Implementation Plan for a Full-Scale Study of Pelagic Populations
Arabian Sea processes are attractive for U.S. GLOBEC studies in two
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.
- 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.
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.
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.
- 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.
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
Evaluations of our specific "hypotheses" may be developed as follows:
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:
- 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.
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.
- 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.
The organizational design for the scientific work would involve teams
with the following designations and duties:
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.
- 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 the aegis 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.
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.).
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
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