Lanternfish

Oceanodromous. Migrating within oceans typically between spawning and different feeding areas, as tunas do. Migrations should be cyclical and predictable and cover more than 100 km.

Dorsal spines (total): 0; Dorsal soft rays (total): 11 - 13; Analspines: 0; Analsoft rays: 13 - 15

Mesopelagic (Ref. 5951).

High-oceanic, found between 425-850 m during the day and between 40-125 m at night (Ref. 4479).

Lampanyctus pusillus

This abundant species grows to 39 mm in the study area, perhaps to 43 mm elsewhere (Hulley, 1981). Lampanyctus pusillus, a bipolar temperate-semisubtropical species, is a ranking myctophid in the North Atlantic subtropical region (Backus et al., 1977). This is demonstrated in the Ocean Acre collections. The species was very abundant near Bermuda, and was among the four most abundant lanternfishes at all three seasons; it was the most abundant lanternfish in late spring. It is represented in the Ocean Acre collections by a total of 4913 specimens; 2545 were caught during the paired seasonal cruises, 1715 of these in discrete-depth samples, of which 1432 were caught in noncrepuscular tows.

DEVELOPMENTAL STAGES.—Postlarvae were 3–14 mm, juveniles 11–27 mm, subadults 20–34 mm, and adults 24–35 mm. Most juveniles smaller than 18 mm could not be sexed. Adult females contained ova as large as 0.6 mm in diameter. Some of the larger females categorized as subadults may have been postspawning adults with regenerating ovaries. Females may be a little larger than males of the same age; at each season females averaged 0.5 mm larger than males for both subadults and adults. This was reversed in juveniles, males of which averaged 0.5–0.8 mm larger than females at each season. However, females were recognized at a smaller size than males; most 14–18 mm juveniles (69 percent) were females.

REPRODUCTIVE CYCLE AND SEASONAL ABUNDANCE.—Lampanyctus pusillus has a one-year life cycle, with only a few individuals surviving into the second year. Spawning occurred all or most of the year, but mostly in winter and spring. As a result of the peak in recruitment over the winter and spring, abundance was greatest in late summer, when the abundance of recruits alone exceeded the total abundance at either of the other two seasons (Table 100). In winter abundance was lowest due to the minimum of spawning in summer and fall and to continuing mortality. Juveniles and subadults were most abundant in late summer, adults in winter and postlarvae in late spring. The abundance of subadults in winter approached that of late summer. At each season more than 75 percent of the catch was juveniles and subadults (Table 100). Judging from the low abundance of 13–18 mm specimens, there were few recent recruits into the population in winter. Most of the winter juveniles probably were spawned in late summer or early fall. Adults, subadults, and perhaps larger juveniles were spawned during or near the previous reproductive peak and were approaching one year in age, at about which time they would spawn and die.

In late spring recruits from the winter–early spring spawn were taken in great abundance and constituted 90 percent of the catch. The peak in spawning appeared to be past, and most of the winter population had died. This mortality was evident in the reduced abundance of subadults and adults (Table 100), and of all specimens over 20 mm, particularly those 23–30 mm. The combined abundance of subadults and adults in late spring was little more than 10 percent of the total abundance in winter, indicating that some 90 percent of the winter population had died by that time.

About 75 percent of the late summer population consisted of juveniles 18–26 mm and subadults 20–28 mm that were spawned during the winter-early spring peak and that appeared as recruits smaller than about 23 mm in late spring. Newer recruits, juveniles smaller than 18 mm, and postlarvae accounted for an additional 22 percent of the late summer population. The residual 3 percent was mostly subadults larger than 28 mm, some of which were undoubtedly spent adults or adults that would soon ripen, spawn, and die.

SEX RATIOS.—The sexes probably are equally abundant at each season, despite females being consistently more numerous than males with sex ratios of 1.1:1 in winter and late spring and 1.3:1 in late summer. Only the last difference, which was due to 15–17 mm juveniles, for which the female to male ratio was 5.1:1, is statistically significant (Table 101). Excluding the 15–17 mm juveniles from the late summer analysis resulted in a female to male ratio of 1.1:1 for the remaining juveniles and for total numbers; neither difference was statistically significant. The difference for 15–17 mm juveniles may be due to females developing faster and, therefore, being recognized at a smaller size than males.

VERTICAL DISTRIBUTION.—Vertical range by day in winter was 33 m and 551–850 m with maximum abundance at 601–650 m, in late spring 52 m and 551–900 m with a maximum at 601–700 m, and in late summer 501–950 m with a maximum at 601–650 m. Nighttime depth range in winter was 40–250 m and 751–800 m with maximum abundance at 101–150 m, in late spring 50–150 m and 551–1000 m with a maximum at 51–100 m, and in late summer 33–1000 m, with a maximum at 33 m and a secondary concentration at 101–150 m (Table 102).

Stage and size stratification were evident at each season, and adults were stratified by sex in winter. During the day only postlarvae were taken at the shallow extreme and only juveniles and postlarvae at the deep extreme at each season. During daytime in late spring, juveniles had a more extensive range than subadults and adults, being caught both at shallower and deeper depths than the older stages. In late summer juveniles and subadults were taken at greater depths than adults. At both of those seasons juveniles, subadults, and adults had their greatest abundance at the same depths. Although the depth ranges of juveniles, subadults, and adults were similar in winter, a few juveniles were taken slightly deeper than the others. Juveniles and subadults were most abundant at 601–650 m and adults at 701–750 m and 551–600 m. All adults taken at 551–600 m were females and 94 percent of those from 701–750 m were males. This stratification of the sexes was not noted at the other two seasons.

Size stratification by day was apparent only in winter and late spring. In late summer most sizes were caught throughout the vertical range. In winter fish larger than 24 mm were taken at 551–750 m, and those 16–24 mm only at 551–650 m. The small catch at 801–850 m consisted of fish 13–15 mm. In late spring both the maximum and mean sizes increased from 551 to 700 m, fish 14–25 mm being caught throughout that range and larger ones (with a single exception) only at 651–700 m. Fish caught at 751–900 m were all 11–19 mm; those smaller than 14 mm were confined to that stratum (Table 102).

At night in late spring and late summer the range of juveniles encompassed that of the other stages; only juveniles were found at the upper depth limit at both seasons and only juveniles and postlarvae (late spring only) at the lower depth limit. At both seasons the depth range of adults was restricted to one or two 50-m intervals. In winter the situation was reversed, with the range of subadults and adults enveloping that of juveniles. In late summer juveniles were most abundant in the upper 50 m and subadults and adults at 101–150 m. In winter juveniles were most abundant at 51–100 m and the two older stages at 101–150 m. Juveniles, subadults, and adults were each most abundant at 51–100 m in late spring, with postlarvae showing a slight concentration at 101–150 m. In late summer an upper layer of juveniles was isolated from the remainder of the population (as was the case for N. valdiviae); at 33 m only juveniles were taken, at 50–70 m no specimens were taken, at 70–80 m no samples were made, at 90 m about 58 percent of the catch consisted of juveniles, at 110 m 51 percent consisted of juveniles. This situation was not evident at the other two seasons. In winter adults and subadults were stratified by sex; 78 percent of those caught at 95 m were females, and 86 percent of those at 200 m were males. The sexes were approximately equally abundant between those depths. The observed segregation of the sexes may be associated with reproduction, since only subadults and adults were involved, and it was observed only in winter, shortly before the breeding peak.

In terms of size, fish that migrated into the upper 50 m in late spring and late summer all were smaller than 23 mm, and the mean size of the catch at that depth was noticeably smaller than at other depths above the day range at both seasons. All specimens caught at day depths at night in late spring were 10–18 mm, whereas all sizes except for 30–33 mm were taken at day depths at night in late summer. In winter most sizes were taken at 51–100 m, but only fish larger than 20 mm were caught at 101–250 m (Table 102).

Postlarvae were stratified by size day and night in late spring and late summer and probably also in winter, when very few were caught. All those caught in the upper 150 m day or night were 4–8 mm, all those caught below 700 m, except for a 5 mm specimen caught at 901–950 m in late summer, were 8–12 mm, and those at intermediate depths (200–500 m) were 6–7 mm. These observations indicate that initial development occurs in the upper 150 m, and that at a size of about 6–7 mm, postlarvae descend to depths below the daytime level of subadults and adults, where transformation occurs, before which significant diel vertical migrations do not occur.

Diel vertical migrations occurred at each season, but not all individuals migrated to shallower depths at night. Nonmigrants were most abundant in late spring and late summer, at which seasons they accounted for 37 and 22 percent of the catch, respectively. A single postlarva was caught at day depths during the night in winter. Migratory behavior is assumed at a size of about 15–16 mm. At night in late spring and late summer juveniles smaller than 14 m were taken only at day depths; those 15–16 mm were taken both at day depths and in the upper 200 m, but mostly the latter. At night larger fish were taken almost exclusively in the upper 100 m in late spring, and were well dispersed vertically in late summer but more abundant in the upper 300 m than at day depths.

Upward migrations commenced no earlier than 2 hours before sunset in winter, 3.5 hours before sunset in late spring, and 2.5 hours before sunset in late summer. Nighttime depths were reached no later than 2.5 hours after sunset at each season, giving evening migrations of no more than 4.5 hours in winter, 6 hours in late spring, and 5 hours in late summer. Using these estimates, approximate minimum rates of upward migrations between day and night centers of abundance were 110 m/hour in winter (650 m to 150 m), 90 m/hour in late spring (650 m to 100 m), and 120 m/hour in late summer (625 m to 33 m).

Night depths were occupied until less than 2 hours before sunrise in winter and late summer and less than 2.5 hours before sunrise in late spring. Day depths were reached no later than 3 hours after sunrise in winter, 2.5 hours after sunrise in late spring, and 1.7 hours after sunrise in late summer. These estimates of maximum times spent in returning to daytime depths yield approximate minimum rates of migration from depths of maximum abundance at night to those during the day of 100 m/hour in winter (150 m to 650 m), 110 m/hour in late spring (100 m to 650 m), and 170 m/hour in late summer (33 m to 625 m).

In both winter and late summer a few specimens were caught at nocturnal depths near or at the time of sunrise, thus the estimate for times of downward migrations at those seasons may be somewhat too large. Estimates of migration times and rates in late summer may be further in error because of the large proportion of nonmigrants, which included most sizes.

At or near sunset in each season and at or near sunrise in late spring, specimens were taken at several intermediate depths, suggesting that the entire population probably does not migrate as a unit.

PATCHINESS.—Clumping was indicated during the day at 551–600 m and 701–750 m in winter, 551–700 m in late spring, and 601–650 m and 801–850 m in late summer. A patchy distribution was indicated at the depth of greatest abundance of all stages, except postlarvae, at each season. In winter, when adults were stratified according to sex, clumping was indicated at the depths of maximum abundance of both sexes.

Patchiness at night was noted at 101–150 m in winter, 51–100 m and 801–850 m in late spring, and 51–100 m and 751–800 m in late summer. Patchiness occurred at the depth of greatest abundance of adults and subadults in winter, of all stages except postlarvae in late spring, and only of nonmigrant juveniles in late summer.

Significant CD values for day samples at 751–800 m in late spring, for night samples in the upper 100 m in winter, and at 33–100 m and 151–300 m in late summer are thought to be due to distributional features other than clumping. Year to year variation in population density may be the cause of the significant CD for the day samples at 751–800 m in late spring. Although the four samples taken at that depth were equally divided between the two cruises, 13 of the 14 fish were caught during one cruise.

At night in winter the 10 samples made in the upper 50 m caught two specimens, which obviously cannot indicate patchiness. Of the samples made at 51–100 m only those from 95–100 m caught L. pusillus. The catches at 95–100 m appeared to be random, and the CD for samples made only at that depth was not significant. The large CD obtained for samples made in the upper 50 m in late summer may or may not indicate patchiness, but there are too few data to be sure. The only two samples (both at 33 m) differed by a factor of six, but the earlier one, which caught fewer specimens, was made shortly after the twilight period and may have captured early migrants. Of the 15 samples made at 151–300 m, 4 were positive and contained a total of 6 specimens. These were not concentrated at any depth, but were all caught at 0300–0400 hours, which was approaching the morning crepuscular period, and suggests that the fishes caught may have been migrating to day depths.

NIGHT:DAY CATCH RATIOS.—Night-to-day catch ratios, including interpolated values, were 0.2:1 in winter, 0.5:1 in late spring, and 0.4:1 in late summer. Except for postlarvae in late spring and late summer, day catches were greater than night catches for each stage at all three seasons. The greatest deviance from a 1:1 ratio for the three oldest stages occurred in winter (Table 103). The observed ratios seem to be due mainly to diel differences in depth range and clumping, rather than diel differences in net avoidance. Although juveniles were responsible for most of the difference at each season, it was not likely that they were better able to avoid the nets than older, presumably stronger swimming fishes. Lampanyctus pusillus was more dispersed vertically at night than by day in late spring and late summer; this may have accounted for some of the difference between night and day catches at those seasons. Another source for the difference in late spring could have been the limited vertical distribution of samples in the upper 100 m; all were made either at the surface, at 50–56 m, or at 91–100 m. If concentrations of L. pusillus existed between those depths, the estimate of abundance would have been too low.

In winter, although the day and night vertical ranges were similar, the total day catch was about 5 times larger than that of the night, and for juveniles the day catch was more than 15 times larger the night catches. Differences of that magnitude probably were due to not sampling the depth of maximum concentration at night in winter. For example, if L. pusillus was most abundant between 100 and 150 m, a significant proportion of the population was not sampled, as there were no discrete-depth noncrepuscular samples made between the two depths.

At each season clumping was indicated to be more prevalent during daytime than at night. This may have contributed to the observed differences in day and night catches.

Lampanyctus pusillus is a species of lanternfish.

Western Atlantic: Grand Bank to 20°N, from Brazil to Argentina.

High-oceanic, found between 425-850 m during the day and between 40-125 m at night.

nektonic

Known from seamounts and knolls

Epipelagic