Lampanyctus festivus Tåning 1928

Lampanyctus festivus

This large myctophid grows to 138 mm (Hulley, 1981); maximum size in the Ocean Acre collections is 110 mm. A bipolar questionably subtropical species, L. festivus is most abundant in the North Atlantic subtropical region (Backus et al., 1977). It is common in the study area, but ranked no higher than twelfth in winter in abundance. The Ocean Acre collections contain 593 specimens; 188 were taken during the paired seasonal cruises, 131 of these in discrete-depth samples, of which 120 were in noncrepuscular tows (Table 23).

DEVELOPMENTAL STAGES.—Postlarvae were 6–23 mm, juveniles 21–65 mm, subadults 63–110 mm, and adult males 85–103 mm. All females larger than 80 mm were examined for reproductive state; none were ripe or nearly ripe. The maximum egg diameter observed was 0.1 mm. The 42 specimens (33–110 mm) that could be sexed were divided equally between males and females. There was no apparent sexual dimorphism.

REPRODUCTIVE CYCLE AND SEASONAL ABUNDANCE.—Lampanyctus festivus probably spawns close to, if not in, the study area. However, until ripe females are taken, the issue is open to question. The parent population apparently spawns over much of the year, with a peak in intensity in winter. The life span is at least two years and probably longer. Abundance was greatest in winter, least in late spring, and intermediate in late summer (Table 92). Postlarvae were most abundant in late spring, juveniles and adults in winter, and subadults in late summer.

Although the lack of ripe females in the Ocean Acre collections casts some doubt on the reproductive status of L. festivus in the study area, the capture of postlarvae and small (less than 25 mm) juveniles suggests that spawning takes place near Bermuda. Young fish were taken over most of the year and probably were present at all times. They were most abundant in late spring and only slightly less abundant in late summer.

In winter recruitment was at a minimum. Very few postlarvae were taken, and most juveniles were larger than 30 mm. Juveniles 30–40 mm dominated the catch. Presumably these represent fish from the previous year's spawning peak. The actual size range of most of the previous year's spawn appears to be 28–57 mm, with few fish larger than 40 mm. The few 63–86 mm specimens taken in late spring probably represent fish two or more years old. The abundance of specimens in excess of 40 mm is too low to note peaks in the size frequency distribution. Size ranges for the various age groups cannot be determined with any certainty.

In late spring postlarvae dominated the small catch, accounting for about 66 percent of the total abundance (Table 92). The relatively low abundance at this season is somewhat puzzling. It probably is due to the relative inefficiency of the 3-m IKMT in sampling postlarvae, which for the most part filter through the net, and specimens larger than about 45 mm. By late summer the abundance of each stage (except postlarvae) increased from its late spring level (Table 92). The increase in juveniles is to be expected, because most postlarvae present in late spring had grown and developed to juveniles. The catch in late spring could be divided into two size groups: recently spawned fish 15–32 mm, and fish larger than 50 mm at least 1.5 years old. Presumably the latter group represented the 28–57 mm fishes of winter at a more advanced age.

VERTICAL DISTRIBUTION.—Depth range by day in winter is 701–850 m with maximum abundance at 701–750 mm, in late spring 51–100 m and 751–800 m (possibly slightly shallower and deeper), and in late summer 501–550 m and 751–1050 m with a maximum at 801–850 m. Vertical range at night in winter is 51–350 m and 851–900 m with maximum abundance at 101–150 m, in late spring 50–150 m and 951–1000 m with a maximum at 50 m, and in late summer 101–300 m and 851–1000 m with a maximum at 251–300 m (Table 93).

Stage and size stratification were evident during the day in late spring and late summer, and by night at each of the three seasons.

During the day only postlarvae were caught above 700 m in late spring and late summer, and deeper than 1000 m in late summer. As a result, mean sizes at those depths were much smaller than those at 751–850 m (Table 93).

At night in winter only juveniles were caught above 150 m and only subadults and adults at 201–350 m. In the upper 200 m size stratification resulted in an increase in the mean and maximum sizes with depth. The only specimen caught at 851–900 m was a 17 mm postlarva. In late spring only postlarvae were taken in the upper 50 m, and the mean size at that depth was much smaller than at other depths. In late summer only postlarvae were caught at 101–150 m and only postlarvae and small juveniles below 850 m. This resulted in smaller mean sizes at those depths than at 151–300 m (Table 93).

Development apparently begins in the upper layers, and transformation takes place at relatively great depths. Postlarvae taken in the upper 120 m are 6–11 mm; those taken at 851–1050 m are 19–23 mm. Four juveniles 21–25 mm (two each day and night) were taken at 901–1000 m. Migrations apparently start at about 25 mm, the size of smallest juvenile size taken in the upper 250 m at night (Table 93).

PATCHINESS.—A patchy distribution was indicated at the depth of maximum abundance by day in each season. Juveniles accounted for all or most of the catch at these depths (Table 93). There was no indication of patchiness at night at any season.

NIGHT: DAY CATCH RATIOS.—Night-to-day catch ratios including interpolated values are 0.6:1 in winter, 3.4:1 in late spring, and 0.4:1 in late summer. Juveniles accounted for most of the diel differences in the catch in winter and late summer, and postlarvae for most of the difference in late spring (Table 94).

The greater catches by day in winter and late summer probably were due to the tendency of juveniles to form aggregations, resulting in large catches when such aggregations were encountered by the net.

In late spring the catch of postlarvae at night accounted for nearly all of the difference between day and night catches. The depth at which most postlarvae were taken at night (50 m) was not sampled during the day. Samples made during the day at 27 m (two samples) and 55 m (one sample) failed to take postlarval L. festivus. In any case, abundance was low.