Appendix C

# Appendix C. Fish Working Group Report

Chair: George Boehlert

Rapporteur: Charlie Miller

Participants: Johannes Kinzer, Don Olson, David Stein, Bernie Zahuranec

The group dealing with fish discussed the general lack of information available on pertinent species at higher trophic levels in the Arabian Sea. It is imperative that all literature and unpublished studies be drawn together to better refine any proposed research. Questions were raised about the source of prey, distribution and abundance, and ultimate fate of the most important species (assumed here to be Benthosema pterotum and Diaphus spp.).

## General Themes and Specific Scientific Questions

Specific hypotheses were posed to serve as a focus of discussion, as follows:

General Hypothesis: The large stocks of myctophid fish and their high growth rates in the Arabian Sea are a function of regional physical conditions.

Specific Hypotheses:

- Myctophids are protected from daytime predation at depth by descent into dysoxic waters.

- High growth is promoted by the unusually high productivity associated with both coastal and open sea upwelling.

- Much myctophid production goes to non-predatory mortality. It should end up on the bottom.

- Myctophid abundance is related to topography.

Tests of these hypotheses will require sampling both the water column and the benthos using a variety of sampling gear. Water column sampling should be done with acoustics for biomass estimation, discrete depth midwater trawling for specimen collection and identification, and larval fish and plankton sampling. Bottom cores should be collected to examine present and historical myctophid abundance as reflected in otolith and/or scale deposits. To examine a wide range of physical conditions, stations should be located on the shelf, slope and in offshore waters. Sampling should be conducted monthly for at least one full year.

Several questions should be answered, including:

- What is the geographic distribution and biomass of Benthosema pterotum (and other important midwater fish species), particularly in the offshore dimension? It seems likely that B. pterotum is largely replaced by Diaphus arabicus beyond the continental slope. Benthosema fibulatum and several other species may be important in the total stock in particular places and seasons. Several ecological analogies may be drawn with the large midwater fish biomass in the Benguela upwelling system (Prosch, 1991; Armstrong and Prosch, 1991).

- What are the mechanisms through which high biomass midwater fish populations are maintained? The genus Benthosema is typically associated with continental slopes or other topography (Reid et al., 1991). Analogous populations of Maurolicus muelleri and Lampanyctodes hectoris exist in the Benguela upwelling system and are proposed to be maintained by interactions of vertical migration with Ekman transport (Hulley and Prosch, 1987), similar to the mechanism proposed by Peterson et al. (1979) for copepods off Oregon. Benthosema pterotum (along with other midwater fishes) may be maintained around offshore banks such as the Murray Ridge through a variety of mechanisms including topographically influenced anticyclonic circulation (Boehlert and Genin, 1987; Loder et al., 1988). Such midwater fish populations may be the principal source of energy to benthic fish populations, as seen on seamounts farther to the south in the Indian Ocean (Parin and Prutko, 1985).

- What is the diel pattern of vertical migration relative to the oxygen minimum layer? Gjosaeter (1984) suggests that only a portion of the B. pterotum population undertakes a diel vertical migration each night, so the frequency of this migration should be carefully examined.

- How does the distribution relate to physical factors, including oxygen? How does the distribution (including variation in biological factors like fecundity, body condition) relate to topography? This latter includes position relative to the continental slope and effects of offshore ridges.

- What are the vital rates (growth rate, reproduction, mortality) of the dominant fish species?

- What is the fate of the annual fish production? If the midwater fish production ends up on the bottom, then a signal of enriched organic matter, as well as otoliths and fish scales, should be evident in the sediments. If varved sediments are available, historical patterns of fish abundance following the techniques of Soutar and Isaacs (1974) or Shackleton (1987) should be conducted. Benthic studies should be coordinated with U.S. JGOFS benthic investigations.

- What role does seasonal predation by larger nekton (squid, tunas) play in removals of some portion of the midwater fish population? An initial effort at determining the regional abundance of these larger nektonic predators should be made from regional fisheries data and any available fisheries surveys (see Stequert and Marsac, 1989; Mohri et al., 1991; and also data reports of the FAO Indo-Pacific Tuna Programme). Should they be abundant seasonally, the extent of larger nekton feeding on mesopelagic fishes (including estimates of total removals) should be determined.

- Why are clupeoids and demersal slope fishes such as gadoids, so common in most productive upwelling areas, apparently at low levels in the Arabian Sea?

## Equipment and Approach Recommendations

The working group discussed how to approach and undertake an extensive and thorough study of the midwater myctophids of the Arabian Sea. We agreed that more than one sampling technique should be applied to the estimation of stock size. For acquisition of specimens and data for the micronekton component, we recommend the following gear:

- An RMT net or equivalent, 25 m2 if possible, although 8 m2 would probably be satisfactory. This would be used to obtain detailed vertical structure of the populations and its relation to the oxygen-depleted layers.

- A medium sized trawl (of order 100 m2) is needed for bulk collections, and checking of acoustic density estimates. It should have a divided catching apparatus at the cod end.

- Acoustics (more below) should be state-of-the-art at the time of the study. The equivalent of a SIMRAD EK-500 now used for research and commercial purposes. The unit should have full recording and integrating capability.

The ideal platform for deployment of these varied gear is a specially equipped dragger/processor ship, possibly chartered from the commercial fishing industry. Suitable ships are not fast, but have high power. In the U.S. suitable ships are mostly in service in the Bering Sea fishery.

The sampling schedule should include the following:

- Monthly or more frequent onshore-offshore surveys on a transect line off Oman, covering a shelf station and 2-3 stations each over the slope and offshore. Each station should include a full 24-hour day of fishing and acoustic work.

- Detailed 48 hour studies for migration and feeding timing and gut content studies at several locations.

- Seasonal geographic surveys to quantify Benthosema and general myctophid populations over the region as a whole. Data shown at the meeting (by Don Olson, from a report by Gjosaeter) suggests a fairly strong seasonal variability in stocks along the south Arabian coast. Perhaps two extended geographic surveys would be sufficient, if coupled to much more dense seasonal coverage along a single transect (probably off Oman). The principal tool for the geographic surveys should be hydroacoustics.

A central goal of ichthyological studies in the Arabian Sea U.S. GLOBEC work is full understanding of myctophid biology and basic ecology. To this end U.S. GLOBEC should support a number of biological studies. We recommend the following minimum list of program components:

- A detailed gut content study coordinated with zooplankton collections. The species of food items should be studied and compared to the zooplankton fauna. At the tropical temperatures immediate preservation of myctophid catches is important to this work. Immunological studies of gut contents should be considered depending upon the adequacy of traditional methods. Evacuation rates and daily ration should be determined.

- Fecundity and gonad cycling should be studied in detail to determine the seasonality and nature of spawning. This should be done in histological detail with quantification of spawning frequency and batch fecundity (see Hunter and Goldberg, 1979). A similar study dealing with midwater fish was conducted by Melo and Armstrong (1991). Sampling and special preservation of materials for this work should be done on the seasonal, diel and 48 hour sampling schedules of the trawl sampling. Depth distribution of egg release should be determined.

- Respiration may well be specially organized in Arabian Sea myctophids, since the daylight period is spent in dysoxic waters. Oxygen debt may build to substantial levels and be tolerated. Special enzymatic mechanisms may exist for extracting oxygen from low concentration waters, for rapid relief of oxygen debt upon return to high oxygen waters, and for tolerating occasional sulfide toxicity. One or more respiratory physiologists should be included in the program.

- Establishing the growth and development timing is essential to advancing the life history analysis of Arabian Sea myctophids. To this end we need detailed calibration of the otolith increments in B. pterotum and other species. Alternative ageing methods (including length-frequency analysis) should be undertaken to confirm otolith estimates as well as to confirm growth rates and individual longevity.

- A study of bioluminescence in myctophids might also be a focus of U.S. GLOBEC studies. Ventral photophores are a prominent feature of myctophid morphology. An investigation of their function in Arabian Sea species could provide some special insights. Does luminescence persist in dysoxic waters at depth during daytime, or is it suppressed by low oxygen levels?

The working group discussed the possibility that all expeditions to the Arabian Sea in the mid-1990's could be equipped with a sophisticated, recording echo integrator system for low frequency (ca. 38 kHz) records of fish abundance. Possibly data could be gathered in a nearly automated fashion and analyzed by U.S. GLOBEC fish program investigators. The WOCE representative present at the meeting emphasized the "not to interfere" basis that would be enforced by investigators from other programs. It was also suggested that calibrated ADCP instruments might make useful records. In general the frequency of these is too high to be useful for evaluation of fish abundance and distribution, but the records of zooplankton abundance would be useful for the overall U.S. GLOBEC research program.

## Recommendations

We recommend that a dedicated ship be requested by U.S. GLOBEC for nekton and plankton research. A dedicated ship is needed in order to ensure adequate sampling (e.g., number, duration and location of stations). A Bering Sea type trawler-processor (135-150 ft) could be chartered for reasonable costs for 6-12 months to enable seasonal sampling along an inshore-offshore transect and geographic surveys. Such a vessel would be capable of working in high wind, rough sea states typical of the region for much of the year. It would be able to accommodate 12-15 scientists and provide ample laboratory space. The ship would include state-of-the-art acoustics and it would have the ability to tow large midwater trawls double warp. It would include a deck crane suitable for moving and deploying small gear, and it would be fitted with a third wire or an acoustic net monitor.

Modifications to the vessel would be installation of a hydrowinch (and A-Frame?), an echo integrator, and a running sea water system. Meteorological deck sensors, modern communications and satellite downlinks, and SAIL systems would be desirable. It would have to be airconditioned.