Rapp. P.-v. Réun. Cons. int. Explor. Mer, 180: 391-398. 1982. Observations on mesopelagic fish off Northwest Africa between 16° and 27°N

Jakob Gjøsæter University of Bergen, Department of Fisheries Biology P.O. Box 1839, 5011 Bergen-Nordnes, Norway

Johan Blindheim Institute of Marine Research P.O. Box 1870-1872, 5011 Bergen-Nordnes, Norway

The mesopelagic fauna off Northwest Africa between 16° and 27°N was studied on a cruise with RV “G. O. Sars” in November/December 1972. It consisted primarily of myctophids, with Diaphus dumerili as the dominant species and D. taaningi ranging next. The abundance was estimated using a 38 kHz echo sounder and electronic integra­ tors. The estimates depend on the procedure for converting integrated echo abun­ dance to fish biomass. There seems to be little doubt that several million tonnes of mesopelagic fish were present in the area. The densest concentrations (a 70 g/m2) were found just off the edge of the continental shelf. A tow with a 1600-mesh pelagic trawl yielded a catch rate of 6 tonnes/hour of mesopelagic fish.

Introduction Material and methods

As part of the CINECA research programme, RV The vessel operated in the area from 1 November to 6 “G. O. Sars” of the Institute of Marine Research, December 1972. Figure 257A shows the course tracks Bergen, Norway, operated in Northwest African from the first half of the cruise when the acoustic sur­ waters in November/December 1972 (Østvedt et al., vey took place. During the second part of the cruise, 1973). The main purpose of the cruise was to study the work was concentrated on biological sampling with commercially exploited fish stocks in the area, applying trawls (Fig. 257B). acoustic survey methods combined with biological A 1600-mesh pelagic trawl, 20 cm stretched meshes, sampling by trawl. This survey also provided some was used to sample mesopelagic fish. Its operational information on the local mesopelagic fauna. horizontal and vertical openings were about 29 and 16 Little work has been done on the mesopelagic fauna m respectively. A covering net in its codend had mesh of this area, although the distribution of species is size 1-4 cm stretched meshes. The depth of the trawl known in general. Few data on abundance are avail­ and the vertical opening were monitored by a net able, and the life histories of the species concerned are sounder. nearly unknown. In the Gulf of Guinea, Voss (1969) For the acoustic survey the main instrument was a 38 found, however, that mesopelagic fish were very abun­ kHz echo sounder combined with analog echo inte­ dant, and he pointed out that they might become an grators. The depth intervals 10-20 m, 20-100 m, 100- important resource for the fisheries. In other upwelling 200 m, and 200-450 m were integrated separately. areas as well, mesopelagic fish have been found in large Integrated echo intensities were logged by a computer concentrations (Gjøsæter, 1978; Gjøsæter and Kawa- and averages over 5 nm were recorded. These 5-mile guschi, 1980). means were split and apportioned to groups of fish The present paper attempts to show that there is a species on the basis of trawl catches and a daily scrutiny large standing stock of mesopelagic fish off Northwest of the echo recordings. Mesopelagic fish were in this Africa, and to present some life history data for the grouping, together with the plankton. For separation dominant species. of mesopelagic fish from zooplankton, it was assumed that the mesopelagic fish make diel vertical migrations

391 Figure 257A, B. Cruise tracks and trawl stations, RV “G. O. Sars”, November/December 1972. 1) Pelagic trawl. 2) Bottom trawl.

between the surface layer, which is their nocturnal 258). From the few data available, it was not possible habitat, and a scattering layer at 300 to 500 m depth to draw isolines for the integrated echo intensities, but where they stay during daylight (Fig. 258). Recordings mean values were worked out in subareas, as shown in above 100 m depth during daylight hours were assumed Figure 259. Only recordings beyond the 150 m depth to be due to zooplankton, and the differences in inte­ were used in the estimate because recordings of grated echo intensities between night and day in the mesopelagic fish were sparse in more shallow waters upper 100 m were taken to be mesopelagic fish. The (Østvedt and Blindheim, 1973; Marchai and Boely, percentages of mesopelagic fish worked out for five 1977). days and nights were 77, 78, 70, 63, and 89 %, with 75 The output M of the echo integrator is linearly % as average. The contribution from mesopelagic fish related to the number of fish per unit area in the inte­ to the echo intensity was therefore taken as 75 % of the grated depth columns (Forbes and Nakken, 1972). The night recordings in the plankton group. Day recordings relationship can be written: were not applied because many of the mesopelagic fish migrated beyond the integrated depth column (Fig. Pa = C M + d (1)

392 Figure 258. Echo recordings showing vertical migration of mesopelagic fish. Depth in m is indicated.

where P.d is number of fish per unit area, C is the number of fish per unit area needed to contribute one unit to the integrated echo intensity, and d is the lowest recordable density (Midttun and Nakken, 1977). The density coefficient C depends on fish species and size, and on the characteristics of the echo sounder/in­ tegrator system used:

c = q cs rb (2)

Here Q is an instrumentation constant, / is fish length and Cs and b are constants for a given species (Nakken, 1975). Usually b is close to 2 (Nakken and Olsen, 1977). This equation gives number of individuals per unit integrator deflection and unit area. The density coefficient, C, for myctophids is not known. It has therefore been necessary to use the coef­ ficient for some other small pelagic fish, and the value worked out for capelin (Mallotus villosus) on “G. O. Sars” in 1972 was applied (Nakken and Dommasnes, 1975). The equation for the density coefficient then becomes:

C = 9-4 106 r 172 (3)

where the mean length of mesopelagic fish in the trawl samples can be substituted for I. The density coefficient was also determined on the basis of a trawl station where the pelagic trawl was operated during the ascent of the fish at sunset (trawl station 333). The trawl was towed for 15 minutes over a distance of 0-6 nm and the surface area of the filtered water column was 32 600 m2. The head line was at 15 m depth and the operational vertical opening was 16 m. The current 5-mile average of the echo intensity in the upper 100 m was 1977 mm/nm. It is assumed that dur­ ing the trawling, which was in the last mile of the 5 Figure 259. Subareas used in abundance assessment. 1) Night recordings used in assessment. 2) Area with dense concentra­ miles averaged, these fish had ascended to the upper 50 tions over the continental slope. 3) Depth contour, 150 m. m (Fig. 260). Supposing fairly even distribution of the

393 . . N & ------I l à

4 &s'o~m —^

Figure 260. Echo recordings during trawl station 333. Depth in m is indicated.

fish in the upper 50 m, the water column filtered would 40 represent an integrated echo intensity of 633 mm/nm. The catch of mesopelagic fish was 1-4 tonnes, and on 3 0 - the assumption that the catch efficiency of the trawl was 100 %, this is equivalent to 148 t/nm2 in the depth column of the trawl. This represents a density coeffi­ S20_ cient of 0-23 t/mm nm2.

1 0 .

Results J :F=R P 20 30 40 50 60 70 80 20 30 40 50 60 Biology LENGTH IN MM Only three samples were available for identification of Figure 261A, B. Length distribution of Diaphus dumerili. A: trawl station 314; B: trawl station 325. species and for biological studies (Table 77; Fig. 257). A sample taken off Cape Blanc was studied and it was found to contain 62 % Myctophum punctatum, 24 % Diaphus dumerili, and 14 % other species. Off St. 15 % D. taaningi. A station off Cape Timiris gave 91 % Louis one station yielded 85 % Diaphus dumerili and D. dumerili and 9 % D. taaningi. At a pelagic trawl station taken off St. Louis, stan­ dard length of D. dumerili ranged between 40 and 90 mm with 50 mm modal length (Fig. 261 A). Mean Table 77. Samples of mesopelagic fish. Station numbers refer to Figure 257. PT: pelagic trawl. BT: bottom trawl. weight of the fish was 1-6 g. Off Cape Timiris a bottom- trawl station yielded D. dumerili with lengths ranging Station Trawl Catch Species between 20 and 70 mm with one mode at 60 mm (Fig. no. (kg/hour) 261 B). 294 PT 100 Myctophum punctatum It has been shown that tropical fish may form daily (62 %), growth rings in the otoliths (Pannella, 1974; Brothers Diaphus dumerili (24 %), et al., 1976), and such rings have been used for a tenta­ D. effulgens, D. taaningi, tive aging of other tropical myctophids (Gjøsæter, Notoscopelus bolini 1978). 314 PT 625 D. dumerili (85 %), Therefore a similar procedure was used to estimate D. taaningi (15 %) the age of Diaphus dumerili. A close correlation was found between number of rings and length of fish (Fig. 325 BT 140 D. dumerili (91 %), D. taaningi (9 %) 262). If these rings are formed daily, they show that maximum size is reached at the age of about one year. 332 PT 227 Myctophidae spp. According to Nafpaktitis (1968) fish in the Gulf of Guinea reach maturity at a size of 44 mm. This corres­ 333 PT 6 000 Myctophidae spp. ponds to an age of about 6 months.

394 8

Figure 264. Otoliths of Notoscopelus bolini.

0 ' 50 ' 100 150 200 250 ' 300 350 ' «io NO. OF ZONES Gillrakers 9+1 + 19 9+1 + 19 9+1 + 18 AO - photophores 9+7 7+6 8+6 Figure 262. Relationship between length and number of pri­ Dorsal fin rays 23 23 23 mary zones in the otoliths of Diaphus dumerili.

The specimens taken were females, and the characters most important for distinguishing between N. kroeyeri D. taaningi ranged second in abundance at the two and N. bolini were therefore absent (Nafpaktitis, southernmost trawl stations. The standard length 1975). The otoliths (Fig. 264) showed, however, that ranged between 45 and 60 mm, with a modal length of the specimens did not belong to N. kroeyeri. 50 mm (Fig. 263). Myctophum punctatum was the most important species off Cape Blanc where it made up 62 % (in number) of the catch. Its length ranged from 40 to 63 A bundance mm, indicating that only young specimens were caught Recordings of mesopelagic fish were made throughout (Zurbrigg and Scott, 1972). the survey area beyond about 150 m depth. The Three specimens of Notoscopelus bolini were caught densest concentrations were recorded just off the edge off Cape Blanc. The following characters are of diag­ of the continental shelf. Figure 265 shows plots of inte­ nostic importance. grated echo intensities on two east-west courses in sub- areas 4 and 5. The bottom depth is also plotted and the concentration off the shelf break is clearly demon­ strated. In the assessment, the near-shelf area where 40 _ these maxima occur is treated separately, as subarea 9. Varying abundance of mesopelagic fish was also observed along the coast. Southwards from 27°N there

3 0 - was a gradual increase and the highest echo intensities were observed off Cape Blanc in subarea 5 (Table 28). Farther southwards the intensities decreased. z An assessment of the biomass of mesopelagic fish tu 20_ O — has been made, based on the density coefficient cn LU worked out from the catch at trawl station 333, and this CL gives a biomass of 5-8 X 106 tonnes for the whole area 10_ (Table 78). A second assessment has been worked out on the basis of the mean integrated echo intensities for the subareas and a density coefficient obtained from Equa­ 0 i —' , 14 - I -U n tion (3). Substituting I in the equation with the mean 30 40 50 60 length of myctophids in the trawl samples, i.e. 5-5 cm, LENGTH IN MM one obtains a density coefficient of 0-50 x 106 indi­ Figure 263. Length distribution of Diaphus taaningi. viduals/mm nm2. Multiplication by the mean weight of

395 3000 Table 78. Extent of subareas in nm2, mean echo intensities in the subareas, standard error of the means (5 x), abundance assessment based on density coefficient from Equation (3) (I), and assessment based on density coefficient from trawl station 333 (II)

Sub Extent Mean S x I II area (nm2) echo (txlO -3) ( tx lO '3) 1000 int.

1 5 620 193 20-0 857 250 2 5 530 227 17-6 993 289 3 9 520 266 14-6 2 001 582 4 19 500 290 8-0 4 467 1 301 5 5 250 506 67-1 2 099 611 o 1000 6 6 820 114 13-4 614 179 7 11 510 83 6-2 755 220 8 9 760 180 16-7 1 388 404 9 9 900 845 146-5 6 609 1 924

Total 83 400 - - 19 783 5 760

‘3000 250 200 150 100 DISTANCE N. MILES fish and plankton, and establishing a density coefficient 3000 for the echo recordings. In the present study only the vertically migrating community was considered. This leaves out the surface plankton and the epipelagic fish. But it also leaves out a component of the mesopelagic fauna that stay at depth during the night (e.g., Schemainda and Ritz- 2000 1000 haupt, 1969). On the other hand it will include some vertically migrating plankton, such as euphausiids and decapods. These groups will, however, give much less echo than fish of comparable biomass (Beamish, 1971), and the trawl catches indicated that they were much scarcer than these fish. 1000 2OOO5; The conversion of integrated echo abundance to fish biomass is also a serious source of error. When fish length is much larger than wavelength a relationship is established (Nakken and Olsen, 1977), but there is doubt about what happens when fish length approaches wavelength (approx. 4 cm for 38 kHz). The present 0 3000 calculations assume that this relationship is applicable 250 200 150 100 to all length groups considered. DISTANCE N. MILES It must also be noted that the density coefficient (Cs) Figure 265. Integrated echo intensities related to bottom is based on capelin. This may introduce bias, although depth (broken line) in subareas 4 and 5. it has been shown that the difference between species has little importance when the fish length is small com­ pared with wavelength (Nakken and Olsen, 1977). myctophids in the trawl samples converts the coeffi­ A density coefficient based on a trawl catch and the cient to 0-79/mm nm2. For the whole survey area this corresponding echo integrator readings is always too gives a biomass of 19-8 x 106 tonnes (Table 78). low. The most important reason for this is that the trawl does not catch with 100 % efficiency. Fishing with various gears in the Arabian Sea showed that the real efficiency of a trawl similar to that used off Northwest Discussion Africa must be lower than 50 %, but it is not possible to estimate how much lower (Anon., 1976). It was also Abundance assessments of the type used in the present supposed that the fish were evenly distributed between study have many serious sources of error. Most impor­ 0 and 50 m, but this assumption is uncertain as well. tant are probably the difficulties in distinguishing be­ The density coefficient based on a trawl catch is tween echoes from mesopelagic fish and from other about one third of that based on calibration on capelin.

396 This discrepancy can be accounted for by the efficiency Blackburn and Nellen (1977) found that Myctophidae of the trawl, and it is therefore unlikely that resonance made up only 0-9 % and M. muelleri 1-7 % of the fish contributes significantly to the echo intensity. larvae caught on a cruise in this area during March- The present study is based on one survey of the area. May 1974. Only a few of these species were observed in As the variance associated with the echo abundance the present samples. computations is fairly high (Table 78) another survey The mesopelagic fauna in oceanic, non-upwelling during the same period could have given a different areas off West Africa show a species composition com­ result. Little is known about the seasonal variation in pletely different from the neritic fauna (Badcock, abundance of mesopelagic fish. 1970). Schemainda and Ritzhaupt (1969) studied the Blackburn (1977), working off Cape Blanc during DSL fauna of the Gulf of Guinea. They found myc- spring 1974, presented an estimate of micronekton tophids to be the most abundant components, but biomass just beyond the edge of the shelf of 0-5 g/m2, larger crustaceans, siphonophores, and squid were also of which about 0-2 g/m2 was fish. The estimate was, present. North of the Canary Islands, Kinzer (1977) however, based on only a few hauls with a micronekton found very few siphonophores in plankton samples net. form the DSL while crustaceans were fairly numerous. Transforming the estimates presented in Table 78 to Although the estimated stock sizes are tentative and g/m2 surface area gives means of 70-200 for subarea 9, should be taken with all reservation, the present study which covers the area near the shelf edge. Using trawl shows that mesopelagic fish may be more important in station 333, which is the only one where the necessary upwelling areas than previously assumed. This conclu­ data are available, one arrives at about 80 g/m2, assum­ sion is supported by sudies in the Arabian Sea (Gjøsæ­ ing that the trawl caught with an efficiency of 50 %. ter, 1978). The mesopelagic fish also seem to have a Schemainda et al. (1975) and Wooster et al. (1976) shallower distribution in the upwelling areas than else­ have shown that upwelling occurs throughout the year where (Voss, 1969; Kinzer, 1977), and this makes them off Cape Blanc, although there is a peak from March easier to catch, even during the daytime. The prospect until June. At about 15°N, upwelling occurs from for utilization of this resource therefore seems promis­ December to May, and north of 25°N the strongest ing. Further studies should, however, be carried out upwelling occurs from June to October. before the true stock size and the production can be Studies on the primary production in the area be­ assessed. tween about 16° and 27°N show conflicting results. Data presented by Lloyd (1971), Cushing (1971), and Østvedt et al. (1973), indicate annual primary produc­ Acknowledgements tion of about 2 X 10s tonnes of carbon. This is also in reasonable agreement with data presented by Berge We should like to thank the cruise leader and the staff (unpublished). On the other hand, Schemainda et al. on the West Africa cruise of RV “G. O. Sars” for help (1975) arrived at an estimate of about 6 x 107 t car­ with data collection, as well as Mr H. Kismul for draw­ bon/year. ing the figures, Dr O. Dragesund and Mr O. J. Østvedt Assuming a carbon/wet-weight ratio of 0-065 and a for helpful comments on the manuscript, and Miss C. transfer coefficient of 10 % between the trophic levels, Hamilton for corrections in the English text. these estimates lead to a production of 3 x 107 and 1 x 107 t wet weight respectively at the first carnivorous level, which is where the mesopelagic fish in question may partly belong. Some of this production takes place above the shelf, where it is inaccessible to the References mesopelagic fish. The estimated abundance of meso­ pelagic fish is in reasonable agreement with the first Anon. 1976. Report on cruise No. 5 of RV “Dr. Fridtjof production estimate, but conflicts with the last one. Nansen”. Indian Ocean Fishery and Development Pro­ Backus et al. (1977) described the area studied as a gramme. Pelagic Fish Assessment Survey North Arabian Sea. FAO, 1-10, 1 tab., 27 figs. (mimeo). faunal region known as the “Mauritanian Upwelling”. Backus, R. H., Craddock, J. E., Haedrich, R. L., and Robi­ Using IKMT they found Diaphus hold, Lepidophanes son, B. H. 1977. Atlantic mesopelagic zoogeography. In guentheri, and Lampanyctus pusillus to be dominant Fishes of the Western North Atlantic, part 7 (1), pp. 266- species; but Diaphus dumerili, D. vanhoeffeni, Cerato- 287. Mem. Sears Found. Mar. Res. Yale University, New Haven. scopelus warmingi, C. maderensis, and Benthosema Badcock, J. 1970. Vertical distribution of mesopelagic fishes glaciale were also important species in this region. collected on the SOND cruise. J. mar. biol. Ass. U. K., 50: Kinzer (1977) found that B. glaciale was by far the most 1001-1044. abundant species during his studies there in February/ Beamish, P. 1971. Acoustic scattering from zooplanktonic organisms. In Proceedings of an International Symposium March 1975. Samyshev and Schetinkin (1971) found D. on Biological Sound Scattering in the Ocean, Rep. No. 005. dumerili, D. taaningi, Lepidophanes guentheri, and M. pp. 474-475. Ed by G. B. Farquhar. Maury Center for muelleri in the same area during February/March 1971. Ocean Science, Washington, D. C.

397 Blackburn, M. 1977. Studies on pelagic animal biomass. In tionship between integrated echo intensity and fish density. Oceanic sound scattering prediction, pp. 283-299. Ed. by ICES CM 1975/B: 26, 8 pp. (mimeo). N. R. Andersen and B. J. Zahuranec. Plenum Press, New Nakken, O., and Dommasnes, A. 1975. The application of an York, 859 pp. echo integration system in investigations on the stock Blackburn, M., and Nellen, W. 1977. Distribution and ecol­ strength of the Barents Sea capelin (Mallotus villotus, Mül­ ogy of pelagic fish studied from eggs and larvae in an ler) 1971-1974. ICES CM 1975/B: 25, 20 pp. (mimeo). upwelling area off Spanish Sahara. Fishery Bull. Fish Wildl. Nakken, O., and Olsen, K. 1977. Target strength measure­ Serv. U. S., 74: 885-896. ments of fish. Rapp. P.-v. Réun. Cons. int. Explor. Mer, Brothers, E. B., Mathews, C. P., and Lasker, R. 1976. Daily 170: 52-69. growth increments in otoliths from larval and adult fishes. Pannella, G. 1974. Otolith growth patterns: an aid in age Fishery Bull. Fish Wildl. Serv. U. S. 74: 1-18. determination in temperate and tropical fishes. In The Cushing, D. H. 1971. Upwelling and the production of fish. aging of fish, pp. 28-39. Ed. by T. B. Bagenal, Unwin Adv. mar. Biol., 9: 255-335. Brothers Ltd., Old Woking, Surrey. Forbes, S. T., and Nakken, O. (Editors). 1972. Manual of Samyshev, E. Z., and Schetinkin, S. V. 1971. Feeding pat­ methods for fishery resources survey and appraisal, part 2. terns of some species of Myctophidae and Maurolicus muel­ The use of acoustic instruments for fish detecting and leri caught in the sound-dispersing layer in the northwestern abundance estimation. FAO, Man. Fish. Sei., (5): 1-138. African area. Annls biol., Copenh., 28: 212-215. Gjøsæter, J. 1978. Aspects of the distribution and ecology of Schemainda, D., Nehring, D., and Schulz, S. 1975. the Myctophidae from the Western and Northern Arabian Ozeanologische Untersuchungen zum Produktionspotential Sea. Dev. Rep. Indian Ocean Programme, (43) Vol. 2: 62- der nordwestafrikanischen Wasserauftriebsregion 1970- 108. 1973. Geod. Geoph. Veröff., 4 (16): 1-85. Gjøsæter, J., and Kawaguschi, K. 1980. Mesopelagic fishes, Schemainda, D ., and Ritzhaupt, H. 1969. Deep scattering Review of world resources. FAO Fish. Tech. Pap., 193: 1- layers and hydrographic structures of the water masses in 151. the Gulf of Guinea. In Proceedings of the Symposium on Kinzer, J. 1977. Observations on feeding habits of the the Oceanography and Fisheries Resources of the Tropical mesopelagic fish Benthosema glaciale (Myctophidae) off Atlantic, pp. 171-177. UNESCO, Paris. NW Africa. In Oceanic sound scattering prediction, pp. Voss, G. L. 1969. The pelagic midwater fauna of the eastern 381-392. Ed by N. R. Andersen and B. J. Zahuranec. tropical Atlantic with special reference to the Gulf of Plenum Press, New York, 859 pp. Guinea. In Proceedings of the Symposium on the Oceano­ Lloyd, I. J. 1971. Primary production off the coast of north­ graphy and Fisheries Resources of the Tropical Atlantic, west Africa. J. cons. int. Explor. Mer, 33: 312-323. pp. 91-99. UNESCO, Paris. Marchai, E., and Boely, T. 1977. Evaluation acoustique des Wooster, W. S., Bakun, A., and McLain, D. R. 1976. The ressources en poissons du plateau continental ouest-africain seasonal upwelling cycle along the eastern boundary of the des iles Bissagos (11°N) à la pointe Stafford (28°N). Cah. North Atlantic. J. mar. Res., 34: 131-141. ORSTOM, Sér. Oceanogr., 15: 139-161. Zurbrigg, R. E., and Scott, W. B. 1972. Evidence for expatri­ Midttun, L., and Nakken, O. 1977. Some results of abund­ ate populations of the lanternfish Myctophum punctatum in ance estimations studies with echo integrators. Rapp. P.-v. the Northwest Atlantic. J. Fish. Res. Bd Can., 29: 1679— Réun. Cons. int. Explor. Mer, 170: 253-258. 1683. Nafpaktitis, B. G. 1968. Lanternfishes of the genera Lobian- Østvedt, O. J., and Blindheim, J. 1973. Studies of the abund­ chia and Diaphus in the North Atlantic. Dana Rept., 73: 1- ance and distribution of fish off West Africa, November- 131. December 1972. ICES CM 1973/J: 24, 16 pp. (mimeo). Nafpaktitis, B. G. 1975. Review of the lanternfish genus Østvedt, O. J., Blindheim, J., Føyn, L., Berge, G., Smed­ Notoscopelus (family Myctophidae) in the North Atlantic stad, O., and Vestnes, G. 1973. Report on a cruise by RV and the Mediterranean. Bull. Mar. Sei., 25: 75-87. “G. O. Sars” to West Africa 23 October - 15 December Nakken, O. 1975. On the problem of determining the rela­ 1972. ICES CM 1973/J: 23, 34 pp. (mimeo).

398