: study of otolith growth rings (useful rings are likely to represent daily or lunar intervals); study of seasonal cohorts developing roughly in phase; experimental rearing (although success with rearing myctophids is very limited). Differences in growth between subregions with different production regimes should shed light on the basis for rapid growth in B. pterotum and other myctophids.
Variations in abundance with topography is a straightforward sampling problem. It will be greatly assisted by deployment of echo-integrating sonar along transects of varying bathymetry.
Examination of sediments may provide clues to the fate of the high primary and myctophid production, although some of the best preserved remains of fish may appear in sediments after their death by predation. Direct evaluation of the fate of 100 million tons of myctophids per year will take some further thought. Because macrourids, hagfish, and amphipods will quickly remove directly deposited bodies, we do not expect to find "windrows" of dead Benthosema lying on the bottom. It is also possible that most of the removal is by predation by scombrids, carangids, mackerels, and any number of pelagic predators.
The alternate hypothesis concerning the relative importance of clupeid and myctophid populations in differing upwelling regimes can only be "tested" by continuing development of pelagic ecosystem theory, by advance of our general understanding of how these ecosystems work. The proposed study will help by expanding and clarifying the myctophid side of the comparison.

Equipment and Approach A fully implemented U.S. GLOBEC study of myctophid stocks in the Arabian Sea will require a dedicated vessel specially equipped for fishery studies. This need not be a particularly large ship, but it should have the following capabilities:

It should be suitable for operation in marginal working conditions, including scientific work off the Arabian coast and under the Findlater jet during the SW monsoon (sea state 6: winds at 22-47 knots, significant wave heights of 6-9 m/mean 7.5, wave period 14 sec).
Accommodations are required for 12-15 scientists
Equipment must include state-of-the-art, low-frequency (38 kHz) acoustic sampling devices; double-warp trawling capability and power to tow and haul 500 m2 (or larger) trawls; processing capacity for handling catches made by such trawls (commercial-scale processing facilities below decks) and winches and booms for deploying smaller oceanographic gear, including CTD and plankton samplers; and a full suite of communications and meteorological instrumentation. Higher frequency (>200 kHz) acoustics (ADCP, dual or split-beam systems) would be useful in providing distributions of zooplankton prey to complement acoustic estimates of fish distributions and abundance.

Ships suitable for this service exist. For example, a Bering Sea-type trawler-processor (135-150 ft. LOA) could be chartered for costs comparable to research vessel costs. For tropical service, air conditioning equipment would have to be added. Suitable trawler-processors also operate off South Africa, including one operating for the South African Sea Fisheries Service. Possibly one of those could be obtained for charter.

The organizational design for the scientific work would involve teams with the following designations and duties:

An Acoustic Survey Team - to gather, collate and interpret geographically and temporally extensive estimates of fish stock biomass by state-of-the-art acoustic methods. The array of possible equipment is now extensive, including 38 Khz acoustic integration systems (Norwegian designs primarily), higher-frequency dual-beam systems, multi-frequency systems, and modified ASW sonar systems now becoming available in the civilian sector. One of the goals of initial work under theÊaegis of U.S. GLOBEC has been improvement of acoustic technology for fishery and plankton work. The Arabian Sea should be an ideal milieu for this technology to show its value.
A Trawl Survey Team - to capture large samples of midwater fish populations, then produce and interpret the data recommended above.
An Ichthyoplankton Survey Team - to evaluate myctophid spawning regions and seasons using egg and larval survey techniques.
A Zooplankton Population Biology Team - to evaluate the general population and production biology of mesozooplankton upon which the myctophid stocks prey. This work should include studies of gelatinous forms (salps, appendicularians, ctenophores), given the evidence in hand that common foods for myctophids are plankters associated with them. This team should come equipped to sample during persistent sea state 6 conditions, which certainly preclude diving.
An Ecophysiology Team - to work on detailed physiology of both myctophid fish and mesozooplankton, particularly those aspects enabling and precluding life in suboxic conditions (cf. Waller, 1989). These include study of aerobic vs. anaerobic metabolic poise, capacity for oxygen exchange at low and extremely low oxygen tension, and determinations of swimming capability and endurance of myctophids under a range of temperature and oxygen conditions. Given the differences between work on fish and zooplankton resulting from differences in size and behavior, separate subgroups should probably address these different groups.

Mesozooplankton

We recommend that the principal focus for an extended Arabian Sea U.S. GLOBEC study be to develop understanding of the prodigious myctophid stocks of the region. Zooplankton studies should be proposed that will supplement and extend this central study of fish. Therefore, the necessary zooplankton studies can be adequately outlined in very brief space. Much is left to the specific interests and creative ability of the proposing investigators.

Investigators proposing Arabian Sea work under the extended plan, as under the minimal plan, should develop their plans around studies of Arabian Sea euphausiids, Calanoides carinatus, and Thalia democratica. Reiterating, these species are certain to be present, likely to be ecologically important, and possibly are significant in the biology of regional fish stocks. Both the euphausiid and copepod species should be amenable to seasonal sampling studies of their population responses to monsoonal variations in upwelling and production. Both species will be suitable for dynamical experimentation, including studies of feeding, fecundity, growth and molting rates, respiration, enzyme rate functions, and adaptation to suboxic conditions.

A topic not addressed in the minimal plan that requires thorough examination in the extended plan, is the dominance of cyclopoid copepods as prey of B. pterotum and D. arabicus (see Kinzer et al., 1993). The importance of the small, numerous cyclopoid copepods to the general economy of the Northern Arabian Sea has been badly neglected to date (although R. Bottger-Schnack is currently investigating the systematics and vertical distribution of cyclopoids in the Northern Arabian Sea using nets with 50 and 100 um mesh; Kinzer, pers. comm.).

Approach

As for the minimal plan, the implementation committee leaves the specific hypotheses around which the research is to be organized to proposing investigators. We only offer the hypotheses above as a general organizing theme. Further, like the minimal plan, all of the specific sampling and experimental methods, station plans, and logistic details are left to proposing investigators. Again like the minimal plan, the Arabian Sea offers an excellent milieu for application of new, U.S. GLOBEC-supported technology in sonar, molecular biology, and other areas. We commend their use to proposing investigators, but the problems proposed for study and the hypotheses proposed for test should determine methods and approach, not U.S. GLOBEC's prior commitment to specific techniques.

Conclusion

Given the system prevalent in U.S. GLOBEC for developing specific research programs, we see no need to specify what should be done beyond this point. That system is for principal investigators to respond to a general implementation plan with specific proposals. Then program managers and review panels assemble the final scientific program and investigator team by selection among proposals and (presumably) by negotiations with and among the proposing workers. If a full U.S. GLOBEC study program is ever to address problems in the Arabian Sea, a program can be developed in that fashion based on the simple outline above. Doubtless, considerable additional biological detail will be considered by proposers, managers, and reviewers as development proceeds. Thus the proposing investigators should expect a shifting target. However, the committee writing this open plan have no doubt that Arabian Sea myctophid fishes, particularly Benthosema pterotum, are among the most exciting target species available in the world for research in pelagic biology. Ancillary studies of zooplankton will generate understanding of how the myctophids fit in the Arabian Sea pelagic ecosystem, and they will make a valuable contribution to planktology in their own right. We recommend this plan (and continued, detailed planning) to the oceanographic community with strong enthusiasm.