Bolinichthys indicus (Nafpaktitis & Nafpaktitis 1969)

Dorsal spines (total): 0; Dorsal soft rays (total): 12 - 14; Analspines: 3; Analsoft rays: 12 - 14

High-oceanic and mesopelagic (Ref. 4479, 75154), found at 500-900 m by day and at 25-300 m by night (Ref. 4479). There is size stratification with depth both day and night (Ref. 4775).

Bolinichthys indicus

This medium-size lanternfish attains a size of 45 mm in the Ocean Acre area. A bipolar subtropical species, B. indicus is a ranking myctophid in the North Atlantic subtropical region (Backus et al., 1977). It is one of the “very abundant” lanternfishes found in the study area, being the most abundant one in winter and among the twelve most abundant during the other two seasons (Table 131). Bolinichthys indicus was represented in the Ocean Acre collections by 3880 specimens; 2272 were caught during the paired seasonal cruises, 1600 of these in discrete-depth samples, 1169 of the latter in noncrepuscular tows (Table 23).

DEVELOPMENTAL STAGES.—Postlarvae were 4–12 mm, juveniles 10–24 mm, subadults 19–40 mm, and adults 27–41 mm. Most juveniles smaller than 17 mm could not be sexed, and most of those larger than 16 mm had small, but recognizable, ovaries or testes. Some larger females (over 30 mm) categorized as subadults may have been postspawning adults with regenerated ovaries. There was no apparent sexual dimorphism in size for any stage.

REPRODUCTIVE CYCLE AND SEASONAL ABUNDANCE.—Bolinichthys indicus apparently has a one-year life cycle, with only a few individuals surviving many months beyond a year. Spawning occurs from mid-spring to late fall with a peak in late summer. This species was most abundant in winter, when it was the top ranking lanternfish, and juveniles and subadults had their greatest abundance. Postlarvae and adults were most abundant in late summer. In late spring total abundance and abundance of all stages was lowest (Table 30).

Adult-size females were caught throughout the year, but enlarged ovaries with eggs greater than 0.2 mm in diameter were observed only from April through November. At all other times large females (over 29 mm) had relatively small ovaries with eggs mostly less than 0.1 mm in diameter. The seasonal distribution of adults with maturing eggs and the great abundance of small juveniles (over 16 mm) in late summer and winter indicate a protracted breeding season over the spring, summer, and fall. Approximately 95 percent of all postlarvae were caught from July through September, suggesting that a peak in spawning intensity occurs from June through August. Occasional postlarvae were taken in January, June, and November. Because the maximum abundance of juveniles occurred in winter, a peak in the numbers of postlarvae would be expected in November or December, but was not indicated by the very few samples made in those months.

In winter more than 98 percent of the population consisted of juveniles and subadults (Table 30). Subadults were spawned early (spring) in the previous breeding season and probably would spawn in the coming spring. Juveniles mostly were spawned the previous fall and would be expected to breed over the coming summer and fall. By far the greatest abundance of juveniles was at this season, indicating that spawning was at a maximum in summer and fall. The few adults caught were all males (Table 31).

Some females categorized as subadults were of adult size (larger than 33 mm). These individuals probably spawned the previous summer or fall at about one year of age and, perhaps, would spawn again the coming spring. The paucity of large fish in winter indicates that most of the population died soon after spawning at about one year of age.

By late spring the spawning season had arrived, and most subadults and the few adults of the winter population had matured, spawned, and died. Recruits from the spring spawn were not yet large enough to be adequately sampled by the gear used. Accordingly, juvenile abundance was at its minimum at this season (Table 30). Juveniles of the winter population now presumably were mostly subadults, but a few were adults. The latter stages comprised nearly 90 percent of the catch in late spring, but their numbers were very small. This low abundance of subadults and adults is puzzling, especially in light of the fact that the abundance of adults in late summer is greater than the total abundance of all stages is late spring (Table 30).

Almost all (97 percent) fish caught in late summer were either smaller than 20 mm or larger than 29 mm. Juvenile recruits from the spring spawn dominated the catch and, together with postlarvae, accounted for about 70 percent of the catch (Table 30). The group of larger fish was made up of the late-spawning one-year-old adults and a few subadults. Large females (greater than 35 mm) categorized as subadults may have spawned already.

SEX RATIOS.—The sexes probably are equally abundant at all seasons. Female-to-male sex ratios were 1.1:1 in winter 1.2:1 in late spring, and 1.0:1 in late summer. None of these ratios differs significantly from equality. Subadult females were more numerous than subadult males at each season, and adult males were more numerous than adult females in each season. The only statistically significant difference from equality was for subadults in winter (Table 31).

VERTICAL DISTRIBUTION.—Day-depth range in winter was 501–850 m with maximum abundance at 601–650 m, in late spring 601–800 m with a maximum at 651–700 m, and in late summer the upper 50 m and 601–1150 m with maxima at 601–650 m and 801–850 m. At night most specimens were taken in the upper 200 m at all seasons, with maxima at 51–100 m in winter and late spring and 101–150 m in late summer. Some fish were caught deeper than 300 m at night in winter and late summer; most were postlarvae and juveniles (Table 32).

Stage and size stratification were evident in both winter and late summer but not in late spring, when most of the catch was from only one 50-m interval both day and night (Table 32). During the day in winter and in late summer, juveniles were found throughout the vertical range, while the older stages were not taken at the shallowest depths. In late summer only postlarvae were caught in the upper 50 m and only postlarvae and juveniles below 500 m (Table 32). In terms of size, only the smallest specimens were caught in the shallowest depth interval during daytime in winter (all smaller than 15 mm) and late summer (all smaller than 17 mm). During the day in winter there was very little overlap in the sizes caught at 501–650 m and from below 700 m. More than 98 percent of the catch from the shallower depths was smaller than 21 mm, while more than 95 percent from the deeper depths was larger than 22 mm. In late summer, except for a 19 mm juvenile, all fish caught below 900 m were 9–11 mm (Table 32).

At night in winter juveniles dominated in the upper 100 m and subadults dominated at 101–150 m. Adults were not found in the upper 100 m. Migrant juveniles were not taken deeper than 200 m, and the older stages occurred as deep as 350 m. In late summer, except for a single specimen from 301–350 m, postlarvae were found only in the upper 50 m and below 850 m. The remainder of the population was well dispersed vertically, and there was no evidence of stage stratification in the migrant element of the population (Table 32).

Size stratification was not as pronounced at night as by day. In winter all fish larger than 30 mm were caught deeper than 100 m, and in late summer fish caught at 151–300 m had a greater mean size than those from shallower depths. The few specimens caught in the upper 50 m in late summer were postlarvae smaller than 9 mm (Table 32).

Postlarvae were stratified according to size in late summer (and probably at other seasons, but there are few data). The smallest ones (4–8 mm) were taken only at the surface or from the upper 50 m both day and night. Those 9–12 mm were taken almost exclusively at 851–1000 m and did not appear to migrate (Table 32). Initial development of postlarvae obviously occurs in the shallow layer; at a size of about 8 mm they descend to the deeper stratum, where they continue to develop and transform into juveniles before undertaking regular vertical migrations.

Diel vertical migrations occurred at all seasons, but only in late spring were all night captures made above day depths. Presumably the entire population at that season consisted of migrants. About 34 percent of the late summer population and 8 percent of the winter population remained at day depths during the night. Nonmigrants were mostly postlarvae and small juveniles (smaller than 13 mm), but included a few subadults. Regular migratory behavior apparently begins at a size of 11–12 mm; all juveniles larger than 12 mm were migrants, but only a few smaller ones were (Table 32).

Upward migrations apparently started between 2 and 3 hours before sunset in late summer and between 1 and 2 hours before sunset in winter. Nocturnal depths were occupied by one hour after sunset in both seasons, giving upward migration times of about 3.5 hours in late summer and 2.5 hours in winter. These estimates of migration times yield minimum rates of 200 m/hr in winter (600 m to 100 m); and 129 m/hr (600 m to 150 m for smaller fish, mean SL 12.2 mm) and 186 m/hr (800 to 150 m for larger fish, mean SL 23.4 mm) in late summer between diurnal and nocturnal depths of maximum abundance.

Morning downward migrations apparently were somewhat faster than the evening ones. Fish were caught at night depths both in winter and late summer less than an hour before sunrise, and possibly even after sunrise (some positive samples started before sunrise and ended after sunrise). Day depths were reached about 2 hours after sunrise in late summer and about 1.5 hours after sunrise in winter, giving migration times of about 2.5 hours in late summer and about 2 hours in winter. Downward migration between night and day depths of maximum abundance is calculated to occur at minimum rates of about 180 m/hr (for smaller fish, mean SL = 12.2 mm) and 260 m/hr (for larger fish, mean SL = 23.4 mm) in late summer and 250 m/hr in winter.

Captures made at several intermediate depths about the times of sunrise and sunset at both seasons indicate that the population did not migrate as an entity.

Migration times and rates could not be determined for B. indicus in late spring because there were no positive samples at day depths near the times of sunset and sunrise.

PATCHINESS.—Patchiness by day was noted only in late summer at 601–650 m and 801–850 m; depths at which juveniles (shallower), and subadults and adults (deeper), respectively, were most abundant (Table 32).

Patchiness was more prevalent at night. Significant clumping was noted in the upper 100 m and at 151–200 m in winter, 51–100 m in late spring, and 51–100 m and 151–200 m in late summer. These were the depths of maximum abundance of juveniles in winter, of juveniles, subadults, and adults in late spring, and of no stage in late summer. Except for adults in winter, the three older stages were each caught at depths where clumping was indicated.

NIGHT:DAY CATCH RATIOS.—Night-to-day catch ratios, including interpolated values, were 0.2:1 in winter, 7.5:1 in late spring, and 0.4:1 in late summer (Table 33). Except for postlarvae in winter and late summer, ratios for the developmental stages followed the overall seasonal trends. The most divergent ratios for all stages except juveniles occurred in late spring when total abundance, as well as that of each stage, was at a minimum (Table 30).

Significant differences in total day and night depth ranges were not apparent at any of the seasons sampled, eliminating compression as a factor in the differential catch rates. If large aggregations were present in narrow depth strata between those sampled, abundance estimates would be too small. Catch data from oblique samples made at night indicate that this was not the case.

Net avoidance may account partially for the observed discrepancy in late spring. Few juveniles were present at that time, and the population sampled (postlarvae not included) had a larger mean size (27.2 mm) than either that in late summer (18.2 mm) or in winter (17.8 mm). Both the mean and maximum sizes of day captures were much smaller than those of night captures (20.3 vs 27.4 mm and 32 vs 40 mm, respectively), suggesting increased diurnal net avoidance by larger subadults and adults in late spring.

Day and night size ranges were almost identical at the other two seasons, and the mean size (not including postlarvae) for night captures was greater than that for day captures in winter and smaller than that for day captures in late summer. These contradictory data indicate that differential net avoidance probably was not the sole cause of the day-night differences in winter and summer.

USA to about 20°N, and from Uruguay to subtropical convergence

High-oceanic and mesopelagic, found at 500-900 m by day and at 25-300 m by night.

nektonic

Known from seamounts and knolls