Benthosema pterotum
B. pterotum seems to grow to a maximum size of about 50 mm, but specimens larger than 45 mm are rare.
Fig. 10 shows the length distribution of B. pterotum caught off Pakistan. The distribution from cruise 3 is bimodal. On cruise 4 juveniles between 5 and 10 mm were caught but not included in Figure 10. Between March 1976 and June 1976 there is an indication of growth, but fish taken in September the same year are smaller again.
Fig. 11 shows that a population of B. pterotum with much lighter colour than those commonly observed (GJØSÆTER in prep.) caught in the Swatch had a larger mean size than the dark coloured fish.
Fish caught in the Gulf of Oman were smaller than those taken off Pakistan (Fig. 12), while those taken in the Gulf of Aden were generally larger (Fig. 13). They also had a bimodal distribution.
The rings in the otoliths commonly regarded as daily growth zones (PANELLA 1974, BROTHERS, MATHEWS & LASKER 1976). were counted in sixteen otoliths from B. pterotum caught during cruise 4. The zones were often indistinct and difficult to count. Fig. 14 shows that there was no increase in number of zones with length of fish, but unfortunately only otoliths from adult fish were available for counting. If the zones are laid down daily, they show that the fish may reach its maximum size in less than half a year, and they may have two generations a year.
The reproduction of this species is studied by MAKHDOOM (in prep.) who has shown that breeding takes place all the year but with maxima in March - June and September -November. These two reproduction periods may explain the bimodality in some of the length distributions. It is therefore suggested that the fish lives more than half a year and the small size of it makes it unreasonable that it should live for more than one year.
The type of stomach contents were studied on cruise 4. Of 120 stomachs with indentifiable contents, 85 contained copepods, 28 various crustacea larvae, 10 euphausiids, 7 gastropods, 7 ostracods and 2 had eaten amphipods.
The sizes of 117 food items from 14 fishes were measured. The mean size was 1.16 mm (sp = 0.52) and the range 0.50 - 2.83 mm.
Records of degree of filling and state of digestion of the stomach contents from fish taken at the diurnal station in the Gulf of Oman are shown in Fig. 15. Stomachs were classified as 1: empty, 2: some contents, 3: half filled, and 4: full or extended. The digestion was classified as 1: newly ingested, 2: partly digested, and 3: much digested. At noon more than 50% of the fish had empty stomachs and only 4% had full stomachs. The digestion was well advanced (3) in more than 90% of the fish.
At the first night station taken two hours after the sunset, 96% of the fish had newly ingested food items and about 55% of the fish had full or extended stomachs. Only 8% were empty.
The stage of digestion increased during the night, and in the morning 62% of the fish contained much digested food and 35% partly digested food. The percentage of fish having full or extended stomachs reached a maximum of about 64% at the second night station (about 4 hours after sunset) and then decreased steadily. Number of half-filled stomachs increased during the latter part of the night and reached a maximum in the morning.
These data suggest that Benthosema pterotum feed most intensively during the first part of the night and little feeding seems to take place during daytime.
B. fibulatum
Length distribution of B. fibulatum from the Somalia coast (Fig. 16) shows a growth in mean length between January 1975 and October 1975. Fish taken in the northern Arabian Sea were much smaller on both cruises, but they also showed growth between the cruises (Fig. 17).
Otolith rings were counted in twelve specimens all caught during cruise 4 (Fig. 18) and showed a rather close relationship to length. The rings were also clearer and more distinct than in B. pterotum. If the zones are laid down daily they suggest a life cycle of one year, although the fish probably may start to breed when half a year old.
The smallest mature fish found during cruise 4 measured 40 mm. Most of the fish caught on cruise 6 were ripe, spawning or had recently spawned. MAKHDOOM (in prep.) found that B. pterotum mainly spawn during April - June and during September - November.
