Lanternfish

Lampanyctus photonotus

This moderately large lanternfish grows to a size of 70 mm in the study area and to 85 mm elsewhere (Hulley, 1981). A tropical-subtropical species, L. photonotus, is a ranking myctophid in the North Atlantic subtropical region (Backus et al., 1977). It is common but not abundant in the Ocean Acre area, being among the 20 most abundant lanternfishes at each of the three seasons, and among the top 10 in winter (Table 131). The Ocean Acre collections contain 1074 specimens; 469 were caught during the paired seasonal cruises, 351 of these in discrete-depth samples, of which 247 were in noncrepuscular tows (Table 23).

DEVELOPMENTAL STAGES.—Postlarvae were 4–20 mm, juveniles 17–37 mm, subadults 30–62 mm, and adults 44–59 mm. Most of the few juveniles smaller than 20 mm had slender thread-like gonads and could not be sexed. Nearly all juveniles greater than 20 mm had recognizable ovaries or testes. Many of the subadult females larger than 45 mm appeared to be spent but had regenerating ovaries. Adult females contained ova as large as 0.4 mm in diameter, but most eggs were 0.2–0.3 mm. There was no apparent sexual dimorphism in size. Adult males have a larger supracaudal luminous gland than adult females (Nafpaktitis et al., 1977).

REPRODUCTIVE CYCLE AND SEASONAL ABUNDANCE.— Lampanyctus photonotus lives for more than one year and probably has a life span of two years. Adult size is reached at the end of the first year, when fish probably spawn for the first time. Spawning occurs from spring to fall or winter, with a peak in spring or early summer.

Abundance was greatest in winter, intermediate in late summer, and lowest in late spring (Table 96). Subadults and juveniles were most abundant in winter, adults in late spring, and postlarvae in late summer. In late summer most of the population was either smaller than 20 mm or larger than 40 mm. The smaller sizes probably are not adequately retained by the net, and the larger ones probably can avoid the net with some degree of success. As a result, abundance in late summer probably was much larger than estimated from the actual catches.

Adult-size females (larger than 45 mm) were caught throughout the year, but few were ripe. Of the eight ripe females taken during the paired seasonal cruises, seven were caught in late spring. The remaining one was caught in late summer. Small juveniles (17–22 mm) were most numerous in October-November collections. The seasonal distribution of ripe females, postlarvae, and small juveniles taken together indicate a spawning peak in late spring to midsummer.

In late spring the scant catch was mostly subadults and adults, with most fish being at least 37 mm in length. The remainder of the catch consisted of 6–10 mm postlarvae. Although the abundance of adults was not great, it was at its yearly peak (Table 96). The size frequency distribution indicated that 3-year classes were represented in the catch: recently spawned fish 6–10 mm, fish 34–49 mm about one year old, and a third group 54–62 mm at least two years old. The absence of juveniles 17–30 mm suggests that little spawning occurred during winter. The relatively low abundance in late spring may have been, at least in part, due to net avoidance, as most of the catch consisted of fish greater than 40 mm.

In late summer nearly 70 percent of the catch consisted of postlarvae 6–20 mm, which were more abundant than all other stages together in late spring (Table 96). The increase in abundance from late spring to late summer was almost exclusively due to recruitment of fish 10–20 mm. Presumably these individuals were spawned in late spring-early summer during the peak in spawning. The remainder of the catch consisted mostly of 17–20 mm juveniles. As most of the juveniles were caught below 850 m at night, they probably were recently metamorphosed individuals. The low abundance of juveniles may mean that most of the recently spawned fish were still postlarvae not adequately sampled by the 3-m IKMT. Small juveniles were abundant in 2-m IKMT samples in October-November. In late summer subadults were mostly larger than 44 mm and may have been spent adults or ripening fish about one year old. More than 80 percent of L. photonotus taken with the Engel trawl in late summer were greater than 40 mm, with a peak in numbers at 45–56 mm. Several of the females examined were ripe or nearly ripe, indicating that spawning continued into late summer. Fish larger than about 40 mm may not have been adequately represented in the late summer discrete-depth collections due to their ability to avoid the net.

By winter, spawning and recruitment were largely at an end. Subadults 30 mm and larger were predominant, accounting for nearly 70 percent of the catch. Most of the remainder consisted of 17–33 mm juveniles. Fish from the current year's spawn were 17–42 mm, with most of those from the peak being greater than 35 mm. Larger fish (47–59 mm) probably were at least 1.5 years old. The maximum abundance in winter was due to sampling recruits from the entire spawning season. Most of these recruits still were smaller than 40 mm, and presumably could not avoid the net as well as larger fishes.

SEX RATIOS.—Males were more numerous than females at each of the three seasons, with ratios of 1.4:1 in winter, 1.1:1 in late spring, and 1.3:1 in late summer (Table 97). Only the ratio in winter was significantly different from equality. Except for adults in late spring and late summer, males of each stage were more numerous than females of the same stages at each season. Although none of the ratios for individual stages differed significantly from equality, the consistently greater number of males than females for subadults and for juveniles in winter, suggests that males may be more abundant than females.

VERTICAL DISTRIBUTION.—Day depth range in winter was 601–850 m (2 specimens 32 mm and 53 mm taken at 1501–1550 m may have been contaminants from previous tow) with maximum abundance at 701–850 m, in late spring 51–100 m and 701–1100 m with no apparent concentration (3 specimens 47–62 mm taken at 301–350 m are thought to have been contaminants), and in late summer 501–550 m and 801–1050 m with a maximum at 1001–1050 m. Vertical range at night in winter was 40–250 m and 801–850 m with maximum abundance at 101–150 m, in late spring 50–100 m, and in late summer 33–250 m and 851–1000 m with a maximum at 951–1000 m (Table 98).

Stage and size stratification were evident day and night in winter and late summer. In late spring too few specimens were taken to be sure if stratification according to stage or size existed.

During the day in winter juveniles had a shallower upper depth limit and were more abundant at shallower depths (601–750 m) than subadults. In late summer postlarvae were caught between 501 m and 1050 m, and the few subadults taken were from 801–850 m; a postlarva was the only specimen caught at 51–100 m. In winter there was an increase in the mean, minimum, and maximum sizes with depth. In late summer individuals 10–20 mm were caught at 501–1050 m and larger specimens only at 801–850 m (Table 98).

At night in winter juveniles and subadults were caught at both shallower and greater depths than the few adults that were taken. Each of the three stages taken was most abundant at 101–150 m. In late summer only postlarvae and small (less than 20 mm) juveniles were taken in the upper 100 m and at 851–1000 m, and only larger juveniles and subadults were captured at 101–250 m. Each of the stages taken was most abundant at a different depth: postlarvae at 951–1000 m, juveniles at 101–150 m and 901–950 m, and subadults at 151–250 m.

Stratification by size in winter was seen in the increase in mean size with depth in the upper 200 m, where most of the catch was made. Individuals larger than 42 mm did not migrate into the upper 100 m and, except for an 18 mm nonmigrant caught at 801–850 m, all specimens smaller than 25 mm were caught in the upper 100 m. Intermediate sizes (25–42 mm) were taken throughout the upper 250 m and comprised most of the catch at 101–150 m. In late summer specimens caught in the upper 100 m and at 851–1000 m were all smaller than 20 mm, and those taken at 101–250 m were 21–59 mm. There was also a size stratification within the 101–250 m stratum; fish caught at 101–150 m were 21–26 mm and those at 151–250 m were 45–59 mm. The mean size at 151–250 m was more than double that at other depths (Table 98).

Postlarvae appeared to be stratified by size both day and night. The few specimens taken in the upper 100 m were all 6–10 mm and, except for a 7 mm postlarva from 851–900 m, all those caught below 800 m were greater than 10 mm. Because these depth relations hold both day and night, postlarvae probably do not undergo diel vertical migrations. Initial development of postlarvae apparently occurs in the upper 100 m, and at about 10 mm they descend to depths in excess of about 800 m, where they continue to develop and transform into juveniles.

Diel vertical migrations occurred at each of the three seasons, but only in late spring were all night captures made above daytime depths. In winter a single nonmigrant juvenile was caught at 801–850 m. Nonmigrants were most abundant in late summer, when 70 percent of the night abundance was due to nonmigrants. Most nonmigrants were postlarvae, with the remainder juveniles. All were 13–19 mm. Regular migratory behavior was assumed at about 20 mm, as all but one migrant were larger than 20 mm and all nonmigrants smaller than 20 mm.

Little could be determined concerning the chronology of vertical migrations. In winter the upper 150 m was occupied by about sunset and the upper 100 m was still occupied between about 0.5–1.5 hours before sunrise. Daytime depths were reached between 1 and 2 hours after sunrise, giving a downward migration time of about 2.5 hours and a rate of migration of about 260 m/hour between night (101–150 m) and day (701–750 m and 801–850 m) depths of maximum abundance. Little could be determined about the time of upward migration in late summer. The upper 100 m was still occupied between 0.2–0.8 hours before sunrise. Day depths were reached no later than 4.0 hours after sunrise, resulting in a migration time of no more than 4.8 hours and a migration rate of no less than 146 m/hour between the night (101–150 m) and day (801–850 m) depths of maximum abundance.

PATCHINESS.—Lampanyctus photonotus probably does not have a patchy distribution at any time, despite significant CD values noted for the upper 100 m at night in winter and at 51–100 m at night in late spring.

At night in winter L. photonotus apparently does not regularly inhabit depths shallower than about 100 m. Only one sample, taken at 40 m, in the upper 96 m was positive for the species. (The sample immediately following the positive sample was taken during the morning crepuscular period and caught two specimens.) This positive sample was responsible for the significant CD value obtained for the 1–50 m interval. Although it is possible that this is an indication of patchiness, several other interpretations also are possible.

For example, L. photonotus may stray above 50 m only occasionally, it may inhabit the upper 50 m only near dawn, or it may occur regularly with uniform density within a narrow depth stratum near 40 m. No specimens were taken between 40 and 96 m, and all samples near 100 m were positive. The significant CD for 51–100 m was due to the concentration near 100 m and the absence of specimens from 51–96 m.

Year to year variation in the catch near 100 m elevated the CD for 51–100 m both in winter and late spring. In both cases samples from one year were all similar to each other in abundance but differed slightly from those of the other year.

NIGHT:DAY CATCH RATIOS.—Night-to-day catch ratios, including interpolated values, were 1.8:1 in winter, 0.9:1 in late spring, and 1.7:1 in late summer (Table 99). Most of the difference in winter was due to subadults and most of that in late summer to postlarvae.

Increased net avoidance by day in winter may account for much of the observed difference in diel catch rates. The night-to-day catch ratio for 35–43 mm fish was 2.8:1, and all but two fish larger than 46 mm were taken at night-today

Few fish were caught either at night or during the day in late spring, and little can be said concerning the night-to-day ratio. Most specimens were larger than 35 mm, sizes that presumably can avoid the net to at least some degree.

In late summer most of the difference in the diel catch rates was due to fish 17–19 mm. The low abundance of specimens this size in day samples almost certainly reflects a bias other than a day-night difference in net avoidance. The abundance of larger fish was quite low both during daytime (0.9 specimens/hour) and at night (1.4 specimens/hour).